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Momentum dependence of ρω mixing in the pion vector form factor and its effect on (g2)μ

  • The inclusion of the ρω mixing effect is essential for a precise description of the pion electromagnetic form factor in the e+eπ+π process, which quantifies the two-pion contribution to the anomalous magnetic moment of muon aμ. In this study, we analyze the momentum dependence of ρω mixing by considering loop contributions at the next-to-leading order in 1/NC expansion within the framework of resonance chiral theory. We revisit a previous study [Y. H. Chen, D. L. Yao, and H. Q. Zheng, Commun. Theor. Phys. 69, 1 (2018)] and consider the contribution arising from the kaon mass splitting in the kaon loops and latest experimental data. We perform two types of fits (with momentum-independent or momentum-dependent ρω mixing amplitude) to describe e+eπ+π and τντ2π data within the energy region of 600900 MeV and decay width of ωπ+π. Furthermore, we compare their results. Our findings indicate that the momentum-independent and momentum-dependent ρω mixing schemes provide appropriate descriptions of the data. However, the momentum-dependent scheme exhibits greater self-consistency, considering the reasonable imaginary part of the mixing matrix element Πρω obtained. Regarding the contribution to the anomalous magnetic moment of the muon, aππμ|[0.6,0.9]GeV, the results obtained from the fits considering the momentum-dependent ρω mixing amplitude are in good agreement with those obtained without incorporating the momentum dependence of ρω mixing, within the margin of errors. Furthermore, based on the fitted values of the relevant parameters, we observe that the decay width of ωπ+πis predominantly influenced by the ρω mixing effect.
  • The anomalous magnetic moment of a muon, denoted as aμ=(gμ2)/2, plays a crucial role in the precision tests of the Standard Model (SM) [1, 2]. The long-standing discrepancy between the SM prediction of aμ and its experimental measurement has recently been updated to 4.2 standard deviations [3, 4], and it has sparked numerous theoretical investigations. The SM uncertainty on aμ is dominated by hadronic vacuum polarization (HVP), with the largest contribution originating from the ππ intermediate states, accounting for over 70% of the HVP contribution. Theoretically, the two-pion low-energy contribution to aμ is expressed as an integral over the modulus squared of the pion electromagnetic form factor, which can be extracted from e+e-annihilation experiments. In principle, the two-pion contribution to aμ can be evaluated accurately as long as the experimental data of e+eπ+πare available everywhere at the required level of precision. Although it is known that a tension exists between the two most precise measurements by BaBar and KLOE Collaborations, the BaBar data lie systematically above the KLOE results in the dominant ρ region. Consequently, considerable efforts have been dedicated to finely describing the pion electromagnetic form factor [511]. In the dominant ρ region of the e+eπ+π process, the isospin-breaking effect due to ρω mixing, which becomes enhanced by the small mass difference between ρ and ω mesons, plays a significant role and should be considered appropriately.

    Usually, the momentum dependence of ρω mixing amplitude is neglected, and a constant mixing amplitude is used to describe e+eπ+πdata due to the narrowness of the ω resonance. The first study on the momentum dependence of ρω mixing amplitude was conducted by Ref. [12]. Based on a quark loop mechanism of ρω mixing, it was determined that the mixing amplitude significantly depends on momentum. Subsequently, the investigation of various loop mechanisms for ρω mixing was initiated in different models, such as the global color model [13], extended Nambu-Jona-Lasinio (NJL) model [14, 15], chiral constituent quark model [16, 17], and hidden local symmetry model [1820]. In our pervious study [21], we examined ρω mixing using a model independent approach through Resonance Chiral Theory (RχT) [22]. Guided by chiral symmetry and large NC expansion, RχT provides us a reliable theoretical framework to study the dynamics with light flavor resonances and pseudo-Goldstone mesons in the intermediate energy region [2328], and it has been successfully applied in the calculation of aμ in the SM [9, 2936]. In Ref. [21], we calculated the one-loop contributions to ρω mixing, which are at the next-to-leading order (NLO) in the 1/NC expansion [28, 3740]. In this study, we update the previous study [21] by incorporating the contribution arising from the kaon mass splitting in the kaon loops.

    Moreover, we focus on analysing the impact of the momentum dependence of ρω mixing on describing the pion vector form factor data and its contribution to aμ. Specifically, we perform two types of fits (with momentum-independent or momentum-dependent ρω mixing amplitude) describing e+eπ+π and and τντ2π data in the energy region of 600–900 MeV, decay width of ωπ+π, and compare their results. The fit results demonstrate that the momentum-independent and momentum-dependent ρω mixing schemes can effectively describe the data, while the momentum-dependent scheme exhibits higher self-consistency due to the reasonable imaginary part of the extracted mixing matrix element Πρω. Regarding the contribution to the anomalous magnetic moment of a muon, aHVP,LOμ[π+π], which is evaluated between 0.6 GeV and 0.9 GeV, the results obtained from fits considering the momentum-dependent ρω mixing amplitude are in good agreement with those from fits that do not include the momentum dependence of ρω mixing, within the margin of errors.

    This paper is organized as follows. In Sec. II, we introduce the description of ρω mixing and elaborate on the calculation of ρω mixing amplitude up to the next-to-leading order in the 1/NC expansion. In Sec. III, the fit results are shown and related phenomenologies are discussed. A summary is provided in Sec. IV.

    In the isospin basis |I,I3, we define the pure isospin states |ρI|1,0 and |ωI|0,0. The mixing between the isospin states of |ρI and |ωI can be implemented by considering the self-energy matrix

    Πμν=Tμν(Πρρ(s)Πρω(s)Πρω(s)Πωω(s)),

    (1)

    with Tμνgμνpμpνp2 and sp2. The none-zero off-diagonal matrix element Πρω(s) contains information on ρω mixing. The mixing between the physical states of ρ0 and ω, is obtainable by introducing the following relation

    (ρ0ω)=C(ρIωI),C=(1ϵ1ϵ21),

    (2)

    where ϵ1 and ϵ2 denote the mixing parameters. The matrix of dressed propagators corresponding to physical states is diagonal [41],

    (1/sρ001/sω)=C(1/sρΠρω/sρsωΠρω/sρsω1/sω)C1,

    (3)

    where abbreviations sρ and sω are defined by the following:

    sρsΠρρ(s)m2ρ,sωsΠωω(s)m2ω.

    (4)

    The information of ρω mixing is encoded in the off-diagonal element of the self-energy matrix, decomposed as follows:

    Πρω(s)=ΔudSρω(s)+4παEρω(s) ,

    (5)

    where Δud=mumd denotes the mass difference between u and d quarks, and α denotes the fine-structure constant. Sρω(s) and Eρω(s) denote the structure functions of the strong and electromagnetic interactions, respectively. In this study, the diagrams in Fig. 1 are calculated in RχT up to NLO in 1/NC expansion.

    Figure 1

    Figure 1.  Feynman diagrams contributing to ρω mixing.

    In RχT, the vector resonances can be described in terms of antisymmetric tensor fields with normalization:

    0|Vμν|V,p=iM1V{pμϵν(p)pνϵμ(p)},

    (6)

    where ϵμ denotes the polarization vector. Here, the vector mesons are collected in a 3×3 matrix as follows:

    Vμν=(12ρ0+12ωρ+K+ρ12ρ0+12ωK0K¯K0ϕ)μν.

    (7)

    The effective Lagrangian for the leading order strong isospin-breaking effect, corresponding to the tree-level contribution diagram (a) in Fig. 1, is as follows [42, 43]:

    Lρω2=λVV6VμνVμνχ+,

    (8)

    with χ+=u+χu++uχ+u and χ=2B0(s+ip). The pseudo-Goldstone bosons originating from the spontaneous breaking of chiral symmetry can be filled nonlinearly into:

    uμ=i(uμuuμu),u=exp(iΦ2F),

    (9)

    with the Goldstone fields

    Φ=(12π0+16ηπ+K+π12π0+16ηK0KˉK026η).

    (10)

    where F denotes the pion decay constant. Considering the mass relations of the vector mesons at O(p2) in terms of the quark counting rule, the value of the coupling constant is determined as follows: λVV6=1/8 [42, 43]. Thus, the tree-level strong contribution can be expressed as follows:

    S(a)ρω=2MV .

    (11)

    The Lagrangian describing the interactions between Vμν and electromagnetic fields or Goldstone bosons are as follows:

    L2(V)=FV22Vμνfμν++iGV2Vμνuμuν ,

    (12)

    with the relevant building blocks defined as follows:

    uμ=i[u+(μ irμ)uu(μilμ)u+],fμν±=uFμνLu+±u+FμνRu.

    (13)

    Here, FμνL,R denote field strength tensors composed of the left and right external sources lμ and rμ, and FV and GV denote real resonance couplings constants. The tree-level electromagnetic contribution from diagram (b) in Fig. 1 can be calculated using the Lagrangian in Eq. (12):

    E(b)ρω=FρFω3.

    (14)

    The physical decay constants Fρ and Fω have been employed in the amplitude, and are differentiated by means of isospin breaking.

    The loop contributions of diagrams (d)–(i) in Fig. 1 have been extensively discussed in our previous study [21]. However, a noteworthy distinction in our current study is the inclusion of the contribution from diagram (c), which arises from the kaon mass splitting within the kaon loops. To ensure comprehensiveness, we present the expressions for the loop contributions in the Appendix A. Furthermore, it should be noted that the ultimate expression for the renormalized mixing amplitude Πρω(p2) is presented in Eq. (A23).

    The mass and width of the ρ meson are conventionally determined by fitting to the experimental data of e+eπ+π and τντ2π [44], where various mechanisms are used to describe ρω mixing effect. To prevent interference due to their ρω mixing mechanisms, we treat mass Mρ and relevant couplings Gρ and Fρ as free parameters in our fit. Regarding its width, the energy-dependent form is constructed in a similar manner as introduced in [45]:

    Γρ(s)=sMρ96πF2[σ3πθ(s4m2π)+12σ3Kθ(s4m2K)],

    (15)

    where σP14m2P/s and θ(s) is the step function.

    With respect to the ω mass, it has been indicated in Refs. [5, 7] that the result determined from e+eπ+π is inconsistent with that from particle data group (PDG) [44], primarily determined by experiments involving e+e3π and e+eπ0γ. Therefore, we treated the ω mass and width as free parameters and estimated them by fitting in our programme. The physical coupling Fω can be determined from the decay width of ωe+e. Using the Lagrangian formula in Eq. (12), the decay width can be derived as follows:

    Γe+eω=4α2πF2ω(2m2e+M2ω)M2ω4m2e27M4ω,

    (16)

    Hence, the expression for Fω can be obtained. Based on the decay widths provided above, sρ and sω in Eq. (4) can be rewritten as

    sρsM2ρ+iMρΓρ(s) ,sωsM2ω+iMωΓω .

    (17)

    The pion form-factor in τντ2π decay, irrelevant to ρω mixing effect, were thoroughly examined in Refs. [36, 4648]:

    Fτπ(s)=(1GρFρsF21sρ)×exp{s96π2F2(Re[A[mπ,Mρ,s]+12A[mK,Mρ,s]])}.

    (18)

    Furthemorme, function

    A(mP,μ,s)=ln(m2P/μ2)+8m2Ps53+σ3Pln(σP+1σP1).

    (19)

    To incorporate isospin-breaking effects, one approach involves multiplying |Fτπ(s)|2 by factor SEWGEM(s), where SEW=1.0233 corresponds to the short distance correction [49]. Additionally, GEM(s) accounts for the long-distance radiative correction, as described in [50]. Specifically, in our fit of τντ2π decay data, we perform the following substitution.

    |Fτπ(s)|2SEWGEM(s)|Fτπ(s)|2.

    (20)

    The pion form-factor in e+e annihilation is as follows:

    Feeπ(s)=[1GρFρsF21sρGρFωs3F21sωΠρω1sρ42aB0Fω(mumd)s3F21sω]×exp{s96π2F2(Re[A[mπ,Mρ,s]+12A[mK,Mρ,s]])}.

    (21)

    As defined in Appendix A.2, parameter a is associated with the combined coupling constant of the direct ωππ interaction. In the first bracket of Eq. (21), the second term corresponds to the contribution from ρππ coupling, third term represents the contribution of ρω mixing, and fourth term corresponds to contribution from the direct isospin-breaking coupling of ω to the pion pair.

    The leading order contribution of ππ(γ) intermediate state to the anomalous magnetic moment of the muon is as follows [51]:

    aππ(γ),LOμ=(αmμ3π)24m2πdsˆK(s)s2Rππ(γ)(s),

    (22)

    where

    Rππ(γ)(s)=3s4πα2σ(0)(e+eππ(γ)) ,

    (23)

    and the kernel function is defined as follows:

    ˆK(s)=3sm2μ[(1+x2)(1+x)2x2(ln(1+x)x+x22)+x22(2x2)+1+x1xx2lnx] ,

    (24)

    with

    x=1βμ(s)1+βμ(s),βμ(s)=14m2μs .

    (25)

    It should be noted that in the formula for aππ(γ),LOμ in Eq. (22), the integration is performed from 4m2π to . In this study, we focus on the momentum dependence of ρω mixing. Therefore, we only describe the pion vector form factor up to 900 MeV. To extend the study by considering higher energies, we must consider the effects of excited resonances, including ρ(1450) and ρ(1700). However, these effects are beyond the scope of this study. It is interesting to note that the 1/s2 enhancement factor in Eq. (22) provides higher weight to the lowest lying resonance ρ(770) that couples strongly to π+π.

    The bare cross section, including final-state radiation, takes the following form [5, 5254]:

    σ(0)(e+eγπ+π(γ))=[1+απη(s)]π|α(s)|23sσ3π(s)|Feeπ(s)|2s+2m2esσe(s),

    (26)

    where

    η(s)=3(1+σ2π(s))2σ2π(s)4logσπ(s)+6log1+σπ(s)2+1+σ2π(s)σπ(s)F(σπ(s))(1σπ(s))(3+3σπ(s)7σ2π(s)+5σ3π(s))4σ3π(s)log1+σπ(s)1σπ(s),F(x)=4Li2(x)+4Li2(x)+2logxlog1+x1x+3Li2(1+x2)3Li2(1x2)+π22,Li2(x)=x0dtlog(1t)t.

    (27)

    The experimental data considered in this study are the pion form factor Feeπ(s) of the e+eπ+π process measured by the OLYA [55], CMD [56], BaBar [57], BESIII [58], KLOE [59], CLEO [60], and SND [61] Collaborations, the form factor Fτπ(s) of τντ2π decay measured by the ALEPH [62] and CLEO [63] Collaborations, and the decay width of ωπ+π [44]. It should be noted that in the experimentally published form factor data Feeπ(s), the vacuum polarization effects have been excluded through the subtraction of the hadronic running of α(s). Thus, in our fitting of the form factor data Feeπ(s), the one-photon-reducible Fig. 1(b) should not be considered. Given that we focus on the analysis of the ρω mixing effect, we only take into account the form factors Feeπ(s) and Fτπ(s) data in the energy region of 600–900 MeV. It should be noted that for the pion form factor Feeπ(s), a tension is observed between the two most precise measurements from BaBar and KLOE in the ρ peak region. However, other measurements align with theirs within the stated uncertainties. This highlights the impact of momentum dependence of ρω mixing, and to avoid the tension between BaBar and KLOE data, we conduct four separate fits. Specifically, in Fits Ia and Ib, we fit all data sets excluding BaBar with momentum-independent Πρω and momentum-dependent Πρω, respectively. In Fits IIa and IIb, we fit all data sets excluding KLOE with momentum-independent Πρω and momentum-dependent Πρω, respectively. Fits Ia and IIa involve eight free parameters: Mρ, Gρ, Fρ, Mω, Γω, a, and the real and imaginary part of constant Πρω. There are nine free parameters in Fits Ib and IIb: Mρ, Gρ, Fρ, Mω, Γω, a, XrW, XrZ, and XrR. As defined in the Appendix A, XrW, XrZ, and XrR are the corresponding parameters for the counterterms.

    In Fig. 2, the fitted results of the fits using momentum-independent Πρω (Fits Ia and IIa) and momentum-dependent Πρω (Fits Ib and IIb) are shown as red dotted lines and black solid lines, respectively. The fitted parameters as well as χ2/d.o.f. are listed in Table 1. It is intriguing to compare the results obtained from fits utilising momentum-independent Πρω and momentum-dependent Πρω for the same datasets. Specifically, we compare Fit Ia and Ib and Fit IIa and IIb. When examining pion form factors |Feeπ(s)|2 and |Fτπ(s)|2, we observe that the differences between the theoretical predictions of the fits using momentum-independent Πρω and the corresponding ones using momentum-dependent Πρω are low. Furthermore, it should be noted that for the pion form factor |Feeπ(s)|2 in Fits Ia and Ib, the theoretical predictions are much higher than the KLOE data at ρ peak, and these deviations contribute significantly to their value of χ2. Thus, we conclude that the momentum-independent Πρω and momentum-dependent Πρω can describe the data well, and the discordances among different collaborations contribute significantly to χ2 values in the fits.

    Figure 2

    Figure 2.  (color online) Fit results of the pion form factor in the e+eπ+π process (left panel) and τντ2π process (right panel) in the energy region of 600–900 MeV. The data of e+e annihilation are considered from OLYA (Gray) [55], CMD (Yellow) [56], BaBar (Blue) [57], BESIII (Green) [58], KLOE (Cyan) [59], CLEO (Magenta) [60], and SND (Orange) [61] Collaborations. The τ decay data are taken from the ALEPH (Orange) [62], and CLEO (Green) [63] collaborations. Fits Ia and Ib fit all data sets excluding BaBar (top), and Fits IIa and IIb fit all data sets excluding KLOE (bottom). Fits Ia and IIa use momentum-independent Πρω and are denoted by the red dashed lines. Fits Ib and IIb use momentum-dependent Πρω and are denoted by the black solid lines. The vertical lines lie at Mρ, Mω, and 2MωMρ (from left to right).

    Table 1

    Table 1.  Fit results of the parameters. Fits Ia and Ib fit all data sets excluding BaBar, and Fits IIa and IIb fit all data sets excluding KLOE. Fits Ia and IIa use momentum-independent Πρω, while Fits Ib and IIb use momentum-dependent Πρω.
    Fit IaFit IbFit IIaFit IIb
    Mρ/MeV775.35±0.10775.68±0.12775.45±0.10775.55±0.11
    Gρ/MeV55.25±0.0955.74±0.0854.21±0.0955.03±0.07
    Fρ/MeV152.65±0.29151.40±0.21155.65±0.23153.38±0.31
    Mω/MeV782.59±0.13782.68±0.12782.39±0.11782.45±0.11
    Γω/MeV8.97±0.279.03±0.268.04±0.168.16±0.17
    a/GeV10.0020±0.01500.0054±0.00100.1066±0.01520.0067±0.0009
    Re(Πρω)/MeV23372±1123799±85
    Im(Πρω)/MeV2296±6694544±704
    XrW/GeV60.141±0.0130.177±0.008
    XrZ/GeV40.195±0.0160.303±0.007
    XrR/GeV20.081±0.0060.133±0.003
    χ2/d.o.f.410.7(2388)=1.79405.8(2389)=1.77392.2(3418)=1.18394.6(3419)=1.19
    aππμ|[0.6,0.9]GeV[×1010]367.72±1.07367.80±2.92375.41±1.03375.29±2.21
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    In the last line of Table 1, we provide the results of aHVP,LOμ[π+π], evaluated between 0.6 GeV and 0.9 GeV. The differences between the results using the momentum-independent Πρω and the results using the momentum-dependent Πρω for the same datasets, namely the differences between Fits Ia and Ib and Fits IIa and IIb, respectively, are negligible.

    In Fig. 3, we plot the real and imaginary parts of the mixing amplitudes Πρω(s) in Fits Ib and IIb. It is determined that the real part is dominant within ρω mixing region. The real part in Fit IIb demonstrates a significant momentum dependence, whereas the real part in Fit Ib displays a smooth momentum dependence. Additionally, it should be noted that the real parts of the two fits nearly reach the same point at s1/2=Mω. In comparison to the real part, the imaginary part is rather small. At s=M2ω, in Fit Ib the mixing amplitude Πρω(M2ω)=(3405.0+62.1i) MeV2, and in Fit IIb Πρω(M2ω)=(3316.3+113.7i) MeV2. The minimal magnitude of the imaginary part aligns with the findings presented in Refs. [64, 65]. However, therein the effect of direct ωIπ+π was not considered. It is worth mentioning that larger imaginary part is obtained in [13, 17] by using global color model and a chiral constituent quark model, respectively. By utilising our fitted parameter results, we proceed to calculate the ratio of the two-pion couplings associated with the isospin-pure ω and ρ.

    Figure 3

    Figure 3.  (color online) Momentum dependence of the mixing amplitudes Πρω(s). The black solid and red dot-dashed lines correspond to the real part of Πρω(s) in Fits Ib and IIb, respectively. The blue dashed and magenta dash-dot-dotted lines correspond to the imaginary part of Πρω(s) in Fits Ib and IIb, respectively. The vertical line lies at Mω.

    G=gωIππgρIππ=42aB0(mumd)Gρ.

    (28)

    The results are G=(2.1±1.1)×103 in Fit Ib, and G=(2.6±1.2)×103 in Fit IIb. It should be noted that the value of G is expected to be of the order α=1/137 in Ref. [65]. The central values of our results of G are in good agreement with the expectation in Ref. [65], while they are lower than other two estimations, namely G=(5.0±1.7model±1.0data)×102 in [66] and G=(3.47±0.64)×102 in [67]. As listed in Table 1, the differences in χ2/d.o.f. between the momentum-dependent fits and momentum-independent fits for the same data sets are minimal. The χ2 of Fit IIa is slightly lower than the χ2 of Fit IIb. However, Fit IIa contains one less fitting parameter than Fit IIb. It can be observed that the magnitude of the imaginary part of Πρω in Fit IIa is significantly greater than those in other three fits. In our framework, the imaginary part of Πρω arises from π0γ and ππ real intermediate states. By considering the decay widths of ωπ0γ and ρπ0γ, the imaginary part of Πρω, contributed from π0γ intermediate state, can be estimated to be approximately −150 MeV2 [7, 65]. If the estimated ratio of the two-pion couplings of the isospin-pure ω and ρ are used: Gα=1/137 [65], then the ππ intermediate state contribution to the imaginary part of Πρω can be obtained in the order of several hundred MeV2. In our momentum-dependent scheme, the imaginary part of Πρω due to π0γ and ππ intermediate states are explicitly computed, and the numerical results of ImΠρω in Fits Ib and IIb are of the order of one hundred MeV2, as expected. However, in the momentum-independent Fits Ia and IIa, the imaginary part of Πρω is a free fitting parameter. As listed in Table 1, the fitted results of ImΠρω and parameter "a" in Fit IIa are unreasonably high. The fitted results of ImΠρω and parameter "a" in Fit Ia exhibit large error bars. Consequently, we conclude that both momentum-independent and momentum-dependent ρω mixing schemes can describe e+eπ+π data well. However, the momentum-dependent ρω mixing scheme is more self-consistent, especially given the reasonable imaginary part of Πρω , which is extracted.

    We wish to emphasize that the direct ωIπ+π coupling is generally an unknown quantity, and it impacts Feeπ(s) in two ways, both through the third term in the first bracket of Eq. (21), appearing as real intermediate state in the contributions to Πρω and through the fourth term in that bracket. Conventionally, the direct ωIπ+π is assumed to be neutralized in e+eπ+π due to the fact that ω and ρ are quasidegenerate and that 2π channel dominates the ρ decay [65]. Theoretical models that do not neglect direct ωIπ+π coupling may be more comprehensive, especially given the availability of high-precision data available currently. It should be noted that in Refs. [5, 7], the pion form factor has been examined in a model-independent way using dispersion theory. Specifically, ρω mixing is subsumed in one parameter ϵω, which should contain a small imaginary part originating from the radiative intermediate states (with an estimated phase of approximately 4 degrees). Furthemrore, given that direct ωIπ+π coupling is not considered in Refs. [5, 7], ϵω term is actually a combination of ρω mixing and direct ωIπ+π. Therefore, it cannot be directly compared to Πρω discussed in this context. (At s=M2ω, our Πρω(M2ω) in Fits Ib and IIb contains negative phase.) Nevertheless, the ratio between the on-ω-mass-shell γωππ transition amplitude and γρππ transition amplitude (without ππ final state interaction) should be model independent. With s=M2ω, the ratio between the second term and first term in Eq. (2.5) of [7] yields Rωρ=Amplitude (γωππ)/Amplitude (γρππ)=(0.178±0.003)×ei(4.66±1.13), using Reϵω=(1.97±0.03)×103 and δϵ=(4.5±1.2) obtained therein. It can be observed that the difference between the phase of Rρω(M2ω) and δϵ is minimal. In this study, the ratio between the sum of the third term and fourth terms and the sum of the first and second terms in the first bracket of Eq. (21) predicts Rωρ=(0.155±0.002)×ei(5.80±1.71) and Rωρ=(0.150±0.002)×ei(3.67±1.71) in Fit Ib and IIb, respectively. It can be observed that our results of Rωρ approximately agree with that in [7].

    Using the central values of the fitted parameters of our best fit (Fit IIb) in Table 1, we calculate the decay width of ωπ+π

    Γωπ+π=1192πF4(M2ω4m2π)32×(exp[M2ω96π2F2(Re[A[mπ,Mρ,M2ω]+12A[mK,Mρ,M2ω]])])2×|82B0(mumd)a+2GρΠρω(M2ω)M2ωM2ρi(MωΓωMρΓρ)|2=0.013(0.29)+(0.22+3.35i)2.

    (29)

    Based on Eq. (29), we can determine that the first term due to direct ωIπ+π is smaller than the second term due to ρω mixing by an order of magnitude. Within 1σ uncertainties, our theoretical value of the branching fraction is B(ωπ+π)=(1.48±0.10)×102, which is consistent with the values provided in PDG [44] and with those reported in the recent dispersive analysis [68].

    Regarding the mass of the ω meson, previous studies [5, 7, 57] indicated that the result extracted from e+eπ+π is substantially lower than the current PDG average [44], which primarily relies on e+e3π and e+eπ0γ experiments. The discrepancy amounts to approximately 1 MeV, corresponding to around 5 σ considering the current precision. It has been observed that the fitted value for Mω and phase of ϵω are strongly correlated [5, 7, 57]. It should be noted that direct ωIπ+π coupling has not been considered in [5, 7, 57]. As indicated in Table 1 above, our fitted results for the mass of ω are in good agreement with the value in PDG: Mω=782.66±0.13 MeV, and this agreement remains unaffected by the inclusion or exclusion of the momentum dependence of Πρω. Furthermore, we can observe that a strong correlation (80%) exists between parameter "a," which quantifies the direct ωIπ+π coupling, and the mass of ω. As mentioned earlier, the direct ωIπ+π coupling influences the imaginary part and real part of the amplitude, and thereby, affects the phase of Rρω(M2ω). It should be noted that the phase of Rρω(M2ω) approximately agrees with the phase of ϵω. Thus, our observations align with with those in Refs. [5, 7, 57]. Hence, a strong correlation exists between the mass of the omega meson and phase of ϵω. Our findings suggest that the inclusion of direct ωIπ+π coupling is likely crucial in the analysis aimed at extracting the ω mass from the e+eπ+π process.

    We utilized the resonance chiral theory to examine ρω mixing. Specifically, we analyzed the impact of the momentum dependence of ρω mixing on describing the pion vector form factor in the e+eπ+π process and its contribution to the anomalous magnetic moment of muon aμ. The incorporation of momentum dependence arises from the calculation of loop contributions, which corresponds to the next-to-leading orders in 1/NC expansion. Based on fitting to the data of e+eπ+π and τντ2π processes within the energy range of 600–900 MeV and decay width of ωπ+π, we determine that theρω mixing amplitude is dominated by its real part, and its imaginary part is relatively small. Although momentum-independent and momentum-dependent ρω mixing schemes yield satisfactory data descriptions, the latter proves to be more self-consistent due to the reasonable imaginary part of the mixing matrix element Πρω. Regarding the contribution to anomalous magnetic moment of muon aππμ|[0.6,0.9]GeV, the results obtained from fits considering the momentum-dependent ρω mixing amplitude align well with those from corresponding fits that exclude the momentum dependence of ρω mixing, within the margin of error. Additionally, we provide the ratio of the isospin-pure ω and ρ two-pion couplings, denoted as G=gωIππ/gρIππ, and observe that ρω mixing plays a crucial role in the decay width of ωπ+π. Furthermore, we ascertain that including direct ωIπ+πcoupling is essential in analyzing the extraction of the mass of the ω meson from the e+eπ+π process.

    We are grateful to Pablo Roig for helpful discussions and valuable suggestions.

    Using ρKˉK and ωKˉK vertexes obtained via the Lagrangian in Eq. (12): iGV/2Vμνuμuν=iGV/F2ρ0μν(μK+νKμK0νˉK0)+iGV/F2ωμν(μK+νK+μK0νˉK0)+..., we can calculate the charged and neutral kaon loops contribution to amplitude

    Πkaon,chargedρω=G2Vp4192F4π2{(16m2K+p2)(λlnm2K+μ2)+538m2K+p2σ3K+ln(σK++1σK+1)},

    and

    Πkaon,neutralρω=G2Vp4192F4π2{(16m2K0p2)(λlnm2K0μ2)+538m2K0p2σ3K0ln(σK0+1σK01)},

    where σP14m2P/p2 and λ1ϵγE+1+ln4π with ϵ=2d2 and γE is the Euler constant.

    The persistence of a non-zero structure function arises from the mass difference between the charged and neutral kaons as described:

    S(c)ρω=1mumd(Πkaon,chargedρω+Πkaon,neutralρω).

    For the isospin-violating vertex of ωIπ+π, we construct the Lagrangian:

    LωIπ+π=a1iVμν{χ+,uμuν}+a2iVμνuμχ+uν=(a112a2)82B0iF2Δudωαβπ+απβ.

    For convenience, we define the combination aa112a2. The ππ-loop contribution to the structure function can be calculated as follows:

    S(d)ρω=2GVB0a12F4π2p4{(16m2πp2)(λlnm2πμ2)+538m2πp2σ3πln(σπ+1σπ1)}.

    According to the Lorentz, P and C invariances, the Lagrangian corresponding to the interaction of ωIρIππ can be constructed as follows:

    LωIρIPP=b1VμνVμν(uαuαχ++χ+uαuα)+b2VμνVμνuαχ+uα+b3Vμνχ+Vμνuαuα+b4VμνuαVμν(χ+uα+uαχ+)+b5VμαVναuμuνχ++VναVμαχ+uνuμ+b6VμαVναuμχ+uν+b7Vμαχ+Vναuμuν+b8VμαVναuνuμχ++VναVμαχ+uμuν+b9VμαVναuνχ+uμ+b10Vμαχ+Vναuνuμ+b11VμαuαVμβuβχ++VμβuαVμαχ+uβ+b12VμαuαVμβχ+uβ+VμβuαVμαuβχ++b13VμαuβVμβuαχ++VμβuβVμαχ+uα+b14VμαuβVμβχ+uα+VμβuβVμαuαχ++g1iVμνVμν(uααχ+αχuα)+g2iVμνuαVμναχ+g3iVμβVμαuβαχ+VμαVμβαχuβ+g4iVμβVμααχuβ+VμαVμβuβαχ+g5iVμβuβVμααχ+VμαuβVμβαχ+λVV6VμνVμνχ+.

    For simplicity, we define the combinations,

    h16b1b2+3b3+b42g1g2,h24b5b6+3b7+4b8b9+3b10+2b11+2b12+2b13+2b142g32g42g5.

    The mass difference between the charged and neutral pions in the internal lines of loops can be disregarded due to its higher-order magnitude beyond our scope of consideration. Consequently, the expanded expression of Lagrangian (A6) can be simplified as follows:

    LωIρIππ=4B0F2h1(mumd)ρIμνωμνπαπα2B0F2λVV6(mumd)ρIμνωμνπ2+4B0F2h2(mumd)ρIμαωναπμπν.

    With the aforementioned Lagrangian, the π-tadpole contribution to ρω mixing can be derived as follows:

    S(e)ρω=m2πB08π2F2{(16λVV6+4h1m2π+h2m2π)×(λlnm2πμ2)+h22m2π}.

    In the loop diagrams (f)-(i), the resonance chiral effective Lagrangian describing vector-photon-pseudoscalar (VJP) and vector-vector-pseudoscalar (VVP) vertices are provided in Ref. [69]:

    LVJP=c1MVϵμνρσ{Vμν,fρα+}αuσ+c2MVϵμνρσ{Vμα,fρσ+}αuν+ic3MVϵμνρσ{Vμν,fρσ+}χ+ic4MVϵμνρσVμν[fρσ,χ+]+c5MVϵμνρσ{αVμν,fρα+}uσ+c6MVϵμνρσ{αVμα,fρσ+}uν+c7MVϵμνρσ{σVμν,fρα+}uα,

    and

    LVVP=d1ϵμνρσ{Vμν,Vρα}αuσ+id2ϵμνρσ{Vμν,Vρσ}χ

    +d3ϵμνρσ{αVμν,Vρα}uσ+d4ϵμνρσ{σVμν,Vρα}uα.

    The couplings involved, or their combinations, can be estimated by matching the leading operator product expansion of VVPGreen function to the same quantity evaluated within RχT. This procedure leads to high energy constraints on the resonance couplings [69]:

    4c3+c1=0,c1c2+c5=0,c5c6=Nc64π2MV2FV,d1+8d2=Nc64π2M2VF2V+F24F2V,d3=Nc64π2M2VF2V+F28F2V.

    Using the the effective vertices stated in Eqs. (A10) and (A11), theπγloop contribution, i.e., the summation of the loops diagrams (f)-(i), can be expressed as

    iΠρωϵρμϵμω=1p2dnk(2π)nik2i(pk)2m2π×[(kp)2ϵμρϵωμk2p2ϵμρϵωμ+p2kϵρkϵω]×{32e23M2VF2[c1(pk)kc2(pk)p4c3m2πc5pk+c6p2]2162FVe23MVF2[1M2ωk2+1M2ρk2]×[d1(pk)2+8d2m2π+2d3pk]×[c1(pk)kc2(pk)p4c3m2πc5pk+c6p2]16F2Ve23F2(M2ρk2)(M2ωk2)×[d1(pk)2+8d2m2π+2d3pk]2}.

    The subsequent calculation is straightforward. However, the result of the extracted electromagnetic structure function EπγρωE(f)ρω+E(g)ρω+E(h)ρω+E(i)ρωis too extensive to present here. It should be noted that in our numerical computation we employ the high energy constraints in Eq. (A12) along with the fitted parameters provided in Ref. [25]. Therefore, all the parameters involved in Eπγρω are known.

    Given that the ω meson predominantly decays into the three-pion state, its two-loop self energy diagram contributes beyond NLO in 1/NC and is not relevant for our current consideration. The self-energy diagrams for ρ meson are depicted in Fig. A1. The Lagrangian required to renormalize ρ meson one-loop self-energy has been provided in Ref. [38],

    Figure A1

    Figure A1.  Feynman diagrams contributing to ρ self-energy.

    L4Y=XY122Vμν{ν,σ}Vμσ+XY24{ν,α}Vμν{σ,α}Vμσ+XY34{σ,α}Vμν{ν,α}Vμσ.

    Specifically, only the combination of couplings XYXY1+XY2+XY3XrY+δXY is relevant for this purpose. Using Lagrangians in Eqs. (12) and (A14), ρ self-energy takes the form:

    Σρ(p2)=G2V48F4π2p4{(16m2πp2)(λlnm2πμ2)8m2πp2σ3πln(σπ+1σπ1)+(16m2Kp2)(λlnm2Kμ2)8m2Kp2σ3Kln(σK+1σK1)+103}XYp4.

    The renormalized ρ mass fulfills:

    M2ρ=M2V+Σρ(M2ρ).

    Given that physical Mρ is finite, the following holds:

    δXY=G2V48F4M2ρπ2(16m2πM2ρ6m2KM2ρ)λ.

    The wave-function renormalization constant of ρ meson is obtained from:

    Zρ=1+Σρ(p2)p2|p2=M2ρ.

    In our calculation of ρω mixing, the tree amplitudes can only absorb the ultraviolet divergence that is proportional to p0. To neutralize O(p2), O(p4), and O(p6) ultraviolet divergence originating from loop contribution S(c)ρω, S(d)ρω, S(e)ρω, and Eπγρω, we construct the counterterms as follows:

    Lct=YAVμνVμνχ+12YBλVλμνVνμχ++YC122Vμν{χ+,{ν,σ}Vμσ}+YC24{ν,α}Vμν{χ+,{σ,α}Vμσ}+YC34{σ,α}Vμν{χ+,{ν,α}Vμσ}+ZAFV22Vμνfμν++ZBFV22Vμν2fμν++ZCFV22Vμν4fμν++ZDFV22Vμν6fμν+.

    We adopt ¯MS1 subtraction scheme and absorb the divergent pieces proportional to λby the bare couplings in the counterterms. Consequently, the remanent finite pieces of counterterms can be expressed as

    Πctρω=XrWp6+XrZp4+XrRp2 ,

    with

    XrW8παFρFω3(ZrD+ZrBZrC) ,XrZ4παFρFω3(2ZrC+ZrB2)+16Mρ(mumd)(YrC1+YrC2+YrC3) ,XrR8παFρFω3ZrB4Mρ(mumd)YrB .

    In summary, at the NLO in 1/NC, the UV-renormalized mixing amplitude is as follows:

    Πrρω(p2)=S(a)ρωZρ+ˉS(c)ρω+ˉS(d)ρω+ˉS(e)ρω+¯Eπγρω(p2)+XrWp6+XrZp4+XrRp2,

    where a bar denotes that the divergences are subtracted.

    As discussed in Ref. [41], the mixing amplitude should vanish as p20. Thus, the final expression of the renormalized mixing amplitude is obtained as follows:

    Πρω(p2)=Πrρω(p2)Πrρω(0) ,

    where an additional finite shift is imposed to guarantee that constraint Πρω(0)=0 is satisfied. It should be noted that due to the finite shift performed in Eq. (A23), our numerical calculation is actually independent of the coupling XrY. In our numerical computation, scale μ will be set to Mρ , and we use (mumd)=2.49 MeV provided by PDG [44].

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Yun-Hua Chen and Meng-Ge Qin. Momentum dependence of ρω mixing in the pion vector form factor and its effect on (g2)μ[J]. Chinese Physics C. doi: 10.1088/1674-1137/ace820
Yun-Hua Chen and Meng-Ge Qin. Momentum dependence of ρω mixing in the pion vector form factor and its effect on (g2)μ[J]. Chinese Physics C.  doi: 10.1088/1674-1137/ace820 shu
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Momentum dependence of ρω mixing in the pion vector form factor and its effect on (g2)μ

  • School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China

Abstract: The inclusion of the ρω mixing effect is essential for a precise description of the pion electromagnetic form factor in the e+eπ+π process, which quantifies the two-pion contribution to the anomalous magnetic moment of muon aμ. In this study, we analyze the momentum dependence of ρω mixing by considering loop contributions at the next-to-leading order in 1/NC expansion within the framework of resonance chiral theory. We revisit a previous study [Y. H. Chen, D. L. Yao, and H. Q. Zheng, Commun. Theor. Phys. 69, 1 (2018)] and consider the contribution arising from the kaon mass splitting in the kaon loops and latest experimental data. We perform two types of fits (with momentum-independent or momentum-dependent ρω mixing amplitude) to describe e+eπ+π and τντ2π data within the energy region of 600900 MeV and decay width of ωπ+π. Furthermore, we compare their results. Our findings indicate that the momentum-independent and momentum-dependent ρω mixing schemes provide appropriate descriptions of the data. However, the momentum-dependent scheme exhibits greater self-consistency, considering the reasonable imaginary part of the mixing matrix element Πρω obtained. Regarding the contribution to the anomalous magnetic moment of the muon, aππμ|[0.6,0.9]GeV, the results obtained from the fits considering the momentum-dependent ρω mixing amplitude are in good agreement with those obtained without incorporating the momentum dependence of ρω mixing, within the margin of errors. Furthermore, based on the fitted values of the relevant parameters, we observe that the decay width of ωπ+πis predominantly influenced by the ρω mixing effect.

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    I.   INTRODUCTION
    • The anomalous magnetic moment of a muon, denoted as aμ=(gμ2)/2, plays a crucial role in the precision tests of the Standard Model (SM) [1, 2]. The long-standing discrepancy between the SM prediction of aμ and its experimental measurement has recently been updated to 4.2 standard deviations [3, 4], and it has sparked numerous theoretical investigations. The SM uncertainty on aμ is dominated by hadronic vacuum polarization (HVP), with the largest contribution originating from the ππ intermediate states, accounting for over 70% of the HVP contribution. Theoretically, the two-pion low-energy contribution to aμ is expressed as an integral over the modulus squared of the pion electromagnetic form factor, which can be extracted from e+e-annihilation experiments. In principle, the two-pion contribution to aμ can be evaluated accurately as long as the experimental data of e+eπ+πare available everywhere at the required level of precision. Although it is known that a tension exists between the two most precise measurements by BaBar and KLOE Collaborations, the BaBar data lie systematically above the KLOE results in the dominant ρ region. Consequently, considerable efforts have been dedicated to finely describing the pion electromagnetic form factor [511]. In the dominant ρ region of the e+eπ+π process, the isospin-breaking effect due to ρω mixing, which becomes enhanced by the small mass difference between ρ and ω mesons, plays a significant role and should be considered appropriately.

      Usually, the momentum dependence of ρω mixing amplitude is neglected, and a constant mixing amplitude is used to describe e+eπ+πdata due to the narrowness of the ω resonance. The first study on the momentum dependence of ρω mixing amplitude was conducted by Ref. [12]. Based on a quark loop mechanism of ρω mixing, it was determined that the mixing amplitude significantly depends on momentum. Subsequently, the investigation of various loop mechanisms for ρω mixing was initiated in different models, such as the global color model [13], extended Nambu-Jona-Lasinio (NJL) model [14, 15], chiral constituent quark model [16, 17], and hidden local symmetry model [1820]. In our pervious study [21], we examined ρω mixing using a model independent approach through Resonance Chiral Theory (RχT) [22]. Guided by chiral symmetry and large NC expansion, RχT provides us a reliable theoretical framework to study the dynamics with light flavor resonances and pseudo-Goldstone mesons in the intermediate energy region [2328], and it has been successfully applied in the calculation of aμ in the SM [9, 2936]. In Ref. [21], we calculated the one-loop contributions to ρω mixing, which are at the next-to-leading order (NLO) in the 1/NC expansion [28, 3740]. In this study, we update the previous study [21] by incorporating the contribution arising from the kaon mass splitting in the kaon loops.

      Moreover, we focus on analysing the impact of the momentum dependence of ρω mixing on describing the pion vector form factor data and its contribution to aμ. Specifically, we perform two types of fits (with momentum-independent or momentum-dependent ρω mixing amplitude) describing e+eπ+π and and τντ2π data in the energy region of 600–900 MeV, decay width of ωπ+π, and compare their results. The fit results demonstrate that the momentum-independent and momentum-dependent ρω mixing schemes can effectively describe the data, while the momentum-dependent scheme exhibits higher self-consistency due to the reasonable imaginary part of the extracted mixing matrix element Πρω. Regarding the contribution to the anomalous magnetic moment of a muon, aHVP,LOμ[π+π], which is evaluated between 0.6 GeV and 0.9 GeV, the results obtained from fits considering the momentum-dependent ρω mixing amplitude are in good agreement with those from fits that do not include the momentum dependence of ρω mixing, within the margin of errors.

      This paper is organized as follows. In Sec. II, we introduce the description of ρω mixing and elaborate on the calculation of ρω mixing amplitude up to the next-to-leading order in the 1/NC expansion. In Sec. III, the fit results are shown and related phenomenologies are discussed. A summary is provided in Sec. IV.

    II.   CALCULATIONS IN RESONANCE CHIRAL THEORY
    • In the isospin basis |I,I3, we define the pure isospin states |ρI|1,0 and |ωI|0,0. The mixing between the isospin states of |ρI and |ωI can be implemented by considering the self-energy matrix

      Πμν=Tμν(Πρρ(s)Πρω(s)Πρω(s)Πωω(s)),

      (1)

      with Tμνgμνpμpνp2 and sp2. The none-zero off-diagonal matrix element Πρω(s) contains information on ρω mixing. The mixing between the physical states of ρ0 and ω, is obtainable by introducing the following relation

      (ρ0ω)=C(ρIωI),C=(1ϵ1ϵ21),

      (2)

      where ϵ1 and ϵ2 denote the mixing parameters. The matrix of dressed propagators corresponding to physical states is diagonal [41],

      (1/sρ001/sω)=C(1/sρΠρω/sρsωΠρω/sρsω1/sω)C1,

      (3)

      where abbreviations sρ and sω are defined by the following:

      sρsΠρρ(s)m2ρ,sωsΠωω(s)m2ω.

      (4)

      The information of ρω mixing is encoded in the off-diagonal element of the self-energy matrix, decomposed as follows:

      Πρω(s)=ΔudSρω(s)+4παEρω(s) ,

      (5)

      where Δud=mumd denotes the mass difference between u and d quarks, and α denotes the fine-structure constant. Sρω(s) and Eρω(s) denote the structure functions of the strong and electromagnetic interactions, respectively. In this study, the diagrams in Fig. 1 are calculated in RχT up to NLO in 1/NC expansion.

      Figure 1.  Feynman diagrams contributing to ρω mixing.

      In RχT, the vector resonances can be described in terms of antisymmetric tensor fields with normalization:

      0|Vμν|V,p=iM1V{pμϵν(p)pνϵμ(p)},

      (6)

      where ϵμ denotes the polarization vector. Here, the vector mesons are collected in a 3×3 matrix as follows:

      Vμν=(12ρ0+12ωρ+K+ρ12ρ0+12ωK0K¯K0ϕ)μν.

      (7)

      The effective Lagrangian for the leading order strong isospin-breaking effect, corresponding to the tree-level contribution diagram (a) in Fig. 1, is as follows [42, 43]:

      Lρω2=λVV6VμνVμνχ+,

      (8)

      with χ+=u+χu++uχ+u and χ=2B0(s+ip). The pseudo-Goldstone bosons originating from the spontaneous breaking of chiral symmetry can be filled nonlinearly into:

      uμ=i(uμuuμu),u=exp(iΦ2F),

      (9)

      with the Goldstone fields

      Φ=(12π0+16ηπ+K+π12π0+16ηK0KˉK026η).

      (10)

      where F denotes the pion decay constant. Considering the mass relations of the vector mesons at O(p2) in terms of the quark counting rule, the value of the coupling constant is determined as follows: λVV6=1/8 [42, 43]. Thus, the tree-level strong contribution can be expressed as follows:

      S(a)ρω=2MV .

      (11)

      The Lagrangian describing the interactions between Vμν and electromagnetic fields or Goldstone bosons are as follows:

      L2(V)=FV22Vμνfμν++iGV2Vμνuμuν ,

      (12)

      with the relevant building blocks defined as follows:

      uμ=i[u+(μ irμ)uu(μilμ)u+],fμν±=uFμνLu+±u+FμνRu.

      (13)

      Here, FμνL,R denote field strength tensors composed of the left and right external sources lμ and rμ, and FV and GV denote real resonance couplings constants. The tree-level electromagnetic contribution from diagram (b) in Fig. 1 can be calculated using the Lagrangian in Eq. (12):

      E(b)ρω=FρFω3.

      (14)

      The physical decay constants Fρ and Fω have been employed in the amplitude, and are differentiated by means of isospin breaking.

      The loop contributions of diagrams (d)–(i) in Fig. 1 have been extensively discussed in our previous study [21]. However, a noteworthy distinction in our current study is the inclusion of the contribution from diagram (c), which arises from the kaon mass splitting within the kaon loops. To ensure comprehensiveness, we present the expressions for the loop contributions in the Appendix A. Furthermore, it should be noted that the ultimate expression for the renormalized mixing amplitude Πρω(p2) is presented in Eq. (A23).

    III.   PHENOMENOLOGICAL DISCUSSION
    • The mass and width of the ρ meson are conventionally determined by fitting to the experimental data of e+eπ+π and τντ2π [44], where various mechanisms are used to describe ρω mixing effect. To prevent interference due to their ρω mixing mechanisms, we treat mass Mρ and relevant couplings Gρ and Fρ as free parameters in our fit. Regarding its width, the energy-dependent form is constructed in a similar manner as introduced in [45]:

      Γρ(s)=sMρ96πF2[σ3πθ(s4m2π)+12σ3Kθ(s4m2K)],

      (15)

      where σP14m2P/s and θ(s) is the step function.

      With respect to the ω mass, it has been indicated in Refs. [5, 7] that the result determined from e+eπ+π is inconsistent with that from particle data group (PDG) [44], primarily determined by experiments involving e+e3π and e+eπ0γ. Therefore, we treated the ω mass and width as free parameters and estimated them by fitting in our programme. The physical coupling Fω can be determined from the decay width of ωe+e. Using the Lagrangian formula in Eq. (12), the decay width can be derived as follows:

      Γe+eω=4α2πF2ω(2m2e+M2ω)M2ω4m2e27M4ω,

      (16)

      Hence, the expression for Fω can be obtained. Based on the decay widths provided above, sρ and sω in Eq. (4) can be rewritten as

      sρsM2ρ+iMρΓρ(s) ,sωsM2ω+iMωΓω .

      (17)

      The pion form-factor in τντ2π decay, irrelevant to ρω mixing effect, were thoroughly examined in Refs. [36, 4648]:

      Fτπ(s)=(1GρFρsF21sρ)×exp{s96π2F2(Re[A[mπ,Mρ,s]+12A[mK,Mρ,s]])}.

      (18)

      Furthemorme, function

      A(mP,μ,s)=ln(m2P/μ2)+8m2Ps53+σ3Pln(σP+1σP1).

      (19)

      To incorporate isospin-breaking effects, one approach involves multiplying |Fτπ(s)|2 by factor SEWGEM(s), where SEW=1.0233 corresponds to the short distance correction [49]. Additionally, GEM(s) accounts for the long-distance radiative correction, as described in [50]. Specifically, in our fit of τντ2π decay data, we perform the following substitution.

      |Fτπ(s)|2SEWGEM(s)|Fτπ(s)|2.

      (20)

      The pion form-factor in e+e annihilation is as follows:

      Feeπ(s)=[1GρFρsF21sρGρFωs3F21sωΠρω1sρ42aB0Fω(mumd)s3F21sω]×exp{s96π2F2(Re[A[mπ,Mρ,s]+12A[mK,Mρ,s]])}.

      (21)

      As defined in Appendix A.2, parameter a is associated with the combined coupling constant of the direct ωππ interaction. In the first bracket of Eq. (21), the second term corresponds to the contribution from ρππ coupling, third term represents the contribution of ρω mixing, and fourth term corresponds to contribution from the direct isospin-breaking coupling of ω to the pion pair.

      The leading order contribution of ππ(γ) intermediate state to the anomalous magnetic moment of the muon is as follows [51]:

      aππ(γ),LOμ=(αmμ3π)24m2πdsˆK(s)s2Rππ(γ)(s),

      (22)

      where

      Rππ(γ)(s)=3s4πα2σ(0)(e+eππ(γ)) ,

      (23)

      and the kernel function is defined as follows:

      ˆK(s)=3sm2μ[(1+x2)(1+x)2x2(ln(1+x)x+x22)+x22(2x2)+1+x1xx2lnx] ,

      (24)

      with

      x=1βμ(s)1+βμ(s),βμ(s)=14m2μs .

      (25)

      It should be noted that in the formula for aππ(γ),LOμ in Eq. (22), the integration is performed from 4m2π to . In this study, we focus on the momentum dependence of ρω mixing. Therefore, we only describe the pion vector form factor up to 900 MeV. To extend the study by considering higher energies, we must consider the effects of excited resonances, including ρ(1450) and ρ(1700). However, these effects are beyond the scope of this study. It is interesting to note that the 1/s2 enhancement factor in Eq. (22) provides higher weight to the lowest lying resonance ρ(770) that couples strongly to π+π.

      The bare cross section, including final-state radiation, takes the following form [5, 5254]:

      σ(0)(e+eγπ+π(γ))=[1+απη(s)]π|α(s)|23sσ3π(s)|Feeπ(s)|2s+2m2esσe(s),

      (26)

      where

      η(s)=3(1+σ2π(s))2σ2π(s)4logσπ(s)+6log1+σπ(s)2+1+σ2π(s)σπ(s)F(σπ(s))(1σπ(s))(3+3σπ(s)7σ2π(s)+5σ3π(s))4σ3π(s)log1+σπ(s)1σπ(s),F(x)=4Li2(x)+4Li2(x)+2logxlog1+x1x+3Li2(1+x2)3Li2(1x2)+π22,Li2(x)=x0dtlog(1t)t.

      (27)

      The experimental data considered in this study are the pion form factor Feeπ(s) of the e+eπ+π process measured by the OLYA [55], CMD [56], BaBar [57], BESIII [58], KLOE [59], CLEO [60], and SND [61] Collaborations, the form factor Fτπ(s) of τντ2π decay measured by the ALEPH [62] and CLEO [63] Collaborations, and the decay width of ωπ+π [44]. It should be noted that in the experimentally published form factor data Feeπ(s), the vacuum polarization effects have been excluded through the subtraction of the hadronic running of α(s). Thus, in our fitting of the form factor data Feeπ(s), the one-photon-reducible Fig. 1(b) should not be considered. Given that we focus on the analysis of the ρω mixing effect, we only take into account the form factors Feeπ(s) and Fτπ(s) data in the energy region of 600–900 MeV. It should be noted that for the pion form factor Feeπ(s), a tension is observed between the two most precise measurements from BaBar and KLOE in the ρ peak region. However, other measurements align with theirs within the stated uncertainties. This highlights the impact of momentum dependence of ρω mixing, and to avoid the tension between BaBar and KLOE data, we conduct four separate fits. Specifically, in Fits Ia and Ib, we fit all data sets excluding BaBar with momentum-independent Πρω and momentum-dependent Πρω, respectively. In Fits IIa and IIb, we fit all data sets excluding KLOE with momentum-independent Πρω and momentum-dependent Πρω, respectively. Fits Ia and IIa involve eight free parameters: Mρ, Gρ, Fρ, Mω, Γω, a, and the real and imaginary part of constant Πρω. There are nine free parameters in Fits Ib and IIb: Mρ, Gρ, Fρ, Mω, Γω, a, XrW, XrZ, and XrR. As defined in the Appendix A, XrW, XrZ, and XrR are the corresponding parameters for the counterterms.

      In Fig. 2, the fitted results of the fits using momentum-independent Πρω (Fits Ia and IIa) and momentum-dependent Πρω (Fits Ib and IIb) are shown as red dotted lines and black solid lines, respectively. The fitted parameters as well as χ2/d.o.f. are listed in Table 1. It is intriguing to compare the results obtained from fits utilising momentum-independent Πρω and momentum-dependent Πρω for the same datasets. Specifically, we compare Fit Ia and Ib and Fit IIa and IIb. When examining pion form factors |Feeπ(s)|2 and |Fτπ(s)|2, we observe that the differences between the theoretical predictions of the fits using momentum-independent Πρω and the corresponding ones using momentum-dependent Πρω are low. Furthermore, it should be noted that for the pion form factor |Feeπ(s)|2 in Fits Ia and Ib, the theoretical predictions are much higher than the KLOE data at ρ peak, and these deviations contribute significantly to their value of χ2. Thus, we conclude that the momentum-independent Πρω and momentum-dependent Πρω can describe the data well, and the discordances among different collaborations contribute significantly to χ2 values in the fits.

      Figure 2.  (color online) Fit results of the pion form factor in the e+eπ+π process (left panel) and τντ2π process (right panel) in the energy region of 600–900 MeV. The data of e+e annihilation are considered from OLYA (Gray) [55], CMD (Yellow) [56], BaBar (Blue) [57], BESIII (Green) [58], KLOE (Cyan) [59], CLEO (Magenta) [60], and SND (Orange) [61] Collaborations. The τ decay data are taken from the ALEPH (Orange) [62], and CLEO (Green) [63] collaborations. Fits Ia and Ib fit all data sets excluding BaBar (top), and Fits IIa and IIb fit all data sets excluding KLOE (bottom). Fits Ia and IIa use momentum-independent Πρω and are denoted by the red dashed lines. Fits Ib and IIb use momentum-dependent Πρω and are denoted by the black solid lines. The vertical lines lie at Mρ, Mω, and 2MωMρ (from left to right).

      Fit IaFit IbFit IIaFit IIb
      Mρ/MeV775.35±0.10775.68±0.12775.45±0.10775.55±0.11
      Gρ/MeV55.25±0.0955.74±0.0854.21±0.0955.03±0.07
      Fρ/MeV152.65±0.29151.40±0.21155.65±0.23153.38±0.31
      Mω/MeV782.59±0.13782.68±0.12782.39±0.11782.45±0.11
      Γω/MeV8.97±0.279.03±0.268.04±0.168.16±0.17
      a/GeV10.0020±0.01500.0054±0.00100.1066±0.01520.0067±0.0009
      Re(Πρω)/MeV23372±1123799±85
      Im(Πρω)/MeV2296±6694544±704
      XrW/GeV60.141±0.0130.177±0.008
      XrZ/GeV40.195±0.0160.303±0.007
      XrR/GeV20.081±0.0060.133±0.003
      χ2/d.o.f.410.7(2388)=1.79405.8(2389)=1.77392.2(3418)=1.18394.6(3419)=1.19
      aππμ|[0.6,0.9]GeV[×1010]367.72±1.07367.80±2.92375.41±1.03375.29±2.21

      Table 1.  Fit results of the parameters. Fits Ia and Ib fit all data sets excluding BaBar, and Fits IIa and IIb fit all data sets excluding KLOE. Fits Ia and IIa use momentum-independent Πρω, while Fits Ib and IIb use momentum-dependent Πρω.

      In the last line of Table 1, we provide the results of aHVP,LOμ[π+π], evaluated between 0.6 GeV and 0.9 GeV. The differences between the results using the momentum-independent Πρω and the results using the momentum-dependent Πρω for the same datasets, namely the differences between Fits Ia and Ib and Fits IIa and IIb, respectively, are negligible.

      In Fig. 3, we plot the real and imaginary parts of the mixing amplitudes Πρω(s) in Fits Ib and IIb. It is determined that the real part is dominant within ρω mixing region. The real part in Fit IIb demonstrates a significant momentum dependence, whereas the real part in Fit Ib displays a smooth momentum dependence. Additionally, it should be noted that the real parts of the two fits nearly reach the same point at s1/2=Mω. In comparison to the real part, the imaginary part is rather small. At s=M2ω, in Fit Ib the mixing amplitude Πρω(M2ω)=(3405.0+62.1i) MeV2, and in Fit IIb Πρω(M2ω)=(3316.3+113.7i) MeV2. The minimal magnitude of the imaginary part aligns with the findings presented in Refs. [64, 65]. However, therein the effect of direct ωIπ+π was not considered. It is worth mentioning that larger imaginary part is obtained in [13, 17] by using global color model and a chiral constituent quark model, respectively. By utilising our fitted parameter results, we proceed to calculate the ratio of the two-pion couplings associated with the isospin-pure ω and ρ.

      Figure 3.  (color online) Momentum dependence of the mixing amplitudes Πρω(s). The black solid and red dot-dashed lines correspond to the real part of Πρω(s) in Fits Ib and IIb, respectively. The blue dashed and magenta dash-dot-dotted lines correspond to the imaginary part of Πρω(s) in Fits Ib and IIb, respectively. The vertical line lies at Mω.

      G=gωIππgρIππ=42aB0(mumd)Gρ.

      (28)

      The results are G=(2.1±1.1)×103 in Fit Ib, and G=(2.6±1.2)×103 in Fit IIb. It should be noted that the value of G is expected to be of the order α=1/137 in Ref. [65]. The central values of our results of G are in good agreement with the expectation in Ref. [65], while they are lower than other two estimations, namely G=(5.0±1.7model±1.0data)×102 in [66] and G=(3.47±0.64)×102 in [67]. As listed in Table 1, the differences in χ2/d.o.f. between the momentum-dependent fits and momentum-independent fits for the same data sets are minimal. The χ2 of Fit IIa is slightly lower than the χ2 of Fit IIb. However, Fit IIa contains one less fitting parameter than Fit IIb. It can be observed that the magnitude of the imaginary part of Πρω in Fit IIa is significantly greater than those in other three fits. In our framework, the imaginary part of Πρω arises from π0γ and ππ real intermediate states. By considering the decay widths of ωπ0γ and ρπ0γ, the imaginary part of Πρω, contributed from π0γ intermediate state, can be estimated to be approximately −150 MeV2 [7, 65]. If the estimated ratio of the two-pion couplings of the isospin-pure ω and ρ are used: Gα=1/137 [65], then the ππ intermediate state contribution to the imaginary part of Πρω can be obtained in the order of several hundred MeV2. In our momentum-dependent scheme, the imaginary part of Πρω due to π0γ and ππ intermediate states are explicitly computed, and the numerical results of ImΠρω in Fits Ib and IIb are of the order of one hundred MeV2, as expected. However, in the momentum-independent Fits Ia and IIa, the imaginary part of Πρω is a free fitting parameter. As listed in Table 1, the fitted results of ImΠρω and parameter "a" in Fit IIa are unreasonably high. The fitted results of ImΠρω and parameter "a" in Fit Ia exhibit large error bars. Consequently, we conclude that both momentum-independent and momentum-dependent ρω mixing schemes can describe e+eπ+π data well. However, the momentum-dependent ρω mixing scheme is more self-consistent, especially given the reasonable imaginary part of Πρω , which is extracted.

      We wish to emphasize that the direct ωIπ+π coupling is generally an unknown quantity, and it impacts Feeπ(s) in two ways, both through the third term in the first bracket of Eq. (21), appearing as real intermediate state in the contributions to Πρω and through the fourth term in that bracket. Conventionally, the direct ωIπ+π is assumed to be neutralized in e+eπ+π due to the fact that ω and ρ are quasidegenerate and that 2π channel dominates the ρ decay [65]. Theoretical models that do not neglect direct ωIπ+π coupling may be more comprehensive, especially given the availability of high-precision data available currently. It should be noted that in Refs. [5, 7], the pion form factor has been examined in a model-independent way using dispersion theory. Specifically, ρω mixing is subsumed in one parameter ϵω, which should contain a small imaginary part originating from the radiative intermediate states (with an estimated phase of approximately 4 degrees). Furthemrore, given that direct ωIπ+π coupling is not considered in Refs. [5, 7], ϵω term is actually a combination of ρω mixing and direct ωIπ+π. Therefore, it cannot be directly compared to Πρω discussed in this context. (At s=M2ω, our Πρω(M2ω) in Fits Ib and IIb contains negative phase.) Nevertheless, the ratio between the on-ω-mass-shell γωππ transition amplitude and γρππ transition amplitude (without ππ final state interaction) should be model independent. With s=M2ω, the ratio between the second term and first term in Eq. (2.5) of [7] yields Rωρ=Amplitude (γωππ)/Amplitude (γρππ)=(0.178±0.003)×ei(4.66±1.13), using Reϵω=(1.97±0.03)×103 and δϵ=(4.5±1.2) obtained therein. It can be observed that the difference between the phase of Rρω(M2ω) and δϵ is minimal. In this study, the ratio between the sum of the third term and fourth terms and the sum of the first and second terms in the first bracket of Eq. (21) predicts Rωρ=(0.155±0.002)×ei(5.80±1.71) and Rωρ=(0.150±0.002)×ei(3.67±1.71) in Fit Ib and IIb, respectively. It can be observed that our results of Rωρ approximately agree with that in [7].

      Using the central values of the fitted parameters of our best fit (Fit IIb) in Table 1, we calculate the decay width of ωπ+π

      Γωπ+π=1192πF4(M2ω4m2π)32×(exp[M2ω96π2F2(Re[A[mπ,Mρ,M2ω]+12A[mK,Mρ,M2ω]])])2×|82B0(mumd)a+2GρΠρω(M2ω)M2ωM2ρi(MωΓωMρΓρ)|2=0.013(0.29)+(0.22+3.35i)2.

      (29)

      Based on Eq. (29), we can determine that the first term due to direct ωIπ+π is smaller than the second term due to ρω mixing by an order of magnitude. Within 1σ uncertainties, our theoretical value of the branching fraction is B(ωπ+π)=(1.48±0.10)×102, which is consistent with the values provided in PDG [44] and with those reported in the recent dispersive analysis [68].

      Regarding the mass of the ω meson, previous studies [5, 7, 57] indicated that the result extracted from e+eπ+π is substantially lower than the current PDG average [44], which primarily relies on e+e3π and e+eπ0γ experiments. The discrepancy amounts to approximately 1 MeV, corresponding to around 5 σ considering the current precision. It has been observed that the fitted value for Mω and phase of ϵω are strongly correlated [5, 7, 57]. It should be noted that direct ωIπ+π coupling has not been considered in [5, 7, 57]. As indicated in Table 1 above, our fitted results for the mass of ω are in good agreement with the value in PDG: Mω=782.66±0.13 MeV, and this agreement remains unaffected by the inclusion or exclusion of the momentum dependence of Πρω. Furthermore, we can observe that a strong correlation (80%) exists between parameter "a," which quantifies the direct ωIπ+π coupling, and the mass of ω. As mentioned earlier, the direct ωIπ+π coupling influences the imaginary part and real part of the amplitude, and thereby, affects the phase of Rρω(M2ω). It should be noted that the phase of Rρω(M2ω) approximately agrees with the phase of ϵω. Thus, our observations align with with those in Refs. [5, 7, 57]. Hence, a strong correlation exists between the mass of the omega meson and phase of ϵω. Our findings suggest that the inclusion of direct ωIπ+π coupling is likely crucial in the analysis aimed at extracting the ω mass from the e+eπ+π process.

    IV.   CONCLUSIONS
    • We utilized the resonance chiral theory to examine ρω mixing. Specifically, we analyzed the impact of the momentum dependence of ρω mixing on describing the pion vector form factor in the e+eπ+π process and its contribution to the anomalous magnetic moment of muon aμ. The incorporation of momentum dependence arises from the calculation of loop contributions, which corresponds to the next-to-leading orders in 1/NC expansion. Based on fitting to the data of e+eπ+π and τντ2π processes within the energy range of 600–900 MeV and decay width of ωπ+π, we determine that theρω mixing amplitude is dominated by its real part, and its imaginary part is relatively small. Although momentum-independent and momentum-dependent ρω mixing schemes yield satisfactory data descriptions, the latter proves to be more self-consistent due to the reasonable imaginary part of the mixing matrix element Πρω. Regarding the contribution to anomalous magnetic moment of muon aππμ|[0.6,0.9]GeV, the results obtained from fits considering the momentum-dependent ρω mixing amplitude align well with those from corresponding fits that exclude the momentum dependence of ρω mixing, within the margin of error. Additionally, we provide the ratio of the isospin-pure ω and ρ two-pion couplings, denoted as G=gωIππ/gρIππ, and observe that ρω mixing plays a crucial role in the decay width of ωπ+π. Furthermore, we ascertain that including direct ωIπ+πcoupling is essential in analyzing the extraction of the mass of the ω meson from the e+eπ+π process.

    ACKNOWLEDGMENTS
    • We are grateful to Pablo Roig for helpful discussions and valuable suggestions.

    APPENDIX A: LOOP CONTRIBUTIONS

      1.   Diagram (c): kaon loops

    • Using ρKˉK and ωKˉK vertexes obtained via the Lagrangian in Eq. (12): iGV/2Vμνuμuν=iGV/F2ρ0μν(μK+νKμK0νˉK0)+iGV/F2ωμν(μK+νK+μK0νˉK0)+..., we can calculate the charged and neutral kaon loops contribution to amplitude

      Πkaon,chargedρω=G2Vp4192F4π2{(16m2K+p2)(λlnm2K+μ2)+538m2K+p2σ3K+ln(σK++1σK+1)},

      and

      Πkaon,neutralρω=G2Vp4192F4π2{(16m2K0p2)(λlnm2K0μ2)+538m2K0p2σ3K0ln(σK0+1σK01)},

      where σP14m2P/p2 and λ1ϵγE+1+ln4π with ϵ=2d2 and γE is the Euler constant.

      The persistence of a non-zero structure function arises from the mass difference between the charged and neutral kaons as described:

      S(c)ρω=1mumd(Πkaon,chargedρω+Πkaon,neutralρω).

    • 2.   Diagram (d): ππ loop

    • For the isospin-violating vertex of ωIπ+π, we construct the Lagrangian:

      LωIπ+π=a1iVμν{χ+,uμuν}+a2iVμνuμχ+uν=(a112a2)82B0iF2Δudωαβπ+απβ.

      For convenience, we define the combination aa112a2. The ππ-loop contribution to the structure function can be calculated as follows:

      S(d)ρω=2GVB0a12F4π2p4{(16m2πp2)(λlnm2πμ2)+538m2πp2σ3πln(σπ+1σπ1)}.

    • 3.   Diagram (e): π-tadpole loop

    • According to the Lorentz, P and C invariances, the Lagrangian corresponding to the interaction of ωIρIππ can be constructed as follows:

      LωIρIPP=b1VμνVμν(uαuαχ++χ+uαuα)+b2VμνVμνuαχ+uα+b3Vμνχ+Vμνuαuα+b4VμνuαVμν(χ+uα+uαχ+)+b5VμαVναuμuνχ++VναVμαχ+uνuμ+b6VμαVναuμχ+uν+b7Vμαχ+Vναuμuν+b8VμαVναuνuμχ++VναVμαχ+uμuν+b9VμαVναuνχ+uμ+b10Vμαχ+Vναuνuμ+b11VμαuαVμβuβχ++VμβuαVμαχ+uβ+b12VμαuαVμβχ+uβ+VμβuαVμαuβχ++b13VμαuβVμβuαχ++VμβuβVμαχ+uα+b14VμαuβVμβχ+uα+VμβuβVμαuαχ++g1iVμνVμν(uααχ+αχuα)+g2iVμνuαVμναχ+g3iVμβVμαuβαχ+VμαVμβαχuβ+g4iVμβVμααχuβ+VμαVμβuβαχ+g5iVμβuβVμααχ+VμαuβVμβαχ+λVV6VμνVμνχ+.

      For simplicity, we define the combinations,

      h16b1b2+3b3+b42g1g2,h24b5b6+3b7+4b8b9+3b10+2b11+2b12+2b13+2b142g32g42g5.

      The mass difference between the charged and neutral pions in the internal lines of loops can be disregarded due to its higher-order magnitude beyond our scope of consideration. Consequently, the expanded expression of Lagrangian (A6) can be simplified as follows:

      LωIρIππ=4B0F2h1(mumd)ρIμνωμνπαπα2B0F2λVV6(mumd)ρIμνωμνπ2+4B0F2h2(mumd)ρIμαωναπμπν.

      With the aforementioned Lagrangian, the π-tadpole contribution to ρω mixing can be derived as follows:

      S(e)ρω=m2πB08π2F2{(16λVV6+4h1m2π+h2m2π)×(λlnm2πμ2)+h22m2π}.

    • 4.   Diagrams (f)-(i): π0γ loops

    • In the loop diagrams (f)-(i), the resonance chiral effective Lagrangian describing vector-photon-pseudoscalar (VJP) and vector-vector-pseudoscalar (VVP) vertices are provided in Ref. [69]:

      LVJP=c1MVϵμνρσ{Vμν,fρα+}αuσ+c2MVϵμνρσ{Vμα,fρσ+}αuν+ic3MVϵμνρσ{Vμν,fρσ+}χ+ic4MVϵμνρσVμν[fρσ,χ+]+c5MVϵμνρσ{αVμν,fρα+}uσ+c6MVϵμνρσ{αVμα,fρσ+}uν+c7MVϵμνρσ{σVμν,fρα+}uα,

      and

      LVVP=d1ϵμνρσ{Vμν,Vρα}αuσ+id2ϵμνρσ{Vμν,Vρσ}χ

      +d3ϵμνρσ{αVμν,Vρα}uσ+d4ϵμνρσ{σVμν,Vρα}uα.

      The couplings involved, or their combinations, can be estimated by matching the leading operator product expansion of VVPGreen function to the same quantity evaluated within RχT. This procedure leads to high energy constraints on the resonance couplings [69]:

      4c3+c1=0,c1c2+c5=0,c5c6=Nc64π2MV2FV,d1+8d2=Nc64π2M2VF2V+F24F2V,d3=Nc64π2M2VF2V+F28F2V.

      Using the the effective vertices stated in Eqs. (A10) and (A11), theπγloop contribution, i.e., the summation of the loops diagrams (f)-(i), can be expressed as

      iΠρωϵρμϵμω=1p2dnk(2π)nik2i(pk)2m2π×[(kp)2ϵμρϵωμk2p2ϵμρϵωμ+p2kϵρkϵω]×{32e23M2VF2[c1(pk)kc2(pk)p4c3m2πc5pk+c6p2]2162FVe23MVF2[1M2ωk2+1M2ρk2]×[d1(pk)2+8d2m2π+2d3pk]×[c1(pk)kc2(pk)p4c3m2πc5pk+c6p2]16F2Ve23F2(M2ρk2)(M2ωk2)×[d1(pk)2+8d2m2π+2d3pk]2}.

      The subsequent calculation is straightforward. However, the result of the extracted electromagnetic structure function EπγρωE(f)ρω+E(g)ρω+E(h)ρω+E(i)ρωis too extensive to present here. It should be noted that in our numerical computation we employ the high energy constraints in Eq. (A12) along with the fitted parameters provided in Ref. [25]. Therefore, all the parameters involved in Eπγρω are known.

    • 5.   Counterterms and renormalized amplitude

    • Given that the ω meson predominantly decays into the three-pion state, its two-loop self energy diagram contributes beyond NLO in 1/NC and is not relevant for our current consideration. The self-energy diagrams for ρ meson are depicted in Fig. A1. The Lagrangian required to renormalize ρ meson one-loop self-energy has been provided in Ref. [38],

      Figure A1.  Feynman diagrams contributing to ρ self-energy.

      L4Y=XY122Vμν{ν,σ}Vμσ+XY24{ν,α}Vμν{σ,α}Vμσ+XY34{σ,α}Vμν{ν,α}Vμσ.

      Specifically, only the combination of couplings XYXY1+XY2+XY3XrY+δXY is relevant for this purpose. Using Lagrangians in Eqs. (12) and (A14), ρ self-energy takes the form:

      Σρ(p2)=G2V48F4π2p4{(16m2πp2)(λlnm2πμ2)8m2πp2σ3πln(σπ+1σπ1)+(16m2Kp2)(λlnm2Kμ2)8m2Kp2σ3Kln(σK+1σK1)+103}XYp4.

      The renormalized ρ mass fulfills:

      M2ρ=M2V+Σρ(M2ρ).

      Given that physical Mρ is finite, the following holds:

      δXY=G2V48F4M2ρπ2(16m2πM2ρ6m2KM2ρ)λ.

      The wave-function renormalization constant of ρ meson is obtained from:

      Zρ=1+Σρ(p2)p2|p2=M2ρ.

      In our calculation of ρω mixing, the tree amplitudes can only absorb the ultraviolet divergence that is proportional to p0. To neutralize O(p2), O(p4), and O(p6) ultraviolet divergence originating from loop contribution S(c)ρω, S(d)ρω, S(e)ρω, and Eπγρω, we construct the counterterms as follows:

      Lct=YAVμνVμνχ+12YBλVλμνVνμχ++YC122Vμν{χ+,{ν,σ}Vμσ}+YC24{ν,α}Vμν{χ+,{σ,α}Vμσ}+YC34{σ,α}Vμν{χ+,{ν,α}Vμσ}+ZAFV22Vμνfμν++ZBFV22Vμν2fμν++ZCFV22Vμν4fμν++ZDFV22Vμν6fμν+.

      We adopt ¯MS1 subtraction scheme and absorb the divergent pieces proportional to λby the bare couplings in the counterterms. Consequently, the remanent finite pieces of counterterms can be expressed as

      Πctρω=XrWp6+XrZp4+XrRp2 ,

      with

      XrW8παFρFω3(ZrD+ZrBZrC) ,XrZ4παFρFω3(2ZrC+ZrB2)+16Mρ(mumd)(YrC1+YrC2+YrC3) ,XrR8παFρFω3ZrB4Mρ(mumd)YrB .

      In summary, at the NLO in 1/NC, the UV-renormalized mixing amplitude is as follows:

      Πrρω(p2)=S(a)ρωZρ+ˉS(c)ρω+ˉS(d)ρω+ˉS(e)ρω+¯Eπγρω(p2)+XrWp6+XrZp4+XrRp2,

      where a bar denotes that the divergences are subtracted.

      As discussed in Ref. [41], the mixing amplitude should vanish as p20. Thus, the final expression of the renormalized mixing amplitude is obtained as follows:

      Πρω(p2)=Πrρω(p2)Πrρω(0) ,

      where an additional finite shift is imposed to guarantee that constraint Πρω(0)=0 is satisfied. It should be noted that due to the finite shift performed in Eq. (A23), our numerical calculation is actually independent of the coupling XrY. In our numerical computation, scale μ will be set to Mρ , and we use (mumd)=2.49 MeV provided by PDG [44].

Reference (69)

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