Ground states and first radial excitations of vector tetraquark states with explicit P-waves via QCD sum rules

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Zhi-Gang Wang. The ground states and first radial excitations of the vector tetraquark states with explicit P-waves via the QCD sum rules[J]. Chinese Physics C. doi: 10.1088/1674-1137/ad5ae5
Zhi-Gang Wang. The ground states and first radial excitations of the vector tetraquark states with explicit P-waves via the QCD sum rules[J]. Chinese Physics C.  doi: 10.1088/1674-1137/ad5ae5 shu
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Ground states and first radial excitations of vector tetraquark states with explicit P-waves via QCD sum rules

    Corresponding author: Zhi-Gang Wang, zgwang@aliyun.com
  • Department of Physics, North China Electric Power University, Baoding 071003, China

Abstract: In this study, we chose the diquark-antidiquark type four-quark currents with an explicit P-wave between the diquark and antidiquark pairs to study the ground states and first radial excitations of the hidden-charm tetraquark states with quantum numbers JPC=1. We also obtained the lowest vector tetraquark masses and made possible assignments of the existing Ystates. There indeed exists a hidden-charm tetraquark state with JPC=1 at an energy of approximately 4.75 GeV as the first radial excitation that accounts for the BESIII data.

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    1.   INTRODUCTION
    • A number of Y states, such as Y(4008), have been observed in the J/ψπ+π channel [1]. Y(4220) and Y(4320) have also been observed in the J/ψπ+π channel [2], Y(4230) has been observed in the ωχc0 channel [3], Y(4260) has been observed in the J/ψπ+π channel [4, 5], Y(4320) has been observed in the ψπ+π channel [6], Y(4360) and Y(4660) have been observed in the ψπ+π channel [7, 8], Y(4390) has been observed in the π+πhc channel [9], Y(4469) has been observed in the D0Dπ+ channel [10], Y(4484) has been observed in the K+KJ/ψ channel [11], Y(4544) has been observed in the ωχc1 channel [12], Y(4630) has been observed in the Λ+cΛc channel [13], Y(4710) has been observed in the K+KJ/ψ channel [14], and Y(4790) has been observed in the D+sDs channel [15].

      Recently, the BESIII collaboration studied the processes e+eωX(3872) and γX(3872) using data samples with an integrated luminosity of 4.5fb1 at center-of-mass energies ranging from 4.66 to 4.95 GeV, and observed that the relatively large cross section for the e+eωX(3872) process is mainly attributed to the enhancement around 4.75 GeV, which may indicate a potential structure in the e+eωX(3872) cross section [16]. If the enhancement is confirmed in the future by enough experimental data, another Y state, Y(4750), may exist.

      Even if Y(4220), Y(4230), and Y(4260) are the same particle; Y(4320), Y(4360), and Y(4390) are the same particle; Y(4469), Y(4484), and Y(4544) are the same particle; Y(4500), Y(4630), and Y(4660) are the same particle; and Y(4710) and Y(4750) are the same particle, the Y states are beyond the compatibility of the traditional quark models. Thus, we have to introduce states such as tetraquark, molecular, and hybrid to make reasonable assignments [1726].

      In Refs. [27, 28], we chose the scalar, pseudoscalar, axialvector, vector, and tensor (anti)diquarks as the elementary constituents to construct the four-quark currents without introducing explicit P-waves and investigated the hidden-charm and hidden-charm-hidden-strange tetraquark states with quantum numbers JPC=1 and 1+ in a comprehensive and consistent manner via the QCD sum rules. We also revisited the assignments of the X/Y states in the hidden-charm tetraquark scenario, as presented in Tables 12, where the subscripts S, P, V (˜V), and A (˜A) stand for the scalar, pseudoscalar, vector, and axialvector (anti)diquarks, respectively.

      YcJPCMY/GeVAssignments
      [uc]P[¯dc]A[uc]A[¯dc]P14.66±0.07? Y(4660)
      [uc]P[¯dc]A+[uc]A[¯dc]P1+4.61±0.07
      [uc]S[¯dc]V+[uc]V[¯dc]S14.35±0.08? Y(4360/4390)
      [uc]S[¯dc]V[uc]V[¯dc]S1+4.66±0.09
      [uc]˜V[¯dc]A[uc]A[¯dc]˜V14.53±0.07? Y(4500)
      [uc]˜V[¯dc]A+[uc]A[¯dc]˜V1+4.65±0.08
      [uc]˜A[¯dc]V+[uc]V[¯dc]˜A14.48±0.08? Y(4500)
      [uc]˜A[¯dc]V[uc]V[¯dc]˜A1+4.55±0.07
      [uc]S[¯dc]˜V[uc]˜V[¯dc]S14.50±0.09? Y(4500)
      [uc]S[¯dc]˜V+[uc]˜V[¯dc]S1+4.50±0.09
      [uc]P[¯dc]˜A[uc]˜A[¯dc]P14.60±0.07
      [uc]P[¯dc]˜A+[uc]˜A[¯dc]P1+4.61±0.08
      [uc]A[¯dc]A14.69±0.08? Y(4660)

      Table 1.  Possible assignments of the hidden-charm tetraquark states; the isospin limit is implied [27].

      YcJPCMY/GeVAssignments
      [sc]P[¯sc]A[sc]A[¯sc]P14.80±0.08? Y(4790)
      [sc]P[¯sc]A+[sc]A[¯sc]P1+4.75±0.08
      [sc]S[¯sc]V+[sc]V[¯sc]S14.53±0.08
      [sc]S[¯sc]V[sc]V[¯sc]S1+4.83±0.09
      [sc]˜V[¯sc]A[sc]A[¯sc]˜V14.70±0.08? Y(4710)
      [sc]˜V[¯sc]A+[sc]A[¯sc]˜V1+4.81±0.09
      [sc]˜A[¯sc]V+[sc]V[¯sc]˜A14.65±0.08? Y(4660)
      [sc]˜A[¯sc]V[sc]V[¯sc]˜A1+4.71±0.08
      [sc]S[¯sc]˜V[sc]˜V[¯sc]S14.68±0.09? Y(4660)
      [sc]S[¯sc]˜V+[sc]˜V[¯sc]S1+4.68±0.09?? X(4630)
      [sc]P[¯sc]˜A[sc]˜A[¯sc]P14.75±0.08
      [sc]P[¯sc]˜A+[sc]˜A[¯sc]P1+4.75±0.08
      [sc]A[¯sc]A14.85±0.09

      Table 2.  Possible assignments of the hidden-charm-hidden-strange tetraquark states [28].

      In Refs. [29, 30], we introduced an explicit P-wave between the diquark and antidiquark pairs to construct hidden-charm four-quark currents and explored the vector tetraquark states systematically via the QCD sum rules. We obtained the lowest vector tetraquark masses reported to date and revisited the assignments of the Y states in the hidden-charm tetraquark scenario, as presented in Table 3, where the angular momenta are S=Sqc+Sˉqˉc and J=L+S.

      |Sqc,Sˉqˉc;S,L;J MY/GeV Assignments
      |0,0;0,1;1 4.24±0.10 Y(4220)
      |1,1;0,1;1 4.28±0.10 Y(4220/4320)
      12(|1,0;1,1;1+|0,1;1,1;1) 4.31±0.10 Y(4320/4390)
      |1,1;2,1;1 4.33±0.10 Y(4320/4390)

      Table 3.  Possible assignments of the hidden-charm tetraquark states with explicit P-waves; the isospin limit is implied [30].

      Tables 13 explicitly show that there is no room for Y(4008) and Y(4750) in the vector hidden-charm tetraquark scenario. If we select X(3872) and Zc(3900) as the lowest tetraquark states with JPC=1++ and 1, respectively [3135], Y(4008) cannot be assigned as a vector tetraquark state owing to the small mass splitting δM100 MeV. If Y(4220/ 4230/4260) can be assigned as the ground state vector tetraquark state, the lowest vector hidden-charm tetraquark state can be obtained by the QCD sum rules, and Y(4750) can be assigned as its first radial excitation according to the mass gap MY(4750)MY(4260)= 0.51 GeV, which happens to be our naive expectation of the mass gap between the ground state and first radial excitation.

      If Y(4750) can be assigned as the first radial excitation of Y(4220/4230/4260), there may exist a spectrum for the first radial excited states, which lie at approximately 4.8 GeV. Therefore, it is interesting to explore such a possibility.

      It is known that the heavy-light diquarks εijkqTjCΓQk have five structures, where i, j, and k are color indexes; CΓ=Cγ5, C, Cγμγ5, Cγμ, and Cσμν for the scalar, pseudoscal ar, vector, axialvector, and tensor diquarks, respectively; and the P-wave is implicitly embodied in the negative parity of the diquarks. We can also introduce an explicit P-wave inside the heavy-light diquarks to obtain εijkqTjCΓμQk, where the derivative μ=μμ embodies the explicit P-wave. Then, the diquarks εijkqTjCΓμQk can be used as the basic building blocks to construct the four-quark currents and study the tetraquark states with JPC=1. We will explore such a possibility in our next study.

      In Refs. [2730], we used the (modified) energy scale formula to obtain suitable energy scales of the QCD spectral densities, enhance the pole contributions, and improve the convergent behaviors of the operator product expansion [36]. This is a unique feature of our research. In this direction, we have also explored the hidden-charm tetraquark states with JPC=0++, 0+, 0, 1+, 2++ [37, 38], hidden-bottom tetraquark states with JPC=0++, 1+, 2++ [39], hidden-charm molecular states with JPC=0++, 1+, 2++ [40], doubly-charm tetraquark (molecular) states with JP=0+, 1+, 2+ [41] ([42]), and hidden-charm pentaquark (molecular) states [43]([44]), and we have assigned the existing exotic states consistently.

      In the isospin limit, the vector tetraquark states with the symbol valence quarks, expressed as

      I=1:cˉcuˉd,cˉcuˉudˉd2,cˉcdˉu,I=0:cˉcuˉu+dˉd2,

      (1)

      have degenerated masses. We explore the cˉcuˉd tetraquark states for simplicity. In particular, we update the analysis of our previous studies [29, 30] and extend these studies to systematically explore the first radial excitations of the vector hidden-charm tetraquark states with the QCD sum rules. We use the modified energy scale formula to properly represent suitable energy scales of the QCD spectral densities and make possible assignments of the existing Y states as well as predictions for the mass spectrum of the first radial excitations at an energy of approximately 4.8 GeV.

      The rest of this paper is organized as follows. We derive the QCD sum rules for the vector tetraquark states in Section II. In Section III, we present the numerical results and discussions. Finally, we present our conclusions in Section IV.

    II.   QCD SUM RULES FOR VECTOR TETRAQUARK STATES
    • We first express the two-point correlation functions Πμν(p) and Πμναβ(p)as

      Πμν(p)=id4xeipx0|T{Jμ(x)Jν(0)}|0,

      (2)

      Πμναβ(p)=id4xeipx0|T{Jμν(x)Jαβ(0)}|0,

      (3)

      where Jμ(x)=J1μ(x), J2μ(x) and J3μ(x),

      J1μ(x)=εijkεimn2uTj(x)Cγ5ck(x)μˉdm(x)γ5CˉcTn(x),

      (4)

      J2μ(x)=εijkεimn2uTj(x)Cγαck(x)μˉdm(x)γαCˉcTn(x),

      (5)

      J3μ(x)=εijkεimn2[uTj(x)Cγμck(x)αˉdm(x)γαCˉcTn(x)+uTj(x)Cγαck(x)αˉdm(x)γμCˉcTn(x)],

      (6)

      Jμν(x)=εijkεimn22[uTj(x)Cγ5ck(x)μˉdm(x)γνCˉcTn(x)+uTj(x)Cγνck(x)μˉdm(x)γ5CˉcTn(x)uTj(x)Cγ5ck(x)νˉdm(x)γμCˉcTn(x)uTj(x)Cγμck(x)νˉdm(x)γ5CˉcTn(x)].

      (7)

      Under charge conjugation transform ˆC, the currents Jμ(x) and Jμν(x) have the following properties:

      ˆCJμ(x)ˆC1=Jμ(x),ˆCJμν(x)ˆC1=Jμν(x),

      (8)

      in other words, they have negative conjugation.

      At the hadron side, we insert a complete set of intermediate hadronic states with the same quantum numbers as the interpolating currents Jμ(x) and Jμν(x) into the correlation functions Πμν(p) and Πμναβ(p), respectively, for proper hadronic representation [4547]. Then, we isolate the ground states and obtain the following expressions:

      Πμν(p)=λ2YM2Yp2(gμν+pμpνp2)+,=ΠY(p2)(gμν+pμpνp2)+,

      (9)

      Πμναβ(p)=λ2YM2Y(M2Yp2)(p2gμαgνβp2gμβgναgμαpνpβgνβpμpα+gμβpνpα+gναpμpβ)+λ2ZM2Z(M2Zp2)(gμαpνpβgνβpμpα+gμβpνpα+gναpμpβ)+,=˜ΠY(p2)(p2gμαgνβp2gμβgναgμαpνpβgνβpμpα+gμβpνpα+gναpμpβ)+˜ΠZ(p2)(gμαpνpβgνβpμpα+gμβpνpα+gναpμpβ),

      (10)

      where we apply the definitions of the pole residues λY and λZ,

      0|Jμ(0)|Y(p)=λYεμ,0|Jμν(0)|Y(p)=λYMYεμναβεαpβ,0|Jμν(0)|Z(p)=λZMZ(εμpνενpμ),

      (11)

      where εμ denotes the polarization vectors of the tetraquark states Y and Z with quantum numbers JPC=1 and 1+, respectively. Next, we explicitly project out the components ΠY(p2) and ΠZ(p2) with the projectors PμναβY and PμναβZ,

      ΠY(p2)=p2˜ΠY(p2)=PμναβYΠμναβ(p),ΠZ(p2)=p2˜ΠZ(p2)=PμναβZΠμναβ(p),

      (12)

      where

      PμναβY=16(gμαpμpαp2)(gνβpνpβp2),PμναβZ=16(gμαpμpαp2)(gνβpνpβp2)16gμαgνβ,

      (13)

      and we take the components ΠY(p2) as we explore the hidden-charm tetraquark states with JPC=1.

      At the QCD side, we accomplish the operator product expansion up to the vacuum condensates of dimension 10 and consider the vacuum condensates that are vacuum expectations of the operators of the orders O(αks) with k1 consistently, i.e., we consider ˉqq, αsGGπ, ˉqgsσGq, ˉqq2, ˉqqαsGGπ, ˉqqˉqgsσGq, ˉqgsσGq2, and ˉqq2αsGGπ. The interested readers can consult Refs. [29, 30] for further details.

      Next, we adopt the quark-hadron duality below the continuum thresholds s0 and s0 and apply the Borel transform with respect to P2=p2 to obtain two QCD sum rules:

      λ2Yexp(M2YT2)=s04m2cdsρQCD(s)exp(sT2),

      (14)

      λ2Yexp(M2YT2)+λ2Yexp(M2YT2)=s04m2cdsρQCD(s)exp(sT2),

      (15)

      where ρQCD(s) denotes the QCD spectral densities obtained through dispersion relation, and s0 and s0 correspond to the ground states Y and first radial excitations Y, respectively.

      We adopt the notations τ=1T2, Dn=(ddτ)n and use the subscripts 1 and 2 to denote Y and Y respectively to simplify the expressions. Next, we rewrite the two QCD sum rules in Eqs. (14)−(15) as

      λ21exp(τM21)=ΠQCD(τ),

      (16)

      λ21exp(τM21)+λ22exp(τM22)=ΠQCD(τ),

      (17)

      where ΠQCD(τ) and ΠQCD(τ) represent the correlation functions below the continuum thresholds s0 and s0, respectively. We derive the QCD sum rules in Eq. (16) with respect to τ to obtain the ground states masses,

      M21=DΠQCD(τ)ΠQCD(τ),

      (18)

      then it is straightforward to obtain the ground state masses and pole residues using the two coupled QCD sum rules; see Eqs. (16) and (18) [29, 30].

      Next, we derive the QCD sum rules in Eq. (17) with respect to τ to obtain

      λ21M21exp(τM21)+λ22M22exp(τM22)=DΠQCD(τ).

      (19)

      From Eqs. (17) and (19), we obtain the QCD sum rules,

      λ2iexp(τM2i)=(DM2j)ΠQCD(τ)M2iM2j,

      (20)

      where ij. Then, we derive the QCD sum rules in Eq. (20) with respect to τ to obtain

      M2i=(D2M2jD)ΠQCD(τ)(DM2j)ΠQCD(τ),M4i=(D3M2jD2)ΠQCD(τ)(DM2j)ΠQCD(τ).

      (21)

      The squared masses M2i obey the equation

      M4ibM2i+c=0,

      (22)

      where

      b=D3D0D2DD2D0DD,c=D3DD2D2D2D0DD,DjDk=DjΠQCD(τ)DkΠQCD(τ),

      (23)

      with subscripts i=1,2 and superscripts j,k=0,1,2,3. Finally, we solve the simple resulting equation and obtain two solutions, i.e., the masses of the ground states and first radial excitations [35, 48, 49],

      M21=bb24c2,

      (24)

      M22=b+b24c2.

      (25)

      We can obtain the ground state masses either from the QCD sum rules in Eq. (18) or in Eq. (24). We prefer the QCD sum rules in Eq. (18) because there are larger ground state contributions and less uncertainties from the continuum threshold parameters. We obtain the masses and pole residues of the first radial excitations from the two coupled QCD sum rules in Eqs. (20) and (25).

    III.   NUMERICAL RESULTS AND DISCUSSIONS
    • We adopted the traditional vacuum condensates ˉqq=(0.24±0.01GeV)3, ˉqgsσGq=m20ˉqq, m20=(0.8±0.1)GeV2, and αsGGπ=0.012±0.003GeV4 at the energy scale μ=1GeV [4547, 50]; chose the modified minimum subtracted mass mc(mc)=(1.275±0.025)GeV from the Particle Data Group [51]; and set mu=md=0. Moreover, we considered the energy scale dependence of the input parameters,

      ˉqq(μ)=ˉqq(1GeV)[αs(1GeV)αs(μ)]12332nf,ˉqgsσGq(μ)=ˉqgsσGq(1GeV)[αs(1GeV)αs(μ)]2332nf,mc(μ)=mc(mc)[αs(μ)αs(mc)]12332nf,αs(μ)=1b0t[1b1b20logtt+b21(log2tlogt1)+b0b2b40t2],

      (26)

      where t=logμ2Λ2QCD, b0=332nf12π, b1=15319nf24π2, b2=(285750339nf+32527n2f)/(128π3), and ΛQCD=210MeV, 292MeV, and 332MeV for the flavors nf=5, 4, and 3, respectively [51, 52]. We chose the flavor nf=4 because we explored the tetraquark states consisting of the valence quarks u, d, and c. We evolved all the input parameters to the suitable energy scales μ to extract the masses of the hidden-charm tetraquark states, which satisfy the modified energy scale formula μ=M2X/Y/Z(2Mc+0.5GeV)2=M2X/Y/Z(4.1GeV)2 , where M is the effective charm quark mass [29, 30].

      In the scenario of tetraquark states, we can tentatively assign X(3915)and X(4500) to be the 1S and 2S states with JPC=0++ [53, 54], respectively; Zc(3900)and Zc(4430) to be the 1S and 2S states with JPC=1+, respectively [31, 33, 35]; Zc(4020) and Zc(4600) to be the 1S and 2S states with JPC=1+, respectively [49, 55]; and X(4140) and X(4685) to be the 1S and 2S states with JPC=1++, respectively [56, 57], where the energy gaps between the 1S and 2S states are approximately 0.570.59GeV.

      In Refs. [29, 30], we chose the continuum threshold parameters as s0=MY+0.550.60±0.10GeV for the hidden-charm tetraquark states with JPC=1; the ground state contribution can be as large as (49%−81%). Compared with the usually chosen pole contributions (40%−60%) [3744], the ground state contributions (49%−81%) in the previous analysis were too large in the QCD sum rules for the multiquark states, which may suffer from contaminations from the first radial excitations. In the present calculations, we chose slightly smaller continuum threshold parameters, s0=MY+0.500.55±0.10GeV, to reduce the ground state contributions and performed a consistent and detailed analysis. Furthermore, the continuum threshold parameters s0 were set as s0=MY+0.40±0.10GeV according to the mass-gap of the ψ and ψ from the Particle Data Group [51].

      We searched for the suitable Borel parameters and continuum threshold parameters via trial and error. The pole contributions (PC) and vacuum condensate contributions (D(n)) are defined as

      PC=s0/s04m2cdsρQCD(s)exp(sT2)4m2cdsρQCD(s)exp(sT2),

      (27)

      and

      D(n)=s0/s04m2cdsρQCD,n(s)exp(sT2)s0/s04m2cdsρQCD(s)exp(sT2),

      (28)

      respectively. Finally, we obtained the Borel windows, continuum threshold parameters, and suitable energy scales and pole contributions, which are listed in Tables 45. From the tables, we can see explicitly that the pole contributions of the ground states (ground states plus first radial excited states) are approximately (40%−60%) ((67%−85%)), similar to the values in our previous studies on other tetraquark states [3744]. The pole dominance criterion is properly satisfied. The contributions from the highest dimensional condensates play a minor role, expressed as |D(10)| < 3% or 1% (< 1% or 1%), for the ground states (the ground states plus first radial excited states). The operator product expansion converges better than that in a previous study of ours [30]. Thus, we can confidently extract reliable tetraquark masses and pole residues.

      |Sqc,Sˉqˉc;S,L;J μ/GeV T2/GeV2 s0/GeV pole(%) D(10)
      |0,0;0,1;1 1.1 2.63.0 4.75±0.10 (40 − 65) < 1%
      |1,1;0,1;1 1.2 2.52.9 4.80±0.10 (39 − 64) < 3%
      12(|1,0;1,1;1+|0,1;1,1;1) 1.3 3.03.4 4.85±0.10 (38 − 60) 1%
      |1,1;2,1;1 1.3 2.73.1 4.85±0.10 (39 − 63) < 1%

      Table 4.  Borel windows T2, continuum threshold parameters s0, energy scales of the QCD spectral densities, contributions of the ground states, and values of D(10).

      |Sqc,Sˉqˉc;S,L;J μ/GeV T2/GeV2 s0/GeV pole(%) D(10)
      |0,0;0,1;1 2.4 2.83.2 5.15±0.10 (67 − 85) 1%
      |1,1;0,1;1 2.5 2.63.0 5.20±0.10 (67 − 86) < 1%
      12(|1,0;1,1;1+|0,1;1,1;1) 2.6 3.03.4 5.25±0.10 (67 − 84) 1%
      |1,1;2,1;1 2.6 2.73.1 5.25±0.10 (68 − 87) 1%

      Table 5.  Borel windows T2, continuum threshold parameters s0, energy scales of the QCD spectral densities, contributions of the ground states plus first radial excitations, and values of D(10).

      |Sqc,Sˉqˉc;S,L;J MY/GeV λY/(102GeV6)
      |0,0;0,1;1 4.24±0.09 2.28±0.42
      |1,1;0,1;1 4.28±0.09 4.80±0.95
      12(|1,0;1,1;1+|0,1;1,1;1) 4.31±0.09 2.94±0.50
      |1,1;2,1;1 4.33±0.09 6.55±1.19

      Table 6.  Masses and pole residues of the ground states.

      |Sqc,Sˉqˉc;S,L;J MY/GeV λY/(102GeV6)
      |0,0;0,1;1 4.75±0.10 8.19±1.23
      |1,1;0,1;1 4.81±0.10 18.3±3.0
      12(|1,0;1,1;1+|0,1;1,1;1) 4.85±0.09 8.63±1.22
      |1,1;2,1;1 4.86±0.10 21.7±3.4

      Table 7.  Masses and pole residues of the first radial excited states.

      From Tables 47, we can see explicitly that the modified energy scale formula can be properly satisfied, and the relations s0=MY+0.500.55±0.10GeV and s0=MY+0.40±0.10GeV are held. Thus, our analysis is consistent.

      In Fig. 1, we plot the masses of the ground states and first radial excitations of the hidden-charm tetraquark states with quantum numbers JPC=1. This figure explicitly shows that flat platforms emerge in the Borel windows. Note also that the uncertainties from the Borel parameters are small.

      Figure 1.  (color online) Masses of the vector tetraquark states with variations of the Borel parameters T2, where A, B, C, and D stand for the |0,0;0,1;1, |1,1;0,1;1, 12(|1,0;1,1;1+|0,1;1,1;1), and |1,1;2,1;1 states, respectively; the lower and upper lines represent the ground states and first radial excitations, respectively.

      Table 8 presents the possible assignments of the vector tetraquark states based on the QCD sum rules. This table explicitly shows that there is room to accommodate Y(4750), i.e., Y(4220/4260) and Y(4750) can be assigned as the ground state and first radial excited state of the Cγ5μγ5C type tetraquark states with JPC=1, respectively. We cannot identify a particle unambiguously with the mass alone. In a future study, we shall investigate the decays of those vector tetraquark candidates with the QCD sum rules to verify the assignments.

      |Sqc,Sˉqˉc;S,L;J MY/GeV Assignments
      |0,0;0,1;1 (1P) 4.24±0.09 Y(4220/4260)
      |0,0;0,1;1 (2P) 4.75±0.10 Y(4750)
      |1,1;0,1;1 (1P) 4.28±0.09 Y(4220/4320)
      |1,1;0,1;1 (2P) 4.81±0.10
      12(|1,0;1,1;1+|0,1;1,1;1) (1P) 4.31±0.09 Y(4320/4390)
      12(|1,0;1,1;1+|0,1;1,1;1) (2P) 4.85±0.09
      |1,1;2,1;1 (1P) 4.33±0.09 Y(4320/4390)
      |1,1;2,1;1 (2P) 4.86±0.10

      Table 8.  Masses of the vector tetraquark states and possible assignments; 1P and 2P denote the ground states and first radial excitations, respectively.

    IV.   CONCLUSIONS
    • In the present study, we chose the diquark-antidiquark type four-quark currents with an explicit P-wave between the diquark and antidiquark pairs to investigate the ground states and first radial excitations of the hidden-charm tetraquark states with quantum numbers JPC=1 via the QCD sum rules. First, we considered the ground states at the hadronic side only and updated the previous analysis by refitting the continuum threshold and Borel parameters. Compared with previous calculations, we obtained better convergent behaviors in the operator product expansion on the QCD side and uniform pole contributions (40%−60%) on the hadronic side. Second, we considered both the ground states and first radial excitations and focused on the first radial excitations, obtaining new predictions. In both present and previous calculations, we used the modified energy scale formula μ=M2X/Y/Z(4.1GeV)2 to select suitable energy scales of the QCD spectral densities to improve the convergent behavior of the operator product expansion and enhance the pole contributions. Overall, we explored and obtained the masses and pole residues of the 1P and 2P vector tetraquark states in a systematic and consistent manner. We obtained the lowest vector tetraquark masses, made possible assignments of the existing Y states, and observed that there indeed exists a hidden-charm tetraquark state with JPC=1 at an energy of approximately 4.75GeV that can account for the BESIII data.

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