## 2021 Vol. 45, No. 7

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2021, 45(7): 073001. doi: 10.1088/1674-1137/abfc38
Abstract:
The prediction of reactor antineutrino spectra will play a crucial role as reactor experiments enter the precision era. The positron energy spectrum of 3.5 million antineutrino inverse beta decay reactions observed by the Daya Bay experiment, in combination with the fission rates of fissile isotopes in the reactor, is used to extract the positron energy spectra resulting from the fission of specific isotopes. This information can be used to produce a precise, data-based prediction of the antineutrino energy spectrum in other reactor antineutrino experiments with different fission fractions than Daya Bay. The positron energy spectra are unfolded to obtain the antineutrino energy spectra by removing the contribution from detector response with the Wiener-SVD unfolding method. Consistent results are obtained with other unfolding methods. A technique to construct a data-based prediction of the reactor antineutrino energy spectrum is proposed and investigated. Given the reactor fission fractions, the technique can predict the energy spectrum to a 2% precision. In addition, we illustrate how to perform a rigorous comparison between the unfolded antineutrino spectrum and a theoretical model prediction that avoids the input model bias of the unfolding method.
2021, 45(7): 073101. doi: 10.1088/1674-1137/abf5c9
Abstract:
In this work, we calculate the sub-leading power contributions to radiative leptonic \begin{document}$D\to\gamma \,\ell \,\nu$\end{document} decay. For the first time, we provide the analytic expressions of next-to-leading power contributions and the error estimation associated with the power expansion of \begin{document}${\cal O}(\Lambda_{\rm QCD}/m_c)$\end{document}. In our calculation, we adopt two different models of the D-meson distribution amplitudes \begin{document}$\phi_{D,\rm I}^+$\end{document} and \begin{document}$\phi_{D,\rm II}^+$\end{document}. Within the framework of QCD factorization as well as the dispersion relation, we evaluate the soft contribution up to the next-to-leading logarithmic accuracy and also consider the higher-twist contribution from the two-particle and three-particle distribution amplitudes. Finally, we find that all the sub-leading power contributions are significant at \begin{document}$\lambda_D(\mu_0) = 354$\end{document} MeV, and the next-to-leading power contributions lead to 143% in \begin{document}$\phi_{D,\rm I}^+$\end{document} and 120% in \begin{document}$\phi_{D,\rm II}^+$\end{document} corrections to leading power vector form factors with \begin{document}$E_{\gamma} = 0.5$\end{document} GeV. As the corrections from the higher-twist and local sub-leading power contributions are enhanced with increasing inverse moment, it is difficult to extract an appropriate inverse moment of the D-meson distribution amplitude. The predicted branching fractions are \begin{document}$(1.88_{-0.29}^{+0.36})\times10^{-5}$\end{document} for \begin{document}$\phi_{D,\rm I}^+$\end{document} and \begin{document}$(2.31_{-0.54}^{+0.65})\times10^{-5}$\end{document} for \begin{document}$\phi_{D,\rm II}^+$\end{document}.
2021, 45(7): 073102. doi: 10.1088/1674-1137/abf72e
Abstract:
We continue our endeavor to investigate lepton number violating (LNV) processes at low energies in the framework of effective field theory (EFT). In this work we study the LNV tau decays \begin{document}$\tau^+\rightarrow \ell^-P_i^{+}P_j^{+}$\end{document}, where \begin{document}$\ell = e,\; \mu$\end{document} and \begin{document}$P^+_{i,j}$\end{document} denote the lowest-lying charged pseudoscalars \begin{document}$\pi^+,\; K^+$\end{document}. We analyze the dominant contributions in a series of EFTs from high to low energy scales, namely the standard model EFT (SMEFT), the low-energy EFT (LEFT), and the chiral perturbation theory (\begin{document}$\chi{{\rm{PT}}}$\end{document}). The decay branching ratios are expressed in terms of the Wilson coefficients of dimension-five and -seven operators in SMEFT and the hadronic low-energy constants. These Wilson coefficients involve the first and second generations of quarks and all generations of leptons; thus, they cannot be explored in low-energy processes such as nuclear neutrinoless double beta decay or LNV kaon decays. Unfortunately, the current experimental upper bounds on the branching ratios are too weak to set useful constraints on these coefficients. Alternatively, if we assume the new physics scale is larger than 1 TeV, the branching ratios are well below the current experimental bounds. We also estimate the hadronic uncertainties incurred in applying \begin{document}$\chi{{\rm{PT}}}$\end{document} to \begin{document}$\tau$\end{document} decays by computing one-loop chiral logarithms and attempt to improve the convergence of chiral perturbation by employing dispersion relations in the short-distance part of the decay amplitudes.
2021, 45(7): 073103. doi: 10.1088/1674-1137/abf827
Abstract:
In this paper, we analyze the dependence of the topological charge density from the overlap operator on the Wilson mass parameter in the overlap kernel by the symmetric multi-probing source (SMP) method. We observe that non-trivial topological objects are removed as the Wilson mass is increased. A comparison of topological charge density calculated by the SMP method using the fermionic definition with that of the gluonic definition by the Wilson flow method is shown. A matching procedure for these two methods is used. We find that there is a best match for topological charge density between the gluonic definition with varied Wilson flow time and the fermionic definition with varied Wilson mass. By using the matching procedure, the proper flow time of Wilson flow in the calculation of topological charge density can be estimated. As the lattice spacing a decreases, the proper flow time also decreases, as expected.
2021, 45(7): 073104. doi: 10.1088/1674-1137/abf829
Abstract:
Measuring vector boson scattering (VBS) precisely is an important step toward understanding the electroweak symmetry breaking of and detecting new physics beyond the standard model (SM). Herein, we propose a neural network that compresses the features of the VBS data into a three-dimensional latent space. The consistency of the SM predictions and experimental data is tested via binned log-likelihood analysis in the latent space. We show that the network is capable of distinguishing different polarization modes of WWjj production in both di- and semi-leptonic channels. The method is also applied to constrain the effective field theory and two Higgs Doublet Model. The results demonstrate that the method is sensitive to general new physics contributing to the VBS.
2021, 45(7): 073105. doi: 10.1088/1674-1137/abf8a2
Abstract:
We present a nonextensive version of the Polyakov-Nambu-Jona-Lasinio model that is based on nonextentive statistical mechanics. This new statistics model is characterized by a dimensionless nonextensivity parameter q that accounts for all possible effects violating the assumptions of the Boltzmann-Gibbs (BG) statistics (for \begin{document}$q\rightarrow 1$\end{document}, it returns to the BG case). Based on the nonextensive Polyakov-Nambu-Jona-Lasinio model, we discussed the influence of nonextensive effects on the curvature of the phase diagram at \begin{document}$\mu=0$\end{document} and especially on the location of the critical end point (CEP). A new and interesting phenomenon we found is that with an increase in q, the CEP position initially shifts toward the direction of larger chemical potential and lower temperature. However, when q is larger than a critical value \begin{document}$q_{c}$\end{document}, the CEP position moves in the opposite direction. In other words, as q increases, the CEP position moves in the direction of smaller chemical potential and higher temperature. This U-turn phenomenon may be important for the search of CEP in relativistic heavy-ion collisions, in which the validity of BG statistics is questionable due to strong fluctuations and long-range correlations, and nonextensive effects begin to manifest themselves. In addition, we calculated the influence of the nonextensive effects on the critical exponents and found that they remain almost constant with q.
2021, 45(7): 073106. doi: 10.1088/1674-1137/abf913
Abstract:
The two-photon-exchange (TPE) effect plays a key role in extracting the form factors (FFs) of the proton. In this work, we discuss several exact properties of the TPE effect in the elastic \begin{document}$ep$\end{document} scattering. By taking four low energy interactions as examples, we analyze kinematical singularities, asymptotic behaviors, and branch cuts of TPE amplitudes. The analytical expressions clearly indicate several exact relations between the dispersion relation (DR) and hadronic model (HM) methods. This suggests that the two methods must be modified to general forms, while novel forms yield the same results. After the modification, new DRs include a non-trivial term with two singularities. Furthermore, new DRs automatically include contributions due to the seagull interaction, meson-exchange effect, contact interactions, and off-shell effect. To analyze the elastic \begin{document}$e^{\pm}p$\end{document} scattering data sets, the new forms must be used.
2021, 45(7): 073107. doi: 10.1088/1674-1137/abfa83
Abstract:
In this study, we choose the scalar and axialvector diquark operators in the color antitriplet as the fundamental building blocks to construct four-quark currents and investigate the diquark-antidiquark type axialvector tetraquark states \begin{document}$c\bar{c}u\bar{s}$\end{document} in the framework of the QCD sum rules. The predicted tetraquark mass \begin{document}$M_Z = 3.99\pm0.09\;\rm{GeV}$\end{document} is in excellent agreement with the experimental value \begin{document}$3985.2^{+2.1}_{-2.0}\pm1.7\;\rm{MeV}$\end{document} from the BESIII collaboration, which supports identifying \begin{document}$Z_{cs}(3985)$\end{document} as the cousin of \begin{document}$Z_c(3900)$\end{document} with quantum numbers \begin{document}$J^{PC} = 1^{+-}$\end{document}. We take into account the light flavor \begin{document}$SU(3)$\end{document} mass-breaking effect to estimate the mass spectrum of the diquark-antidiquark type hidden-charm tetraquark states with strangeness according to previous studies.
2021, 45(7): 073108. doi: 10.1088/1674-1137/abfb50
Abstract:
Top partners are well motivated in many new physics models. Usually, vector like quarks, \begin{document}$T_{\rm L,R}$\end{document}, are introduced to circumvent the quantum anomaly. Therefore, it is crucial to probe their interactions with standard model particles. However, flavor changing neutral couplings are always difficult to detect directly in current and future experiments. In this paper, we demonstrate how to constrain the flavor changing neutral Yukawa coupling \begin{document}$Tth$\end{document} indirectly, via the di-Higgs production. We consider the simplified model, including a pair of gauge singlet \begin{document}$T_{\rm L,R}$\end{document}. Under the perturbative unitarity and experimental constraints, we select \begin{document}$m_T=400~{\rm{GeV}},s_{\rm L}=0.2$\end{document}, and \begin{document}$m_T=$\end{document}\begin{document}$800~{\rm{GeV}},s_{\rm L}=0.1$\end{document} as benchmark points. After the analysis on the amplitude and evaluation of the numerical cross sections, we infer that the present constraints from di-Higgs production have already surpassed the unitarity bound because of the \begin{document}$(y_{\rm L,R}^{tT})^4$\end{document} behavior. For the case of \begin{document}$m_T=400~{\rm{GeV}}$\end{document} and \begin{document}$s_{\rm L}=0.2$\end{document}, \begin{document}${\rm{Re}}y_{\rm L,R}^{tT}$\end{document} and \begin{document}${\rm{Im}}y_{\rm L,R}^{tT}$\end{document} can be bounded optimally in the range \begin{document}$(-0.4, 0.4)$\end{document} at the HL-LHC with \begin{document}$2\sigma$\end{document} CL. For the case of \begin{document}$m_T=800~{\rm{GeV}}$\end{document} and \begin{document}$s_{\rm L}=0.1$\end{document}, \begin{document}${\rm{Re}}y_{\rm L,R}^{tT}$\end{document} and \begin{document}${\rm{Im}}y_{L,R}^{tT}$\end{document} can be bounded optimally in the range \begin{document}$(-0.5, 0.5)$\end{document} at the HL-LHC with \begin{document}$2\sigma$\end{document} CL. The anomalous triple Higgs coupling \begin{document}$\delta_{hhh}$\end{document} can also affect the constraints on \begin{document}$y_{\rm L,R}^{tT}$\end{document}. Finally, we determine that the top quark electric dipole moment can provide stronger \begin{document}$y_{\rm L,R}^{tT}$\end{document} bounds in the off-axis regions for some scenarios.
2021, 45(7): 073109. doi: 10.1088/1674-1137/abfb5f
Abstract:
We study the impact of steady, homogeneous, and external parallel electric and magnetic field strengths (\begin{document}$eE\parallel eB$\end{document}) on the chiral symmetry breaking-restoration and confinement-deconfinement phase transition. We also sketch the phase diagram of quantum chromodynamics (QCD) at a finite temperature T and in the presence of background fields. The unified formalism for this study is based on the Schwinger-Dyson equations, symmetry preserving vector-vector contact interaction model of quarks, and an optimal time regularization scheme. At \begin{document}$T = 0$\end{document}, in the purely magnetic case (i.e., \begin{document}$eE\rightarrow 0$\end{document}), we observe the well-known magnetic catalysis effect. However, in a pure electric field background (\begin{document}$eB\rightarrow 0$\end{document}), the electric field tends to restore the chiral symmetry and deconfinement above the pseudo-critical electric field \begin{document}$eE^{\chi, C}_c$\end{document}. In the presence of both \begin{document}$eE$\end{document} and \begin{document}$eB$\end{document}, we determine the magnetic catalysis effect in the particular region where \begin{document}$eB$\end{document} dominates over \begin{document}$eE$\end{document}, whereas we observe the chiral inhibition (or electric chiral rotation) effect when \begin{document}$eE$\end{document} overshadows eB. At finite T, in the pure electric field case, the phenomenon of inverse electric catalysis appears to exist in the proposed model. Conversely, for a pure magnetic field background, we observe the magnetic catalysis effect in the mean-field approximation and inverse magnetic catalysis with \begin{document}$eB$\end{document}-dependent coupling. The combined effects of \begin{document}$eE$\end{document} and \begin{document}$eB$\end{document} on the pseudo-critical \begin{document}$T^{\chi, C}_c$\end{document} yields an inverse electromagnetic catalysis, with and without an \begin{document}$eB$\end{document}-dependent effective coupling of the model. The findings of this study agree well with the already predicted results obtained via lattice simulations and other reliable effective models of QCD.
2021, 45(7): 073110. doi: 10.1088/1674-1137/abfbca
Abstract:
We extend the hotspot model to include the virtuality dependence and use it to study the exclusive and dissociative \begin{document}${J}/{\psi}$\end{document} production combined with the dipole amplitude in the target rapidity representation. We determined that virtuality takes effect on a number of hotspots, thus providing a better description of the \begin{document}${J}/\mathrm{\psi}$\end{document} production data at HERA. The collinear improved Balitsky-Kovchegove equation in the target rapidity representation is numerically solved and used to fit the \begin{document}${J}/\mathrm{\psi}$\end{document} experimental data with a series of hotspot sizes. We infer that virtuality significantly influences the number and size of hotspots. The expression \begin{document}$\chi^2/d.o.f=1.0183$\end{document} resulting from the fit with the collinear improved dipole amplitude in the target rapidity representation is more reasonable than the corresponding \begin{document}$\chi^2/d.o.f=1.3995$\end{document} originating from the leading order fit, which indicates that the collinear improved evolution equation in the target rapidity representation can provide a relatively good depiction of the exclusive and dissociative HERA data.
2021, 45(7): 073111. doi: 10.1088/1674-1137/abfc36
Abstract:
Deep learning has been widely and actively used in various research areas. Recently, in gauge/gravity duality, a new deep learning technique called AdS/DL (Deep Learning) has been proposed. The goal of this paper is to explain the essence of AdS/DL in the simplest possible setups, without resorting to knowledge of gauge/gravity duality. This perspective will be useful for various physics problems: from the emergent spacetime as a neural network to classical mechanics problems. For prototypical examples, we choose simple classical mechanics problems. This method is slightly different from standard deep learning techniques in the sense that we not only have the right final answers but also obtain physical understanding of learning parameters.
2021, 45(7): 073112. doi: 10.1088/1674-1137/abfd28
Abstract:
Last year, the \begin{document}$\Xi(1620)$\end{document} state, which is cataloged in the Particle Data Group (PDG) with only one star, was reported again in the \begin{document}$\Xi^{-}\pi^{+}$\end{document} final state by the Belle Collaboration. Its properties, such as the spectroscopy and decay width, cannot be simply explained in the context of conventional constituent quark models. This inspires an active discussion on the structure of this resonance. In this paper, we study the radiative decays of the newly observed \begin{document}$\Xi(1620)$\end{document} assuming that it is a meson-baryon molecular state of \begin{document}$\Lambda\bar{K}$\end{document} and \begin{document}$\Sigma\bar{K}$\end{document} with spin-parity \begin{document}$J^P = 1/2^{-}$\end{document} developed in our previous study. The partial decay widths of the \begin{document}$\Lambda\bar{K}-\Sigma\bar{K}$\end{document} molecular state into \begin{document}$\Xi\gamma$\end{document} and \begin{document}$\Xi\pi\gamma$\end{document} final states through hadronic loops are evaluated using effective Lagrangians. The partial widths for \begin{document}$\Xi(1620)^0\to\gamma\Xi$\end{document} is evaluated to be approximately \begin{document}$118.76-174.21$\end{document} keV, which may be accessible for the LHCb experiment. If \begin{document}$\Xi(1620)$\end{document} is a \begin{document}$\Lambda\bar{K}-\Sigma\bar{K}$\end{document} molecule, the radiative transition strength \begin{document}$\Xi(1620)^0\to\gamma\bar{K}\Lambda$\end{document} is considerably small and the decay width is of the order of 0.01 eV. Future experimental measurements of these processes can be useful to test the molecule interpretations of the \begin{document}$\Xi(1620)$\end{document}.
2021, 45(7): 073113. doi: 10.1088/1674-1137/abfe51
Abstract:
We present a case study for the doubly charged Higgs boson \begin{document}$H^{\pm\pm}$\end{document} pair production in \begin{document}$e^+e^-$\end{document} and pp colliders with their subsequent decays to four charged leptons. We consider the Higgs Triplet Model (\begin{document}${\texttt{HTM}}$\end{document}), which is not restricted by the custodial symmetry, and the Minimal Left-Right Symmetric Model (\begin{document}${\texttt{MLRSM}}$\end{document}). These models include scalar triplets with different complexities of scalar potentials and, because of experimental restrictions, completely different scales of non-standard triplet vacuum expectation values. In both models, a doubly charged Higgs boson \begin{document}$H^{\pm\pm}$\end{document} can acquire a mass of hundreds of gigaelectronvolts, which can be probed at the HL-LHC, future \begin{document}$e^+e^-$\end{document}, and hadron colliders. We take into account a comprehensive set of constraints on the parameters of both models coming from neutrino oscillations, LHC, \begin{document}$e^+e^-$\end{document}, and low-energy lepton flavor violating data and assume the same mass of \begin{document}$H^{\pm\pm}$\end{document}. Our finding is that the \begin{document}$H^{\pm\pm}$\end{document} pair production in lepton and hadron colliders is comparable in both models, though more pronounced in the \begin{document}${\texttt{MLRSM}}$\end{document}. We show that the decay branching ratios can be different within both models, leading to four distinguishable lepton signals, and that the strongest are \begin{document}$4\mu$\end{document} events yielded by the \begin{document}${\texttt{MLRSM}}$\end{document}. Typically, we find that the \begin{document}${\texttt{MLRSM}}$\end{document} signals are one order of magnitude larger those in the \begin{document}${\texttt{HTM}}$\end{document}. For example, the \begin{document}$pp \to 4\mu$\end{document} \begin{document}${\texttt{MLRSM}}$\end{document} signal for 1 TeV \begin{document}$H^{\pm \pm}$\end{document} mass results in a clearly detectable significance of \begin{document}$S \simeq 11$\end{document} for the HL-LHC and \begin{document}$S \simeq 290$\end{document} for the FCC-hh. Finally, we provide quantitative predictions for the dilepton invariant mass distributions and lepton separations, which help to identify non-standard signals.
2021, 45(7): 073114. doi: 10.1088/1674-1137/abf828
Abstract:
In this study, we perform a detailed analysis on the same-sign dilepton signature in the inert doublet model. Focusing on the low dark matter mass region, we randomly scan the corresponding parameter space. Viable samples allowed by various constraints are obtained, and among them are twenty benchmark points that are selected for further study on collider signature. At hadron colliders, the same-sign dilepton signature is produced via \begin{document}$pp\to W^{\pm *}W^{\pm *}jj \to H^\pm H^\pm jj$\end{document} with the leptonic decay mode \begin{document}$H^\pm \to HW^\pm (\to l^\pm \nu)$\end{document}, where H represents the dark matter candidate. We investigate the testability of this signal at the high-luminosity LHC (HL-LHC) and the proposed 27 TeV high-energy LHC (HE-LHC). According to our simulation, the HL-LHC with \begin{document}${\cal{L}}=3\;{\rm{ab}}^{-1}$\end{document} can barely probe this signal. Meanwhile, for the HE-LHC with \begin{document}${\cal{L}}=15\;{\rm{ab}}^{-1}$\end{document}, it is promising to obtain a \begin{document}$5\sigma$\end{document} significance when \begin{document}$250\;{\rm{GeV}}\lesssim m_{H^\pm}-m_H\lesssim 300$\end{document} GeV with dark matter mass \begin{document}$m_H\sim 60$\end{document} or 71 GeV.
2021, 45(7): 074101. doi: 10.1088/1674-1137/abf5ca
Abstract:
Cross-section data of the 185Re(n,2n)184mRe, 185Re(n,2n)184gRe, 185Re(n,α)182m1+m2+gTa, 187Re(n, 2n)186g,(m)Re, 187Re(n,α)184Ta, and 187Re(n,p)187W reactions were measured at four neutron energies, namely 13.5, 14.1, 14.4, and 14.8 MeV, by means of the activation technique, relative to the reference cross-section values of the 93Nb(n,2n)92mNb reaction. The neutrons were generated from the T(d,n)4He reaction at the K-400 Neutron Generator at China Academy of Engineering Physics. The induced γ activities were measured using a high-resolution γ-ray spectrometer equipped with a coaxial high-purity germanium detector. The excitation functions of the six above-mentioned nuclear reactions at neutron energies from the threshold to 20 MeV were calculated by adopting the nuclear theoretical model program system Talys-1.9 with the relevant parameters properly adjusted. The measured cross sections were analyzed and compared with previous experiments conducted by other researchers, and with the evaluated data of BROND-3.1, ENDF/B-VIII.0, JEFF-3.3, and the theoretical values based on Talys-1.9. The new measured results agree with those of previous experiments and the theoretical excitation curve at the corresponding energies. The theoretical excitation curves based on Talys-1.9 generally match most of experimental data well.
2021, 45(7): 074102. doi: 10.1088/1674-1137/abf645
Abstract:
The large values and constituent-quark-number scaling of the elliptic flow of low-\begin{document}$p_T$\end{document} D mesons imply that charm quarks, initially produced through hard processes, might be partially thermalized through strong interactions with quark-gluon plasma (QGP) in high-energy heavy-ion collisions. To quantify the degree of thermalization of low-\begin{document}$p_T$\end{document} charm quarks, we compare the \begin{document}$D^0$\end{document} meson spectra and elliptic flow from a hydrodynamic model to experimental data as well as transport model simulations. We use an effective charm chemical potential at the freeze-out temperature to account for the initial charm quark production from hard processes and assume that they are thermalized in the local comoving frame of the medium before freeze-out. \begin{document}$D^0$\end{document} mesons are sampled statistically from the freeze-out hyper-surface of the expanding QGP as described by the event-by-event (3+1)D viscous hydrodynamic model CLVisc. Both the hydrodynamic and transport models can describe the elliptic flow of \begin{document}$D^0$\end{document} mesons at \begin{document}$p_T<3$\end{document} GeV/c as measured in Au+Au collisions at \begin{document}$\sqrt{s_{NN}} = 200$\end{document} GeV. Though the experimental data on \begin{document}$D^0$\end{document} spectra are consistent with the hydrodynamic result at small \begin{document}$p_T\sim 1$\end{document} GeV/c, they deviate from the hydrodynamic model at high transverse momentum,\begin{document}$p_T>2$\end{document} GeV/c. The diffusion and parton energy loss mechanisms in the transport model can describe the measured spectra reasonably well within the theoretical uncertainty. Our comparative study indicates that charm quarks only approach local thermal equilibrium at small \begin{document}$p_T$\end{document}, even though they acquire sizable elliptic flow comparable to light-quark hadrons at both small and intermediate \begin{document}$p_T$\end{document}.
2021, 45(7): 074103. doi: 10.1088/1674-1137/abf6c3
Abstract:
First-principle calculations based on the density functional theory (DFT) method are adopted to investigate the influence of a strong electric field on the 7Be half-life. Accordingly, electronic structures of Be and BeO are examined in the presence of a homogeneous electric field. The electron density at the nucleus is estimated upon the geometry optimization. Our computations for the Be metal indicate a 0.02% increase in the decay rate of the 7Be nucleus, corresponding to a 0.02% decrease in the 7Be half-life, both at 5.14 V/Å (0.1 a.u.). Furthermore, it is determined that the decay rate of 7Be is not considerably altered up to 3.6 V/Å in the BeO structure. Our results show that the screening energy of the electron can be dependent on the applied electric field strength. Furthermore, we predict variations in the Coulomb potential at the 7Be nucleus due to electric field application.
2021, 45(7): 074104. doi: 10.1088/1674-1137/abf8a3
Abstract:
Neutrons tunneling to the classically forbidden (CF) region in the neutron-rich nucleus \begin{document}$^{68} {\rm{Ca}}$\end{document} are investigated in the Skyrme Hartree-Fock (HF) and Hartree-Fock-Bogoliubov (HFB) models. The definition of the CF region is examined in the HF model by using different single-particle potentials for the bound states. In the HFB model, the weakly bound and continuum states could also contribute to the neutrons in the CF region due to the pairing correlation. Their asymptotic wave functions are carefully calculated by the Green’s function method.
2021, 45(7): 074105. doi: 10.1088/1674-1137/abf99a
Abstract:
The coexistence of neutron-neutron (n-n), proton-proton (p-p), and neutron-proton (n-p) pairings is investigated by adopting an effective density-dependent contact pairing potential. These three types of pairings can coexist only if the n-p pairing is stronger than the n-n and p-p pairings for the isospin asymmetric nuclear matter. In addition, the existence of n-n and p-p pairs might enhance n-p pairings in asymmetric nuclear matter when the n-p pairing strength is significantly stronger than the n-n and p-p ones. Conversely, the n-p pairing is reduced by the n-n and p-p pairs when the n-p pairing interaction approaches n-n and p-p pairings.
2021, 45(7): 074106. doi: 10.1088/1674-1137/abf99b
Abstract:
We explore the effects of the density dependence of symmetry energy on the crust-core phase transition and dynamical instabilities in cold and warm neutron stars in the relativistic mean field (RMF) theory with point-coupling interactions using the Vlasov approach. The role of neutrino trapping is also considered. The crust-core transition density and pressure, distillation effect, and cluster size and growth rates are discussed. The present work shows that the slope of symmetry energy at saturation, temperature, and neutrino trapping have non-negligible effects.
2021, 45(7): 074107. doi: 10.1088/1674-1137/abfa84
Abstract:
To explore the properties of neutron-rich nuclei with approximately 40 protons, the density-dependent point coupling (DD-PC1) effective interaction parameter is adopted in the relativistic mean-field theory with the complex momentum representation (RMF-CMR). The calculated two-neutron separation energy (S2n) and root-mean-square (rms) radii support the halo structure that appear in Mo and Ru isotopic chains. Besides, the neutron skin structures appear in Kr and Sr isotopes. The conclusions drawn are also supported by the single-particle energy levels and their occupancy probability and density distribution. Particularly, the energy levels, which reduce to bound states or are approximately 0 MeV with a small orbital angular momentum, are suggested to provide the primary contribution to increasing the neutron radius. Moreover, the single-particle energy levels significantly reflect the shell structure. In addition, the neutron drip line nuclei for Kr, Sr, Mo, and Ru elements are proposed via the changes in S2n.
2021, 45(7): 074108. doi: 10.1088/1674-1137/abfaf2
Abstract:
A nuclear mass formula based on the macroscopic microscopic approach is proposed, in which the number of model parameters is reduced compared with other macroscopic microscopic models. The root mean square (RMS) deviation with respect to 2314 training sets (measured nuclear masses) is reduced to 0.447 MeV, and the calculated value of each nucleus is no more than 0.8% different from the experimental value. The single and two nucleon separation energies and the shell gaps are calculated to test the model. The shell corrections and double magic number of superheavy nuclei are also analyzed.
2021, 45(7): 074109. doi: 10.1088/1674-1137/abfb51
Abstract:
Three typical Pauli blocking algorithms in quantum molecular dynamics type models are investigated in the nuclear matter, the nucleus, and heavy ion collisions. In nuclear matter, the blocking ratios obtained with the three algorithms are underestimated by 13%-25% compared to the corresponding analytical values. For a finite nucleus, spurious collisions occur around the surface of the nucleus owing to the defects of the Pauli blocking algorithms. In the simulations of heavy ion collisions, the uncertainty of stopping power arising from the different Pauli blocking algorithms is less than 5%. Furthermore, the in-medium effects of nucleon-nucleon (NN) cross sections on the nuclear stopping power are discussed. Our results show that the transport model calculations with free NN cross sections result in the stopping power decreasing with beam energy when the beam energy is less than 300 MeV/u. To increase or decrease the values of the stopping power, the transport model calculations need enhanced or suppressed model dependent in-medium NN cross sections that are expected to be smaller than the true in-medium NN cross sections.
2021, 45(7): 074110. doi: 10.1088/1674-1137/abfd29
Abstract:
The moments and moment products of conserved charges are believed to be sensitive to critical fluctuations, which have been adopted in determining the QCD critical point. Using a dynamical multiphase transport model, we reproduce the centrality and energy dependences of moments and moment products of net-charge multiplicity distributions in Au+Au collisions, measured by the Beam Energy Scan program at the RHIC. No non-monotonic energy dependence is observed. We infer that the moment products develop during the dynamical evolution of heavy-ion collisions. The observed difference based on the expectation of the Poisson baseline indicates a positive two-particle correlation between positively and negatively charged particles, which can arise from different dynamical processes at different stages. Therefore, to adopt moments and moment products of net-charge multiplicity distributions in determining the QCD critical point of relativistic heavy-ion collisions, it is essential to consider the dynamical evolution.
2021, 45(7): 075001. doi: 10.1088/1674-1137/abf6c2
Abstract:
We present a systematic determination of the responses of PandaX-II, a dual phase xenon time projection chamber detector, to low energy recoils. The electron recoil (ER) and nuclear recoil (NR) responses are calibrated, respectively, with injected tritiated methane or 220Rn source, and with 241Am-Be neutron source, in an energy range from 1-25 keV (ER) and 4-80 keV (NR), under the two drift fields, 400 and 317 V/cm. An empirical model is used to fit the light yield and charge yield for both types of recoils. The best fit models can describe the calibration data significantly. The systematic uncertainties of the fitted models are obtained via statistical comparison to the data.
2021, 45(7): 075101. doi: 10.1088/1674-1137/abf6c4
Abstract:
Considering a grand canonical ensemble, we study the phase structures and transitions of RN black holes surrounded by quintessence dark energy on two different boundary conditions, namely AdS space and a Dirichlet wall. For AdS space, under the condition of fixed temperature and potential, as the temperature increases for lower potential, the black hole undergoes a first-order phase transition, while for higher potential, no phase transition occurs. There are two different regions in the parameter space. For the Dirichlet wall, on which the temperature and potential are fixed, the state parameter of quintessence \begin{document}$\omega=-2/3$\end{document} is analyzed in detail. Then, three different physically allowed regions in the parameter space of the black hole are well studied. As the temperature rises, first-order and second-order phase transitions may occur. In this case, there are nine regions in the parameter space, which is evidently distinct from the case of AdS space.
2021, 45(7): 075102. doi: 10.1088/1674-1137/abf72d
Abstract:
We review steady spherically symmetric accretion onto a renormalization group improved Schwarzschild space-time, which is a solution to an asymptotically safe theory (AS) containing high-derivative terms. We use a Hamiltonian dynamical system approach for the analysis of the accretion of four types of isothermal test fluids: ultra-stiff fluid, ultra-relativistic fluid, radiation fluid, and sub-relativistic fluid. An important outcome of our study is that, contrary to the claim in a recent work, there are physical solutions for the accretion of an ultra-relativistic fluid in AS, which include subsonic, supersonic, and transonic regimes. Furthermore, we study quantum corrections to the known stability of the accretion in general relativity (GR). To this end, we use a perturbative procedure based on the continuity equation with the mass accretion rate being the perturbed quantity. Two classes of perturbations are studied: standing and traveling waves. We find that quantum gravity effects either enhance or diminish the stability of the accretion depending on the type of test fluid and the radial distance to the central object.
2021, 45(7): 075103. doi: 10.1088/1674-1137/abf9ff
Abstract:
Primordial black holes have been considered attractive dark matter candidates, whereas some of the predictions rely heavily on the near-horizon physics that remains to be tested experimentally. As a concrete alternative, thermal 2-2-holes closely resemble black holes without event horizons. Being a probable endpoint of gravitational collapse, they provide a solution to the information loss problem but also naturally result in stable remnants. Previously, we have considered primordial 2-2-hole remnants as dark matter. Owing to the strong constraints from a novel phenomenon associated with remnant mergers, only small remnants with mass approximate to the Planck mass can constitute all dark matter. In this paper, we examine the scenario in which the majority of dark matter consists of particles produced by the evaporation of primordial 2-2-holes, whereas the remnant contribution is secondary. The products with sufficiently light mass may contribute to the number of relativistic degrees of freedom in the early universe, which we also calculate. Moreover, 2-2-hole evaporation can produce particles that are responsible for the baryon asymmetry. We observe that baryogenesis through direct B-violating decays or through leptogenesis can both be realized. Overall, the viable parameter space for the Planck remnant scenario is similar to that of primordial black holes with Planck remnants. However, heavier remnants result in different predictions, and the viable parameter space remains large even when the remnant abundance is small.