## 2020 Vol. 44, No. 5

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2020, 44(5): 053101. doi: 10.1088/1674-1137/44/5/053101
Abstract:
The sea quark contributions to the nucleon electromagnetic form factors of the up, down and strange quarks are studied with the nonlocal chiral effective Lagrangian. Both octet and decuplet intermediate states are included in the one loop calculations. Compared with the strange quark form factors, although their signs are the same, the absolute value of the light quark form factors are much larger. For both the electric and magnetic form factors, the contribution of the d quark is larger than of the u quark. The current lattice simulations of the light sea quark form factors are in between our results for the u and d quarks.
2020, 44(5): 053102. doi: 10.1088/1674-1137/44/5/053102
Abstract:
We study the semileptonic \begin{document}$B/B_s \to (D^{(*)},D_s^{(*)}) l\nu_l$\end{document} decays in the framework of the Standard Model (SM), by employing the perturbative QCD (PQCD) factorization formalism combined with the lattice QCD input for the relevant transition form factors. We calculate the branching ratios \begin{document}${\cal B}(B_{(s)} \to D_{(s)}^{(*)} l \nu_l )$\end{document} with \begin{document}$l = (e,\mu,\tau)$\end{document}, the ratios of the branching fractions \begin{document}$R(D^{(*)})$\end{document} and \begin{document}$R(D_s^{(*)} )$\end{document}, and the physical observables \begin{document}$P_\tau(D_{(s)}^{(*)})$\end{document}, \begin{document}$F_L(D^*_{(s)})$\end{document} and \begin{document}$A_{FB}(\tau)$\end{document}. The “PQCD+Lattice” predictions for \begin{document}${\cal B}(B \to D^{(*)} l\nu_l)$\end{document} and \begin{document}$R(D^{(*)})$\end{document} agree with the available experimental measurements within errors. For the ratios \begin{document}$R(D_s)$\end{document} and \begin{document}$R(D_s^*)$\end{document}, the "PQCD+Lattice" predictions agree with the other predictions. For \begin{document}$P_\tau(D^*)$\end{document} and \begin{document}$F_L(D^*)$\end{document}, our theoretical predictions agree with the measured values within errors. Our theoretical predictions of the semileptonic \begin{document}$B/B_s$\end{document} decays considered could be tested in the near future by the LHCb and Belle II experiments.
2020, 44(5): 053103. doi: 10.1088/1674-1137/44/5/053103
Abstract:
This study extends the investigation of quantum dissipative effects of a cosmological scalar field by taking into account cosmic expansion and contraction. Cheung, Drewes, Kang, and Kim calculated the effective action and quantum dissipative effects of a cosmological scalar field in a recent work, where analytical expressions for the effective potential and damping coefficient were presented using a simple scalar model with quartic interactions, and the work was conducted using Minkowski-space propagators in loop diagrams. In this work, we incorporate the Hubble expansion and contraction of the cosmic background and focus on the thermal dynamics of a scalar field in a regime where the effective potential changes slowly. Given that the Hubble parameter, H, attains a small but non-zero value, we carry out calculations to the first order in H. If we set H = 0, all results match those in flat spacetime. Interestingly, we must integrate over the resonances, which in turn leads to an amplification of the effects of a non-zero H. This is an intriguing phenomenon, which cannot be uncovered in flat spacetime. The implications on particle creations in the early universe will be studied in a forthcoming study.
2020, 44(5): 054101. doi: 10.1088/1674-1137/44/5/054101
Abstract:
In this paper a pair of observables are proposed as alternative ways, by examining the fluctuation of net momentum-ordering of charged pairs, to study the charge separation induced by the Chiral Magnetic Effect (CME) in relativistic heavy ion collisions. They are, the out-of-plane to in-plane ratio of fluctuation of the difference between signed balance functions measured in pair’s rest frame, and the ratio of it to similar measurement made in the laboratory frame. Both observables have been studied with simulations including flow-related backgrounds, and for the first time, backgrounds that are related to resonance's global spin alignment. The two observables have similar positive responses to signal, and opposite, limited responses to identifiable backgrounds arising from resonance flow and spin alignment. Both observables have also been tested with two realistic models, namely, a multi-phase transport (AMPT) model and the anomalous-viscous fluid dynamics (AVFD) model. These two observables, when cross examined, will provide useful insights in the study of CME-induced charge separation.
2020, 44(5): 054102. doi: 10.1088/1674-1137/44/5/054102
Abstract:
Heavy quarks play an important role in probing the properties of strongly interacting quark-gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions. We study the interactions of single heavy (charm) quarks and correlated charm and anticharm (\begin{document}$c\bar c$\end{document}) quark pairs with the medium constituents of QGP by performing fireball+Langevin simulations of the pertinent Brownian motion with elastic collisions. Besides studying the traditional observables, the nuclear modification factor and the elliptic flow of single heavy quarks in QGP for different thermal relaxation rates, we also study the broadening of the azimuthal correlations of charm and anticharm quark pairs in the QGP medium for different relaxation rates and transverse momenta classes. We quantified the smearing of \begin{document}$c\bar c$\end{document} pair azimuthal correlations with an increasing thermal relaxation rate: while the (nearly) back-to-back correlations among \begin{document}$c\bar c$\end{document} pairs are almost completely washed out at low transverse momentum (pT), these correlations at high pT largely survive the pair diffusion. This provides a novel observable for diagnosing the properties of QGP.
2020, 44(5): 054103. doi: 10.1088/1674-1137/44/5/054103
Abstract:
The complex-scaled Green's function (CGF) method is employed to explore the single-proton resonance in 15F. Special attention is paid to the first excited resonant state 5/2+, which has been widely studied in both theory and experiments. However, past studies generally overestimated the width of the 5/2+ state. The predicted energy and width of the first excited resonant state 5/2+ by the CGF method are both in good agreement with the experimental value and close to Fortune's new estimation. Furthermore, the influence of the potential parameters and quadruple deformation effects on the resonant states are investigated in detail, which is helpful to the study of the shell structure evolution.
2020, 44(5): 054104. doi: 10.1088/1674-1137/44/5/054104
Abstract:
We investigate the \begin{document}$\eta \bar{K}K^*$\end{document} three-body system in order to look for possible \begin{document}$I^G(J^{PC}) = 0^+(1^{-+})$\end{document} exotic states in the framework of the fixed-center approximation of the Faddeev equation. We assume the scattering of \begin{document}$\eta$\end{document} on a clusterized system \begin{document}$\bar{K}K^*$\end{document}, which is known to generate \begin{document}$f_1(1285)$\end{document}, or a \begin{document}$\bar{K}$\end{document} in a clusterized system \begin{document}$\eta K^*$\end{document}, which is shown to generate \begin{document}$K_1(1270)$\end{document}. In the case of \begin{document}$\eta$\end{document}-\begin{document}$(\bar{K}K^*)_{f_1(1285)}$\end{document} scattering, we find evidence of a bound state \begin{document}$I^G(J^{PC}) = 0^+(1^{-+})$\end{document} below the \begin{document}$\eta{f_1(1285)}$\end{document} threshold with a mass of around 1700 MeV and a width of about 180 MeV. Considering \begin{document}$\bar{K}$\end{document}-\begin{document}$(\eta K^*)_{K_1(1270)}$\end{document} scattering, we obtain a bound state \begin{document}$I(J^{P}) = 0(1^{-})$\end{document} just below the \begin{document}$\bar{K}{K_1(1270)}$\end{document} threshold with a mass of around 1680 MeV and a width of about 160 MeV.
2020, 44(5): 054105. doi: 10.1088/1674-1137/44/5/054105
Abstract:
We analyze the transverse momentum dependence of HBT radii in relativistic heavy-ion collisions using several source models. Results indicate that the single-particle space-momentum angle distribution plays an important role in the transverse momentum dependence of HBT radii. In a cylinder source, we use several formulas to describe the transverse momentum dependence of HBT radii and the single-particle space-momentum angle distribution. We also make a numerical connection between them in the transverse plane.
2020, 44(5): 054106. doi: 10.1088/1674-1137/44/5/054106
Abstract:
We derive a simple Woods-Saxon-type form for potentials between \begin{document}$Y=\Xi, \Omega$\end{document}, and \begin{document}$\alpha$\end{document} using a single-folding potential method, based on a separable Y-nucleon potential. The potentials \begin{document}$\Xi+\alpha$\end{document} and \begin{document}$\Omega+\alpha$\end{document} are accordingly obtained using the ESC08c Nijmegens \begin{document}$\Xi N$\end{document} potential (in \begin{document}$^{3}S_{1}$\end{document} channel) and HAL QCD collaboration \begin{document}$\Omega N$\end{document} interactions (in lattice QCD), respectively. In deriving the potential between Y and \begin{document}$\alpha$\end{document}, the same potential between Y and N is employed. The binding energy, scattering length, and effective range of the Y particle on the alpha particle are approximated by the resulting potentials. The depths of the potentials in \begin{document}$\Omega \alpha$\end{document} and \begin{document}$\Xi \alpha$\end{document} systems are obtained at \begin{document}$-61$\end{document} MeV and \begin{document}$-24.4$\end{document} MeV, respectively. In the case of the \begin{document}$\Xi \alpha$\end{document} potential, a fairly good agreement is observed between the single-folding potential method and the phenomenological potential of the Dover-Gal model. These potentials can be used in 3-,4- and 5-body cluster structures of \begin{document}$\Omega$\end{document} and \begin{document}$\Xi$\end{document} hypernuclei.
2020, 44(5): 054107. doi: 10.1088/1674-1137/44/5/054107
Abstract:
We used the cluster structure properties of the 212Po to estimate the neutron skin thickness of 208Pb. For this purpose, we considered two important components: (a) alpha decay is a low energy phenomenon; therefore, one can expect that the mean-field, which can explain the ground state properties of 212Po, does not change during the alpha decay process. (b) 212Po has a high alpha cluster-like structure, two protons and two neutrons outside its core nucleus with a double magic closed-shell, and the cluster model is a powerful formalism for the estimation of alpha decay preformation factor of such nuclei. The slope of the symmetry energy of 208Pb is estimated to be \begin{document}$75\pm25$\end{document} MeV within the selected same mean-fields and Skyrme forces, which can simultaneously satisfy the ground-state properties of parent and daughter nuclei, as their neutron skin thicknesses are consistent with experimental data.
2020, 44(5): 054108. doi: 10.1088/1674-1137/44/5/054108
Abstract:
To explain the experimental observation that the fusion cross-section of a proton-halo nucleus with a heavy target nucleus is not enhanced as expected, the shielding hypothesis was proposed, where the proton-halo nucleus is polarized and the valence proton shielded by the core. In the frame of the improved quantum molecular dynamics model, the fusion reaction 17F on 208Pb around the Coulomb barrier is simulated. The existence of the shielding effect is verified by the microscopic dynamics simulations. Its influence on the effective interaction potential is also investigated.
2020, 44(5): 054109. doi: 10.1088/1674-1137/44/5/054109
Abstract:
A microscopic approach is employed to study the optical potential for the 7Li-nucleus interaction system without any free parameters. It is obtained by folding the microscopic optical potentials of the constituent nucleons of 7Li over their density distributions. We employ an isospin-dependent nucleon microscopic optical potential, which is based on the Skyrme nucleon-nucleon effective interaction and derived using the Green's function method, as the nucleon optical potential. The harmonic oscillator shell model is used to describe the internal wave function of 7Li and obtain the nucleon density distribution. The 7Li microscopic optical potential is used to predict the reaction cross-sections and elastic scattering angular distributions for the target range from 27Al to 208Pb and energy range below 450 MeV. Generally, the results can reproduce the measured data reasonably well. In addition, the microscopic optical potential is comparable to a global phenomenological optical potential by fitting the presently existing measured data.
2020, 44(5): 054110. doi: 10.1088/1674-1137/44/5/054110
Abstract:
The energy per particle BA in nuclear matter is calculated up to high baryon density in the whole isospin asymmetry range from symmetric matter to pure neutron matter. The results, obtained in the framework of the Brueckner-Hartree-Fock approximation with two- and three-body forces, confirm the well-known parabolic dependence on the asymmetry parameter β = (NZ)/A (β2 law) that is valid in a wide density range. To investigate the extent to which this behavior can be traced back to the properties of the underlying interaction, aside from the mean field approximation, the spin-isospin decomposition of BA is performed. Theoretical indications suggest that the β2 law could be violated at higher densities as a consequence of the three-body forces. This raises the problem that the symmetry energy, calculated according to the β2 law as a difference between BA in pure neutron matter and symmetric nuclear matter, cannot be applied to neutron stars. One should return to the proper definition of the nuclear symmetry energy as a response of the nuclear system to small isospin imbalance from the Z = N nuclei and pure neutron matter.
2020, 44(5): 055101. doi: 10.1088/1674-1137/44/5/055101
Abstract:
We study the prospects of using the low-redshift and high-redshift black hole shadows as new cosmological standard rulers for measuring cosmological parameters. We show that, using the low-redshift observation of the black hole shadow of \begin{document}${\rm M87}^\star$\end{document}, the Hubble constant can be independently determined with a precision of about 13% as \begin{document}$H_0=70\pm 9$\end{document} km \begin{document}${\rm s}^{-1}$\end{document} \begin{document}${\rm Mpc}^{-1}$\end{document}. The high-redshift observations of super-massive black hole shadows may accurately determine a combination of parameters \begin{document}$H_0$\end{document} and \begin{document}${\Omega_{m}}$\end{document}, and we show by a simple simulation that combining them with the type Ia supernovae observations would give precise measurements of the cosmological parameters.
2020, 44(5): 055102. doi: 10.1088/1674-1137/44/5/055102
Abstract:
We study quasinormal modes (QNMs) of charged black holes in the Einstein-Maxwell-Weyl (EMW) gravity by adopting the test scalar field perturbation. We find that the imaginary part of QNM frequencies is consistently negative for different angular parameters l, indicating that these modes always decay and are therefore stable. We do not observe a linear relationship between the QNM frequency ω and parameter p for these black holes, as their charge Q causes a nonlinear effect. We evaluate the massive scalar field perturbation in charged black holes and find that random long lived modes (i.e., quasiresonances) could exist in this spectrum.
2020, 44(5): 055103. doi: 10.1088/1674-1137/44/5/055103
Abstract:
The first law of black hole thermodynamics has been shown to be valid in the extended phase space. However, the second law and the weak cosmic censorship conjecture have not been investigated extensively. We investigate the laws of thermodynamics and the weak cosmic censorship conjecture of an AdS black hole with a global monopole in the extended phase space in the case of charged particle absorption. It is shown that the first law of thermodynamics is valid, while the second law is violated for the extremal and near-extremal black holes. Moreover, we find that the weak cosmic censorship conjecture is valid only for the extremal black hole, and that it can be violated for the near-extremal black holes, which is different from the previous results.
2020, 44(5): 055104. doi: 10.1088/1674-1137/44/5/055104
Abstract:
The quantum electrodynamics (QED) in a spatially flat (1+3)-dimensional Friedmann-Lemaître-Robertson-Walker (FLRW) space-time with a Milne-type scale factor is outlined focusing on the amplitudes of the allowed processes in the first order perturbations. The definition of the transition rates is reconsidered such that an appropriate angular behavior of the probability for creation of an electron-positron pair from a photon is obtained, which has a similar rate as the creation of a photon and an electron-positron pair from vacuum. It is shown that these processes are allowed only in the first order perturbations, since the photon emission or absorption by an electron or positron are forbidden.