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2020 Vol. 44, No. 10

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PARTICLES AND FIELDS
A compact analytical approximation for a light sterile neutrino oscillation in matter
Bao-Biao Yue, Wei Li, Jia-Jie Ling, Fan-Rong Xu
2020, 44(10): 103001. doi: 10.1088/1674-1137/ababfa
Abstract:
The existence of light sterile neutrinos is a long-standing question in particle physics. Several experimental “anomalies” might be explained by introducing eV mass scaled light sterile neutrinos. Many experiments are actively searching for such light sterile neutrinos through neutrino oscillation. For long baseline experiments, the matter effect should be treated carefully for precise calculation of the neutrino oscillation probabilities. However, this is usually time-consuming or analytically complex. In this manuscript, we adopt a Jacobi-like method to diagonalize the Hermitian Hamiltonian matrix and derive analytically simplified neutrino oscillation probabilities for 3 (active) + 1 (sterile)-neutrino mixing for a constant matter density. These approximations can reach a considerably high numerical accuracy while retaining their analytical simplicity and fast computing speed. This would be useful for current and future long baseline neutrino oscillation experiments.
Equation of state and chiral transition in soft-wall AdS/QCD with a more realistic gravitational background
Zhen Fang, Yue-Liang Wu
2020, 44(10): 103101. doi: 10.1088/1674-1137/abab90
Abstract:
We construct an improved soft-wall AdS/QCD model with a cubic coupling term of the dilaton and the bulk scalar field. The background fields in this model are solved by the Einstein-dilaton system with a nontrivial dilaton potential, which has been shown to reproduce the equation of state from the lattice QCD with two flavors. The chiral transition behaviors are investigated in the improved soft-wall AdS/QCD model with the solved gravitational background, and the crossover transition can be realized. Our study provides the possibility to address the deconfining and chiral phase transitions simultaneously in the bottom-up holographic framework.
Analysis of the strong decays of Pc(4312) as a pentaquark molecular state with QCD sum rules
Zhi-Gang Wang, Xu Wang
2020, 44(10): 103102. doi: 10.1088/1674-1137/ababf7
Abstract:
In this article, we tentatively assign \begin{document}$P_c(4312)$\end{document} to be the \begin{document}$\bar{D}\Sigma_c$\end{document} pentaquark molecular state with the spin-parity \begin{document}$J^P={\frac{1}{2}}^-$\end{document} , and discuss the factorizable and non-factorizable contributions in the two-point QCD sum rules for the \begin{document}$\bar{D}\Sigma_c$\end{document} molecular state in detail to prove the reliability of the single pole approximation in the hadronic spectral density. We study its two-body strong decays with the QCD sum rules, and special attention is paid to match the hadron side with the QCD side of the correlation functions to obtain solid duality. We obtain the partial decay widths \begin{document}$\Gamma\left(P_c(4312)\to \eta_c p\right)=0.255\,\,{\rm{MeV}}$\end{document} and \begin{document}$\Gamma\left(P_c(4312)\to J/\psi p\right)=9.296^{+19.542}_{-9.296}\,\,{\rm{MeV}}$\end{document} , which are compatible with the experimental value of the total width, and support assigning \begin{document}$P_c(4312)$\end{document} to be the \begin{document}$\bar{D}\Sigma_c$\end{document} pentaquark molecular state.
Logarithmic Regge pole
S. D. Campos
2020, 44(10): 103103. doi: 10.1088/1674-1137/ababf8
Abstract:
This work presents the subtraction procedure and the Regge cut in the logarithmic Regge pole approach. The subtraction mechanism leads to the same asymptotic behavior as previously obtained in the non-subtraction case. The Regge cut, in contrast, introduces a clear role to the non-leading contributions for the asymptotic behavior of the total cross-section. From these results, some simple parameterization is introduced to fit the experimental data for the proton-proton and antiproton-proton total cross-section above some minimum value up to the cosmic-ray. The fit parameters obtained are used to present predictions for the \begin{document}$ \rho(s)$\end{document} -parameter as well as to the elastic slope \begin{document}$ B(s)$\end{document} at high energies.
Studying the localized CP violation and the branching fraction of the ${\bar{B}^0\rightarrow K^-\pi^+\pi^+\pi^-}$ decay
Jing-Juan Qi, Zhen-Yang Wang, Jing Xu, Xin-Heng Guo
2020, 44(10): 103104. doi: 10.1088/1674-1137/abac00
Abstract:
In this work, we study the localized \begin{document}$ CP $\end{document} violation and the branching fraction of the four-body decay \begin{document}$ \bar{B}^0\rightarrow K^-\pi^+\pi^-\pi^+ $\end{document} by employing a quasi-two-body QCD factorization approach. Considering the interference of \begin{document}$ \bar{B}^0\rightarrow \bar{K}_0^*(700)\rho^0(770)\rightarrow K^-\pi^+\pi^-\pi^+ $\end{document} and \begin{document}$ \bar{B}^0\rightarrow \bar{K}^*(892)f_0(500)\rightarrow K^-\pi^+\pi^-\pi^+ $\end{document} channels, we predict \begin{document}$ \mathcal{A_{CP}}(\bar{B}^0\rightarrow K^-\pi^+\pi^-\pi^+)\in [0.15,0.28] $\end{document} and \begin{document}$ {\cal{B}}(\bar{B}^0\rightarrow K^-\pi^+\pi^-\pi^+)\in[1.73,5.10]\times10^{-7} $\end{document}, respectively, which shows that the interference mechanism of these two channels can induce the localized \begin{document}$ CP $\end{document} violation to this four-body decay. Meanwhile, within the two quark model framework for the scalar mesons \begin{document}$ f_0(500) $\end{document} and \begin{document}$ \bar{K}_0^*(700) $\end{document}, we calculate the direct CP violations and branching fractions of the \begin{document}$ \bar{B}^0\rightarrow \bar{K}_0^*(700)\rho^0(770) $\end{document} and \begin{document}$ \bar{B}^0\rightarrow \bar{K}^*(892)f_0(500) $\end{document} decays, respectively. The corresponding results are \begin{document}$ \mathcal{A_{CP}}(\bar{B}^0\rightarrow \bar{K}_0^*(700)\rho^0(770)) \in [0.20, 0.36] $\end{document}, \begin{document}$ \mathcal{A_{CP}}(\bar{B}^0\rightarrow \bar{K}^*(892)f_0(500))\in [0.08, 0.12] $\end{document}, \begin{document}${\cal{B}} (\bar{B}^0\rightarrow \bar{K}_0^*(700) \rho^0(770)\in [6.76, 18.93]\times10^{-8}$\end{document} and \begin{document}$ {\cal{B}} (\bar{B}^0\rightarrow \bar{K}^*(892)f_0(500))\in [2.66, 4.80]\times10^{-6} $\end{document}, indicating that the \begin{document}$ CP $\end{document} violations of these two-body decays are both positive and the branching fractions quite different. These studies provide a new way to investigate the aforementioned four-body decay and can be helpful in clarifying the configuration of the structure of the light scalar meson.
NUCLEAR PHYSICS
Measurement of away-side broadening with self-subtraction of flow in Au+Au collisions at ${{\sqrt{s_{NN}} = 200}}$ GeV
J. Adam, L. Adamczyk, J. R. Adams, J. K. Adkins, G. Agakishiev, M. M. Aggarwal, Z. Ahammed, I. Alekseev, D. M. Anderson, A. Aparin, E. C. Aschenauer, M. U. Ashraf, F. G. Atetalla, A. Attri, G. S. Averichev, V. Bairathi, K. Barish, A. Behera, R. Bellwied, A. Bhasin, J. Bielcik, J. Bielcikova, L. C. Bland, I. G. Bordyuzhin, J. D. Brandenburg, A. V. Brandin, J. Butterworth, H. Caines, M. Calderón de la Barca Sánchez, D. Cebra, I. Chakaberia, P. Chaloupka, B. K. Chan, F-H. Chang, Z. Chang, N. Chankova-Bunzarova, A. Chatterjee, D. Chen, J. H. Chen, X. Chen, Z. Chen, J. Cheng, M. Cherney, M. Chevalier, S. Choudhury, W. Christie, X. Chu, H. J. Crawford, M. Csanád, M. Daugherity, T. G. Dedovich, I. M. Deppner, A. A. Derevschikov, L. Didenko, X. Dong, J. L. Drachenberg, J. C. Dunlop, T. Edmonds, N. Elsey, J. Engelage, G. Eppley, S. Esumi, O. Evdokimov, A. Ewigleben, O. Eyser, R. Fatemi, S. Fazio, P. Federic, J. Fedorisin, C. J. Feng, Y. Feng, P. Filip, E. Finch, Y. Fisyak, A. Francisco, L. Fulek, C. A. Gagliardi, T
2020, 44(10): 104001. doi: 10.1088/1674-1137/ab97a9
Abstract:
High transverse momentum (\begin{document}$ p_T $\end{document}) particle production is suppressed owing to the parton (jet) energy loss in the hot dense medium created in relativistic heavy-ion collisions. Redistribution of energy at low-to-modest \begin{document}$ p_T $\end{document} has been difficult to measure, owing to large anisotropic backgrounds. We report a data-driven method for background evaluation and subtraction, exploiting the away-side pseudorapidity gaps, to measure the jetlike correlation shape in Au+Au collisions at \begin{document}$\sqrt{s_{{NN}}} = 200$\end{document} GeV in the STAR experiment. The correlation shapes, for trigger particles \begin{document}$ p_T>3\;{\rm{GeV}}/{\rm{c}} $\end{document} and various associated particle \begin{document}$ p_T $\end{document} ranges within \begin{document}$ 0.5<p_T<10\;{\rm{GeV}}/{\rm{c}} $\end{document}, are consistent with Gaussians, and their widths increase with centrality. The results indicate jet broadening in the medium created in central heavy-ion collisions.
Quenching of ${ \gamma_{0}}$ transition results from 2p-1h doorway mechanism by p-wave neutron excitation
Tao-Feng Wang, Xiao-Ting Yang, T. Katabuchi, Zi-Ming Li, Zhi-Bo Xu, G. N. Kim, T. I. Ro, Ying-Lu Han, Li-Hua Zhu, M. Igashira
2020, 44(10): 104002. doi: 10.1088/1674-1137/abab8b
Abstract:
The ratio of \begin{document}$\gamma$\end{document} transition-intensities from the initial capture state to low-lying states may represent the model-independent \begin{document}$\gamma$\end{document}-strength function, which reflects the effects of different neutron-capture reaction mechanisms. The extraordinary quenching of the \begin{document}$\gamma_{0}$\end{document} transition from the p-wave neutron radiative capture in \begin{document}$^{57}$\end{document}Fe is observed, for the first time, from the pronounced enhancement of the \begin{document}$\gamma$\end{document}-strength function ratios \begin{document}$f_{\gamma_{1}}/f_{\gamma_{0}}$\end{document} and \begin{document}$f_{\gamma_{2}}/f_{\gamma_{0}}$\end{document}. The 2p-1h doorway excitation leads to suppression of the \begin{document}$\gamma_{0}$\end{document} transition to the ground state and the enhancement of the \begin{document}$\gamma_{1}$\end{document} and \begin{document}$\gamma_{2}$\end{document} transitions to the first and second excited states, respectively. The \begin{document}$fp$\end{document} sub-shells supply the exact number of spaces required for the 2p-1h configuration, which features the neutron capture mechanism in the vicinity of A = 55.
The angular distributions of elastic scattering of 12,13C+Zr
Cui-Hua Rong, Gao-Long Zhang, Lin Gan, Zhi-Hong Li, L. C. Brandão, E. N. Cardozo, M. R. Cortes, Yun-Ju Li, Jun Su, Sheng-Quan Yan, Sheng Zeng, Gang Lian, Bing Guo, You-Bao Wang, Wei-Ping Liu, J. Lubian
2020, 44(10): 104003. doi: 10.1088/1674-1137/abab8d
Abstract:
To obtain the neutron spectroscopic amplitudes for \begin{document}${}^{90-96}$\end{document}Zr overlaps, experimental data of elastic scattering with small experimental errors and precise optical potentials were analyzed. In this study, the elastic scattering angular distributions of \begin{document}${}^{12,13}$\end{document}C + \begin{document}${}^{A} {\rm{Zr}}$\end{document} (A = 90, 91, 92, 94, 96) were measured using the high-precision Q3D magnetic spectrometer in the Tandem accelerator. The São Paulo potential was used for the optical potential. The optical model and coupled channel calculations were compared with the experimental data. The theoretical results were found to be very close to the experimental data. In addition, the possible effects of the couplings to the inelastic channels of the \begin{document}${}^A {\rm{Zr}}$\end{document} targets and \begin{document}${}^{12, 13}$\end{document}C projectiles on the elastic scattering were studied. It was observed that the couplings to the inelastic channels of the \begin{document}${}^{12,13}$\end{document}C projectiles could improve the agreement with the experimental data, while the inelastic couplings to the target states are of minor importance. The effect of the one-neutron stripping in the \begin{document}${}^{13}$\end{document}C+\begin{document}${}^A {\rm{Zr}}$\end{document} elastic scattering was also studied. The one-neutron stripping channel in \begin{document}${}^{13}$\end{document}C + \begin{document}${}^A {\rm{Zr}}$\end{document} was found to be not relevant and did not affect the elastic scattering angular distributions. Our results also show that in the reactions with the considered zirconium isotopes, the presence of the extra neutron in \begin{document}${}^{13}$\end{document}C does not influence the reaction mechanism, which is governed by the collective excitation of the \begin{document}${}^{12}$\end{document}C core.
Possibility of observation of hidden-bottom pentaquark resonances in bottomonium photoproduction on protons and nuclei near threshold
E. Ya. Paryev
2020, 44(10): 104101. doi: 10.1088/1674-1137/aba5f8
Abstract:
We study the \begin{document}$\Upsilon(1S)$\end{document} meson photoproduction on protons and nuclei at near-threshold center-of-mass energies below 11.4 GeV (or at the corresponding photon laboratory energies \begin{document}$E_{\gamma}$\end{document} below 68.8 GeV). We calculate the absolute excitation functions for the nonresonant and resonant photoproduction of \begin{document}$\Upsilon(1S)$\end{document} mesons off protons at incident photon laboratory energies of 63-68 GeV by considering direct (\begin{document}${\gamma}p \to {\Upsilon(1S)}p$\end{document}) and two-step (\begin{document}${\gamma}p \to P^+_b(11080) \to {\Upsilon(1S)}p$\end{document}, \begin{document}${\gamma}p \to P^+_b(11125) \to {\Upsilon(1S)}p$\end{document}, \begin{document}${\gamma}p \to P^+_b(11130) \to {\Upsilon(1S)}p$\end{document}) \begin{document}$\Upsilon(1S)$\end{document} production channels within different scenarios for the nonresonant total cross section of the elementary reaction \begin{document}${\gamma}p \to {\Upsilon(1S)}p$\end{document} and for branching ratios of the decays \begin{document}$P^+_b(11080) \to {\Upsilon(1S)}p$\end{document}, \begin{document}$P^+_b(11125) \to {\Upsilon(1S)}p$\end{document}, and \begin{document}$P^+_b(11130) \to {\Upsilon(1S)}p$\end{document}. We also calculate an analogous function for the photoproduction of \begin{document}$\Upsilon(1S)$\end{document} mesons on the 12C and 208Pb target nuclei in the near-threshold center-of-mass beam energy region of 9.0-11.4 GeV by considering the respective incoherent direct (\begin{document}${\gamma}N \to {\Upsilon(1S)}N$\end{document}) and two-step (\begin{document}${\gamma}p \to P^+_b(11080) \to {\Upsilon(1S)}p$\end{document}, \begin{document}${\gamma}p \to P^+_b(11125) \to {\Upsilon(1S)}p$\end{document}, \begin{document}${\gamma}p \to P^+_b(11130) \to {\Upsilon(1S)}p$\end{document} and \begin{document}${\gamma}n \to P^0_b$\end{document}\begin{document}$ (11080) \to{\Upsilon(1S)}n $\end{document}, \begin{document}${\gamma}n \to P^0_b(11125) \to {\Upsilon(1S)}n$\end{document}, \begin{document}${\gamma}n \to P^0_b(11130) \to {\Upsilon(1S)}n$\end{document}) \begin{document}$\Upsilon(1S)$\end{document}) production processes using a nuclear spectral function approach. We demonstrate that a detailed scan of the\begin{document}$\Upsilon(1S)$\end{document} total photoproduction cross section on proton and nuclear targets in the near-threshold energy region in future high-precision experiments at the proposed high-luminosity electron-ion colliders EIC and EicC in the US and China should provide a definite result for or against the existence of the nonstrange hidden-bottom pentaquark states\begin{document}$P_{bi}^+$\end{document} and \begin{document}$P_{bi}^0$\end{document} (\begin{document}$i$\end{document}=1, 2, 3) as well as clarify their decay rates.
Calculations of the α-decay properties of Z = 120, 122, 124, 126 isotopes
Zhishuai Ge, Gen Zhang, Shihui Cheng, Yu. S. Tsyganov, Feng-Shou Zhang
2020, 44(10): 104102. doi: 10.1088/1674-1137/abab00
Abstract:
The \begin{document}$\alpha$\end{document}-decay properties of even-Z nuclei with Z = 120, 122, 124, 126 are predicted. We employ the generalized liquid drop model (GLDM), Royer's formula, and universal decay law (UDL) to calculate the \begin{document}$\alpha$\end{document}-decay half-lives. By comparing the theoretical calculations with the experimental data of known nuclei from Fl to Og, we confirm that all the employed methods can reproduce the \begin{document}$\alpha$\end{document}-decay half-lives well. The preformation factor \begin{document}$P_{\alpha}$\end{document} and \begin{document}$\alpha$\end{document}-decay energy \begin{document}$Q_{\alpha}$\end{document} show that \begin{document}$^{298,304,314,316,324,326,338,348}$\end{document}120, \begin{document}$^{304,306,318,324,328,338}$\end{document}122, and \begin{document}$^{328,332,340,344}$\end{document}124 might be stable. The \begin{document}$\alpha$\end{document}-decay half-lives show a peak at Z = 120, N = 184, and the peak vanishes when Z = 122, 124, 126. Based on detailed analysis of the competition between \begin{document}$\alpha$\end{document}-decay and spontaneous fission, we predict that nuclei nearby N = 184 undergo \begin{document}$\alpha$\end{document}-decay. The decay modes of \begin{document}$^{287-339}$\end{document}120, \begin{document}$^{294-339}$\end{document}122, \begin{document}$^{300-339}$\end{document}124, and \begin{document}$^{306-339}$\end{document}126 are also presented.
New measurements and reanalysis of 14N elastic scattering on 10B target
Marzhan Nassurlla, N. Burtebayev, T. Kh. Sadykov, I. Boztosun, N. Amangeldi, D. Alimov, Zh. Kerimkulov, J. Burtebayeva, Maulen Nassurlla, A. Kurakhmedov, S.B. Sakuta, Mesut Karakoc, Awad A. Ibraheem, K.W. Kemper, Sh. Hamada
2020, 44(10): 104103. doi: 10.1088/1674-1137/abab89
Abstract:
The angular distributions of elastic scattering of 14N ions on 10B targets have been measured at incident beam energies of 21.0 and 24.5 MeV. Angular distributions at higher energies 38–94.0 MeV (previously measured) were also included in the analysis. All data were analyzed within the framework of the optical model and the distorted waves Born approximation method. The observed rise in cross sections at large angles was interpreted as a possible contribution of the α-cluster exchange mechanism. Spectroscopic amplitudes SA2 and SA4 for the configuration 14N→ 10B +α were extracted. Their average values are 0.58±0.10 and 0.81±0.12 for SA2 and SA4, respectively, suggesting that the exchange mechanism is a major component of the elastic scattering for this system. The energy dependence of the depths for the real and imaginary potentials was found.
$ {{ \Lambda}{ \Lambda}{ N} { -} { \Xi} { N}{ N}} $ S wave resonance
H. Garcilazo, A. Valcarce
2020, 44(10): 104104. doi: 10.1088/1674-1137/abab8c
Abstract:
We use an existing model of the \begin{document}$ \Lambda\Lambda N - \Xi NN $\end{document} three-body system based on two-body separable interactions to study the \begin{document}$ (I,J^P) = (1/2,1/2^+) $\end{document} three-body channel. For the \begin{document}$ \Lambda\Lambda $\end{document}, \begin{document}$ \Xi N $\end{document}, and \begin{document}$ \Lambda\Lambda - \Xi N $\end{document} amplitudes, we have constructed separable potentials based on the most recent results of the HAL QCD Collaboration. They are characterized by the existence of a resonance just below or above the \begin{document}$ \Xi N $\end{document} threshold in the H-dibaryon channel, \begin{document}$ (i,j^p) = (0,0^+) $\end{document}. A three-body resonance appears 2.3 MeV above the \begin{document}$ \Xi d $\end{document} threshold. We show that if the \begin{document}$ \Lambda\Lambda - \Xi N $\end{document} H-dibaryon channel is not considered, the \begin{document}$ \Lambda\Lambda N - \Xi NN $\end{document} S wave resonance disappears. Thus, the possible existence of a \begin{document}$ \Lambda\Lambda N - \Xi NN $\end{document} resonance would be sensitive to the \begin{document}$ \Lambda\Lambda - \Xi N $\end{document} interaction. The existence or nonexistence of this resonance could be evidenced by measuring, for example, the \begin{document}$ \Xi d $\end{document} cross section.
Transverse momentum balance and angular distribution of ${{b\bar{b}}}$ dijets in Pb + Pb collisions
Wei Dai, Sa Wang, Shan-Liang Zhang, Ben-Wei Zhang, Enke Wang
2020, 44(10): 104105. doi: 10.1088/1674-1137/abab8f
Abstract:
In this study, the production of inclusive b-jet and \begin{document}$b\bar{b}$\end{document} dijets in Pb + Pb collisions has been investigated by considering the in-medium evolution of heavy and light quarks simultaneously. The initial hard processes of inclusive b-jet and \begin{document}$b\bar{b}$\end{document} dijets production are described using a next-to-leading order (NLO) plus parton shower Monte Carlo (MC) event generator, SHERPA, which can be well matched with the experimental data in p + p collisions. The framework uses the Langevin transport model to describe the evolution of the bottom quark. Furthermore, the collisional energy loss and higher-twist description are considered to determine the radiative energy loss from both the bottom and light quarks. We compare the theoretical simulation of the inclusive jet and b-jet \begin{document}$R_{\rm AA}$\end{document} in the Pb + Pb collisions at \begin{document}$\sqrt{s_{ NN}}=2.76$\end{document} TeV with the experimental data and present the theoretical simulation of the momentum balance of the \begin{document}$b\bar{b}$\end{document} dijet in the Pb + Pb collisions at \begin{document}$5.02$\end{document} TeV along with recent CMS data for the first time. A similar trend to that seen in inclusive dijets is observed in \begin{document}$b\bar{b}$\end{document} dijets; the distribution of the production shifts to smaller \begin{document}$x_{\rm J}$\end{document} owing to the jet quenching effect. Finally, we report the prediction of the normalized azimuthal angle distribution of the \begin{document}$b\bar{b}$\end{document} dijet in the Pb + Pb collisions at \begin{document}$5.02$\end{document} TeV. The medium-induced energy loss effect of the \begin{document}$b\bar{b}$\end{document} dijets will generally suppress its production; however, the same side (\begin{document}$\Delta \phi \to 0$\end{document} region) suffers more energy loss than the far side (\begin{document}$\Delta \phi \to \pi$\end{document} region), thus leading to suppression on the same side and enhancement on the far side in the normalized azimuthal angle distribution in A + A collisions.
Massless limit of transport theory for massive Fermions
Xingyu Guo
2020, 44(10): 104106. doi: 10.1088/1674-1137/ababf9
Abstract:
We studied the \begin{document}$ m = 0 $\end{document} limit of different components of Wigner functions for massive fermions. Comparing with the chiral kinetic theory, we separated the vanishing and non-vanishing parts of vector and axial-vector components, up to the first order of \begin{document}$ \hbar $\end{document}. Then, we discussed the possible physical meaning of the vanishing and non-vanishing parts and their different behaviors at thermal equilibrium.
PARTICLE AND NUCLEAR ASTROPHYSICS AND COSMOLOGY
Quantum ekpyrotic mechanism in Fermi-bounce curvaton cosmology
Andrea Addazi, Antonino Marciano
2020, 44(10): 105101. doi: 10.1088/1674-1137/aba58d
Abstract:
Within the context of the Fermi-bounce curvaton mechanism, we analyze the one-loop radiative corrections to the four-fermion interaction, generated by the non-dynamical torsion field in the Einstein-Cartan-Holst-Sciama-Kibble theory. We show that contributions that arise from the one-loop radiative corrections modify the energy-momentum tensor, mimicking an effective Ekpyrotic fluid contribution. Therefore, we call this effect quantum Ekpyrotic mechanism. This leads to the dynamical washing out of anisotropic contributions to the energy-momentum tensor, without introducing any new extra Ekpyrotic fluid. We discuss the stability of the bouncing mechanism and derive the renormalization group flow of the dimensional coupling constant ξ, checking whether any change of its sign takes place towards the bounce. This enforces the theoretical motivations in favor of the torsion curvaton bounce cosmology as an alternative candidate to the inflation paradigm.
Durgapal IV model considering the minimal geometric deformation approach
Francisco Tello-Ortiz, Ángel Rincón, Piyali Bhar, Y. Gomez-Leyton
2020, 44(10): 105102. doi: 10.1088/1674-1137/aba5f7
Abstract:
The present article reports the study of local anisotropic effects on Durgapal's fourth model in the context of gravitational decoupling via the minimal geometric deformation approach. To achieve this, the most general equation of state relating the components of the \begin{document}$\theta$\end{document}−sector is imposed to obtain the decoupler function \begin{document}$f(r)$\end{document}. In addition, certain properties of the obtained solution, such as the behavior of the salient material content threading the stellar interior; causality and energy conditions; hydrostatic balance through the modified Tolman−Oppenheimer−Volkoff conservation equation and stability mechanism against local anisotropies using the adiabatic index; sound velocity of the pressure waves; convection factor; and the Harrison−Zeldovich−Novikov procedure, are investigated to check whether the model is physically admissible or not. Regarding the stability analysis, it is found that the model presents unstable regions when the sound speed of the pressure waves and convection factor are used in distinction with the adiabatic index and Harrison−Zeldovich−Novikov case. To produce a more realistic picture, the numerical data for some known compact objects were determined and different values of the parameter \begin{document}$\alpha$\end{document} were considered to compare with the GR case, i.e., \begin{document}$\alpha=0$\end{document}.
Multi-pole dark energy
Chao-Jun Feng, Xiang-Hua Zhai, Xin-Zhou Li
2020, 44(10): 105103. doi: 10.1088/1674-1137/aba5f9
Abstract:
A scalar field with a pole in its kinetic term is often used to study cosmological inflation; it can also play the role of dark energy, which is called the pole dark energy model. We propose a generalized model where the scalar field may have two or even multiple poles in the kinetic term, and we call it the multi-pole dark energy. We find that the poles can place some restrictions on the values of the original scalar field with a non-canonical kinetic term. After the transformation to the canonical form, we get a flat potential for the transformed scalar field even if the original field has a steep one. The late-time evolution of the universe is obtained explicitly for the two pole model, while dynamical analysis is performed for the multiple pole model. We find that it does have a stable attractor solution, which corresponds to the universe dominated by the potential of the scalar field.
Running vacuum model in a non-flat universe
Chao-Qiang Geng, Yan-Ting Hsu, Lu Yin, Kaituo Zhang
2020, 44(10): 105104. doi: 10.1088/1674-1137/abab86
Abstract:
We investigate observational constraints on the running vacuum model (RVM) of \begin{document}$\Lambda=3\nu (H^{2}+K/a^2)+c_0$\end{document} in a spatially curved universe, where \begin{document}$\nu$\end{document} is the model parameter, \begin{document}$K$\end{document} corresponds to the spatial curvature constant, \begin{document}$a$\end{document} represents the scalar factor, and \begin{document}$c_{0}$\end{document} is a constant defined by the boundary conditions. We study the CMB power spectra with several sets of \begin{document}$\nu$\end{document} and \begin{document}$K$\end{document} in the RVM. By fitting the cosmological data, we find that the best fitted \begin{document}$\chi^2$\end{document} value for RVM is slightly smaller than that of \begin{document}$\Lambda$\end{document}CDM in the non-flat universe, along with the constraints of \begin{document}$\nu\leqslant O(10^{-4})$\end{document} (68% C.L.) and \begin{document}$|\Omega_K=-K/(aH)^2|\leqslant O(10^{-2})$\end{document} (95% C.L.). In particular, our results favor the open universe in both \begin{document}$\Lambda$\end{document}CDM and RVM. In addition, we show that the cosmological constraints of \begin{document}$\Sigma m_{\nu}=0.256^{+0.224}_{-0.234}$\end{document} (RVM) and \begin{document}$\Sigma m_{\nu}=0.257^{+0.219}_{-0.234}$\end{document} (\begin{document}$\Lambda$\end{document}CDM) at 95% C.L. for the neutrino mass sum are relaxed in both models in the spatially curved universe.
Jet quenching parameter from a soft wall AdS/QCD model
Xiangrong Zhu, Zi-qiang Zhang
2020, 44(10): 105105. doi: 10.1088/1674-1137/abab87
Abstract:
We study the effect of chemical potential and nonconformality on the jet quenching parameter in a holographic QCD model with conformal invariance broken by background dilaton. The presence of chemical potential and nonconformality both increase the jet quenching parameter, thus enhancing the energy loss, consistently with the findings of the drag force.
Exploring physical properties of compact stars in ${ f(R,T)}$ -gravity: An embedding approach
Ksh. Newton Singh, Abdelghani Errehymy, Farook Rahaman, Mohammed Daoud
2020, 44(10): 105106. doi: 10.1088/1674-1137/abab88
Abstract:
Solving field equations exactly in \begin{document}$f(R,T)-$\end{document} gravity is a challenging task. To do so, many authors have adopted different methods such as assuming both the metric functions and an equation of state (EoS) and a metric function. However, such methods may not always lead to well-behaved solutions, and the solutions may even be rejected after complete calculations. Nevertheless, very recent studies on embedding class-one methods suggest that the chances of arriving at a well-behaved solution are very high, which is inspiring. In the class-one approach, one of the metric potentials is estimated and the other can be obtained using the Karmarkar condition. In this study, a new class-one solution is proposed that is well-behaved from all physical points of view. The nature of the solution is analyzed by tuning the \begin{document}$f(R,T)-$\end{document} coupling parameter \begin{document}$\chi$\end{document} , and it is found that the solution leads to a stiffer EoS for \begin{document}$\chi=-1$\end{document} than that for \begin{document}$\chi=1$\end{document} . This is because for small values of \begin{document}$\chi$\end{document} , the velocity of sound is higher, leading to higher values of \begin{document}$M_{\rm max}$\end{document} in the \begin{document}$M-R$\end{document} curve and the EoS parameter \begin{document}$\omega$\end{document} . The solution satisfies the causality condition and energy conditions and remains stable and static under radial perturbations (static stability criterion) and in equilibrium (modified TOV equation). The resulting \begin{document}$M-R$\end{document} diagram is well-fitted with observed values from a few compact stars such as PSR J1614-2230, Vela X-1, Cen X-3, and SAX J1808.4-3658. Therefore, for different values of \begin{document}$\chi$\end{document} , the corresponding radii and their respective moments of inertia have been predicted from the \begin{document}$M-I$\end{document} curve.
Revisiting black hole thermodynamics in massive gravity: charged particle absorption and infalling shell of dust
Shi-Qian Hu, Bo Liu, Xiao-Mei Kuang, Rui-Hong Yue
2020, 44(10): 105107. doi: 10.1088/1674-1137/abab8a
Abstract:
In this study, we apply two methods to consider the variation of massive black holes in both normal and extended thermodynamic phase spaces. The first method considers a charged particle being absorbed by the black hole, whereas the second considers a shell of dust falling into it. With the former method, the first and second laws of thermodynamics are always satisfied in the normal phase space; however, in the extended phase space, the first law is satisfied but the validity of the second law of thermodynamics depends upon the model parameters. With the latter method, both laws are valid. We argue that the former method's violation of the second law of thermodynamics may be attributable to the assumption that the change of internal energy of the black hole is equal to the energy of the particle. Finally, we demonstrate that the event horizon always ensures the validity of weak cosmic censorship in both phase spaces; this means that the violation of the second law of thermodynamics, arising under the aforementioned assumption, does not affect the weak cosmic censorship conjecture. This further supports our argument that the assumption in the first method is responsible for the violation and requires deeper treatment.
The effective potential originating from swampland and the non-trivial Brans-Dicke coupling
Qi Li, Jing Li, Yongxiang Zhou, Xun Xue
2020, 44(10): 105108. doi: 10.1088/1674-1137/abab8e
Abstract:
The effective vacuum energy density contributed by the non-trivial contortion distribution and the bare vacuum energy density can be viewed as the energy density of the auxiliary quintessence field potential. We find that the negative bare vacuum energy density from string landscape leads to a monotonically decreasing quintessence potential while the positive one from swampland leads to the metastable or stable de Sitter-like potential. Moreover, the non-trivial Brans-Dicke like coupling between the quintessence field and gravitation field is necessary in the latter case.
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