2019 Vol. 43, No. 11

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Searching for neutrino-less double beta decay of 136Xe with PandaX-II liquid xenon detector
Kaixiang Ni, Yihui Lai, Abdusalam Abdukerim, Wei Chen, Xun Chen, Yunhua Chen, Xiangyi Cui, Yingjie Fan, Deqing Fang, Changbo Fu, Lisheng Geng, Karl Giboni, Franco Giuliani, Linhui Gu, Xuyuan Guo, Ke Han, Changda He, Di Huang, Yan Huang, Yanlin Huang, Zhou Huang, Peng Ji, Xiangdong Ji, Yonglin Ju, Kun Liang, Huaxuan Liu, Jianglai Liu, Wenbo Ma, Yugang Ma, Yajun Mao, Yue Meng, Parinya Namwongsa, Jinhua Ning, Xuyang Ning, Xiangxiang Ren, Changsong Shang, Lin Si, Andi Tan, Anqing Wang, Hongwei Wang, Meng Wang, Qiuhong Wang, Siguang Wang, Xiuli Wang, Zhou Wang, Mengmeng Wu, Shiyong Wu, Jingkai Xia, Mengjiao Xiao, Pengwei Xie, Binbin Yan, Jijun Yang, Yong Yang, Chunxu Yu, Jumin Yuan, Dan Zhang, Hongguang Zhang, Tao Zhang, Li Zhao, Qibin Zheng, Jifang Zhou, Ning Zhou, Xiaopeng Zhou
2019, 43(11): 113001. doi: 10.1088/1674-1137/43/11/113001
We report the Neutrino-less Double Beta Decay (NLDBD) search results from PandaX-II dual-phase liquid xenon time projection chamber. The total live time used in this analysis is 403.1 days from June 2016 to August 2018. With NLDBD-optimized event selection criteria, we obtain a fiducial mass of 219 kg of natural xenon. The accumulated xenon exposure is 242 kg·yr, or equivalently 22.2 kg·yr of 136Xe exposure. At the region around 136Xe decay Q-value of 2458 keV, the energy resolution of PandaX-II is 4.2%. We find no evidence of NLDBD in PandaX-II and establish a lower limit for decay half-life of 2.1 \begin{document}$ \times 10^{23} $\end{document} yr at the 90% confidence level, which corresponds to an effective Majorana neutrino mass \begin{document}$m_{\beta \beta} < (1.4 - 3.7)$\end{document} eV. This is the first NLDBD result reported from a dual-phase xenon experiment.
The X(4140) and X(4160) resonances in the $ {{e^+e^-\to \gamma J/\psi \phi}}$ reaction
En Wang, Ju-Jun Xie, Li-Sheng Geng, Eulogio Oset
2019, 43(11): 113101. doi: 10.1088/1674-1137/43/11/113101
We investigate the \begin{document}$ J/\psi \phi $\end{document} invariant mass distribution in the \begin{document}$ e^+e^-\to \gamma J/\psi\phi $\end{document} reaction at a center-of-mass energy of \begin{document}$ \sqrt{s} = 4.6 $\end{document} GeV measured by the BESIII collaboration, which concluded that no significant signals were observed for \begin{document}$ e^+e^- \to \gamma X(4140) $\end{document} because of the low statistics. We show, however, that the \begin{document}$ J/\psi \phi $\end{document} invariant mass distribution is compatible with the existence of the \begin{document}$ X(4140) $\end{document} state, appearing as a peak, and a strong cusp structure at the \begin{document}$ D^*_s\bar{D}^*_s $\end{document} threshold, resulting from the molecular nature of the \begin{document}$ X(4160) $\end{document} state, which provides a substantial contribution to the reaction. This is consistent with our previous analysis of the \begin{document}$ B^+\to J/\psi\phi K^+ $\end{document} decay measured by the LHCb collaboration. We strongly suggest further measurements of this process with more statistics to clarify the nature of the \begin{document}$ X(4140) $\end{document} and \begin{document}$ X(4160) $\end{document} resonances.
Simplified dark matter models with loop effects in direct detection and the constraints from indirect detection and collider search
Tong Li, Peiwen Wu
2019, 43(11): 113102. doi: 10.1088/1674-1137/43/11/113102
We reexamine the simplified dark matter (DM) models with fermionic DM particle and spin-0 mediator. The DM-nucleon scattering cross sections in these models are low-momentum suppressed at tree-level, but receive sizable loop-induced spin-independent contribution. We perform one-loop calculations for scalar-type and twist-2 DM-quark operators, and complete two-loop calculations for scalar-type DM-gluon operator. Analyzing the loop-level contribution from new operators, we find that future direct detection experiments could be sensitive to a fraction of the parameter space. The indirect detection and collider search also provide complementary constraints on these models.
Drag force on heavy quarks from holographic QCD
Yuanhui Xiong, Xingzheng Tang, Zhongjie Luo
2019, 43(11): 113103. doi: 10.1088/1674-1137/43/11/113103
We study the drag force of a relativistic heavy quark using a holographic QCD model with conformal invariance broken by a background dilaton. The effects of the chemical potential and the confining scale on this quantity are analyzed. The drag force in this model is shown to be larger than that of \begin{document}${\cal N}=4$\end{document} supersymmetric Yang-Mills (SYM) plasma. In particular, the inclusion of the chemical potential and confining scale both enhance the drag force, in agreement with earlier findings. Moreover, we discuss how the chemical potential and confining scale influence the diffusion coefficient.
Probing the top quark flavor-changing couplings at CEPC
Liaoshan Shi, Cen Zhang
2019, 43(11): 113104. doi: 10.1088/1674-1137/43/11/113104
We propose to study the flavor properties of the top quark at the future Circular Electron Positron Collider (CEPC) in China. We systematically consider the full set of 56 real parameters that characterize the flavor-changing neutral interactions of the top quark, which can be tested at CEPC in the single top production channel. Compared with the current bounds from the LEP2 data and the projected limits at the high-luminosity LHC, we find that CEPC could improve the limits of the four-fermion flavor-changing coefficients by one to two orders of magnitude, and would also provide similar sensitivity for the two-fermion flavor-changing coefficients. Overall, CEPC could explore a large fraction of currently allowed parameter space that will not be covered by the LHC upgrade. We show that the c-jet tagging capacity at CEPC could further improve its sensitivity to top-charm flavor-changing couplings. If a signal is observed, the kinematic distribution as well as the c-jet tagging could be exploited to pinpoint the various flavor-changing couplings, providing valuable information about the flavor properties of the top quark.
A possible explanation of the threshold enhancement in the process $ {{e^+e^-\rightarrow \Lambda\bar{\Lambda} }}$
Li-Ye Xiao, Xin-Zhen Weng, Xian-Hui Zhong, Shi-Lin Zhu
2019, 43(11): 113105. doi: 10.1088/1674-1137/43/11/113105
Inspired by the recent measurement of the process \begin{document}$ e^+e^-\rightarrow \Lambda\bar{\Lambda} $\end{document}, we calculate the mass spectrum of the \begin{document}$ \phi $\end{document} meson with the GI model. For the excited vector strangeonium states \begin{document}$ \phi(3S,\; 4S,\; 5S,\; 6S) $\end{document} and \begin{document}$ \phi(2D,\; 3D,\; 4D,\; 5D) $\end{document}, we investigate the electronic decay width with the Van Royen-Weisskopf formula, and the partial widths of the \begin{document}$ \Lambda\bar{\Lambda} $\end{document}, \begin{document}$ \Xi^{-(*)}\bar{\Xi}^+ $\end{document}, and \begin{document}$ \Sigma^{+(*)}\bar{\Sigma}^{-(*)} $\end{document} decay modes with the extended quark-pair creation model. We find that the electronic decay width of the \begin{document}$ D $\end{document}-wave vector strangeonium is about \begin{document}$ 3\sim8 $\end{document} times larger than of the \begin{document}$ S $\end{document}-wave vector strangeonium. Around 2232 MeV, the partial decay width of the \begin{document}$ \Lambda\bar{\Lambda} $\end{document} mode can be up to several MeV for \begin{document}$ \phi(3^3S_1) $\end{document}, while the partial \begin{document}$ \Lambda\bar{\Lambda} $\end{document} decay width of \begin{document}$ \phi(2^3D_1) $\end{document} is \begin{document}$ {\cal{O}}(10^{-3}) $\end{document} keV. If the threshold enhancement reported by the BESIII collaboration arises from the strangeonium meson, this state is very likely the \begin{document}$ \phi(3^3S_1) $\end{document} state. We also note that the \begin{document}$ \Lambda\bar{\Lambda} $\end{document} and \begin{document}$ \Sigma^{+}\bar{\Sigma}^{-} $\end{document} partial decay widths of the states \begin{document}$ \phi(3^3D_1) $\end{document} and \begin{document}$ \phi(4^3S_1) $\end{document} are several MeV, which is sufficient to be observed in future experiments.
Alternative approach to thermodynamic phase transitions
Seyed Hossein Hendi, Shahram Panahiyan, Behzad Eslam Panah, Mubasher Jamil
2019, 43(11): 113106. doi: 10.1088/1674-1137/43/11/113106
One of the major open problems in theoretical physics is the lack of a consistent quantum gravity theory. Recent developments in our knowledge on thermodynamic phase transitions of black holes and their van der Waals-like behavior may provide an interesting quantum interpretation of classical gravity. Studying different methods of investigating phase transitions can extend our understanding of the nature of quantum gravity. In this paper, we present an alternative theoretical approach for finding thermodynamic phase transitions in the extended phase space. Unlike the standard methods based on the usual equation of state involving temperature, our approach uses a new quasi-equation constructed from the slope of temperature versus entropy. This approach addresses some of the shortcomings of the other methods and provides a simple and powerful way of studying the critical behavior of a thermodynamical system. Among the applications of this approach, we emphasize the analytical demonstration of possible phase transition points and the identification of the non-physical range of horizon radii for black holes.
Tensor force effect on the exotic structure of neutron-rich Ca isotopes
Yan-Zhao Wang, Xue-Dou Su, Chong Qi, Jian-Zhong Gu
2019, 43(11): 114101. doi: 10.1088/1674-1137/43/11/114101
The structure of neutron-rich Ca isotopes is studied in the spherical Skyrme-Hartree-Fock-Bogoliubov (SHFB) approach with SLy5, SLy5+T, and 36 sets of TIJ parametrizations. The calculated results are compared with the available experimental data for the average binding energies, two-neutron separation energies and charge radii. It is found that the SLy5+T, T31, and T32 parametrizations reproduce best the experimental properties, especially the neutron shell effects at N = 20, 28 and 32, and the recently measured two-neutron separation energy of 56Ca. The calculations with the SLy5+T and T31 parametrizations are extended to isotopes near the neutron drip line. The neutron giant halo structure in the very neutron-rich Ca isotopes is not seen with these two interactions. However, depleted neutron central densities are found in these nuclei. By analyzing the neutron mean-potential, the reason for the bubble-like structure formation is given.
Giant dipole resonance parameters from photoabsorption cross-sections
Yuan Tian, Xi Tao, Jimin Wang, Xianbo Ke, Ruirui Xu, Zhigang Ge
2019, 43(11): 114102. doi: 10.1088/1674-1137/43/11/114102
The structural effect is believed to have no influence on the decay properties of medium and heavy-mass nuclei at excitation energies above the pairing gap. These properties can be described by statistical properties using so-called photon strength functions for different multipolarities, and directly related to the photoabsorption cross-section (\begin{document}$\sigma_{\rm abs}$\end{document}). \begin{document}$\sigma_{\rm abs}$\end{document} is dominated by the electric giant dipole resonance at \begin{document}$\gamma$\end{document} energy \begin{document}$\epsilon_\gamma \leqslant 40$\end{document} MeV. In this study, we construct two kinds of systematic giant dipole resonance parameters by fitting the experimental photoabsorption cross-sections. One is based on the microscopic relativistic quasiparticle random phase approximation approach, whereas the other is estimated by the phenomenological models within the Lorentzian representation. Both of them are demonstrated ot efficiently describe the experimental photoabsorption cross-sections available for medium to heavy-mass nuclei, and they can obtain more reliable predictions for the unknown nuclear system.
A pattern for the flavor dependent quark-antiquark interaction
Muyang Chen, Lei Chang
2019, 43(11): 114103. doi: 10.1088/1674-1137/43/11/114103
A flavor dependent kernel is constructed based on the rainbow-ladder truncation of the Dyson-Schwinger and Bethe-Salpeter equations in quantum chromodynamics. The quark-antiquark interaction is composed of a flavor dependent infrared part and a flavor independent ultraviolet part. Our model gives a successful and unified description of the light, heavy and heavy-light ground state pseudoscalar and vector mesons. Our model shows, for the first time, that the infrared enhanced quark-antiquark interaction is stronger and wider for lighter quarks.
The contribution of the first forbidden transitions to the nuclear β-decay half-life
Ji-Lin You, Xiao-Ping Zhang, Qi-Jun Zhi, Zhong-Zhou Ren, Qing-Dong Wu
2019, 43(11): 114104. doi: 10.1088/1674-1137/43/11/114104
\begin{document}$ \beta $\end{document}-decay half-life is a key quantity for nuclear structure and nucleosynthesis studies. There exist large uncertainties in the contributions of allowed and forbidden transitions to the total \begin{document}$ \beta $\end{document}-decay life, which limits the resolution of the predicted \begin{document}$ \beta $\end{document}-decay half-life. We systematically study the contribution of the first forbidden (FF) transitions to the \begin{document}$ \beta^{-} $\end{document}-decay half-life, and quantify it with a formula based on simple physics considerations. We also propose a new formula for calculation of the \begin{document}$ \beta^{-} $\end{document}-decay half-life that includes the FF contribution. It is shown that the inclusion of the contribution of FF transitions significantly improves the precision of calculations of the \begin{document}$ \beta^{-} $\end{document}-decay half-life. By fitting of the RQRPA results for neutron-rich \begin{document}$ Z = 47 $\end{document}, 57 isotopes and \begin{document}$ N = 80 $\end{document}, 94 isotones, the formula for the contribution of the FF transitions gives similar results as the RQRPA calculations. However, because of limited experimental data for the branching ratios of unstable nuclei, the fit parameters are not fully constrained. Therefore, the proposed formula for the \begin{document}$ \beta^{-} $\end{document}-decay half-life is more suitable for calculations of half-lives than of the FF contributions. The formula could be used to predict the \begin{document}$ \beta^{-} $\end{document}-decay half-life in nuclear structure studies as well as nucleosynthesis calculations in stars.
Non-relativistic expansion of single-nucleon Dirac equation: Comparison between Foldy-Wouthuysen transformation andsimilarity renormalization group
Yixin Guo, Haozhao Liang
2019, 43(11): 114105. doi: 10.1088/1674-1137/43/11/114105
By following the Foldy-Wouthuysen (FW) transformation of the Dirac equation, we derive the exact analytic expression up to the 1/M4 order for general cases in the covariant density functional theory. The results are compared with the corresponding ones derived from another novel non-relativistic expansion method, the similarity renormalization group (SRG). Based on this comparison, the origin of the difference between the results obtained with the FW transformation and the SRG method is explored.
Isospin-sensitive observables as a probe of proton transition momentum in the HMT
Fang Zhang, Hai-Bo Peng, Jiang-Tao Zhao
2019, 43(11): 114106. doi: 10.1088/1674-1137/43/11/114106
Based on the IBUU transport model, the effect of proton transition momentum on collective flows is studied in 40Ca + 40Ca,112Sn + 112Sn, and 197Au + 197Au collisions at an incident beam energy of 400 MeV/A with impact parameter \begin{document}$ b = 6~{\rm fm} $\end{document}. It is found that in a neutron rich system, the difference between neutron and proton elliptic flow is largely affected by the proton transition momentum. At beam energies around (and particularly below) the pion production threshold, the \begin{document}$ \pi^{-}/\pi^{+} $\end{document} ratio is greatly sensitive to proton transition momentum in asymmetric nuclear matter. This study may help us to understand the nucleon momentum distribution in nuclei, which is important for the equation of state of asymmetric nuclear matter, such as neutron stars.
The charge-dependent Bonn potentials with pseudovector pion-nucleon coupling
Chencan Wang, Jinniu Hu, Ying Zhang, Hong Shen
2019, 43(11): 114107. doi: 10.1088/1674-1137/43/11/114107
In order to use high-precision realistic nucleon-nucleon (NN) potentials in relativistic many-body problems, new versions of the charge-dependent Bonn (CD-Bonn) NN potential are constructed with pseudovector pion-nucleon coupling, instead of pseudoscalar coupling used in the original CD-Bonn potential as given by Machleidt2) . To describe precisely the charge dependence in the NN scattering data, two effective scalar mesons are introduced, whose coupling constants with nucleons are independently determined for each partial wave and for the total angular momentum \begin{document}$ J\leqslant 4 $\end{document}. The coupling constants between the vector and pseudovector mesons and a nucleon are identical in all channels. Three revised CD-Bonn potentials with the pseudovector pion-nucleon coupling (pvCD-Bonn) are generated by fitting the Nijmegen PWA phase shift data and the deuteron binding energy with different pion-nucleon coupling strengths. The potentials reproduce the phase shifts in the spin-single channels and the low-energy NN scattering parameters very well, but result in significantly different mixing parameters in the spin-triplet channels. The D-state probabilities for the deuteron range from 4.22% to 6.05%, demonstrating that the potentials contain different components of the tensor force, which is useful when considering the role of the tensor force in nuclear few- and many-body systems.
Multifractal spectrum and spectral behavior of calcium and titanium isotopes based on nuclear shell model
V. Razazi, S. Behnia, J. Ziaei
2019, 43(11): 114108. doi: 10.1088/1674-1137/43/11/114108
We investigate the effect of valence space nucleons on the multifractal analysis (MFA) and spectral analysis of calcium and titanium isotopes. The multifractality of wavefunctions is characterized by its associated singularity spectrum f(α) and generalized dimension Dq. The random matrix theory (RMT) has been employed in the study of properties of the distribution of energy levels. In particular, we find that the number of nucleons and two-body residual interactions particularly affect the singularity and energy level spectra.