## 2020 Vol. 44, No. 2

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2020, 44(2): 021001. doi: 10.1088/1674-1137/44/2/021001
Abstract:
From April to July 2018, a data sample at the peak energy of the \begin{document}$\varUpsilon \left( {4{\rm{S}}} \right)$\end{document} resonance was collected with the Belle II detector at the SuperKEKB electron-positron collider. This is the first data sample of the Belle II experiment. Using Bhabha and digamma events, we measure the integrated luminosity of the data sample to be (\begin{document}$496.3 \pm 0.3 \pm 3.0)\;{\rm pb}^{-1}$\end{document}, where the first uncertainty is statistical and the second is systematic. This work provides a basis for future luminosity measurements at Belle II.
2020, 44(2): 022001. doi: 10.1088/1674-1137/44/2/022001
Abstract:
The production of \begin{document}$\varXi _{cc}^ {++}$\end{document} baryons in proton-proton collisions at a centre-of-mass energy of \begin{document}$\sqrt{s}=13\;{\rm{TeV}}$\end{document} is measured in the transverse-momentum range \begin{document}$4 < p_{\rm{T}} <15\;{\rm{GeV}}/c$\end{document} and the rapidity range \begin{document}$2.0 <y <4.5$\end{document}. The data used in this measurement correspond to an integrated luminosity of \begin{document}$1.7\;{\rm{fb}}^{-1}$\end{document}, recorded by the LHCb experiment during 2016. The ratio of the \begin{document}$\varXi _{cc}^ {++}$\end{document} production cross-section times the branching fraction of the \begin{document}$\varXi _{cc}^ {++} \to \varLambda _c^ + K^-\pi^+ \pi^+$\end{document} decay relative to the prompt \begin{document}$\varLambda _c^ +$\end{document} production cross-section is found to be \begin{document}$(2.22\pm 0.27 \pm 0.29)\times 10^{-4}$\end{document}, assuming the central value of the measured \begin{document}$\varXi _{cc}^ {++}$\end{document} lifetime, where the first uncertainty is statistical and the second systematic.
2020, 44(2): 023101. doi: 10.1088/1674-1137/44/2/023101
Abstract:
The thermalization process of the holographic entanglement entropy (HEE) of an annular domain is investigated in the Vaidya-AdS geometry. We determine numerically the Hubeny-Rangamani-Takayanagi (HRT) surface, which may be a hemi-torus or two disks, depending on the ratio of the inner radius to the outer radius of the annulus. More importantly, for some fixed ratio of the two radii, the annulus undergoes a phase transition, or a double phase transition, during thermalization from a hemi-torus to a two-disk configuration, or vice versa. The occurrence of various phase transitions is determined by the ratio of the two radii of the annulus. The rate of entanglement growth is also investigated during the thermal quench. The local maximal rate of entanglement growth occurs in the region with a double phase transition. Finally, if the quench process is sufficiently slow, which may be controlled by the thickness of the null shell, the region with a double phase transition vanishes.
2020, 44(2): 023102. doi: 10.1088/1674-1137/44/2/023102
Abstract:
The constituent quark model is used to compute the ground and excited state masses of QQQ baryons containing either c or b quarks. The quark model parameters previously used to describe the properties of charmonium and bottomonium states were used in this analysis. The non-relativistic three-body bound state problem is solved by means of the Gaussian expansion method which provides sufficient accuracy and simplifies the subsequent evaluation of the matrix elements. Several low-lying states with quantum numbers \begin{document}$J^P=\frac{1}{2}^\pm, \frac{3}{2}^\pm, \frac{5}{2}^\pm$\end{document} and \begin{document}$\frac{7}{2}^+$\end{document} are reported. We compare the results with those obtained by the other theoretical formalisms. There is a general agreement for the mass of the ground state in each sector of triply heavy baryons. However, the situation is more puzzling for the excited states, and appropriate comments about the most relevant features of our comparison are given.
2020, 44(2): 023103. doi: 10.1088/1674-1137/44/2/023103
Abstract:
We study the contributions of intermediate bottomonium-like \begin{document}$Z_b$\end{document} states and the bottom meson loops in the heavy quark spin flip transitions \begin{document}$\Upsilon(4S) \to h_b(1P,2P) \pi^+\pi^-$\end{document}. Depending on the constructive or destructive interferences between the \begin{document}$Z_b$\end{document}-exchange and the bottom meson loops mechanisms, we predict two possible branching ratios for each process: BR\begin{document}$_{\Upsilon(4S) \to h_b(1P)\pi^+\pi^-}\simeq\big(1.2^{+0.8}_{-0.4}\times10^{-6}\big)$\end{document} or \begin{document}$\big( 0.5^{+0.5}_{-0.2}\times10^{-6}\big)$\end{document}, and \begin{document}${{BR}}_{\Upsilon(4S) \to h_b(2P)\pi^+\pi^-}\simeq \big(7.1^{+1.7}_{-1.1}\times10^{-10}\big)$\end{document} or \begin{document}$\big( 2.4^{+0.2}_{-0.1}\times10^{-10}\big)$\end{document}. The contribution of the bottom meson loops is found to be considerably larger than that of the \begin{document}$Z_b$\end{document}-exchange in the \begin{document}$\Upsilon(4S) \to h_b(1P) \pi\pi$\end{document} transitions, while its decay rates are not comparable to those of heavy quark spin conserved \begin{document}$\Upsilon(4S) \to \Upsilon(1S,2S) \pi\pi$\end{document} processes. We also predict the contribution of the charm meson loops in the branch fractions of \begin{document}$\psi(3S,4S) \to h_c(1P)\pi\pi$\end{document}.
2020, 44(2): 023104. doi: 10.1088/1674-1137/44/2/023104
Abstract:
We study the semileptonic decays \begin{document}$B_c^- \to (\eta_c, J/\psi) l ^- \bar{\nu}_l$\end{document} using the PQCD factorization approach with the newly defined distribution amplitudes of the \begin{document}$B_c$\end{document} meson and a new kind of parametrization for extrapolating the form factors which takes into account the recent lattice QCD results. We find the following main results: (a) the PQCD predictions of the branching ratios of the \begin{document}$B_c \to (\eta_c,J/\psi) l \bar{\nu}$\end{document} decays are smaller by about 5%-16% when the lattice results are taken into account in the extrapolation of the relevant form factors; (b) the PQCD predictions of the ratio \begin{document}$R_{\eta_c}, R_{ J/\psi}$\end{document} and of the longitudinal polarization \begin{document}$P_{\tau}$\end{document} are \begin{document}$R_{\eta_c}=0.34\pm 0.01, R_{J/\psi}=0.28\pm 0.01$\end{document}, \begin{document}$P_{\tau}(\eta_c) = 0.37\pm 0.01$\end{document} and \begin{document}$P_{\tau}(J/\psi) = -0.55 \pm 0.01$\end{document}; and (c) after including the lattice results, the theoretical predictions slightly change: \begin{document}$R_{\eta_c}=0.31\pm 0.01$\end{document}, \begin{document}$R_{ J/\psi}=0.27\pm 0.01$\end{document}, \begin{document}$P_{\tau}( \eta_c) = 0.36 \pm 0.01$\end{document} and \begin{document}$P_{\tau}( J/\psi) = -0.53\pm 0.01$\end{document}. The theoretical predictions of \begin{document}$R_{ J/\psi}$\end{document} agree with the measurements within the errors. The other predictions could be tested by the LHCb experiment in the near future.
2020, 44(2): 023105. doi: 10.1088/1674-1137/44/2/023105
Abstract:
Dark sector may couple to the Standard Model via one or more mediator particles. We discuss two types of mediators: the dark photon \begin{document}$A^{\prime}$\end{document} and the dark scalar mediator \begin{document}$\phi$\end{document}. The total cross-sections and various differential distributions of the processes \begin{document}$e^{+} e^{-} \rightarrow q \bar{q} A^{\prime}$\end{document} and \begin{document}$e^{+} e^{-} \rightarrow q \bar{q} \phi$\end{document} (\begin{document}$q = u,\; d,\; c,\; s$\end{document} and b quarks) are discussed. We focus on the study of the invisible \begin{document}$A^{\prime}$\end{document} due to the cleaner background at future \begin{document}$e^{+} e^{-}$\end{document} colliders. It is found that the kinematic distributions of the two-jet system could be used to identify (or exclude) the dark photon and the dark scalar mediator, as well as to distinguish between them. We further study the possibility of a search for dark photons at a future CEPC experiment with \begin{document}$\sqrt{s} = 91.2$\end{document} GeV and 240 GeV. With CEPC running at \begin{document}$\sqrt{s} =$\end{document} 91.2 GeV, it would be possible to perform a decisive measurement of the dark photon (20 GeV \begin{document}$< m_{A^{\prime}} <$\end{document} 60 GeV) in less than one operating year. The lower limits of the integrated luminosity for the significance \begin{document}$S/\sqrt{B} =$\end{document} 2\begin{document}$\sigma$\end{document}, 3\begin{document}$\sigma$\end{document} and 5\begin{document}$\sigma$\end{document} are presented.
2020, 44(2): 024001. doi: 10.1088/1674-1137/44/2/024001
Abstract:
Elastic scattering of 10Be on a 208Pb target was measured at \begin{document}$E_{\rm Lab}$\end{document} = 127 MeV, which corresponds to three times the Coulomb barrier. The secondary 10Be beam was produced at the Radioactive Ion Beam Line in Lanzhou of the Heavy-Ion Research Facility in Lanzhou. The angular distribution of elastic scattering in the 10Be + 208Pb system shows a typical Fresnel diffraction peak. Optical model analysis of the angular distribution was performed using the Woods-Saxon, double-folding and global potentials. With the global potential, different density distributions were used. The results indicate that different density distributions for the projectile induce distinct effects in the angular distribution.
2020, 44(2): 024002. doi: 10.1088/1674-1137/44/2/024002
Abstract:
The level structure in neutron-deficient nucleus 91Ru was investigated via the 58Ni (36Ar, 2p1n\begin{document}$\gamma$\end{document}) 91Ru reaction at a beam energy of 111 MeV. Charged particles, neutrons, and \begin{document}$\gamma$\end{document}-rays were emitted in this reaction and detected by the DIAMANT CsI ball, Neutron Wall, and the EXOGAM Ge clover array, respectively. In addition to the previously reported levels in 91Ru, new low-to-medium spin states were observed. Angular correlation and linear polarization measurements were performed to unambiguously determine spins and parities of the excited states in 91Ru. The low-spin states of 91Ru exhibit a scheme of multi-quasiparticle excitations, which is very similar to that of the neighboring \begin{document}$N=47$\end{document} isotone. These excitations have been interpreted in terms of the shell model. The calculations performed in the configuration space \begin{document}$(p_{3/2},f_{5/2},p_{1/2},g_{9/2})$\end{document} reproduce the experimental excitation energies reasonably well, supporting the interpretation of the newly assigned positive-parity states in terms of the three quasiparticle configurations \begin{document}$\pi(g_{9/2})^{-2} \nu(g_{9/2})^{-1}$\end{document} and \begin{document}$\nu (g_{9/2})^{-3}$\end{document}.
2020, 44(2): 024101. doi: 10.1088/1674-1137/44/2/024101
Abstract:
We investigate the mass spectrum of the \begin{document}$ss \bar s \bar s$\end{document} tetraquark states in the relativized quark model. By solving the Schrödinger-like equation with the relativized potential, the masses of S- and P-wave \begin{document}$ss \bar s \bar s$\end{document} tetraquarks are obtained. The screening effects are also taken into account. It is found that the resonant structure \begin{document}$X(2239)$\end{document} observed in the \begin{document}$e^+e^- \to K^+K^-$\end{document} process by the BESIII collaboration can be assigned as a P-wave \begin{document}$1^{--}$\end{document} \begin{document}$ss \bar s \bar s$\end{document} tetraquark state. Furthermore, the radiative transition and the strong decay behavior of this structure are also estimated, which can provide helpful information for future experimental searches.
2020, 44(2): 024102. doi: 10.1088/1674-1137/44/2/024102
Abstract:
The coupled \begin{document}$\Lambda\Lambda nn-\Xi^- pnn$\end{document} system was studied to investigate whether the inclusion of channel coupling is able to bind the \begin{document}$\Lambda\Lambda nn$\end{document} system. We use a separable potential three-body model of the coupled \begin{document}$\Lambda\Lambda nn - \Xi^- pnn$\end{document} system and a variational four-body calculation with realistic interactions. Our results exclude the possibility of a \begin{document}$\Lambda\Lambda nn$\end{document} bound state by a large margin. Instead, we found a \begin{document}$\Xi^- t$\end{document} quasibound state above the \begin{document}$\Lambda\Lambda nn$\end{document} threshold.
2020, 44(2): 024103. doi: 10.1088/1674-1137/44/2/024103
Abstract:
The averaged jet charge characterizes the electric charge of the initiating parton and provides a powerful tool to distinguish quark jets from gluon jets. We predict, for the first time, the medium modification of the averaged jet charge in the heavy-ion collisions at the LHC, where jet productions in p+p collisions are simulated by PYTHIA6, and the parton energy loss in QGP is calculated with two Monte Carlo models of jet quenching: PYQUEN and JEWEL. We found that the distribution of averaged jet charge is significantly suppressed by initial state isospin effects due to the participation of neutrons with zero electric charge during nuclear collisions. The considerable enhancement of the averaged jet charge in central Pb+Pb collisions is observed relative to peripheral collisions, since the jet quenching effect is more pronounced in central collisions. The distinct feature of the averaged jet charge between quark and gluon jets, along with the sensitivity of medium modifications on the jet charge to flavor dependence of the parton energy loss, could be very useful to discriminate the energy loss pattern between quark and gluon jets in heavy-ion collisions.
2020, 44(2): 024104. doi: 10.1088/1674-1137/44/2/024104
Abstract:
Considering the magnetic field response of the QGP medium, we perform a systematical study of the chiral magnetic effect (CME), and make a comparison with the experimental results for the background-subtracted correlator H at the energies of the RHIC Beam Energy Scan (BES) and the LHC energy. The CME signals from our computations show a centrality trend and beam energy dependence that are qualitatively consistent with the experimental measurements of the charge dependent correlations. The time evolution of the chiral electromagnetic current at the RHIC and LHC energies is systematically studied. The dependence of the time-integrated current signal on the beam energy \begin{document}$\sqrt{s}$\end{document} with different centralities is investigated. Our phenomenological analysis shows that the time-integrated electromagnetic current is maximal near the collision energy \begin{document}$\sqrt{s} \approx 39$\end{document} GeV . The qualitative trend of the induced electromagnetic current is in agreement with the CME experimental results at the RHIC and LHC energies.
2020, 44(2): 024105. doi: 10.1088/1674-1137/44/2/024105
Abstract:
Based on the Froissart-Martin theorem, the Regge theory and the possible Odderon exchange, the total cross-section \begin{document}$\sigma_{\rm tot}$\end{document} and the ratio of the real to imaginary parts of the forward scattering amplitude \begin{document}$\rho$\end{document} in the \begin{document}$pp$\end{document} and \begin{document}$\bar{p}p$\end{document} elastic collisions in the TOTEM energy region are studied in the FPO model. We consider the contributions of the Froissart bound and of the Pomeron, Reggeon and Odderon exchange terms in the scattering amplitude of the \begin{document}$pp$\end{document} and \begin{document}$\bar{p}p$\end{document} elastic collisions. Using the Odderon intercept \begin{document}$\alpha_{\rm O}(0)=0.5$\end{document}, our theoretical predictions are in good agreement with the recent results of the TOTEM experiment. These results show that the Odderon, corresponding to the odd elastic scattering amplitude, is likely to exist.
2020, 44(2): 024106. doi: 10.1088/1674-1137/44/2/024106
Abstract:
The \begin{document}$\Delta$\end{document} mass dependence of the M matrix and its influence on the \begin{document}$N\Delta \to NN$\end{document} cross-sections are investigated in the one-boson exchange model. Our calculations show that the \begin{document}$\Delta$\end{document} mass dependence of the momentum of the outgoing \begin{document}$\Delta$\end{document} and the M matrix affects the calculations of \begin{document}$\sigma_{N\Delta\to NN}$\end{document}, especially around the threshold energy.
2020, 44(2): 025101. doi: 10.1088/1674-1137/44/2/025101
Abstract:
We discuss the effects of quantum fluctuations spewed by a black hole on its deflection angle. The Gauss-Bonnet theorem (GBT) is exploited with quantum corrections through the extended uncertainty principle (EUP), and the corresponding deflection angle is obtained. Moreover, we have attempted to broaden the scope of our work by subsuming the effects of plasma medium on the deflection angle. To demonstrate the degree of difference, the acquired results are compared with the prevailing findings.