## 2019 Vol. 43, No. 2

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2019, 43(2): 023001. doi: 10.1088/1674-1137/43/2/023001
Abstract:
As a Higgs factory, the CEPC (Circular Electron-Positron Collider) project aims at precision measurements of the Higgs boson properties. A baseline detector concept, APODIS (A PFA Oriented Detector for the HIggS factory), has been proposed for the CEPC CDR (Conceptual Design Report) study. We explore the Higgs signatures for this baseline design with \begin{document}$\nu\bar{\nu}$\end{document} Higgs events. The detector performance for reconstructing charged particles, photons and jets is quantified with \begin{document}$H \to \mu\mu, \gamma\gamma$\end{document} and jet final states, respectively. The resolutions of reconstructed Higgs boson mass are comparable for the different decay modes with jets in the final states. We also analyze the \begin{document}$H \to WW$\end{document} * and ZZ* decay modes, where a clear separation between different decay cascades is observed.
2019, 43(2): 023101. doi: 10.1088/1674-1137/43/2/023101
Abstract:
Lepton number violation processes can be induced by the Majorana neutrino exchange, which provide evidence for the Majorana nature of neutrinos. In addition to the natural explanation of the small neutrino masses, Type-I seesaw mechanism predicts the existence of Majorana neutrinos. The aim of this work is to study the B meson rare decays \begin{document}$B^{+} \to K^{(*)+}\mu^+\mu^-$\end{document} in the Standard Model and its extensions, and then to investigate the same-sign decay process \begin{document}$B^{+}\to K^{(*)-}\mu^{+}\mu^+$\end{document} . The corresponding dilepton invariant mass distributions are calculated. It is found that the dilepton angular distributions could shed light on the properties of new interactions induced by Majorana neutrinos.
2019, 43(2): 023102. doi: 10.1088/1674-1137/43/2/023102
Abstract:
We confront the perturbativity problem in the real scalar quintuplet minimal dark matter model. In the original model, the quintuplet quartic self-coupling inevitably hits a Landau pole at a scale ~1014 GeV, far below the Planck scale. In order to push up this Landau pole scale, we extend the model with a fermionic quintuplet and three fermionic singlets which couple to the scalar quintuplet via Yukawa interactions. Involving such Yukawa interactions at a scale ~1010 GeV can not only keep all couplings perturbative up to the Planck scale, but can also explain the smallness of neutrino masses via the type-I seesaw mechanism. Furthermore, we identify the parameter regions favored by the condition that perturbativity and vacuum stability are both maintained up to the Planck scale.
2019, 43(2): 023103. doi: 10.1088/1674-1137/43/2/023103
Abstract:
BESIII data show a particular angular distribution for the decay of \begin{document}$J/\psi$\end{document} and \begin{document}$\psi(2S)$\end{document} mesons into \begin{document}$\Lambda \overline \Lambda$\end{document} and \begin{document}$\Sigma^0 {\overline \Sigma}{}^0$\end{document} hyperons: the angular distribution of the decay \begin{document}${{\psi(2S)}} \to {\Sigma^0\overline{\Sigma}{}^0}$\end{document} exhibits an opposite trend with respect to the other three channels: \begin{document}$J/\psi \to \Lambda \overline \Lambda$\end{document} , \begin{document}$J/\psi \to \Sigma^0 {\overline \Sigma}{}^0$\end{document} and \begin{document}$\psi(2S) \to \Lambda \overline \Lambda$\end{document} . We define a model to explain the origin of this phenomenon.
2019, 43(2): 023104. doi: 10.1088/1674-1137/43/2/023104
Abstract:
Whether an additional light scalar exists is an interesting topic in particle physics beyond the Standard Model (SM), as we do not know as yet the nature of physics beyond the SM in the low mass region in view of the inconsistent results of the ATLAS and CMS collaborations in their search for light resonances around 95 GeV in the diphoton channel. We study a light scalar in the Minimal Dilaton Model (MDM). Under the theoretical and the latest experimental constraints, we sort the selected data samples into two scenarios according to the diphoton rate of the light scalar: the large-diphoton scenario (with \begin{document}$\sigma_{\gamma\gamma}/{\rm SM}\gtrsim0.2$\end{document} ) and the small-diphoton scenario (with \begin{document}$\sigma_{\gamma\gamma}/{\rm SM}\lesssim0.2$\end{document} ), which are favored by the CMS and ATLAS results, respectively. We compare the two scenarios, test the characteristics of the model parameters, the scalar couplings, production and decay, and consider how they could be further discerned at colliders. We draw the following conclusions for the two scenarios: (i) The large-diphoton scenario has in general a small Higgs-dilaton mixing angle ( \begin{document}$|\sin\theta_S|\lesssim0.2$\end{document} ) and a small dilaton vacuum expectation value (VEV) \begin{document}$f$\end{document} ( \begin{document}$0.5\lesssim\eta\equiv v/f\lesssim1$\end{document} ), and the small-diphoton scenario has large mixing ( \begin{document}$|\sin\theta_S|\gtrsim0.4$\end{document} ) or large VEV ( \begin{document}$\eta\equiv v/f\lesssim0.3$\end{document} ). (ii) The large-diphoton scenario in general predicts small \begin{document}$s\gamma\gamma$\end{document} coupling ( \begin{document}$|C_{s\gamma\gamma}/{\rm SM}|\lesssim0.3$\end{document} ) and large \begin{document}$sgg$\end{document} coupling ( \begin{document}$0.6\lesssim|C_{sgg}/{\rm SM}|\lesssim1.2$\end{document} ), while the small-diphoton scenario predicts small \begin{document}$sgg$\end{document} coupling ( \begin{document}$|C_{sgg}/{\rm SM}|\lesssim0.5$\end{document} ). (iii) The large-diphoton scenario can interpret the small diphoton excess seen by CMS at its central value, when \begin{document}$m_s\simeq95$\end{document} GeV, \begin{document}$\eta\simeq0.6$\end{document} and \begin{document}$|\sin\theta_S|\simeq0$\end{document} . (iv) The large-diphoton scenario in general predicts a negative correlation between the Higgs couplings \begin{document}$|C_{h\gamma\gamma}/{\rm SM}|$\end{document} and \begin{document}$|C_{hgg}/{\rm SM}|$\end{document} , while the small-diphoton scenario predicts that both couplings are smaller than 1, or \begin{document}$|C_{h\gamma\gamma}/{\rm SM}|\lesssim0.9 \lesssim|C_{hgg}/{\rm SM}|$\end{document} .
2019, 43(2): 023105. doi: 10.1088/1674-1137/43/2/023105
Abstract:
The thermodynamics of strongly interacting matter near the critical end point are investigated in a holographic QCD model, which can describe the QCD phase diagram in \begin{document}$T-\mu$\end{document} plane qualitatively. Critical exponents along different axes ( \begin{document}$\alpha,\beta,\gamma,\delta$\end{document} ) are extracted numerically. It is given that \begin{document}$\alpha\approx 0$\end{document} , \begin{document}$\beta\approx 0.54$\end{document} , \begin{document}$\gamma \approx 1.04$\end{document} , and \begin{document}$\delta \approx 2.97$\end{document} , which is similar to the three-dimensional Ising mean-field approximation and previous holographic QCD model calculations. We also discuss the possibilities to go beyond the mean field approximation by including the full back-reaction of the chiral dynamics in the holographic framework.
2019, 43(2): 023106. doi: 10.1088/1674-1137/43/2/023106
Abstract:
Using the covariant light-front approach with conventional vertex functions, we estimate the production rates of D -wave charmed/charmed-strange mesons via the \begin{document}$B_{(s)}$\end{document} semileptonic decays. As the calculated production rates are significant, it seems possible to experimentally search for D -wave charmed/charmed-strange mesons via semileptonic decays, which may provide an additional approach for exploring D -wave charmed/charmed-strange mesons.
2019, 43(2): 024101. doi: 10.1088/1674-1137/43/2/024101
Abstract:
We systematically study different production sources of light nuclei in ultra-relativistic heavy-ion collisions with a new method, an exclusive quark combination model + an inclusive hadron recombination model. We take deuterons and 3He produced in Pb-Pb collisions at \begin{document}$\sqrt{s_{NN}}=2.76$\end{document} TeV as examples to show the contribution of different production sources by studying their rapidity densities \begin{document}${\rm d}N/{\rm d}y$\end{document} , yield ratios and transverse momentum ( \begin{document}$p_T$\end{document} ) spectra just after hadronization and at the final kinetic freeze-out. We find that about a half of \begin{document}$d$\end{document} and a fourth of 3He created just after hadronization can survive after the hadronic evolution process. Nucleons from \begin{document}$\Delta$\end{document} resonance decays make a much larger contribution to the regeneration of light nuclei at the hadronic phase stage, and this contribution is about 77% and 90% for \begin{document}$d$\end{document} and 3He, respectively, calculated at the final kinetic freeze-out. In addition, we give an explanation for the constant behaviors of yield ratios \begin{document}$d/p$\end{document} and 3He \begin{document}$/p$\end{document} as a function of the averaged charged multiplicity in Pb-Pb collisions and also provide a possible explanation for the observation that \begin{document}$d/p$\end{document} in Pb-Pb collisions is larger by a factor of about two than in pp collisions at LHC energies.
2019, 43(2): 024102. doi: 10.1088/1674-1137/43/2/024102
Abstract:
With quark-antiquark annihilation and creation in the first Born approximation, we study the reactions: \begin{document}$K \bar {K} \to K \bar {K}^\ast$\end{document} , \begin{document}$K \bar{K} \to K^* \bar{K}$\end{document} , \begin{document}$\pi K \to \pi K^\ast$\end{document} , \begin{document}$\pi K \to \rho K$\end{document} , \begin{document}$\pi \pi \to K \bar{K}^\ast$\end{document} , \begin{document}$\pi \pi \to K^\ast \bar{K}$\end{document} , \begin{document}$\pi \pi \to K^\ast \bar{K}^\ast$\end{document} , \begin{document}$\pi \rho \to K \bar{K}$\end{document} , \begin{document}$\pi \rho \to K^\ast \bar{K}^\ast$\end{document} , \begin{document}$\rho \rho \to K^\ast \bar{K}^\ast$\end{document} , \begin{document}$K \bar{K}^\ast \to \rho \rho$\end{document} , and \begin{document}$K^* \bar{K} \to \rho \rho$\end{document} . Unpolarized cross sections for the reactions are obtained from transition amplitudes that are composed of mesonic quark-antiquark relative-motion wave functions and the transition potential for quark-antiquark annihilation and creation. Using a quark-antiquark potential that is equivalent to the transition potential, we prove that the total spin of the two final mesons may not be equal to the total spin of the two initial mesons. Based on flavor matrix elements, cross sections for some isospin channels of reactions can be obtained from other isospin channels of the reactions. Remarkable temperature dependence of the cross sections was observed.
2019, 43(2): 024103. doi: 10.1088/1674-1137/43/2/024103
Abstract:
We propose a dynamical description of the 136Xe + p spallation at 1000 MeV/nucleon with the aim of probing the mechanism which rules the production of intermediate-mass fragments (IMF). The isospin-dependent quantum molecular dynamics (IQMD) model is used to describe the dynamical process of spallation until hot fragments with excitation energy less than a certain value Estop are formed. The statistical code GEMINI is applied to simulate the light-particle evaporation from hot fragments. It is found that IMF production is well described by the model when Estop = 2 MeV/nucleon is used. Comparison of the calculated mean neutron-to-proton ratio and the experimental data indicates that Estop should be 3 MeV/nucleon.
2019, 43(2): 024104. doi: 10.1088/1674-1137/43/2/024104
Abstract:
Based on the neutron and proton degrees of freedom, low-lying energy levels, \begin{document}$E2$\end{document} , \begin{document}$M1$\end{document} , and \begin{document}$E0$\end{document} transition strengths of nucleus 124Te have been calculated by the neutron-proton interacting boson model. The calculated results are reasonably consistent with the experimental data. By comparing the key observables of the states at the critical point of \begin{document}${\rm U}_{\pi \nu}(5)$\end{document} - \begin{document}${\rm O}_{\pi \nu}(6)$\end{document} transition with the experimental data and calculated results, we show that the 124Te is a possible nucleus at the critical point of the second-order phase transition from vibration to unstable rotation, and such a critical point exhibits slight triaxial rotation. The 0 \begin{document}$_2^ +$\end{document} state of 124Te can be interpreted as the lowest state of the first-excited family of the intrinsic levels in the critical point symmetry.
2019, 43(2): 024105. doi: 10.1088/1674-1137/43/2/024105
Abstract:
The in-medium \begin{document}$NN\rightarrow N\Delta$\end{document} cross sections and its differential cross sections in isospin asymmetric nuclear medium are investigated in the framework of the one-boson exchange model by including isovector mesons, i.e., \begin{document}$\delta$\end{document} and \begin{document}$\rho$\end{document} mesons. Our results show that the in-medium \begin{document}$NN\rightarrow N\Delta$\end{document} cross sections are suppressed when the density increases, and the differential cross sections become isotropic with an increase in the density around the \begin{document}$\Delta$\end{document} threshold energy. The isospin splitting on the medium correction factor, \begin{document}$R=\sigma_{ NN\rightarrow N\Delta}^*/\sigma_{NN\rightarrow N\Delta}^{\rm{free}}$\end{document} is observed for different channels of \begin{document}$NN\to N\Delta$\end{document} , especially around the threshold energy for all the effective Lagrangian parameters. By analyzing the selected effective Lagrangian parameters, our results show that the larger effective mass is, the weaker medium correction \begin{document}$R$\end{document} is.
2019, 43(2): 025101. doi: 10.1088/1674-1137/43/2/025101
Abstract:
We study a spherically symmetric spacetime made of an anisotropic fluid whose radial equation-of-state is given by \begin{document}$p_1 = -\rho$\end{document} . This case allows analytic solutions and is a good example for studying the static configuration of a black hole plus matter. For a given equation-of-state parameter \begin{document}$w_2 = p_2/\rho$\end{document} for angular directions, we find the exact solutions of the Einstein equation described by two parameters. We classify the solutions into six types based on the behavior of the metric function. Depending on the parameters, the solutions can have event and cosmological horizons. One of the solution types corresponds to a generalization of the Reissner-Nordström black hole, the thermodynamic properties for which are obtained in a simple form. The solutions are stable under radial perturbations.
2019, 43(2): 025102. doi: 10.1088/1674-1137/43/2/025102
Abstract:
We investigate the power spectra of the CMB temperature and matter density in the running vacuum model (RVM) with the time-dependent cosmological constant of \begin{document}$\Lambda = 3 \nu H^2 + \Lambda_0$\end{document} , where \begin{document}$H$\end{document} is the Hubble parameter. In this model, dark energy decreases in time and decays to both matter and radiation. By using the Markov chain Monte Carlo method, we constrain the model parameter \begin{document}$\nu$\end{document} as well as the cosmological observables. Explicitly, we obtain \begin{document}$\nu \leqslant 1.54 \times 10^{-4}$\end{document} (68% confidence level) in the RVM with the best-fit \begin{document}$\chi^2_{\mathrm{RVM}} = 13968.8$\end{document} , which is slightly smaller than \begin{document}$\chi^2_{\Lambda \mathrm{CDM}} = 13969.8$\end{document} in the \begin{document}$\Lambda{\rm{CDM}}$\end{document} model of \begin{document}$\nu=0$\end{document} .