## 2019 Vol. 43, No. 5

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2019, 43(5): 053101. doi: 10.1088/1674-1137/43/5/053101
Abstract:
We initiate the study of exotic Higgs decays to long-lived particles (LLPs) at proposed future lepton colliders, focusing on scenarios with displaced hadronic final states. Our analysis entails a realistic tracker-based search strategy involving the reconstruction of displaced secondary vertices and the imposition of selection cuts appropriate for eliminating the largest irreducible backgrounds. The projected sensitivity is broadly competitive with that of the LHC and potentially superior at lower LLP masses. In addition to forecasting branching ratio limits, which may be freely interpreted in a variety of model frameworks, we interpret our results in the parameter space of a Higgs portal Hidden Valley and various incarnations of neutral naturalness, illustrating the complementarity between direct searches for LLPs and precision Higgs coupling measurements at future lepton colliders.
2019, 43(5): 053102. doi: 10.1088/1674-1137/43/5/053102
Abstract:
In this study, we explore the entanglement of free spin-\begin{document}$\displaystyle\frac{1}{2}$\end{document}, spin-1, and spin-2 fields. We start with an example involving Majorana fields in 1+1 and 2+1 dimensions. Subsequently, we perform the Bogoliubov transformation and express the vacuum state with a particle pair state in the configuration space, which is used to calculate the entropy. This clearly demonstrates that the entanglement entropy originates from the particles across the boundary. Finally, we generalize this method to free spin-1 and spin-2 fields. These higher free massless spin fields have well-known complications owing to gauge redundancy. We deal with the redundancy by gauge-fixing in the light-cone gauge. We show that this gauge provides a natural tensor product structure in the Hilbert space, while surrendering explicit Lorentz invariance. We also use the Bogoliubov transformation to calculate the entropy. The area law emerges naturally by this method.
2019, 43(5): 053103. doi: 10.1088/1674-1137/43/5/053103
Abstract:
The observed hardening of the spectra of cosmic ray protons and helium nuclei is studied within the model of nonlinear diffusive shock acceleration of supernova remnants (SNRs). In this model, the injected particles with energies below the spectral " knee” are assumed to be described by two populations with different spectral indexes around 200 GeV. The high-energy population is dominated by the particles with energies above 200 GeV released upstream of the shock of SNR, and the low-energy population is attributed to the particles with energies below 200 GeV released downstream of the shock of SNR. In this scenario, the spectral hardening of cosmic ray protons and helium nuclei observed by PAMELA, AMS-02, and CREAM experiments can be reproduced.
2019, 43(5): 054001. doi: 10.1088/1674-1137/43/5/054001
Abstract:
The multi-layer computing model is developed to calculate wide-angle neutron spectra, in the range from 0° to 180° with a 5° step, produced by bombarding a thick beryllium target with deuterons. The double-differential cross-sections (DDCSs) for the 9Be(d, xn) reaction are calculated using the TALYS-1.8 code. They are in agreement with the experimental data, and are much better than the PHITS-JQMD/GEM results at 15° , 30° , 45° and 60° neutron emission angles for deuteron energy of 10.0 MeV. In the TALYS-1.8 code, neutron contributions from direct reactions (break-up, stripping and knock-out reactions) are controlled by adjustable parameters, which describe the basic characteristics of typical direct reactions and control the relative intensity and the position of the ridgy hillock at the tail of DDCSs. It is found that the typical calculated wide-angle neutron spectra for different neutron emission angles and neutron angular distributions agree quite well with the experimental data for 13.5 MeV deuterons. The multi-layer computing model can reproduce the experimental data reasonably well by optimizing the adjustable parameters in the TALYS-1.8 code. Given the good agreement with the experimental data, the multi-layer computing model could provide better predictions of wide-angle neutron energy spectra, neutron angular distributions and neutron yields for the 9Be(d, xn) reaction neutron source.
2019, 43(5): 054101. doi: 10.1088/1674-1137/43/5/054101
Abstract:
The proton and neutron cross-shell excitations across the Z = 50 shell are investigated in the southwest quadrant of 132Sn by large-scale shell-model calculations with extended pairing and multipole-multipole force. The model space allows proton (neutron) core excitations, and both the low- and high-energy states for 130In are well described, as found by comparison with the experimental data. The monopole effects between the proton orbit \begin{document}$g_{9/2}$\end{document} and neutron orbit \begin{document}$g_{7/2}$\end{document} are studied as the new monopole correction that perfectly reproduces the first 1+ level in 130In. The energy interval of proton (neutron) core excitations in 130In lies in the range of 4.5−6.5 (2.0−4.1) MeV, and the high energy yrast states are predicted as neutron core excitations. The \begin{document}$\beta$\end{document} decays are calculated among the A=130 nuclei of 130In, 130Sn and 130Cd.
2019, 43(5): 054102. doi: 10.1088/1674-1137/43/5/054102
Abstract:
The constituent counting rule, determining the scaling behavior of the transition amplitudes in an exclusive process at high energies, is applied to probe the internal structure of the newly observed \begin{document}$d^*(2380)$\end{document} resonance. Several selected exclusive processes at high energies for the production of \begin{document}$d^*$\end{document} are discussed. Results of two structural scenarios for \begin{document}$d^*(2380)$\end{document}, a hexaquark dominant compact system in the quark degrees of freedom, and a \begin{document}$\pi N\Delta$\end{document} three-body bound state in the hadronic degrees of freedom, are analyzed and compared. A rather remarkable difference between the results of these two scenarios for the mentioned processes are addressed.
2019, 43(5): 054103. doi: 10.1088/1674-1137/43/5/054103
Abstract:
We study the quark-antiquark scattering phase shift and meson spectral function in the pion superfluid described by the Nambu-Jona-Lasinio model. Meson mixing in the pion superfluid dramatically changes the full scattering phase shift and significantly broadens the spectral function of some collective modes.
2019, 43(5): 054104. doi: 10.1088/1674-1137/43/5/054104
Abstract:
A quark coalescence model, based on semi-relativistic molecular dynamics with color interactions among quarks, is presented and applied to pp collisions. A phenomenological potential with two tunable parameters is introduced to describe the color interactions between quarks and antiquarks. The interactions drive the process of hadronization that finally results in different color neutral clusters, which can be identified as hadrons based on some criteria. A Monte Carlo generator PYTHIA is used to generate quarks in the initial state of hadronization, and different values of tunable parameters are used to study the final state distributions and correlations. Baryon-to-meson ratio, transverse momentum spectra, pseudorapidity distributions and forward-backward multiplicity correlations of hadrons produced in the hadronization process, obtained from this model with different parameters, are compared with those from PYTHIA.
2019, 43(5): 054105. doi: 10.1088/1674-1137/43/5/054105
Abstract:
This study investigates the optimal projectile/target combination for the production of new neutron-deficient isotopes of superheavy nuclei (SHN). To this end, the dependence of the evaporation residue cross-section (ERCS) used to synthesize SHN on the mass asymmetry and the isospin of colliding nuclei are analyzed within the dinuclear system (DNS) concept. The predicted ERCSs for the production of new neutron-deficient isotopes of SHN were found to be quite large with the 36S projectile, and the cross-section of SHN decreases slowly with the charge of compound nuclei owing to the increase in their survival probability, \begin{document}$W_{{\rm sur}}$\end{document}. Wsur is not canceled by the decreasing probability, PCN, that the system will evolve from a touching configuration to the compound nucleus in competition with the quasifission process.
2019, 43(5): 054106. doi: 10.1088/1674-1137/43/5/054106
Abstract:
Within the framework of the UrQMD model, by tracing the number of initial quarks in protons, we study the elliptic flow of protons with 3, 2, 1, 0 initial quarks and anti-protons in Au+Au collisions at \begin{document}$\sqrt{s_{{\rm NN}}}$\end{document} = 7.7, 11.5, 39, 200 GeV. The difference of elliptic flow between protons with 2, 1, 0 initial quarks and anti-protons is smaller than 0, or consistent with 0, respectively. The difference of elliptic flow between transported protons (with 3 initial quarks) and anti-protons is larger than 0 at 7.7, 11.5 and 39 GeV. This is in good agreement with the STAR results at 7.7 and 11.5 GeV, but overestimates the STAR results at 39 GeV. The yield of transported protons with 3 initial quarks is smaller than of protons with 2 and 1 initial quarks, and \begin{document}$v_{2}$\end{document} of all protons is much smaller than the STAR results. The observation of the difference of elliptic flow between transported protons and anti-protons in the UrQMD model partly explains the \begin{document}$v_{2}$\end{document} difference between protons and anti-protons observed in the Beam Energy Scan program at the Relativistic Heavy Ion Collider (RHIC).
2019, 43(5): 054107. doi: 10.1088/1674-1137/43/5/054107
Abstract:
The particle-number-conserving method based on the cranked shell model is used to investigate the antimagnetic rotation band in 104Pd. The experimental moments of inertia and reduced B(E2) transition probabilities are reproduced well. The J(2)/B(E2) ratios are also discussed. The occupation probability of each orbital close to the Fermi surface and the contribution of each major shell to the total angular momentum alignment as function of rotational frequency are analyzed. The backbending mechanism of the ground state band in 104Pd is understood clearly and the configuration of the antimagnetic rotation after backbending is clarified. In addition, the crossing of a four quasiparticle state with this antimagnetic rotation band is also predicted. By examining the closing of the angular momenta of four proton holes towards the neutron angular momentum, the " two-shears-like” mechanism for this antimagnetic rotation is investigated and two stages of antimagnetic rotation in 104Pd are clearly seen.
2019, 43(5): 054108. doi: 10.1088/1674-1137/43/5/054108
Abstract:
Inasmuch as the hydrostatic structure of the interior of neutron stars uniquely depends on the equation of state (EOS), the inverse constraints on EOS from astrophysical observations have been an important method for revealing the properties of high density matter. Currently, most EOS for neutron star matter are given in tabular form, but these numerical tables can have quite different resolution. To guarantee both the accuracy and efficiency in computing the Tolman-Oppenheimer-Volkoff equations, a concise standard for generating EOS tables with suitable resolution is investigated. It is shown that EOS tables with 50 points logarithmic-uniformly distributed in the supra-nuclear density segment [\begin{document}$\rho_{0}, 10 \rho_{0}$\end{document}], where \begin{document}$\rho_{0}$\end{document} is the nuclear saturation density, correspond to the interpolation induced errors of ~0.02% for the gravitational mass \begin{document}$M$\end{document} and ~0.2% for the tidal deformability \begin{document}$\Lambda$\end{document}.
2019, 43(5): 054109. doi: 10.1088/1674-1137/43/5/054109
Abstract:
We discuss the sign and energy dependence of second to tenth order susceptibilities of the baryon number, charge number, and strangeness for the analysis of critical conditions in heavy ion collisions in the LHC and RHIC by applying a modified Nambu-Jona-Lasinio model. This model is fitted to the quark condensate of the lattice QCD result at finite temperature and zero baryon chemical potential. The presence of a critical point made these susceptibilities deviate considerably from a Hadron-Resonance-Gas model that shows no criticality. The sign, magnitude, and energy dependence of these higher order fluctuations hint towards the existence and location of a critical point that could be discovered in future heavy ion collision experiments.
2019, 43(5): 054110. doi: 10.1088/1674-1137/43/5/054110
Abstract:
Jet shape measurements are employed to explore the microscopic evolution mechanisms of parton-medium interaction in ultra-relativistic heavy-ion collisions. In this study, jet shape modifications are quantified in terms of the fragmentation function \begin{document}$F(z)$\end{document}, relative momentum \begin{document}$p_{T}^{\rm rel}$\end{document}, density of charged particles \begin{document}$\rho(r)$\end{document}, jet angularity \begin{document}$girth$\end{document}, jet momentum dispersion \begin{document}$p_{T}^{\rm disp}$\end{document}, and \begin{document}$LeSub$\end{document} for proton-proton (pp) collisions at 0.9, 2.76, 5.02, 7, and 13 TeV, as well as for lead-lead collisions at 2.76 TeV and 5.02 TeV by JEWEL. A differential jet shape parameter \begin{document}$D_{girth}$\end{document} is proposed and studied at a smaller jet radius \begin{document}$r<0.3$\end{document}. The results indicate that the medium has the dominant effect on jet shape modification, which also has a weak dependence on the center-of-mass energy. Jet fragmentation is enhanced significantly at very low \begin{document}$z<0.02$\end{document}, and fragmented jet constituents are linearly spread to larger jet-radii for \begin{document}$p_{T}^{\rm rel}<1$\end{document}. The waveform attenuation phenomena is observed in \begin{document}$p_{T}^{\rm rel}$\end{document}, \begin{document}$girth$\end{document}, and \begin{document}$D_{girth}$\end{document} distributions. The results obtained for \begin{document}$D_{girth}$\end{document}from \begin{document}${\rm pp}$\end{document} to \begin{document}${\rm Pb+Pb}$\end{document}, where the wave-like distribution in \begin{document}${\rm pp}$\end{document} collision is ahead of \begin{document}${\rm Pb+Pb}$\end{document} collisions at small jet-radii, indicates a strong medium effect.