Highlights
• Sound velocity in dense stellar matter with strangeness and compact stars
2021, 45(5): 055104. doi: 10.1088/1674-1137/abea0d
The phase state of dense matter in the intermediate density range ($\sim$1-10 times the nuclear saturation density) is both intriguing and unclear and can have important observable effects in the present gravitational wave era of neutron stars. As matter density increases in compact stars, the sound velocity is expected to approach the conformal limit ($c_s/c=1/\sqrt{3}$) at high densities and should also fulfill the causality limit ($c_s/c<1$). However, its detailed behavior remains a prominent topic of debate. It was suggested that the sound velocity of dense matter could be an important indicator of a deconfinement phase transition, where a particular shape might be expected for its density dependence. In this work, we explore the general properties of the sound velocity and the adiabatic index of dense matter in hybrid stars as well as in neutron stars and quark stars. Various conditions are employed for the hadron-quark phase transition with varying interface tension. We find that the expected behavior of the sound velocity can also be achieved by the nonperturbative properties of the quark phase, in addition to a deconfinement phase transition. Moreover, it leads to a more compact star with a similar mass. We then propose a new class of quark star equation of states, which can be tested by future high-precision radius measurements of pulsar-like objects.
• Relative probabilities of breakup channels in reactions of 6,7Li with 209Bi at energies around and above the Coulomb barrier
2021, 45(5): 054104. doi: 10.1088/1674-1137/abe3ee
Coincidence measurements of breakup fragments in reactions of ${^{6, 7}{\rm{Li}}}$ with ${^{209}{\rm{Bi}}}$ at energies around and above the Coulomb barrier were carried out using a large solid-angle covered detector array. Through the Q values along with the relative energies of the breakup fragments, different breakup components (prompt breakups and delayed breakups) and different breakup modes ($\alpha + t$, $\alpha + d$, $\alpha + p$, and $\alpha + \alpha$) are distinguished. A new breakup mode, $\alpha + t$, is observed in ${^{6}{\rm{Li}}}$-induced reactions at energies above the Coulomb barrier. Correlations between breakup modes and breakup components as well as their variations with the incident energy are investigated. The results will help us better understand the breakup effects of weakly bound nuclei on the suppression of a complete fusion, particularly for the above-barrier energies.
• Categorization of two-loop Feynman diagrams in the ${{\cal O}{\mathit{\boldsymbol{(\alpha^2)}}}}$ correction to ${{\mathit{\boldsymbol{e^+}}}{{\mathit{\boldsymbol{e^-}}}} {\mathit{\boldsymbol{\rightarrow ZH}}}}$
2021, 45(5): 053102. doi: 10.1088/1674-1137/abe84d
The $e^+e^- \rightarrow ZH$ process is the dominant process for the Higgs boson production at the future Higgs factory. In order to match the analysis on the Higgs properties with highly precise experiment data, it will be crucial to include the theoretical prediction to the full next-to-next-to-leading order electroweak effect in the production rate $\sigma(e^+e^-\rightarrow ZH)$. In this inspiring work, we categorize the two-loop Feynman diagrams of the ${\cal O}(\alpha^2)$ correction to $e^+e^- \rightarrow ZH$ into 6 categories according to relevant topological structures. Although 25377 diagrams contribute to the ${\cal O}(\alpha^2)$ correction in total, the number of the most challenging diagrams with seven denominators is 2250, which contain only 312 non-planar diagrams with 155 independent types. This categorization could be a valuable reference for the complete calculation in future.
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• Effects of electric fields on 7Be half-life
Published: 2021-05-12, doi: 10.1088/1674-1137/abf6c3
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First-principle calculations based on the density functional theory (DFT) method are adopted to investigate the influence of a strong electric field on the 7Be half-life. Accordingly, electronic structures of Be and BeO are examined in the presence of a homogeneous electric field. The electron density at the nucleus is estimated upon the geometry optimization. Our computations for the Be metal indicate a 0.02% increase in the decay rate of the 7Be nucleus, corresponding to a 0.02% decrease in the 7Be half-life, both at 5.14 V/Å (0.1 a.u.). Furthermore, it is determined that the decay rate of 7Be is not considerably altered up to 3.6 V/Å in the BeO structure. Our results show that the screening energy of the electron can be dependent on the applied electric field strength. Furthermore, we predict variations in the Coulomb potential at the 7Be nucleus due to electric field application.
• Thermodynamic instability of 3D Einstein-Born-Infeld AdS black holes
Published: 2021-05-11, doi: 10.1088/1674-1137/abf1dc
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Super-entropic black holes possess finite-area but noncompact event horizons and violate the reverse isoperimetric inequality. It has been conjectured that such black holes always have negative specific heat at constant volume $C_{V}$ or negative specific heat at constant pressure $C_{P}$ whenever $C_{V}>0$, making them unstable in extended thermodynamics. In this paper, we describe a test of this instability conjecture with a family of nonlinear electrodynamic black holes, namely 3D Einstein-Born-Infeld (EBI) AdS black holes. Our results show that when nonlinear electrodynamics effects are weak, the instability conjecture is valid. However, the conjecture can be violated in some parameter region when nonlinear electrodynamics effects are strong enough. This observation thus provides a counter example to the instability conjecture, which suggests that super-entropic black holes may be thermodynamically stable.
• Hydrodynamic description of D meson production in high-energy heavy-ion collisions
Published: 2021-05-11, doi: 10.1088/1674-1137/abf645
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The large values and constituent-quark-number scaling of the elliptic flow of low-$p_T$ D mesons imply that charm quarks, initially produced through hard processes, might be partially thermalized through strong interactions with quark-gluon plasma (QGP) in high-energy heavy-ion collisions. To quantify the degree of thermalization of low-$p_T$ charm quarks, we compare the $D^0$ meson spectra and elliptic flow from a hydrodynamic model to experimental data as well as transport model simulations. We use an effective charm chemical potential at the freeze-out temperature to account for the initial charm quark production from hard processes and assume that they are thermalized in the local comoving frame of the medium before freeze-out. $D^0$ mesons are sampled statistically from the freeze-out hyper-surface of the expanding QGP as described by the event-by-event (3+1)D viscous hydrodynamic model CLVisc. Both the hydrodynamic and transport models can describe the elliptic flow of $D^0$ mesons at $p_T<3$ GeV/c as measured in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV. Though the experimental data on $D^0$ spectra are consistent with the hydrodynamic result at small $p_T\sim 1$ GeV/c, they deviate from the hydrodynamic model at high transverse momentum,$p_T>2$ GeV/c. The diffusion and parton energy loss mechanisms in the transport model can describe the measured spectra reasonably well within the theoretical uncertainty. Our comparative study indicates that charm quarks only approach local thermal equilibrium at small $p_T$, even though they acquire sizable elliptic flow comparable to light-quark hadrons at both small and intermediate $p_T$.
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2021 Vol. 45, No. 5
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