## 2019 Vol. 43, No. 6

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2019, 43(6): 063101. doi: 10.1088/1674-1137/43/6/063101
Abstract:
Within the framework of the Dyson-Schwinger equations and by means of Multiple Reflection Expansion, we study the effect of finite volume on the chiral phase transition in a sphere, and discuss in particular its influence on the possible location of the critical end point (CEP). According to our calculations, when we take a sphere instead of a cube, the influence of finite volume on phase transition is not as significant as previously calculated. For instance, as the radius of the spherical volume decreases from infinite to 2 fm, the critical temperature \begin{document}$T_{c}$\end{document} , at zero chemical potential and finite temperature, drops only slightly. At finite chemical potential and finite temperature, the location of CEP shifts towards smaller temperature and higher chemical potential, but the amplitude of the variation does not exceed 20%. As a result, we find that not only the size of the volume but also its shape have a considerable impact on the phase transition.
2019, 43(6): 063102. doi: 10.1088/1674-1137/43/6/063102
Abstract:
One of the main problems in particle physics is to understand the origin and nature of dark matter. An exciting possibility is to consider that dark matter belongs to a new complex but hidden sector. In this paper, we assume the existence of a strongly interacting dark sector consisting of a new scalar doublet and new vector resonances, in accordance with the model recently proposed by our group. Since it was found in the previous work that it is very challenging to find the new vector resonances at the LHC, here we study the possibility of finding them at the future Compact Linear Collider (CLIC) running at \begin{document}$\sqrt{s}=3$\end{document} TeV. We consider two distinct scenarios. In the first, when the non-standard scalars are heavy, the dark resonance is intense enough to make its discovery possible at CLIC when the resonance mass is in the range [2000, 3000] GeV. In the second scenario, when the non-standard scalars are light, the new vector boson is too broad to be recognized as a resonance, and is not detectable except when the mass of the scalars is close to (but smaller than) half of the resonance mass and the scale of the dark sector is high. In all positive cases, less than a tenth of the maximum integrated luminosity is needed to reach the discovery level. Finally, we also comment on the mono-\begin{document}$Z$\end{document} production.
2019, 43(6): 064001. doi: 10.1088/1674-1137/43/6/064001
Abstract:
The potential-driving model is used to describe the driving potential distribution and to calculate the pre-neutron emission mass distributions for different incident energies in the \begin{document}${}^{237} \rm{Np(n,f)}$\end{document} reaction. The potential-driving model is implemented in Geant4 and used to calculate the fission-fragment yield distributions, kinetic energy distributions, fission neutron spectrum and the total nubar for the \begin{document}${}^{237} \rm{Np(n,f)}$\end{document} reaction. Compared with the built-in G4ParaFissionModel, the calculated results from the potential-driving model are in better agreement with the experimental data and evaluated data. Given the good agreement with the experimental data, the potential-driving model in Geant4 can describe well the neutron-induced fission of actinide nuclei, which is very important for the study of neutron transmutation physics and the design of a transmutation system.
2019, 43(6): 064101. doi: 10.1088/1674-1137/43/6/064101
Abstract:
The Faddeev AGS equations for the coupled-channels \begin{document}$\bar{K}NN-\pi\Sigma{N}$\end{document} system with quantum numbers I = 1/2 and S = 0 are solved. Using separable potentials for the \begin{document}$\bar{K}N-\pi\Sigma$\end{document} interaction, we calculate the transition probability for the \begin{document}$(Y_{K})_{I=0}+N\rightarrow\pi\Sigma{N}$\end{document} reaction. The possibility to observe the trace of the \begin{document}$K^{-}pp$\end{document} quasi-bound state in \begin{document}$\pi\Sigma{N}$\end{document} mass spectra was studied. Various types of chiral-based and phenomenological potentials are used to describe the \begin{document}$\bar{K}N-\pi\Sigma$\end{document} interaction. Finally, we show that we can observe the signature of the \begin{document}$K^{-}pp$\end{document} quasi-bound state in the mass spectra, as well as the trace of branch points in the observables.
2019, 43(6): 064102. doi: 10.1088/1674-1137/43/6/064102
Abstract:
The Efimov (Thomas) trimers in excited 12C nuclei, for which no observation exists yet, are discussed by means of analyzing the experimental data of 70(64)Zn(64Ni) + 70(64)Zn(64Ni) reactions at the beam energy of E/A = 35 MeV/nucleon. In heavy ion collisions, \begin{document}$\alpha$\end{document}-particles interact with each other and can form complex systems such as 8Be and 12C. For the 3 \begin{document}$\alpha$\end{document}-particle systems, multi-resonance processes give rise to excited levels of 12C. The interaction between any two of the 3 \begin{document}$\alpha$\end{document}-particles provides events with one, two or three 8Be. Their interfering levels are clearly seen in the minimum relative energy distributions. Events with the three \begin{document}$\alpha$\end{document}-particle relative energies consistent with the ground state of 8Be are observed with the decrease of the instrumental error for the reconstructed 7.458 MeV excitation level in 12C, which was suggested as the Efimov (Thomas) state.
2019, 43(6): 064103. doi: 10.1088/1674-1137/43/6/064103
Abstract:
Significant enhancements of \begin{document}$J/\psi$\end{document} production at very low transverse momenta were recently observed by the ALICE and STAR collaborations in peripheral hadronic A+A collisions. The anomalous excess points to coherent photon-nucleus interactions in violent hadronic heavy-ion collisions, which were conventionally studied only in ultra-peripheral collisions. Assuming that the coherent photoproduction is the underlying mechanism responsible for the excess observed in peripheral A+A collisions, its contribution in p+p collisions with nuclear overlap, i.e. non-single-diffractive collisions, is of particular interest. In this paper, we perform a calculation of exclusive \begin{document}$J/\psi$\end{document} photoproduction in non-single-diffractive p+p collisions at the RHIC and LHC energies based on the pQCD motivated parametrization using the world-wide experimental data, which could be further employed to improve the precision of the phenomenological calculations for photoproduction in A+A collisions. The differential rapidity and transverse momentum distributions of \begin{document}$J/\psi$\end{document} from photoproduction are presented. In comparison with the \begin{document}$J/\psi$\end{document} production from hadronic interactions, we find that the contribution of photoproduction is negligible.
2019, 43(6): 064104. doi: 10.1088/1674-1137/43/6/064104
Abstract:
Within an effective Lagrangian approach and resonance model, we study the \begin{document}$\gamma p \to a_1(1260)^+ n$\end{document} and \begin{document}$\gamma p \to \pi^+\pi^+\pi^- n$\end{document} reactions via the \begin{document}$\pi$\end{document}-exchange mechanism. For the \begin{document}$\gamma p \to \pi^+\pi^+\pi^- n$\end{document} reaction, we perform a calculation of the differential and total cross-sections by considering the contributions of the \begin{document}$a_1(1260)$\end{document} intermediate resonance decaying into \begin{document}$\rho \pi$\end{document} and then into \begin{document}$\pi^+\pi^+\pi^-$\end{document}. Besides, the non-resonance process is also considered. With a lower mass of \begin{document}$a_1(1260)$\end{document}, the experimental data for the invariant \begin{document}$\pi^+\pi^+\pi^-$\end{document} mass distributions can be fairly well reproduced. For the \begin{document}$\gamma p \to a_1(1260)^+ n$\end{document} reaction, with the model parameters, the total cross-section is of the order of 10 μb at the photon beam energy \begin{document}$E_{\gamma}$\end{document}~2.5 GeV. It is expected that the model calculations in this work could be tested by future experiments.
2019, 43(6): 064105. doi: 10.1088/1674-1137/43/6/064105
Abstract:
The multinucleon transfer reaction in the collisions of 40Ca+ 124Sn at \begin{document}$E_{ \rm{c.m.}}=128.5$\end{document} MeV is investigated using the improved quantum molecular dynamics model. The measured angular distributions and isotopic distributions of the products are reproduced reasonably well by the calculations. The multinucleon transfer reactions of 40Ca + 112Sn, 58Ni + 112Sn, 106Cd + 112Sn, and 48Ca + 112Sn are also studied. This demonstrates that the combinations of neutron-deficient projectile and target are advantageous for the production of exotic neutron-deficient nuclei near N, Z = 50. The charged particles' emission plays an important role at small impact parameters in the de-excitation processes of the system. The production cross sections of the exotic neutron-deficient nuclei in multinucleon transfer reactions are much larger than those measured in the fragmentation and fusion-evaporation reactions. Several new neutron-deficient nuclei can be produced in the 106Cd + 112Sn reaction. The corresponding production cross sections for the new neutron-deficient nuclei, 101, 112Sb, 103Te, and 106, 107I, are 2.0 nb, 4.1 nb, 6.5 nb, 0.4 \begin{document}$\mu$\end{document}b and 1.0 \begin{document}$\mu$\end{document}b, respectively.
2019, 43(6): 064106. doi: 10.1088/1674-1137/43/6/064106
Abstract:
The newly observed isomer and ground-state band in the odd-Z neutron-rich rare-earth nucleus 163Eu are investigated by using the cranked shell model (CSM), with pairing treated by the particle-number conserving (PNC) method. This is the first time detailed theoretical investigations are performed of the observed 964(1) keV isomer and ground-state rotational band in 163Eu. The experimental data are reproduced very well by the theoretical results. The configuration of the 964(1) keV isomer is assigned as the three-particle state \begin{document}$\displaystyle\frac{13}{2}^{-}\left(\nu\displaystyle\frac{7}{2}^{+}[633]\otimes\nu\displaystyle\frac{1}{2}^{-}[521]\otimes\pi\displaystyle\frac{5}{2}^{+}[413]\right)$\end{document}. More low-lying multi-particle states are predicted in 163Eu. Due to its significant effect on the nuclear mean field, the high-order \begin{document}$\varepsilon_{6}$\end{document} deformation plays an important role in the energy and configuration assignment of the multi-particle states. Compared to its neighboring even-even nuclei 162Sm and 164Gd, there is a 10%~15% increase of \begin{document}$J^{(1)}$\end{document} of the one-particle ground-state band in 163Eu. This is explained by the pairing reduction due to the blocking of the nucleon on the proton \begin{document}$\pi\displaystyle\frac{5}{2}^{+}$\end{document}[413] orbital in 163Eu.
2019, 43(6): 064107. doi: 10.1088/1674-1137/43/6/064107
Abstract:
In this study, we investigate the ion-ball screening model (model (I)), focused on the screening electrostatic potential per electron under the Wigner-Seitz approximation and the Q-value correction. By considering the changes of the Coulomb free energy and the effects of strong electron screening (SES) on the Q-value and the Coulomb chemical potential, we discuss the linear-response screening model (model (II)). We also analyze the influence of the SES on the \begin{document}$\beta^-$\end{document} decay antineutrino energy loss rate by considering the corrections of the Q-value, the electron chemical potential, and electron energy, as well as the shell and pair effects. The antineutrino energy loss rate is found to increase by two orders of magnitude (e.g., the SES enhancement factor reaches 651.9 for model (II)) due to the SES effect.
2019, 43(6): 064108. doi: 10.1088/1674-1137/43/6/064108
Abstract:
Nuclear matrix elements (NME) and phase space factors (PSF) entering the half-life formulas of the double-beta decay (DBD) process are two key quantities whose accurate computation still represents a challenge. In this study, we propose a new approach of calculating these, namely the direct computation of their product as an unique formula. This procedure allows a more coherent treatment of the nuclear approximations and input parameters appearing in both quantities and avoids possible confusion in the interpretation of DBD data due to different individual expressions adopted for PSF and NME (and consequently their reporting in different units) by different authors. Our calculations are performed for both two neutrino (\begin{document}$2\nu\beta\beta$\end{document}) and neutrinoless (\begin{document}$0\nu\beta\beta$\end{document}) decay modes, for five nuclei of the most experimental interest. Further, using the most recent experimental limits for \begin{document}$0\nu\beta\beta$\end{document} decay half-lives, we provide new constraints on the light mass neutrino parameter. Finally, by separating the factor representing the axial-vector constant to the forth power in the half-life formulas, we advance suggestions on how to reduce the errors introduced in the calculation by the uncertain value of this constant, exploiting the DBD data obtained from different isotopes and/or decay modes.
2019, 43(6): 064109. doi: 10.1088/1674-1137/43/6/064109
Abstract:
In our previous study, the deduced Langevin equation has been applied to investigate the isoscalar giant monopole resonance. In the current study, the framework is extended to study the isovector giant dipole resonance (IVGDR). The potential well in the IVGDR is calculated by separating the neutron and proton densities based on the Hartree-Fock ground state. Subsequently, the Langevin equation is solved self-consistently, resulting in the centroid energy of the IVGDR without width. The symmetry energy around the density of 0.02 fm−3 contributes the most to the potential well in the IVGDR. By comparison with the updated experimental data of IVGDR energies in spherical nuclei, the calculations within 37 sets of Skyrme functionals suggest the symmetry energy to be in the range of 8.13-9.54 MeV at a density of 0.02 fm−3.
2019, 43(6): 064110. doi: 10.1088/1674-1137/43/6/064110
Abstract:
A production representation of partial-wave S matrix is utilized to construct low-energy elastic pion-nucleon scattering amplitudes from cuts and poles on complex Riemann sheets. Among them, the contribution of left-hand cuts is estimated using the \begin{document}${\cal{O}}\left( {{p^3}} \right)$\end{document} results obtained in covariant baryon chiral perturbation theory within the extended-on-nass-shell scheme. By fitting to data on partial-wave phase shifts, it is indicated that the existences of hidden poles in S11 and P11 channels, as conjectured in our previous paper [Eur. Phys. J. C, 78(7): 543 (2018)], are firmly established. Specifically, the pole mass of the S11 hidden resonance is determined to be (895±81)−(164±23)i MeV, whereas, the virtual pole in the P11 channel locates at (966±18) MeV. It is found that analyses at the \begin{document}${\cal{O}}\left( {{p^3}} \right)$\end{document} level improves significantly the fit quality, comparing with the previous \begin{document}${\cal{O}}\left( {{p^2}} \right)$\end{document} one. Quantitative studies with cautious physical discussions are also conducted for the other S- and P-wave channels.
2019, 43(6): 064111. doi: 10.1088/1674-1137/43/6/064111
Abstract:
The interaction of the pseudoscalar meson and the baryon octet is investigated by solving the Bethe-Salpeter equation in the unitary coupled-channel approximation. In addition to the Weinberg-Tomozawa term, the contribution of the \begin{document}$s-$\end{document} and \begin{document}$u-$\end{document} channel potentials in the S-wave approximation are taken into account. In the sector of isospin \begin{document}$I=1/2$\end{document} and strangeness \begin{document}$S=0$\end{document}, a pole is detected in a reasonable region of the complex energy plane of \begin{document}$\sqrt{s}$\end{document} in the center-of-mass frame by analyzing the behavior of the scattering amplitude, which is higher than the \begin{document}$\eta N$\end{document} threshold and lies on the third Riemann sheet. Thus, it can be regarded as a resonance state and might correspond to the \begin{document}$N(1535)$\end{document} particle of the Particle Data Group (PDG) review. The coupling constants of this resonance state to the \begin{document}$\pi N$\end{document}, \begin{document}$\eta N$\end{document}, \begin{document}$K \Lambda$\end{document} and \begin{document}$K \Sigma$\end{document} channels are calculated, and it is found that this resonance state couples strongly to the hidden strange channels. Apparently, the hidden strange channels play an important role in the generation of resonance states with strangeness zero. The interaction of the pseudoscalar meson and the baryon octet is repulsive in the sector of isospin \begin{document}$I=3/2$\end{document} and strangeness \begin{document}$S=0$\end{document}, so that no resonance state can be generated dynamically.
2019, 43(6): 065101. doi: 10.1088/1674-1137/43/6/065101
Abstract:
We propose a novel mechanism for the production of gravitational waves in the early Universe that originates from the relaxation processes induced by the QCD phase transition. While the energy density of the quark-gluon mean-field is monotonously decaying in real time, its pressure undergoes a series of violent oscillations at the characteristic QCD time scales that generate a primordial multi-peaked gravitational waves signal in the radio frequencies’ domain. The signal is an echo of the QCD phase transition that is accessible by planned measurements at the FAST and SKA telescopes.