## 2020 Vol. 44, No. 12

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2020, 44(12): 123001. doi: 10.1088/1674-1137/abb4d8
Abstract:
The Circular Electron Positron Collider (CEPC), proposed as a future Higgs boson factory, will operate at a center-of-mass energy of 240 GeV and will accumulate 5.6 ab−1 of integrated luminosity in 7 years. In this study, we estimate the upper limit of BR(\begin{document}$H \rightarrow$\end{document} inv) for three independent channels, including two leptonic channels and one hadronic channel, at the CEPC. Based on the full simulation analysis, the upper limit of BR(\begin{document}$H \rightarrow$\end{document} inv) could reach 0.26% at the 95% confidence level. In the Stand Model (SM), the Higgs boson can only decay invisibly via \begin{document}$H\rightarrow ZZ^\ast\rightarrow\nu\overline{\nu}\nu\overline{\nu}$\end{document}, so any evidence of invisible Higgs decays that exceed BR(\begin{document}$H \rightarrow$\end{document} inv) of the SM will indicate a phenomenon that is beyond the SM (BSM). The invariant mass resolution of the visible hadronic decay system \begin{document}$ZH(Z \rightarrow qq$\end{document}, \begin{document}$H \rightarrow$\end{document} inv) is simulated, and the physics requirement at the CEPC detector for reaching this is given.
2020, 44(12): 123101. doi: 10.1088/1674-1137/abb4c8
Abstract:
We calculate the spinor helicity amplitudes of anomalous \begin{document}$H\to ZZ \to 4\ell$\end{document} decay. After embedding these analytic formulas into the MCFM package, we study the interference effects between the anomalous \begin{document}$gg\to H\to ZZ \to 4\ell$\end{document} process and the SM processes, which are indispensable in the Higgs off-shell region. Subsequently, the constraints on the anomalous couplings are estimated using LHC experimental data.
2020, 44(12): 123102. doi: 10.1088/1674-1137/abb4cb
Abstract:
It is believed that there are more fundamental gauge symmetries beyond those described by the Standard Model of particle physics. The scales of these new gauge symmetries are usually too high to be reachable by particle colliders. Considering that the phase transition (PT) relating to the spontaneous breaking of new gauge symmetries to the electroweak symmetry might be strongly first order, we propose considering the stochastic gravitational waves (GW) arising from this phase transition as an indirect way of detecting these new fundamental gauge symmetries. As an illustration, we explore the possibility of detecting the stochastic GW generated from the PT of \begin{document}${\bf{B}}-{\bf{L}}$\end{document} in the space-based interferometer detectors. Our study demonstrates that the GW energy spectrum is reachable by the LISA, Tianqin, Taiji, BBO, and DECIGO experiments only for the case where the spontaneous breaking of \begin{document}${\bf{B}}-{\bf{L}}$\end{document} is triggered by at least two electroweak singlet scalars.
2020, 44(12): 123103. doi: 10.1088/1674-1137/abb4cc
Abstract:
In this work, we propose the possible assignment of the newly observed \begin{document}$X(2239)$\end{document}, as well as \begin{document}$\eta(2225)$\end{document}, as a molecular state from the interaction of a baryon \begin{document}$\Lambda$\end{document} and an antibaryon \begin{document}$\bar{\Lambda}$\end{document}. With the help of effective Lagrangians, the \begin{document}$\Lambda\bar{\Lambda}$\end{document} interaction is described within the one-boson-exchange model with \begin{document}$\eta$\end{document}, \begin{document}$\eta'$\end{document}, \begin{document}$\omega$\end{document}, \begin{document}$\phi$\end{document}, and \begin{document}$\sigma$\end{document} exchanges considered. After inserting the potential kernel into the quasipotential Bethe-Salpeter equation, the bound states from the \begin{document}$\Lambda\bar{\Lambda}$\end{document} interaction can be studied by searching for the pole of the scattering amplitude. Two loosely bound states with spin parities \begin{document}$I^G(J^{PC})=0^+(0^{-+})$\end{document} and \begin{document}$0^-(1^{--})$\end{document} appear near the threshold with almost the same parameter. The \begin{document}$0^-(1^{--})$\end{document} state can be assigned to \begin{document}$X(2239)$\end{document} observed at BESIII, which is very close to the \begin{document}$\Lambda\bar{\Lambda}$\end{document} threshold. The scalar meson \begin{document}$\eta(2225)$\end{document} can be interpreted as a \begin{document}$0^+(0^{-+})$\end{document} state from the \begin{document}$\Lambda\bar{\Lambda}$\end{document} interaction. The annihilation effect is also discussed through a coupled-channel calculation plus a phenomenological optical potential. It provides large widths to two bound states produced from the \begin{document}$\Lambda\bar{\Lambda}$\end{document} interaction. The mass of the \begin{document}$1^-$\end{document} state is slightly larger than the mass of the \begin{document}$0^-$\end{document} state after including the annihilation effect, which is consistent with our assignment of these two states as \begin{document}$X(2239)$\end{document} and \begin{document}$\eta(2225)$\end{document}, respectively. The results suggest that further investigation is required to understand the structures near the \begin{document}$\Lambda\bar{\Lambda}$\end{document} threshold, such as \begin{document}$X(2239)$\end{document}, \begin{document}$\eta(2225)$\end{document}, and \begin{document}$X(2175)$\end{document}.
2020, 44(12): 123104. doi: 10.1088/1674-1137/abb4ce
Abstract:
Motivated by the problem of expanding the single-trace tree-level amplitude of Einstein-Yang-Mills theory to the BCJ basis of Yang-Mills amplitudes, we present an alternative expansion formula in gauge invariant vector space. Starting from a generic vector space consisting of polynomials of momenta and polarization vectors, we define a new sub-space as a gauge invariant vector space by imposing constraints on the gauge invariant conditions. To characterize this sub-space, we compute its dimension and construct an explicit gauge invariant basis from it. We propose an expansion formula in this gauge invariant basis with expansion coefficients being linear combinations of the Yang-Mills amplitude, manifesting the gauge invariance of both the expansion basis and coefficients. With the help of quivers, we compute the expansion coefficients via differential operators and demonstrate the general expansion algorithm using several examples.
2020, 44(12): 123105. doi: 10.1088/1674-1137/abb4d2
Abstract:
Vector boson scattering at the Large Hadron Collider (LHC) is sensitive to anomalous quartic gauge couplings (aQGCs). In this study, we investigate the aQGC contribution to \begin{document}$W \gamma jj$\end{document} production at the LHC with \begin{document}$\sqrt{s}=13$\end{document} TeV in the context of an effective field theory (EFT). The unitarity bound is applied as a cut on the energy scale of this production process, which is found to have significant suppressive effects on signals. To enhance the statistical significance, we analyze the kinematic and polarization features of the aQGC signals in detail. We find that the polarization effects induced by aQGCs are unique and can discriminate the signals from the SM backgrounds well. With the proposed event selection strategy, we obtain the constraints on the coefficients of dimension-8 operators with current luminosity. The results indicate that the process \begin{document}$pp \to W \gamma jj$\end{document} is powerful for searching for the \begin{document}$O_{M_{2,3,4,5}}$\end{document} and \begin{document}$O_{T_{5,6,7}}$\end{document} operators.
2020, 44(12): 123106. doi: 10.1088/1674-1137/abb6df
Abstract:
In this paper, we summarize the existing methods of solving the evolution equation of the leading-twist \begin{document}$B$\end{document}-meson LCDA. Then, in the Mellin space, we derive a factorization formula with next-to-leading-logarithmic (NLL) resummation for the form factors \begin{document}$F_{A,V}$\end{document} in the \begin{document}$B \to \gamma \ell\nu$\end{document} decay at leading power in \begin{document}$\Lambda/m_b$\end{document}. Furthermore, we investigate the power suppressed local contributions, factorizable non-local contributions (which are suppressed by \begin{document}$1/E_\gamma$\end{document} and \begin{document}$1/m_b$\end{document}), and soft contributions to the form factors. In the numerical analysis, which employs the two-loop-level hard function and the jet function, we find that both the resummation effect and the power corrections can sizably decrease the form factors. Finally, the integrated branching ratios are also calculated for comparison with future experimental data.
2020, 44(12): 124001. doi: 10.1088/1674-1137/abb4d3
Abstract:
The cross sections for the 94Zr(n,d*)93m+gY, 96Zr(n,γ)97Z, 96Zr(n,2n)95Zr, 90Zr(n,α)87mSr, 94Zr(n,α)91Sr,90Zr(n,p)90mY, 92Zr(n,p)92Y, and 94Zr(n,p)94Y reactions have been measured in the neutron energy range of 13.5-14.8 MeV by means of the activation technique. The neutrons were produced via the D-T reaction. A high-purity germanium detector with high energy resolution was used to measure the induced γ activities. In combination with the nuclear reaction theoretical models, the excitation curves of the above-mentioned eight nuclear reactions within the incident neutron energy range from the threshold to 20 MeV were obtained by adopting the nuclear theoretical model program system Talys-1.9. The resulting experimental cross sections were analyzed and compared with the experimental data from published studies. Calculations were performed using Talys-1.9 and are in agreement with our experimental results, previous experimental values, as well as results of the theoretical excitation curves at the corresponding energies. The theoretical excitation curves generally match the experimental data well.
2020, 44(12): 124101. doi: 10.1088/1674-1137/abb4ca
Abstract:
The atomic mass table presents zones where the structure of the states changes rapidly as a function of the neutron or proton number. Among them, notable examples are the A ≈ 100 Zr region, the Pb region around the neutron midshell (N = 104), and the N ≈ 90 rare-earth region. The observed phenomena can be understood in terms of either shape coexistence or quantum phase transitions. The objective of this study is to find an observable that can distinguish between both shape coexistence and quantum phase transitions. As an observable to be analyzed, we selected the two-neutron transfer intensity between the 0+ states in the parent and daughter nuclei. The framework used for this study is the Interacting Boson Model (IBM), including its version with configuration mixing (IBM-CM). To generate wave functions of isotope chains of interest needed for calculating transfer intensities, previous systematic studies using IBM and IBM-CM were used without changing the parameters. The results of two-neutron transfer intensities are presented for Zr, Hg, and Pt isotopic chains using IBM-CM. Moreover, for Zr, Pt, and Sm isotopic chains, the results are presented using IBM with only a single configuration, i.e., without using configuration mixing. For Zr, the two-neutron transfer intensities between the ground states provide a clear observable, indicating that normal and intruder configurations coexist in the low-lying spectrum and cross at A = 98 → 100. This can help clarify whether shape coexistence induces a given quantum phase transition. For Pt, in which shape coexistence is present and the regular and intruder configurations cross for the ground state, there is almost no impact on the value of the two-neutron transfer intensity. Similar is the situation with Hg, where the ground state always has a regular nature. For the Sm isotope chain, which is one of the quantum phase transition paradigms, the value of the two-neutron transfer intensity is affected strongly.
2020, 44(12): 124102. doi: 10.1088/1674-1137/abb4cf
Abstract:
Half-lives of α decay for Z≥ 84 nuclei are calculated based on the WKB theory applied for a phenomenological potential barrier composed of a centrifugal contribution and a screened electrostatic interaction represented by a Hulthen potential. For favored decays, the model has a single adjustable parameter associated with the screening of the electrostatic potential. The description of half lives for unfavored decays requires an additional hindrance term. A good agreement with experimental data is obtained in all considered cases. The evolution of the screening parameter for each nucleus revealed its dependence on shell filling. The model is also used for theoretical predictions on a few nuclei with uncertain or incomplete decay information.
2020, 44(12): 124103. doi: 10.1088/1674-1137/abb4d0
Abstract:
We construct a new global optical model potential to describe the elastic scattering of 12C. The experimental data of elastic-scattering angular distributions and total reaction cross sections for targets from 24Mg to 209Bi are considered below 200 MeV within the framework of the optical model. The results calculated using the derived global optical potential are then compared with the existing experimental data. The reliability of the global optical potential is further tested by predicting the elastic scattering data out of the mass and energy ranges, within which the global potential parameters are determined, and reasonable results are also obtained.
2020, 44(12): 124104. doi: 10.1088/1674-1137/abb4d1
Abstract:
Using bare Argonne V4' (AV4'), V6' (AV6'), and V8' (AV8') nucleon–nucleon (\begin{document}$NN$\end{document}) interactions, the nuclear equations of state (EOSs) for neutron matter are calculated with the unitary correlation operator and high-momentum pair methods. Neutron matter is described using a finite particle number approach with magic number \begin{document}$N=66$\end{document} under a periodic boundary condition. The central short-range correlation originating from the short-range repulsion in the \begin{document}$NN$\end{document} interaction is treated by the unitary correlation operator method (UCOM), and the tensor correlation and spin-orbit effects are described by the two-particle two-hole (2p2h) excitations of nucleon pairs, where the two nucleons with a large relative momentum are regarded as a high-momentum (HM) pair. With increasing 2p2h configurations, the total energy per particle of the neutron matter is well-converged under this UCOM+HM framework. Comparing the results calculated with AV4', AV6', and AV8' \begin{document}$NN$\end{document} interactions, we demonstrate the effects of the short-range correlation, tensor correlation, and spin-orbit coupling on the density dependence of the total energy per particle of neutron matter. Moreover, the contribution of each Hamiltonian component to the total energy per particle is discussed. The EOSs of neutron matter calculated within the present UCOM+HM framework agree with the calculations of six microscopic many-body theories, especially the auxiliary field-diffusion Monte Carlo calculations.
2020, 44(12): 124105. doi: 10.1088/1674-1137/abb4d4
Abstract:
Scrupulous theoretical study of 8Be nucleus states, both clustered and non-clustered, is performed over a wide range of excitation energies. The quantities that characterize the degree of the alpha-clustering of these states, i.e., spectroscopic factors, cluster form factors, and alpha-decay widths, are computed in the framework of the developed accurate ab initio approach. Other basic properties of the 8Be spectrum, including the binding and excitation energies and the mean values of the isospin, are calculated simultaneously. In the majority of instances, the results of the computation turn out to be in good agreement with the spectroscopic data. A number of predictions are made, and corresponding verification experiments are proposed. Prospects of the developed approach for nuclear spectroscopy are demonstrated.
2020, 44(12): 124106. doi: 10.1088/1674-1137/abb4d7
Abstract:
By adopting the adiabatic assumption in the cooling process, we discuss a novel mechanism of \begin{document}$\Upsilon(1S)$\end{document} suppression that occurs due to the fast heating process at the early stage of the fireball, instead of its finite decay width in a finite temperature medium generated by heavy ion collisions. We calculate the transition probability after the fast heating dissociation as a function of the temperature of the medium and the nuclear modification factor in central collisions and find that the suppression is not negligible at RHIC, even if the width of \begin{document}$\Upsilon(1S)$\end{document} becomes zero.
2020, 44(12): 124107. doi: 10.1088/1674-1137/abb657
Abstract:
Fragment production in spallation reactions yields key infrastructure data for various applications. Based on the empirical SPACS parameterizations, a Bayesian-neural-network (BNN) approach is established to predict the fragment cross sections in proton-induced spallation reactions. A systematic investigation has been performed for the measured proton-induced spallation reactions of systems ranging from intermediate to heavy nuclei systems and incident energies ranging from 168 MeV/u to 1500 MeV/u. By learning the residuals between the experimental measurements and SPACS predictions, it is found that the BNN-predicted results are in good agreement with the measured results. The established method is suggested to benefit the related research on nuclear astrophysics, nuclear radioactive beam sources, accelerator driven systems, proton therapy, etc.
2020, 44(12): 124108. doi: 10.1088/1674-1137/abba13
Abstract:
In this work, the characteristics of \begin{document}$2\nu\beta\beta$\end{document} decays for six nuclei (\begin{document}$^{36}$\end{document}Ar, \begin{document}$^{46}$\end{document}Ca, \begin{document}$^{48}$\end{document}Ca, \begin{document}$^{50}$\end{document}Cr, \begin{document}$^{70}$\end{document}Zn, and \begin{document}$^{136}$\end{document}Xe) in a mass range from \begin{document}$A = 36$\end{document} to \begin{document}$A = 136$\end{document} are studied within the nuclear shell model (NSM) framework. Calculations are presented for the half-lives, nuclear matrix elements (NMEs), phase space factors (\begin{document}$G_{2\nu}$\end{document}), and convergence of the NMEs. The theoretical results agree well with the experimental data. In addition, we predict the half-lives of \begin{document}$2\nu\beta\beta$\end{document} decays for four nuclei. We focus on the convergence of the NMEs by analyzing the number of contributing intermediate \begin{document}$1^{+}$\end{document} states (\begin{document}$N_{\rm{C}}$\end{document}) for the nuclei of interest. We assume that \begin{document}$N_{\rm{C}}$\end{document} is safely determined when the accumulated NMEs saturate 99.7% of the final calculated magnitude. From the calculations of the involved nuclei, we discover a connection between \begin{document}$N_{\rm{C}}$\end{document} and the total number of intermediate \begin{document}$1^{+}$\end{document} states (\begin{document}$N_{\rm{T}}$\end{document}). According to the least squares fit, we conclude that the correlation is \begin{document}$N_{\rm{C}}=\left( 10.8\pm 1.2\right) \times N_{\rm{T}}^{\left( 0.29\pm 0.02\right)}$\end{document}.
2020, 44(12): 124109. doi: 10.1088/1674-1137/abbb83
Abstract:
Weakly bound states often occur in nuclear physics. To precisely understand their properties, the coupling to the continuum should be worked out explicitly. As the first step, we use a simple nuclear model in the continuum and on a lattice to investigate the influence of a third particle on a loosely bound state of a particle and a heavy core. Our approach is consistent with the Lüscher formalism.
2020, 44(12): 125001. doi: 10.1088/1674-1137/abb658
Abstract:
We report the dark matter search results obtained using the full 132 ton·day exposure of the PandaX-II experiment, including all data from March 2016 to August 2018. No significant excess of events is identified above the expected background. Upper limits are set on the spin-independent dark matter-nucleon interactions. The lowest 90% confidence level exclusion on the spin-independent cross section is 2.2 × 10−46 cm2 at a WIMP mass of 30 GeV/c2.
2020, 44(12): 125101. doi: 10.1088/1674-1137/abb4c9
Abstract:
Recently, the non-trivial solutions for 4-dimensional black holes of Einstein-Gauss-Bonnet gravity had been discovered. In this paper, considering a charged particle entering into a 4-dimensional Gauss-Bonnet-Maxwell black hole, we calculate the black hole thermodynamic properties using the Hamilton-Jacobi equation. In the normal phase space, the cosmological constant and Gauss-Bonnet parameter are fixed, the black hole satisfies the first and second laws of thermodynamics, and the weak cosmic censorship conjecture (WCCC) is valid. On the other hand, in the case of extended phase space, the cosmological constant and Gauss-Bonnet parameter are treated as thermodynamic variables. The black hole also satisfies the first law of thermodynamics. However, the increase or decrease in the black hole's entropy depends on some specific conditions. Finally, we observe that the WCCC is violated for the near-extremal black holes in the extended phase space.
2020, 44(12): 125102. doi: 10.1088/1674-1137/abb4cd
Abstract:
To solve the cosmological constant fine tuning problem, we investigate an \begin{document}$(n+1)$\end{document}-dimensional generalized Randall-Sundrum brane world scenario with two \begin{document}$(n-1)$\end{document}-branes instead of two 3-branes. Adopting an anisotropic metric ansatz, we obtain the positive effective cosmological constant \begin{document}$\Omega_{\rm eff}$\end{document} of order \begin{document}$10^{-124}$\end{document} and only require a solution \begin{document}$\simeq50-80$\end{document}. Meanwhile, both the visible and hidden branes are stable because their tensions are positive. Therefore, the fine tuning problem can be solved quite well. Furthermore, the Hubble parameter \begin{document}$H_{1}(z)$\end{document} as a function of redshift \begin{document}$z$\end{document} is in good agreement with the cosmic chronometers dataset. The evolution of the universe naturally shifts from deceleration to acceleration. This suggests that the evolution of the universe is intrinsically an extra-dimensional phenomenon. It can be regarded as a dynamic model of dark energy that is driven by the evolution of the extra dimensions on the brane.
2020, 44(12): 125103. doi: 10.1088/1674-1137/abb4d5
Abstract:
Global symmetry can guarantee the stability of dark matter particles (DMps). However, the nonminimal coupling between dark matter (DM) and gravity can break the global symmetry of DMps, which in turn leads to their decay. Under the framework of nonminimal coupling between scalar singlet dark matter (ssDM) and gravity, it is worth exploring the extent to which the symmetry of ssDM is broken. It is suggested that the total number of decay products of ssDM cannot exceed current observational constraints. Along these lines, the data obtained with satellites such as Fermi-LAT and AMS-02 suggest that the scale of ssDM global symmetry breaking can be limited. Because the mass of many promising DM candidates is likely to be in the GeV-TeV range, we determine reasonable parameters for the ssDM lifetime within this range. We find that when the mass of ssDM is around the electroweak scale (246 GeV), the corresponding 3\begin{document}$\sigma$\end{document} lower limit of the lifetime of ssDM is \begin{document}$5.3\times10^{26}$\end{document} s. Our analysis of ssDM around the electroweak scale encompasses the most abundant decay channels of all mass ranges so that the analysis of the behavior of ssDM under the influence of gravity is more comprehensive.
2020, 44(12): 125104. doi: 10.1088/1674-1137/abb4d6
Abstract:
According to a corrected dispersion relation proposed in the study on the string theory and quantum gravity theory, the Rarita-Schwinger equation was precisely modified, which resulted in the Rarita-Schwinger-Hamilton-Jacobi equation. Using this equation, the characteristics of arbitrary spin fermion quantum tunneling radiation from non-stationary Kerr-de Sitter black holes were determined. A number of accurately corrected physical quantities, such as surface gravity, chemical potential, tunneling probability, and Hawking temperature, which describe the properties of black holes, were derived. This research has enriched the research methods and enabled increased precision in black hole physics research.
2020, 44(12): 125105. doi: 10.1088/1674-1137/abb532
Abstract:
We study light rays in the static and spherically symmetric gravitational field of the null aether theory (NAT). To this end, we employ the Gauss-Bonnet theorem to compute the deflection angle formed by a NAT black hole in the weak limit approximation. Using the optical metrics of the NAT black hole, we first obtain the Gaussian curvature and then calculate the leading terms of the deflection angle. Our calculations indicate how gravitational lensing is affected by the NAT field. We also illustrate that the bending of light stems from global and topological effects.
2020, 44(12): 125106. doi: 10.1088/1674-1137/abb656
Abstract:
We investigated the tendency in the variations of CFT2 when a rotating AdS3 black hole changes because of the fluxes transferred by the scattering of a massive scalar field according to the anti-de Sitter (AdS)/conformal field theory (CFT) correspondence. The conserved quantities of the black hole are definitely constrained by the extremal condition. Moreover, the laws of thermodynamics provide a direction for the changes in the conserved quantities. Therefore, the black hole cannot be extremal under the scattering; this is naturally preferred. According to the relationship between the rotating AdS3 black hole and dual CFT2, we find that such changes in the black hole constrain the variations in the eigenstates of dual CFT2. Furthermore, the tendency in the variations is closely related to the laws of thermodynamics.