Highlights
• Lattice study of two-photon decay widths for scalar and pseudo-scalar charmonium
2020, 44(8): 083108. doi: 10.1088/1674-1137/44/8/083108
This exploratory study computes two-photon decay widths of pseudo-scalar ($\eta_c$) and scalar ($\chi_{c0}$) charmonium using two ensembles of $N_f = 2$ twisted mass lattice QCD gauge configurations. The simulation is performed using two lattice ensembles with lattice spacings $a = 0.067$ fm with size $32^3\times{64}$ and $a = 0.085$ fm with size $24^3\times{48}$. The decay widths for the two charmonia are obtained within the expected ballpark, but are however smaller than the experimental ones. Possible reasons for these discrepancies are discussed.
• Expected LHAASO sensitivity to decaying dark matter signatures from dwarf galaxies gamma-ray emission
2020, 44(8): 085001. doi: 10.1088/1674-1137/44/8/085001
As a next-generation complex extensive air shower array with a large field of view, the large high altitude air shower observatory (LHAASO) is very sensitive to the very-high-energy gamma rays from ~300 GeV to 1 PeV and may thus serve as an important probe for the heavy dark matter (DM) particles. In this study, we make a forecast for the LHAASO sensitivities to the gamma-ray signatures resulting from DM decay in dwarf spheroidal satellite galaxies (dSphs) within the LHAASO field of view. Both individual and combined limits for 19 dSphs incorporating the uncertainties of the DM density profile are explored. Owing to the large effective area and strong capability of the photon-proton discrimination, we find that LHASSSO is sensitive to the signatures from decaying DM particles above ${\cal{O}}(1)$ TeV. The LHAASO sensitivity to the DM decay lifetime reaches ${\cal{O}} (10^{26}) \sim {\cal{O}} (10^{28})$ s for several decay channels at the DM mass scale from 1 TeV to 100 TeV.
• Top quark pair production at complete NLO accuracy with NNLO+NNLL′ corrections in QCD
2020, 44(8): 083104. doi: 10.1088/1674-1137/44/8/083104
We describe predictions for top quark pair differential distributions at hadron colliders, by combining the next-to-next-to-leading order quantum chromodynamics calculations and next-to-leading order electroweak corrections with double resummation at the next-to-next-to-leading logarithmic accuracy of threshold logarithms and small-mass logarithms. To the best of our knowledge, this is the first study to present such a combination, which incorporates all known perturbative information. Numerical results are presented for the invariant-mass distribution, transverse-momentum distribution, and rapidity distributions.
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• Equation of state and chiral transition in soft-wall AdS/QCD with a more realistic gravitational background
Published: 2020-08-11, doi: 10.1088/1674-1137/abab90
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We construct an improved soft-wall AdS/QCD model with a cubic coupling term of the dilaton and the bulk scalar field. The background fields in this model are solved by the Einstein-dilaton system with a nontrivial dilaton potential, which has been shown to reproduce the equation of state from the lattice QCD with two flavors. The chiral transition behaviors are investigated in the improved soft-wall AdS/QCD model with the solved gravitational background, and the crossover transition can be realized. Our study provides the possibility to address the deconfining and chiral phase transitions simultaneously in the bottom-up holographic framework.
• Studying the localized CP violation and the branching fraction of the ${\bar{B}^0\rightarrow K^-\pi^+\pi^+\pi^-}$ decay
Published: 2020-08-11, doi: 10.1088/1674-1137/abac00
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In this work, we study the localized $CP$ violation and the branching fraction of the four-body decay $\bar{B}^0\rightarrow K^-\pi^+\pi^-\pi^+$ by employing a quasi-two-body QCD factorization approach. Considering the interference of $\bar{B}^0\rightarrow \bar{K}_0^*(700)\rho^0(770)\rightarrow K^-\pi^+\pi^-\pi^+$ and $\bar{B}^0\rightarrow \bar{K}^*(892)f_0(500)\rightarrow K^-\pi^+\pi^-\pi^+$ channels, we predict $\mathcal{A_{CP}}(\bar{B}^0\rightarrow K^-\pi^+\pi^-\pi^+)\in [0.15,0.28]$ and ${\cal{B}}(\bar{B}^0\rightarrow K^-\pi^+\pi^-\pi^+)\in[1.73,5.10]\times10^{-7}$, respectively, which shows that the interference mechanism of these two channels can induce the localized $CP$ violation to this four-body decay. Meanwhile, within the two quark model framework for the scalar mesons $f_0(500)$ and $\bar{K}_0^*(700)$, we calculate the direct CP violations and branching fractions of the $\bar{B}^0\rightarrow \bar{K}_0^*(700)\rho^0(770)$ and $\bar{B}^0\rightarrow \bar{K}^*(892)f_0(500)$ decays, respectively. The corresponding results are $\mathcal{A_{CP}}(\bar{B}^0\rightarrow \bar{K}_0^*(700)\rho^0(770)) \in [0.20, 0.36]$, $\mathcal{A_{CP}}(\bar{B}^0\rightarrow \bar{K}^*(892)f_0(500))\in [0.08, 0.12]$, ${\cal{B}} (\bar{B}^0\rightarrow \bar{K}_0^*(700) \rho^0(770)\in [6.76, 18.93]\times10^{-8}$ and ${\cal{B}} (\bar{B}^0\rightarrow \bar{K}^*(892)f_0(500))\in [2.66, 4.80]\times10^{-6}$, indicating that the $CP$ violations of these two-body decays are both positive and the branching fractions quite different. These studies provide a new way to investigate the aforementioned four-body decay and can be helpful in clarifying the configuration of the structure of the light scalar meson.
• A massless scalar field in Robertson-Walker spacetimes: Adiabatic regularization and Green’s function
Published: 2020-08-11, doi: 10.1088/1674-1137/44/9/095104
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We study adiabatic regularization of a coupling massless scalar field in general spatially flat Robertson-Walker (RW) spacetimes. For the conformal coupling, the 2nd-order regularized power spectrum and 4th-order regularized stress tensor are zero, and no trace anomaly exists in general RW spacetimes. This is a new result that exceeds those found in de Sitter space. For the minimal coupling, the regularized spectra are also zero in the radiation-dominant and matter-dominant stages, as well as in de Sitter space. The vanishing of these adiabatically regularized spectra is further confirmed by direct regularization of the Green's function. For a general coupling and general RW spacetimes, the regularized spectra can be negative under the conventional prescription. At a higher order of regularization, the spectra will generally become positive, but will also acquire IR divergence, which is inevitable for a massless field. To avoid the IR divergence, the inside-horizon regularization is applied. Through these procedures, nonnegative UV- and IR-convergent power spectrum and spectral energy density will eventually be achieved.
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2020 Vol. 44, No. 8
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