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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.