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Nuclear Theory·nucl-th

Thursday·June 25, 2020

7 papers—1 primary·6 cross-listed

All7Primary1Cross-lists6
  1. 02

    [Submitted on 22 Jun 2020] (cross-list from hep-ph)

    Excess of soft dielectrons and photons

    I.M. Dremin🇷🇺

    Spectra of unbound electron-positron pairs (dielectrons, in brief) and photons from decays of parapositronia produced in ultraperipheral collisions of electrically charged objects are calculated. Their shapes at energies of the NICA collider are demonstrated. Soft dielectrons and photons are abundantly produced. The relevance of these processes to the astrophysical problem of cooling electron-positron pairs and the intense emission of 511 keV photons from the Galactic center is discussed.

    Comments:
    10 pages, 2 Figures
    Subjects:
    High Energy Physics — Phenomenology (hep-ph); High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics — Experiment (hep-ex); Nuclear Experiment (nucl-ex); Nuclear Theory (nucl-th)
    arXiv:
    2006.12033 [pdf]
    Universe(2020)·7 citations
  2. 03

    [Submitted on 23 Jun 2020] (cross-list from hep-th)

    Dynamical chemistry: non-equilibrium effective actions for reactive fluids

    Michael J. Landry🇺🇸

    We present two approaches for describing chemical reactions taking place in fluid phase. The first method mirrors the usual derivation of the hydrodynamic equations of motion by relating conserved---or to account for chemical reactions, non-conserved---currents to local-equilibrium parameters. The second method involves a higher-brow approach in which we attack the same problem from the perspective of non-equilibrium effective field theory (EFT). Non-equilibrium effective actions are defined using the in-in formalism on the Schwinger-Keldysh contour and are therefore capable of describing thermal fluctuations and dissipation as well as quantum effects. The non-equilibrium EFT approach is especially powerful as all terms in the action are fully specified by the symmetries of the system; in particular the second law of thermodynamics does not need to be included by hand, but is instead derived from the action itself. We find that the equations of motion generated by both methods agree, but the EFT approach yields certain advantages. To demonstrate some of these advantages we construct a quadratic action that is valid to very small distance scales---much smaller than the scales at which ordinary hydrodynamic theories break down. Such an action captures the full thermodynamic and quantum behavior of reactions and diffusion at quadratic order. Finally, taking the low-frequency and low-wavenumber limit, we reproduce the linearized version of the well-known reaction-diffusion equations as a final coherence check.

    Comments:
    11 pages
    Subjects:
    High Energy Physics — Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Nuclear Theory (nucl-th); Fluid Dynamics (physics.flu-dyn)
    arXiv:
    2006.13220 [pdf]
    J.Stat.Mech.(2022)·16 citations
  3. 04

    [Submitted on 24 Jun 2020] (cross-list from hep-lat)

    Chiral phase structure of three flavor QCD in a background magnetic field

    Heng-Tong Ding🇨🇳 · Christian Schmidt🇩🇪 · Akio Tomiya🇺🇸 · Xiao-Dan Wang🇨🇳

    We investigate the chiral phase structure of three flavor QCD in a background magnetic field using the standard staggered action and the Wilson plaquette gauge action. We perform simulations on lattices with a temporal extent of and four spatial extents of and 24. We choose a smaller-than-physical quark mass in lattice spacing as such that there exists a crossover transition at vanishing magnetic fields, and adopt two values of magnetic field strength in lattice spacing and 2. We find that the transition becomes stronger in the presence of a background magnetic field, and turns into a first order as seen from the volume scaling of the order parameter susceptibility as well as the metastable states in the time history of the chiral condensate. On the other hand, the chiral condensate and transition temperature always increase with even within the regime of a first order phase transition. This suggests that the discrepancy in the behavior of chiral condensates and transition temperature as a function of between earlier lattice studies using larger-than-physical pion masses with standard staggered fermions and those using physical pions with improved staggered fermions is mainly due to lattice cutoff effects.

    Comments:
    23 pages, 9 figures
    Subjects:
    High Energy Physics — Lattice (hep-lat); Nuclear Theory (nucl-th)
    arXiv:
    2006.13422 [pdf]
    PRD(2020)·38 citations
  4. 05

    [Submitted on 24 Jun 2020] (cross-list from hep-th)

    Holographic quarkyonic matter

    Nicolas Kovensky🇬🇧 · Andreas Schmitt🇬🇧

    We point out a new configuration in the Witten-Sakai-Sugimoto model, allowing baryons in the pointlike approximation to coexist with fundamental quarks. The resulting phase is a holographic realization of quarkyonic matter, which is predicted to occur in QCD at a large number of colors, and possibly plays a role in real-world QCD as well. We find that holographic quarkyonic matter is chirally symmetric and that, for large baryon chemical potentials, it is energetically preferred over pure nuclear matter and over pure quark matter. The zero-temperature transition from nuclear matter to the quarkyonic phase is of first order in the chiral limit and for a realistic pion mass. For pion masses far beyond the physical point we observe a quark-hadron continuity due to the presence of quarkyonic matter.

    Comments:
    38 pages, 8 figures; v2: improvements in the text, references added, version published in JHEP
    Subjects:
    High Energy Physics — Theory (hep-th); High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
    arXiv:
    2006.13739 [pdf]
    JHEP(2020)·60 citations
  5. 06

    [Submitted on 24 Jun 2020] (cross-list from hep-ph)

    An improved anisotropic hydrodynamics ansatz

    Huda Alalawi🇺🇸 · Michael Strickland🇺🇸

    We introduce an improved form for the anisotropic hydrodynamics distribution function which explicitly takes into account the free-streaming and equilibrating contributions separately. We demonstrate that with this improvement one can better reproduce exact results available in the literature for the evolution of moments of the distribution function, in particular, for moments which contain no powers of the longitudinal momentum in their definition (m=0 moments). Using the resulting dynamical equations, we extract the non-equilibrium attractor associated with our improved aHydro ansatz and demonstrate that the improvement also allows one to better reproduce the exact dynamical attractor obtained using kinetic theory in the relaxation time approximation, particularly at early rescaled times and for m=0 moments.

    Comments:
    22 pages, 4 figures
    Subjects:
    High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
    arXiv:
    2006.13834 [pdf]
    PRC(2020)·16 citations
  6. 07

    [Submitted on 24 Jun 2020] (cross-list from hep-ph)

    Chiral Symmetry in the Confinement Phase of QCD

    S. D. Campos (Universidade Federal de São Carlos)🇧🇷

    Based on the Pomeranchuk theorem, one constructs the parameter to measure the difference between experimental data for the particle-particle and particle-antiparticle total cross section at the same energy. The experimental data for the proton-proton and proton-antiproton total cross section were used to show that, at the same energy, this parameter tends to zero as the collision energy grows. Furthermore, one assumes a classical description of the total cross section, dividing it into a finite number of non-interacting disjoint cells, each one containing a quark-antiquark pair subject to the confinement potential. Near the minimum of the total cross section, one associates with the entropy generated by these cells, analogous to the XY model. Using both the Quigg-Rosner and Cornell confinement potentials and neglecting other energy contributions, one can calculate the internal energy of the hadron. One obtains that both the entropy and internal energy possess the same logarithmic dependence on the spatial separation between the pairs in the cell. The Helmholtz free energy is used to estimate the transition temperature, which is far from the temperature widely related to the Quark-Gluon Plasma.

    Comments:
    Change in the title and minor changes along the text. Final version accepted for publication in Mod. Phys. Lett. A
    Subjects:
    High Energy Physics — Phenomenology (hep-ph); Nuclear Theory (nucl-th)
    arXiv:
    2006.13885 [pdf]
    Mod.Phys.Lett.A(2021)·1 citation

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first authorsco-authorsvia INSPIRE