Baryonic sources of thermal photons
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Regular Article - Theoretical Physics
Baryonic sources of thermal photons Nathan P. M. Holt1,2,a , Ralf Rapp2,b 1 2
Piedmont College, Demorest, GA 30535, USA Department of Physics and Astronomy, Cyclotron Institute, Texas A&M University, College Station, TX 77843, USA
Received: 31 July 2020 / Accepted: 6 November 2020 / Published online: 20 November 2020 © Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Communicated by Evgeni Kolomeitsev
Abstract Thermal radiation of photons and dileptons from hadronic matter plays an essential role in understanding electromagnetic emission spectra in high-energy heavyion collisions. In particular, baryons and anti-baryons have been found to be strong catalysts for electromagnetic radiation, even at collider energies where the baryon chemical potential is small. Here, we conduct a systematic analysis of π - and ω-meson-induced reactions off a large set of baryon states. The interactions are based on effective hadronic Lagrangians where the parameters are quantitatively constrained by empirical information from vacuum decay branchings and scattering data, and gauge invariance is maintained by suitable regularization procedures. The thermal emission rates are computed using kinetic theory but can be directly compared to previous calculations using hadronic many-body theory. The comparison to existing calculations in the literature reveals our newly identified contributions to be rather significant.
1 Introduction Electromagnetic (EM) radiation from the fireballs formed in heavy-ion collisions offers a wide range of insights into the properties of matter governed by quantum chromodynamics (QCD). Low-mass dilepton spectra, at invariant masses M 1 GeV, directly probe how the spectral functions (SFs) of vector mesons (most notably the ρ-meson) transit from massive and confined degrees of freedom in the QCD vacuum into a rather structureless spectrum at high temperature and density, suggestive of a quark-antiquark continuum [1]. At intermediate masses, M 1.5 GeV, continuum radiation is chiefly emitted from the early phases of the fireball, and its inverse slope serves as an excellent thermometer of the medium [2]. While invariant-mass spectra are, a e-mail:
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b e-mail:
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by definition, unaffected by any Doppler blueshift caused by the collective expansion of the fireball, this is no longer the case for the transverse-momentum (qT ) spectra of both photons and dileptons. Temperature extractions from the qT spectra therefore require a deconvolution of the radial flow of the emitting fireball cells. The spectra of “direct” photons (obtained after subtracting long-lived final-state hadron decays) measured at RHIC and the LHC not only exhibit excess yields indicating a robust signal of thermal radiation, but also carry a significant asymmetry in the azimuthal emission angle (φ) in the transverse plane, commonly associated with the “elliptic flow” of the underlying hydrodynamic medium
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