COHERENT PHOTON-POMERON AND PHOTON-PHOTON INTERACTIONS IN ULTRA-PERIPHERAL COLLISIONS AT RHIC

Transverse momentum spectra of π+, p, Λ, Ξ or Ξ¯+, Ω or Ω¯+ and deuteron (d) in different centrality intervals in nucleus–nucleus collisions at the center of mass energy are analyzed by the blast wave model with Boltzmann Gibbs statistics. We extracted the kinetic freezeout temperature, transverse flow velocity and kinetic freezeout volume from the transverse momentum spectra of the particles. It is observed that the non-strange and strange (multi-strange) particles freezeout separately due to different reaction cross-sections. While the freezeout volume and transverse flow velocity are mass dependent, they decrease with the resting mass of the particles. The present work reveals the scenario of a double kinetic freezeout in nucleus–nucleus collisions. Furthermore, the kinetic freezeout temperature and freezeout volume are larger in central collisions than peripheral collisions. However, the transverse flow velocity remains almost unchanged from central to peripheral collisions.

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Constraining the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 2.76 and 5.02 TeV

Journal of High Energy Physics ◽

10.1007/jhep09(2020)160 ◽

2020 ◽

Vol 2020 (9) ◽

Author(s):

S. Acharya ◽

◽

D. Adamová ◽

A. Adler ◽

J. Adolfsson ◽

...

Keyword(s):

Charged Particles ◽

Magnetic Effect ◽

Anisotropic Flow ◽

Charge Conservation ◽

Particle Correlations ◽

Azimuthal Correlations ◽

Chiral Magnetic Effect ◽

Local Charge ◽

Peripheral Collisions ◽

Dependent Signal

Abstract Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity (η) and transverse momentum (pT) ranges |η| < 0.8 and 0.2 < pT< 5 GeV/c. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators γ1, 1 = 〈cos(φα + φβ − 2Ψ2)〉 and γ1, − 3 = 〈cos(φα − 3φβ + 2Ψ2)〉. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured γ1,1. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form γ1, 2 = 〈cos(φα + 2φβ − 3Ψ3)〉, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of γ1,1.

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Photon–Photon Interactions and the Opacity of the Universe in Gamma Rays

Universe ◽

10.3390/universe7050146 ◽

2021 ◽

Vol 7 (5) ◽

pp. 146

Author(s):

Alberto Franceschini

Keyword(s):

Gamma Rays ◽

Personal Account ◽

Fundamental Physics ◽

Extragalactic Background Light ◽

Substantial Modification ◽

Global Coherence ◽

Photon Interactions ◽

Available Information ◽

The Universe ◽

Present Understanding

We discuss the topic of the transparency of the Universe in gamma rays due to extragalactic background light, and its cosmological and physical implications. Rather than a review, this is a personal account on the development of 30 years of this branch of physical science. Extensive analysis of the currently available information appears to us as revealing a global coherence among the astrophysical, cosmological, and fundamental physics data, or, at least, no evident need so far of substantial modification of our present understanding. Deeper data from future experiments will verify to what extent and in which directions this conclusion should be modified.

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