Scaling properties of direct photon yields in heavy ion collisions

Vladimir Khachatryan,Michał Praszałowicz

doi:10.1140/epjc/s10052-020-8219-9

Vladimir Khachatryan, Michał Praszałowicz

Open Access

https://doi.org/10.1140/epjc/s10052-020-8219-9

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Abstract

A recent analysis from the PHENIX collaboration of available direct photon measurement results in collisions of various systems such as Au+Au, Cu+Cu, and Pb+Pb, at different beam energies ranging from 39 to 2760 GeV, has shown a universal, within experimental uncertainties, multiplicity scaling, in which direct photon p_{T}-spectra for transverse momenta up to 2 GeV/c are scaled with charged hadron pseudorapidity density at midrapidity raised to power alpha =1.25. On the other hand, those direct photon p_{T}-spectra also exhibit geometrical scaling in the similar p_{T} range. Assuming power-law dependence of the scaled photon spectra for both scaling laws, we formulate two independent conditions for the power alpha , which overshoot experimental data by sim 10% on average. We discuss possible sources that might improve this estimate.

Highlights

Low energy WA98 data [10] have mostly upper bounds in the relevant pT ≤ 2 GeV/c region
It is an experimental observation in heavy ion collisions (HIC), which shows that direct photon invariant yields follow one universal curve within experimental statistical and systematic uncertainties for various colliding species and system sizes at different center-of-mass energies, when scaled by charged hadron multiplicities at midrapidity raised to the power of α = 1.25 [5]: 1 d Nch/dη|η≈0 α d Nγ d2 pT dy where G is a universal energy- and multiplicity-independent function of pT2, and Q0 ∼ 1 GeV/c
One possibility might be the apparent violation of geometrical scaling (GS) that has been mentioned in Sec. 2, namely the energy dependence of ST

Summary

Introduction

It means that the scaling properties of an initial state are preserved and even built up by the QCD evolution This may explain the fact that the geometrical scaling is observed in the hadronic pT -spectra and even more so in the case of photons. In the present paper we formulate two conditions that allow one to calculate the power α of MS in terms of the parameters that enter the theoretical parametrization of GS, assuming power-law dependence of the scaled photon spectra These two conditions result in two predictions for α: 1.34 and 1.40, i.e. We discuss possible corrections that might improve this theoretical prediction of α

Basics of geometrical scaling

Charged particle multiplicity

Relating the scaling laws

Findings

Discussion and conclusions