Scaling properties of direct photon yields in heavy ion collisions
- PDF / 586,005 Bytes
- 7 Pages / 595.276 x 790.866 pts Page_size
- 16 Downloads / 171 Views
Regular Article - Theoretical Physics
Scaling properties of direct photon yields in heavy ion collisions Vladimir Khachatryan1,a , Michał Praszałowicz2,b 1 2
Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, USA Institute of Theoretical Physics, Jagiellonian University, S. Łojasiewicza 11, 30-348 Kraków, Poland
Received: 23 April 2020 / Accepted: 7 July 2020 / Published online: 24 July 2020 © The Author(s) 2020
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 pT -spectra for transverse momenta up to 2 GeV/c are scaled with charged hadron pseudorapidity density at midrapidity raised to power α = 1.25. On the other hand, those direct photon pT -spectra also exhibit geometrical scaling in the similar pT range. Assuming power-law dependence of the scaled photon spectra for both scaling laws, we formulate two independent conditions for the power α, which overshoot experimental data by ∼ 10% on average. We discuss possible sources that might improve this estimate.
1 Introduction Measurements of direct photons provide unique opportunities in probing and studying the properties and evolution of the matter produced in heavy ion collisions (HIC). These photons are defined to be produced from all the sources except for hadronic decays. Since they hardly interact with the “fireball” of quarks and gluons due to a small interaction cross section with the medium, the information they carry from the time of their production is not washed out by final state interactions. Experimentally measured low momentum direct photon pT √ spectra by PHENIX (in Au+Au at s N N = 200 GeV and √ 62.4 GeV, in Cu+Cu at s N N = 200 GeV) [1–7] and ALICE √ (in Pb+Pb at s N N = 2760 GeV) [8] collaborations in HIC are enhanced with respect to Ncoll (number of binary nucleon collisions) scaled reference yield (measured or calculated) in p+p collisions. Low momentum direct photon measurements by STAR collaboration show less enhancement [9]. Earlier a e-mail:
[email protected]
b e-mail:
[email protected] (corresponding author)
low energy WA98 data [10] have mostly upper bounds in the relevant pT ≤ 2 GeV/c region. Direct photons in HIC also show large anisotropy (elliptic flow) [7,11,12]. Figure 1 shows several data sets of direct photon pT spectra at low- (< 1 GeV/c) and intermediate- (from 1 GeV/c up to ∼ 5 GeV/c) pT regions. There have been many theoretical attempts to reproduce the photon yields shown in Fig. 1 and flow coefficients (that are not discussed here) with, however, mixed success. Hydrodynamical simulations of the fireball evolution [13–16], calculations in the framework of the elliptic-fireball expansion scenario [17–19], PartonHadron-String Dynamics transport approach [20–23], as well as the spec
Data Loading...
