Production of photons in relativistic heavy-ion collisions

In this work it is shown that the use of a hydrodynamical model of heavy-ion collisions which incorporates recent developments, together with updated photon emission rates, greatly improves agreement with both ALICE and PHENIX measurements of direct photons, supporting the idea that thermal photons are the dominant source of direct photon momentum anisotropy. The event-by-event hydrodynamical model uses the impact parameter dependent Glasma model (IP-Glasma) initial states and includes, for the first time, both shear and bulk viscosities, along with second-order couplings between the two viscosities. The effect of both shear and bulk viscosities on the photon rates is studied, and those transport coefficients are shown to have measurable consequences on the photon momentum anisotropy.

Figure 1

The temperature dependence of the bulk-viscosity-to-entropy-density ratio, as used in this work. The points on the low temperature side are the results of a calculation from Ref. [32], while the high-temperature results are from Ref. [33].

Figure 2

Top panel: Direct photon spectrum measured in $\sqrt{s_{NN}}=200$ GeV proton-proton collisions at RHIC compared with perturbative QCD calculations at different scales $Q$. Bottom panel: Normalized perturbative QCD calculations; see details in the main text.

Figure 3

Ideal QGP and hadronic photon rate near the cross-over region.

Figure 4

The result of a hydrodynamic calculation of direct photon spectra, for Au-Au collisions at RHIC, in the 0–20% (top panel) and 20–40% (bottom panel) centrality ranges. The different curves are explained in the text; the data are from Ref. [11].

Figure 5

The result of a hydrodynamic calculation of direct photon spectra, for Au-Au collisions at RHIC, in minimum bias centrality range. The data are from Refs. [11, 71].

Figure 6

Hydrodynamic calculation of the direct photon $v_2$, for Au-Au collisions at RHIC, in the 0–20% (top panel) and 20–40% (bottom panel) centrality range. The data are from Ref. [12].

Figure 7

The direct photon spectrum for Pb-Pb collisions at the LHC 0–20% (top panel) and 20–40% (bottom panel) centrality range. The different curves are explained in the text, and the data are from the ALICE Collaboration [15].

Figure 8

The direct photon $v_2$ at 0–40% centrality. Data are from the ALICE Collaboration [14, 64].

Figure 9

Effect of bulk viscosity on the direct photon spectrum (top panel) and $v_2$ (bottom panel) in Pb-Pb collisions at $\sqrt{s_{NN}}=2760\mathrm{GeV}$. ALICE measurements [13, 14, 64] are shown for reference.

Figure 10

Effect of viscosity corrections to the photon emission rates for the direct photon spectrum (top panel) and $v_2$ (bottom panel) in Pb-Pb collisions at $\sqrt{s_{NN}}=2760\mathrm{GeV}$.

Figure 11

Event-average space-time volume ${〈d{V}_4/dy〉}_T$ (left) and event-average flow velocity ${\langle u^{\tau }\rangle }_T$ (right) for hydrodynamical model with and without bulk viscosity in Pb-Pb collisions at $\sqrt{s_{NN}}=2760\mathrm{GeV}$.

Figure 12

Importance of different hadronic photon production channels on the direct photon spectrum (top) and $v_2$ (bottom) in Pb-Pb collisions at $\sqrt{s_{NN}}=2760\mathrm{GeV}$.

Figure 13

Direct photon spectrum (top) and $v_2$ (bottom) evaluated with different QGP and hadronic photon emission rates, in Pb-Pb collisions at $\sqrt{s_{NN}}=2760\mathrm{GeV}$. See text for details.

Figure 14

Flow anisotropy of the medium with respect to the reaction plane (top) and momentum anisotropy of thermal photons (bottom), for different temperature ranges, in Pb-Pb collisions at $\sqrt{s_{NN}}=2760\mathrm{GeV}$.

Figure 15

Importance of postparticlization photon production on the photon spectrum (top) and $v_2$ (bottom) in Pb-Pb collisions at $\sqrt{s_{NN}}=2760\mathrm{GeV}$.