Effect of Isolation Criteria on Prompt Photon Production in Relativistic Nuclear Collisions

TL;DR

The paper investigates how isolation criteria affect the separation of direct and fragmentation photons in relativistic nuclear collisions using the JETPHOX framework. It employs two isolation prescriptions, fixed $p_T^{\rm cut}=2$ GeV and dynamic $p_T^{\rm cut}=0.1\,p_T$ with a cone radius $R=0.4$, to evaluate prompt-photon cross sections in $pp$ and $AA$ collisions across RHIC and LHC energies, incorporating CT14 PDFs, EPS09 nPDFs, and BFG II FFs. A central finding is that fragmentation is substantial at $p_T<15$ GeV and that dynamic isolation suppresses this component more effectively than fixed cuts, with the suppression strength depending on beam energy and $p_T$ rather than system size. The results underscore the need for consistent isolation definitions when comparing data to theory and inform future measurements of prompt photons, their backgrounds, and related observables such as $v_2$ and $R_{AA}$.

Abstract

Prompt photon measurements in relativistic nuclear collisions serve as an essential comparative basis for heavy ion studies enabling the separation of medium induced effects. However, the identification of prompt photons is experimentally challenging due to substantial backgrounds from photons produced in hadron decays and jet fragmentation. Appropriate isolation criteria are applied to suppress these background contributions. We analyze prompt photon spectra using the JETPHOX framework to quantify the relative contributions of fragmentation and direct production mechanisms to the total photon yield. We perform a systematic study of the impact of isolation criteria on prompt photon production in relativistic nuclear collisions with emphasis on their dependence on beam energy and photon transverse momentum. The fragmentation contribution is found to be substantially large particularly for $p_T < 15$ GeV and the isolation criterion plays a crucial role in the analysis of prompt photons in that $p_T$ region. A dynamical isolation criterion suppresses the fragmentation component more effectively than a fixed one in this region. Furthermore, the isolation criterion shows a stronger dependence on beam energy and photon $p_T$ than on system size. These observations emphasize the importance of employing carefully selected and consistent isolation criteria when comparing experimental data with theoretical calculations especially for observables sensitive to fragmentation.

Figures (15)

• Figure 1: Prompt photon cross section in $pp$ collisions at $\sqrt{s}=7$ TeV for two different isolation criteria and for the case without any isolation cut along with ALICE data alice_pp_7TeV in the rapidity range $|\eta|<0.27$.
• Figure 2: Prompt photon cross section in $Pb+Pb$ collisions at $\sqrt{s_{NN}}=5.02$ TeV for two isolation criteria and for the case without any isolation cut along with CMS data cms_PbPb_5TeV in the rapidity range $|\eta|<1.44$.
• Figure 3: Prompt photon cross section in $pp$ collisions at $\sqrt{s}=510$ GeV for two different isolation criteria and for the case without any isolation cut along with PHENIX data phenix_pp_510GeV in the rapidity range $|\eta|<0.25$. The isolation cone radius $R$ is taken as 0.5, same as in the PHENIX data analysis.
• Figure 4: Relative contributions of direct (D/(D+F)) and fragmentation (F/(D+F)) parts in total prompt production for isolation criterion [upper panel] $p_T^{\rm cut} = 2$ GeV and [lower panel] $p_T^{\rm cut} = 0.1 p_T$ for $pp$ collisions at various center of mass energies for $|\eta| < 0.5$ using JETPHOX jp1.
• Figure 5: Relative contributions of direct (D/(D+F)) and fragmentation (F/(D+F)) parts in total prompt production for isolation criterion [upper panel] $p_T^{\rm cut} = 2 \ p_T$ and [lower panel] $p_T^{\rm cut} = 0.1 p_T$ for heavy ion collisions at different center of mass energies for $|\eta| < 0.5$ using JETPHOX jp1.