[Paper Review] Prompt photons in heavy ion collisions at the LHC: A ''multi-purpose'' observable

François Arleo|ArXiv.org|Jul 16, 2007

High-Energy Particle Collisions Research1 references3 citations

TL;DR

This paper proposes prompt photons in LHC heavy-ion collisions as a multi-purpose probe to extract nuclear parton distribution functions and medium-modified fragmentation functions. By analyzing photon production ratios in p–Pb and Pb–Pb collisions, the study demonstrates that isolated photon yields at mid-rapidity are sensitive to nuclear gluon and structure function modifications, with significant suppression observed in Pb–Pb due to parton energy loss, providing a clean signature for medium effects beyond initial-state nuclear effects.

ABSTRACT

I emphasize in this contribution how prompt photons can be used to probe nuclear parton densities as well as medium-modified fragmentation functions in heavy ion collisions. Various predictions in p-A and A-A collisions at LHC energies are given.

Motivation & Objective

• To investigate prompt photons as a multi-purpose observable for probing nuclear parton distribution functions (nPDFs) in p–A and A–A collisions at LHC energies.
• To assess the sensitivity of isolated photon production ratios to nuclear modifications of gluon and quark structure functions.
• To explore the impact of parton energy loss in dense nuclear matter on prompt photon yields in Pb–Pb collisions.
• To evaluate the potential of photon-hadron momentum correlations as a probe of vacuum and medium-modified fragmentation functions.

Proposed method

• The nuclear modification ratio $ R_{pA}(x_\perp, y) $ is computed as the ratio of photon production cross sections in p–A to p–p collisions, serving as a proxy for nuclear effects.
• An analytic approximation $ R^{\text{approx}}(x_\perp, y) $ is derived, relating $ R_{pA} $ to the ratio of nuclear parton densities $ R^A_F $ and $ R^A_G $, particularly at $ y=0 $ and $ y=3 $.
• The nDSg nuclear PDFs are used to compute $ R_{pA} $ for isolated photons in p–Pb collisions at $ \sqrt{s_{\text{NN}}}=8.8 $ TeV.
• Photon quenching in Pb–Pb collisions is modeled by including parton energy loss, with a cutoff scale $ \omega_c = 50 $ GeV.
• Momentum-imbalance distributions between prompt photons and leading hadrons (e.g., $ \gamma $–$ \pi^0 $) are evaluated as a probe of fragmentation functions in vacuum and medium.
• Comparisons are made between predictions including only nPDFs and those including both nPDFs and energy loss effects to isolate medium-induced suppression.

Experimental results

Research questions

• RQ1 To what extent can prompt photon production ratios in p–A collisions constrain nuclear parton distribution functions at high $ x $ and low $ Q^2 $?
• RQ2 How does parton energy loss in the quark-gluon plasma affect the yield of prompt photons in Pb–Pb collisions?
• RQ3 Can photon-hadron momentum correlations provide a clean probe of medium-modified fragmentation functions?
• RQ4 How well does the analytic approximation $ R^{\text{approx}} $ reproduce the full $ R_{pA} $ ratio in p–Pb collisions?

Key findings

• The $ R_{pA} $ ratio for isolated photons in p–Pb collisions at $ \sqrt{s_{\text{NN}}}=8.8 $ TeV shows good agreement with the analytic approximation $ R^{\text{approx}}_{y=0} $, validating its sensitivity to nuclear gluon and structure function modifications.
• A significant suppression of inclusive photon yields in Pb–Pb collisions at $ \sqrt{s_{\text{NN}}}=5.5 $ TeV is observed when parton energy loss is included, unlike predictions based solely on nPDFs.
• The ratio $ R_{pA} $ at mid-rapidity ($ y=0 $) is primarily sensitive to the nuclear modification of the gluon density and the $ F_2 $ structure function.
• The $ R^{\text{approx}} $ approximation at $ y=3 $ is shown to be well described by $ R^A_G(x_\perp e^{-y}) $, linking photon production to the modified gluon distribution.
• The inclusion of energy loss leads to a clear suppression in $ R_{\text{PbPb}} $, distinguishing medium effects from initial-state nuclear effects.
• Photon-hadron momentum-imbalance distributions are predicted to be sensitive to medium-modified fragmentation functions, offering a future experimental probe.