[Paper Review] Photons at PHENIX

Baldo Sahlmueller|arXiv (Cornell University)|Apr 30, 2009

High-Energy Particle Collisions Research1 references3 citations

TL;DR

This paper presents PHENIX's measurements of direct photons in heavy-ion collisions, using two methods to isolate direct photons from thermal and decay sources. It reports the first observation of excess direct photons at low transverse momentum (pT) in Au+Au collisions beyond p+p expectations via internal conversion, and measures direct photon azimuthal anisotropy (v2), providing insight into medium interactions and thermal radiation in the quark-gluon plasma.

ABSTRACT

Direct photons are a powerful probe to study the properties of the unique matter created in ultrarelativistic heavy-ion collisions. They carry information on the various stages of a heavy-ion collision. At different transverse momenta (pT), different production processes dominate the direct photon yield in heavy-ion collisions. Photons at high pT can be used to study initial hard scattering processes while photons at low and intermediate pT provide direct information on the hot and dense medium created in such collisions since they origin predominantly from jet-medium interactions and from thermal radiation from the medium itself. PHENIX has measured direct-photon yields over a broad pT and energy range in different collision systems such as Au+Au and Cu+Cu, allowing systematic studies of the behavior of direct photons in heavy-ion collisions. Two different methods have been used to measure the direct photons. An excess of direct photons in Au+Au collisions at low pT beyond the expectation from p+p collisions was measured for the first time via internal conversion. Furthermore, the azimuthal anisotropy parameter v2 has been measured. The direct photon v2 sheds light on the different processes contributing to the production of direct photons. The p+p data at the same energy provide a baseline for understanding the heavy-ion data, but are also interesting in their own right, e.g. for testing pQCD calculations or - as RHIC collides polarized protons - for constraining models on the gluon contribution to the proton spin.

Motivation & Objective

• To study the properties of the hot, dense medium formed in ultrarelativistic heavy-ion collisions using direct photons as probes.
• To disentangle contributions from initial hard scattering and medium interactions by measuring direct photons across a broad pT range.
• To establish a baseline using p+p collisions for comparison with heavy-ion data, especially for testing pQCD and proton spin structure.
• To measure the direct photon azimuthal anisotropy (v2) to probe the dynamics of photon production mechanisms in the medium.
• To observe and quantify excess direct photons at low pT in Au+Au relative to p+p collisions, indicating medium modification.

Proposed method

• Using internal conversion of photons to electron-positron pairs to identify and isolate direct photons in the low-pT region.
• Measuring direct photon yields across a broad range of transverse momenta (pT) in Au+Au and Cu+Cu collisions at RHIC energies.
• Comparing direct photon spectra in heavy-ion collisions with those from p+p collisions to extract excess direct photons.
• Measuring the azimuthal anisotropy (v2) of direct photons to probe the medium's influence on photon production.
• Applying kinematic and topological reconstruction techniques to distinguish direct photons from secondary photons from meson decays.
• Using p+p data at the same center-of-mass energy as a reference to subtract known sources and isolate medium-induced contributions.

Experimental results

Research questions

• RQ1 What is the origin of the excess direct photon yield observed at low pT in Au+Au collisions compared to p+p collisions?
• RQ2 How do jet-medium interactions and thermal radiation contribute to the direct photon spectrum in heavy-ion collisions?
• RQ3 What is the value of the direct photon azimuthal anisotropy (v2), and what does it reveal about the production mechanisms in the medium?
• RQ4 To what extent do pQCD calculations describe the direct photon production in p+p collisions at RHIC energies?
• RQ5 How does the direct photon v2 compare to that of hadrons, and what does this imply about the thermalization and collectivity of the medium?

Key findings

• An excess of direct photons at low transverse momentum (pT) was observed in Au+Au collisions beyond the expectation from p+p collisions, confirmed via internal conversion.
• The direct photon yield at low pT shows a significant enhancement compared to p+p data, indicating contributions from medium interactions and thermal radiation.
• The direct photon azimuthal anisotropy (v2) was measured, providing evidence for the medium's influence on photon production dynamics.
• The measured v2 of direct photons is consistent with contributions from both jet-medium interactions and thermal emission from the quark-gluon plasma.
• The p+p data serve as a critical baseline for understanding heavy-ion results and validating pQCD-based predictions.
• The results support the interpretation that direct photons at intermediate pT are sensitive probes of the hot and dense medium formed in heavy-ion collisions.