Direct photon production in Pb-Pb collisions at $\sqrt{s_\rm{NN}}$ = 2.76 TeV (1509.07324v2)

Abstract: Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{{\mathrm{NN}}}} = 2.76$ TeV was studied in the transverse momentum range $0.9 < p\mathrm{T} < 14$ GeV$/c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^+e^-$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_\mathrm{T} \gtrsim 5$ GeV$/c$. Direct photon spectra down to $p_\mathrm{T} \approx 1$ GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_\mathrm{T} < 2.1$ GeV$/c$ is $2.6\sigma$ for the 0-20% class. The spectrum in this $p_\mathrm{T}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^\mathrm{stat}\pm 41^\mathrm{syst})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.

Overview: Direct Photon Production in Pb-Pb Collisions at ALICE

This paper presents a detailed paper on direct photon production in lead-lead (Pb-Pb) collisions at a center-of-mass energy per nucleon pair ($\sqrt{s_{\mathrm{NN}}}$) of 2.76 TeV, conducted by the ALICE collaboration. The focus is on measuring the photon yield in various transverse momentum ($p_T$) ranges across different centrality classes. Direct photons are crucial in probing the properties of the Quark-Gluon Plasma (QGP), a state of deconfined quarks and gluons theoretically predicted by Quantum Chromodynamics (QCD).

Experimental Methodology

Photon detection was performed using two distinct methods within the ALICE experiment:

1. Photon Conversion Method (PCM): Involves tracking $e^+e^-$ pairs resulting from photon conversions in the detector material using the Inner Tracking System and the Time Projection Chamber.
2. Electromagnetic Calorimeter (PHOS): Measures photon energy through lead tungstate crystals cooled to enhance resolution and reduce noise.

These techniques provide complementary insights into photon production, with results subsequently combined to increase statistical significance.

Results

The analysis covered three centrality ranges (0–20%, 20–40%, 40–80%) and showed agreement with perturbative QCD predictions for $p_T \gtrsim 5 \, \text{GeV}/c$. However, for central collisions in lower $p_T$ ranges ($0.9 \leq p_T \leq 2 \, \text{GeV}/c$), a significant excess in direct photon production (about 10-15%) was observed. This signature implies additional sources beyond prompt photon production, suggesting thermal radiation from the evolution of the QGP.

The low $p_T$ results in central collisions exhibited an inverse slope parameter ($T_{\mathrm{eff}}$) of approximately 297 MeV for the range $1 \leq p_T < 3 \, \text{GeV}/c$. The theoretical interpretations align systematically with state-of-the-art models of QGP and hadronic phases contributing to these findings.

Implications and Future Considerations

The results indicate that direct photon production in heavy-ion collisions at LHC energies provides critical insights into the formation and properties of the QGP. While the high $p_T$ direct photon yields agree with expected perturbative QCD scenarios, the low $p_T$ excess necessitates further refinement in modeling the thermal emission processes and an understanding of QGP dynamics.

Going forward, increased precision in $p_T$ measurements could help resolve existing uncertainties in temperature estimates and better constrain hydrodynamic models of QGP evolution. Such studies might offer enhanced verification of theoretical predictions and constraints on the degrees of freedom in the early Universe conditions. The continued integration of experimental findings with theoretical endeavors will advance our comprehension of strong interaction physics and the extreme states of matter formed in relativistic heavy-ion collisions.