Enhanced production of direct photons in Au+Au collisions at sqrt(s_NN)=200 GeV and implications for the initial temperature

Abstract: The production of low mass e+e- pairs for m_{e+e-} < 300 MeV/c^2 and 1 < p_T <5 GeV/c is measured in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. Enhanced yield above hadronic sources is observed. Treating the excess as internal conversions, the invariant yield of direct photons is deduced. In central Au+Au collisions, the excess of direct photon yield over p+p is exponential in transverse momentum, with inverse slope T = 221 +/- 19 (stat) +/- 19 (syst) MeV. Hydrodynamical models with initial temperatures ranging from 300--600 MeV at times of ~ 0.6 - 0.15 fm/c after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at ~ 170 MeV.

• The paper demonstrates a novel method using internal conversion of virtual photons to isolate direct photon signals in Au+Au collisions.
• It reveals an exponential photon spectrum with a thermal slope parameter of 221 ± 19 MeV, indicating high initial temperatures.
• Results support hydrodynamic model predictions of rapid QGP formation and thermalization in central heavy ion collisions.

An Expert Analysis of "Enhanced production of direct photons in Au+Au collisions at √sNN=200 GeV and implications for the initial temperature"

The paper "Enhanced production of direct photons in Au+Au collisions at √sNN=200 GeV and implications for the initial temperature" explores the production of direct photons during heavy ion collisions, specifically Au+Au, at the Relativistic Heavy Ion Collider (RHIC). The key focus of this study is to analyze the excess production of direct photons, considered as thermal radiation emitted from the partonic phase, to infer the temperature of the matter formed immediately after such collisions.

Measurement of Direct Photons

In high-energy heavy ion collisions, direct photons provide a vital probe of the initial conditions of the created dense matter, as they emerge directly from the collision zone without subsequent interaction. Direct photon measurement, however, is compounded by substantial background from hadronic sources. The authors employed a new method of estimating direct photons using internal conversion to virtual photons, which transform into low mass electron-positron pairs.

Key Findings

• Photon Measurement and Background Subtraction: The study succeeds in improving the signal-to-background ratio for direct photons by utilizing internal conversions of virtual photons and focusing on invariant masses above the $\pi^0$ meson threshold. This method reduced the hadronic background by 80%.
• Thermal Photon Emission: The results showcase a significant surplus of direct photon production in central Au+Au over peripheral or $p+p$ collisions. This surplus follows an exponential distribution in transverse momentum, characteristic of thermal emission with an inverse slope parameter $T=221 \pm 19^{\rm stat} \pm 19^{\rm syst}$ MeV for central collisions.
• Hydrodynamic Model Validation: These results were analyzed within the context of hydrodynamical models that predict initial temperatures in the range of 300–600 MeV. The observed photon production aligns qualitatively with these models, suggesting a rapid thermalization of the quark-gluon plasma.

Theoretical Implications

The lattice QCD predicts a phase transition into quark-gluon plasma (QGP) near 170 MeV. The inferred initial temperatures from photon spectra beyond this transition temperature point towards the realization of QGP in such heavy ion collisions. It represents a direct measurement confirming the high-energy density and thermal nature of the matter formed in these nuclear collisions.

Future Prospects

This study highlights potential avenues of refining photon detection techniques further. Enhancements in detection techniques and methodologies will pave the way for more direct and precise initial temperature measurements. Additionally, continuing experimental and theoretical synergy is necessary to deepen the understanding of thermal quantum chromodynamics (QCD) matter and its properties under extreme conditions.

In conclusion, the paper's findings contribute significantly to the body of evidence supporting QGP formation at RHIC, underscoring the paper's import in advancing the comprehension of high-energy nuclear physics and the early Universe conditions.