Nonprompt direct-photon production in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

TL;DR

This paper reports on the measurement of direct photons in gold-gold collisions at 200 GeV, revealing an excess of nonprompt photons at low transverse momentum and their scaling behavior with collision size, providing insights into early collision stages.

Contribution

The study introduces a measurement of nonprompt direct-photon spectra in Au+Au collisions at 200 GeV, highlighting their scaling and potential sensitivity to early collision dynamics.

Findings

Excess of direct photons observed for pT<6 GeV/c.

Nonprompt photon yields follow a power-law scaling with system size.

Inverse slope of photon spectrum increases with pT, indicating early-stage sensitivity.

Abstract

The measurement of the direct-photon spectrum from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV is presented by the PHENIX collaboration using the external-photon-conversion technique for 0\%--93\% central collisions in a transverse-momentum ($p_T$) range of 0.8--10 GeV/$c$. An excess of direct photons, above prompt-photon production from hard-scattering processes, is observed for $p_T<6$ GeV/$c$. Nonprompt direct photons are measured by subtracting the prompt component, which is estimated as $N_{\rm coll}$-scaled direct photons from $p$$+$$p$ collisions at 200 GeV, from the direct-photon spectrum. Results are obtained for $0.8<p_T<6.0$ GeV/$c$ and suggest that the spectrum has an increasing inverse slope from ${\approx}0.2$ to 0.4 GeV/$c$ with increasing $p_T$, which indicates a possible sensitivity of the measurement to photons from earlier stages of the evolution of the collision. In addition, like the direct-photon production, the $p_T$-integrated nonprompt direct-photon yields also follow a power-law scaling behavior as a function of collision-system size. The exponent, $\alpha$, for the nonprompt component is found to be consistent with 1.1 with no apparent $p_T$ dependence.