Low-momentum direct photon measurement in Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV

TL;DR

This study measures low-momentum direct photons in Cu+Cu collisions at 200 GeV, revealing an excess over known backgrounds and consistent behavior with Au+Au collisions, providing insights into the hot medium created in such collisions.

Contribution

First measurement of low-momentum direct photons in Cu+Cu collisions at 200 GeV, showing an excess over backgrounds and similar scaling as in Au+Au collisions.

Findings

Observed photon excess for pT<4 GeV/c in Cu+Cu collisions.

Inverse slopes of the photon spectra are approximately 285 and 333 MeV/c.

Photon rapidity density scales with charged particle multiplicity.

Abstract

We have measured direct photons for $p_T<5~$GeV/$c$ in minimum bias and 0\%--40\% most central events at midrapidity for Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The $e^{+}e^{-}$ contribution from quasi-real direct virtual photons has been determined as an excess over the known hadronic contributions in the $e^{+}e^{-}$ mass distribution. A clear enhancement of photons over the binary scaled $p$$+$$p$ fit is observed for $p_T<4$ GeV/$c$ in Cu$+$Cu data. The $p_T$ spectra are consistent with the Au$+$Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the $p$$+$$p$ baseline are 285$\pm$53(stat)$\pm$57(syst)~MeV/$c$ and 333$\pm$72(stat)$\pm$45(syst)~MeV/$c$ for minimum bias and 0\%--40\% most central events, respectively. The rapidity density, $dN/dy$, of photons demonstrates the same power law as a function of $dN_{\rm ch}/d\eta$ observed in Au$+$Au at the same collision energy.