# Low-momentum direct-photon measurement in Cu + Cu collisions at $\sqrt{{s}_{NN}}=200\phantom{\rule{4pt}{0ex}}\mathrm{GeV}$

15 November 2018

Abstract: We measured direct photons for ${p}_{T}<5\phantom{\rule{3.33333pt}{0ex}}\mathrm{GeV}/c$ in minimum bias and 0%–40% most-central events at midrapidity for $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{{s}_{{}_{NN}}}=200\phantom{\rule{4pt}{0ex}}\mathrm{GeV}$. The ${e}^{+}{e}^{-}$ contribution from quasireal 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 ${p}_{T}<4\phantom{\rule{4pt}{0ex}}\mathrm{GeV}/c$ in $\mathrm{Cu}+\mathrm{Cu}$ data. The ${p}_{T}$ spectra are consistent with the $\mathrm{Au}+\mathrm{Au}$ 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 $285±53\left(\mathrm{stat}\right)±57\left(\mathrm{syst}\right)\phantom{\rule{4pt}{0ex}}\mathrm{MeV}/c$ and $333±72\left(\mathrm{stat}\right)±45\left(\mathrm{syst}\right)\phantom{\rule{4pt}{0ex}}\mathrm{MeV}/c$ 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 $d{N}_{\mathrm{ch}}/d\eta$ observed in $\mathrm{Au}+\mathrm{Au}$ at the same collision energy.

Published in: Physical Review C 98 (2018)