Direct photon measurement via internal conversions in Cu+Cu collisions at $\sqrt{s_{NN}} = 200\,\mathrm{GeV}$ in PHENIX

The measurement of direct photons on hot and dense matter created in heavy ion collisions can provide thermodynamic information, but their measurement in the low $p_T$ region is very challenging due to large hadronic background. However, internal conversion method allows access to low and intermediate $p_T$ direct photons. In an earlier measurement in Au+Au collisions, direct photon invariant yield was obtained in the pair mass region $M_{ee} < 300MeV/c^{2}$ for transverse momentum $1 < p_{T} < 5GeV/c$. In the case of Au+Au central collisions, direct photon $p_T$ spectra have been fitted to an exponential with inverse slope parameter $T = 221\pm23(stat)\pm18(syst)MeV$, which is higher than critical temperature $T_{c} = 170MeV$ predicted by Lattice QCD~[1]. Therefore, it is interesting to also measure direct photons and the temperature of hot and dense matter created in Cu+Cu collisions, a smaller system that might be expected to reach a lower temperature. The latest status of the measurement is reported. \\[4pt] [1] K.Adcox et al., Nucl.Phys. A757,184 (2005)