Direct Photon Interferometry in Bi–Bi Collision at $$\sqrt {{{s}_{{NN}}}} $$ = 9.2 GeV

We consider Bose–Einstein correlations of direct photons in central Bi–Bi collisions at $$\sqrt {{{s}_{{NN}}}} $$ = 9.2 GeV. Two-photon correlations are calculated in UrQMD hybrid model. Two equations of state, one with the first-order QCD phase transition and the other with the pure hadron gas, are considered to study the effects of the formation of quark-gluon plasma (QGP) and mixed phase on the observables. The extracted correlation radii in out-side-long parametrization demonstrate significant increasing in outward and longitudinal directions for the scenario including QGP, while correlations in side direction are not sensitive to the equation of the state.