Direct Photons in Nuclear Collisions at FAIR Energies

Using the extrapolation of existing data estimations of prompt photon production at FAIR energies have been made. At y = yc.m. the rapidity density of prompt photons with pt > 1.5 GeV/c per central Au+Au event at 25 AGeV is estimated as ∼ 10 . With the planed beam intensity 10 per second and 1% interaction probability, for 10% of most central events one can expect the prompt photon rate ∼ 10 photons per second. Direct photons from the hadron scenario of ion collisions generated by the Hadron-String-Dynamics (HSD) transport approach with implemented meson scatterings πρ → πγ, ππ → ργ have been analyzed. Photons from short-living resonances (e.g. ω → πγ) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the direct photon spectrum at pt = 0.5 − 1 GeV/c. At the FAIR energy 25 AGeV in Au+Au central collisions the HSD generator predicts, as a lower estimate, γdirect/γπ0 ≃ 0.5% in the region pt = 0.5− 1 GeV/c. At pt = 1.5 − 2 GeV/c γprompt/γπ0 ≃ 2%. Thermal direct photons have been evaluated with the Bjorken HydroDynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct photon spectrum is very sensitive to the initial temperature parameter T0 of the model. The 10 MeV increase in the T0 value leads to ∼ 2 times higher photon yield. PACS 24.10.Lx, 25.20.Lj, 25.75.-q ∗Talk at the session of Russian Academy of Sciences, ITEP, Moscow, 26 30 November 2007