Direct Photons in Nuclear Collisions at FAIR Energies

TL;DR

This paper estimates prompt and direct photon production in nuclear collisions at FAIR energies, analyzing contributions from hadronic processes and thermal sources, and compares model predictions with extrapolated data.

Contribution

It provides the first detailed estimates of direct photon yields at FAIR energies using transport and hydrodynamic models, highlighting the sensitivity to initial conditions.

Findings

Prompt photon rate at 25 AGeV is approximately 100 photons/sec.

Hadronic decay contributions significantly affect low p_t photon spectra.

Thermal photon yields are highly sensitive to initial temperature T_0.

Abstract

Using the extrapolation of existing data estimations of prompt photon production at FAIR energies have been made. At $y=y_{c.m.}$ the rapidity density of prompt photons with $p_{t}>$ 1.5 GeV/c per central Au+Au event at 25 AGeV is estimated as $\sim 10^{-4}$ . With the planed beam intensity $10^{9}$ per second and 1% interaction probability, for 10% of most central events one can expect the prompt photon rate $\sim 10^{2}$ 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 $\pi\rho\to\pi\gamma, \pi\pi\to\rho\gamma$ have been analyzed. Photons from short-living resonances (e.g. $\omega \to \pi^{0} \gamma$) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the direct photon spectrum at $p_{t}=0.5 - 1$ GeV/c. At the FAIR energy 25 AGeV in Au+Au central collisions the HSD generator predicts, as a lower estimate, $\gamma_{direct}/\gamma_{\pi^{0}} \simeq$ 0.5% in the region $p_{t}=0.5 - 1$ GeV/c. At $p_{t}=1.5 - 2$ GeV/c $\gamma_{prompt}/\gamma_{\pi^{0}} \simeq$ 2%. Thermal direct photons have been evaluated with the Bjorken Hydro-Dynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct photon spectrum is very sensitive to the initial temperature parameter $T_{0}$ of the model. The 10 MeV increase in the $T_{0}$ value leads to $\sim$ 2 times higher photon yield.