Excess low energy photon pairs from pion annihilation at the chiral phase transition

Abstract

The photon pair production by pion annihilation in a hot and dense medium at the chiral phase transition is investigated within a chiral quark model. As a direct consequence of this transition the $\sigma$ meson appears as a bound state in the domain of temperatures and chemical potentials where the condition $M_{\sigma }(T,\mu )\approx 2M_{\pi }(T,\mu )$ is fulfilled. This effect results in a strong enhancement of the cross section for the pion annihilation process $2\pi \to 2\gamma$ compared with the vacuum case. The calculation of the photon pair production rate as function of the invariant mass shows a strong enhancement and narrowing of the $\sigma$ meson resonance at threshold due to chiral symmetry restoration.

Comments: 15 pages, LaTeX, 6 figures, Phys. Lett. B