Estimating $\gamma \gamma$ absorption for UHE photons with lepton and hadron production

Abstract

Surprisingly, the contribution to the cosmic opacity to UHE ($E>10^{18}\mathrm{eV}$) $\gamma$-rays has been systematically computed so far for the $\gamma \gamma \to e^{+}{e}^{-}$ and $\gamma \gamma \to e^{+}{e}^{-}{e}^{+}{e}^{-}$ processes alone. We go a step further by systematically evaluating the additional opacity brought about by other leptons and hadrons. We find that the dominant channels are those leading to the production of ${\mu }^{\pm }$ and hadrons (mainly ${\pi }^{\pm }$, ${\pi }^0$, $K^{\pm }$, $K^0$, $\eta$). For nearly the GZK radius, the photon survival probability becomes smaller by about a factor of two with respect to current estimates.

Comments: 9 pages, 4 figures, revised version