Neutrinos as the source of ultrahigh energy cosmic rays in extra dimensions

Abstract

If neutrinos are to be identified with the primary source of ultrahigh energy cosmic rays (UHECR), their interaction on relic neutrinos is of great importance in understanding their long intergalactic journey. In theories with large compact dimensions, the exchange of a tower of massive spin-2 gravitons (Kaluza-Klein excitations) gives an extra contribution to $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\rightarrow}f\overline{f}$ and $\ensuremath{\gamma}\ensuremath{\gamma}$ processes along with the opening of a new channel for the neutrinos to annihilate with the relic cosmic neutrino background $\ensuremath{\nu}\overline{\ensuremath{\nu}}\ensuremath{\rightarrow}{G}_{\mathrm{kk}}$ to produce bulk gravitons in the extra dimensions. This will affect their attenuation. We compute the contribution of these Kaluza-Klein excitations to the above processes and find that for parameters of the theory constrained by supernova cooling, the contribution does indeed become the dominant contribution above $\sqrt{s}\ensuremath{\simeq}300\mathrm{GeV}.$