High energy neutrino, photon, and cosmic ray fluxes from VHS cosmic strings

Abstract

Decaying topological defects, in particular cosmic strings, can produce a significant flux of high energy neutrinos, photons and cosmic rays. According to the prevailing understanding of cosmic string dynamics in an expanding Universe, the network of long strings loses its energy first into string loops, which in turn give off most of their energy as gravitational radiation. However, it has been suggested by Vincent, Hindmarsh, and Sakellariadou (VHS) that particle emission may be the dominant energy loss channel for the long string network. In this case, the predicted flux of high energy particles would be much larger. Here we calculate the predicted flux of high energy gamma rays, neutrinos and cosmic ray antiprotons and protons as a function of the scale of symmetry breaking $\ensuremath{\eta}$ at which the strings are produced and as a function of the initial energy ${m}_{J}$ of the particle jets which result from the string decay. Assuming the validity of the VHS scenario, we find that due to the interactions with the cosmic radiation backgrounds all fluxes but the neutrino flux are suppressed at the highest energies. This indicates that the observed events above the GZK cutoff can only be accounted for in this scenario if the primary particle is a neutrino and $\ensuremath{\eta}$ is somewhat less than the GUT scale, i.e. $\ensuremath{\eta}\ensuremath{\lesssim}{10}^{23} \mathrm{eV}.$ The domain of parameter space corresponding to GUT-scale symmetry breaking is excluded also by the current observations below the GZK cutoff. A new aspect of this work is the calculation of the spectrum of the tau neutrinos directly produced in the decay of the X particles. This significantly increases the tau neutrino signal at high energies in all cosmic string scenarios.