Constraints on ultrahigh-energy cosmic ray mass composition using cosmogenic neutrinos
July 22, 2019
Ultrahigh-energy cosmic rays (UHECRs), with energy 1 EeV and above, propagate over cosmological
distances, rendering them susceptible to interactions with the cosmic photon backgrounds that
produce secondary particles, viz., neutrinos and gamma-rays. The sources, as well as the mass
composition of UHECRs, can be constrained by probing these cosmogenic fluxes that extend to
energies exceeding 1 EeV. The neutrinos, being weakly interacting, travel unhindered and
undeflected by extragalactic or Galactic magnetic fields. We fit the observed UHECR spectrum
as measured by the Pierre Auger Observatory (PAO) by simulating the propagation of UHECRs
of various mass compositions at injection, from different source distributions. We also calculate
the cosmogenic neutrino spectrum for the same UHECR parameters, fitting the PAO data.
The neutrino spectrum varies depending on the UHECR mass composition and source properties.
Although the currently operating detectors do not reach the necessary sensitivity for observing
cosmogenic neutrinos in all cases, a few parameter sets producing relatively high fluxes are already
constrained by the flux upper limit imposed by 9-yrs of IceCube data. We also explore
possibilities to constrain the UHECR abundance fraction of light nuclei at injection by identifying
the fluxes of individual neutrino flavors in future detectors. Although the contribution of
neutron beta decay to the cosmogenic neutrino spectrum is insignificant, it shifts the ratio of the
fluxes of individual flavors obtained at earth from their constant values; thus serving as a discriminator
between different mass composition models. The interactions leading to the production of
neutrinos also leave their imprint on the observed UHECR spectrum. This can further explain the
origin of cutoff in the UHECR spectrum at the highest energies.
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