Prof Subir Sarkar

Abstract:

Cosmogenic neutrinos originate from photo-hadronic interactions of cosmic ray protons with the cosmic microwave background (CMB). The neutrino production rate can be constrained through the accompanying electrons, positrons and gamma-rays that quickly cascade on the CMB and intergalactic magnetic fields to lower energies and generate a gamma ray background in the GeV-TeV region. Bethe-Heitler pair production by protons also contributes to the cascade and can tighten the neutrino constraints in models where extragalactic cosmic rays begin to dominate over the galactic component at a relatively low "crossover" energy. We investigate this issue in the light of the recent Fermi-LAT measurements of the diffuse extragalactic gamma ray background and illustrate by a fit to the HiRes spectrum how the prediction of the cosmogenic neutrino flux in all-proton models varies with the crossover energy. The neutrino flux is required to be smaller when the gamma-ray bound is applied, nevertheless such models are still consistent with HiRes and Fermi-LAT if one properly takes into account the energy uncertainty of cosmic ray measurements. The presently allowed flux is within reach of the IceCube neutrino telescope and other dedicated radio experiments.

Abstract:

The Pierre Auger (cosmic ray) Observatory provides a laboratory for studying fundamental physics at energies far beyond those available at colliders. The Observatory is sensitive not only to hadrons and photons, but can in principle detect ultrahigh energy neutrinos in the cosmic radiation. Interestingly, it may be possible to uncover new physics by analyzing characteristics of the neutrino flux at the Earth. By comparing the rate for quasi-horizontal, deeply penetrating air showers triggered by all types of neutrinos, with the rate for slightly upgoing showers generated by Earth-skimming tau neutrinos, we determine the ratio of events which would need to be detected in order to signal the existence of new non-perturbative interactions beyond the TeV-scale in which the final state energy is dominated by the hadronic component. We use detailed Monte Carlo simulations to calculate the effects of interactions in the Earth and in the atmosphere. We find that observation of 1 Earth-skimming and 10 quasi-horizontal events would exclude the standard model at the 99% confidence level. If new non-perturbative physics exists, a decade or so would be required to find it in the most optimistic case of a neutrino flux at the Waxman-Bahcall level and a neutrino-nucleon cross-section an order of magnitude above the standard model prediction.

Abstract:

The extraction of a 'haze' from the WMAP microwave skymaps is based on subtraction of known foregrounds, viz. free-free (bremsstrahlung), thermal dust and synchrotron, each traced by other skymaps. While the 408 MHz all-sky survey is used for the synchrotron template, the WMAP bands are at tens of GHz where the spatial distribution of the radiating cosmic ray electrons ought to be quite different because of the energy-dependence of their diffusion in the Galaxy. The systematic uncertainty this introduces in the residual skymap is comparable to the claimed haze and can, for certain source distributions, have a very similar spectrum and latitudinal profile and even a somewhat similar morphology. Hence caution must be exercised in interpreting the 'haze' as a physical signature of, e.g., dark matter annihilation in the Galactic centre.