UHECR mass composition at highest energies from anisotropy of their arrival directions

TL;DR

This paper introduces a new method to estimate ultra-high energy cosmic ray (UHECR) mass composition using anisotropy in their arrival directions, which is robust against magnetic field uncertainties and effective at GZK energies.

Contribution

The paper presents a novel statistical approach based on event deflections to determine UHECR mass composition from arrival direction data.

Findings

Method is robust to magnetic field variations.

Effective at GZK energy range with low statistics.

Can discriminate between different composition models.

Abstract

We propose a new method for the estimation of ultra-high energy cosmic ray (UHECR) mass composition from a distribution of their arrival directions. The method employs a test statistic (TS) based on a characteristic deflection of UHECR events with respect to the distribution of luminous matter in the local Universe. Making realistic simulations of the mock UHECR sets, we show that this TS is robust to the presence of galactic and non-extreme extra-galactic magnetic fields and sensitive to the mass composition of events in a set. This allows one to constrain the UHECR mass composition by comparing the TS distribution of a composition model in question with the data TS, and to discriminate between different composition models. While the statistical power of the method depends somewhat on the MF parameters, this dependence decreases with the growth of statistics. The method shows good performance even at GZK energies where the estimation of UHCER mass composition with traditional methods is complicated by a low statistics.