The proton and helium anomalies in the light of the Myriad model

TL;DR

This paper introduces the Myriad model for cosmic ray propagation, treating sources as point-like events, and uses the generalized central limit theorem to explain proton and helium flux anomalies with probabilistic flux distributions.

Contribution

It provides a novel probabilistic framework for cosmic ray fluxes using the Myriad model and stable laws, addressing the infinite variance problem.

Findings

Fluxes follow a heavy-tailed stable distribution.

Local sources are unlikely to fully explain anomalies under realistic parameters.

The model quantifies the probability of observed flux anomalies.

Abstract

A hardening of the proton and helium fluxes is observed above a few hundreds of GeV/nuc. The distribution of local sources of primary cosmic rays has been suggested as a potential solution to this puzzling behavior. Some authors even claim that a single source is responsible for the observed anomalies. But how probable these explanations are? To answer that question, our current description of cosmic ray Galactic propagation needs to be replaced by the Myriad model. In the former approach, sources of protons and helium nuclei are treated as a jelly continuously spread over space and time. A more accurate description is provided by the Myriad model where sources are considered as point-like events. This leads to a probabilistic derivation of the fluxes of primary species, and opens the possibility that larger-than-average values may be observed at the Earth. For a long time though, a major obstacle has been the infinite variance associated to the probability distribution function which the fluxes follow. Several suggestions have been made to cure this problem but none is entirely satisfactory. We go a step further here and solve the infinite variance problem of the Myriad model by making use of the generalized central limit theorem. We find that primary fluxes are distributed according to a stable law with heavy tail, well-known to financial analysts. The probability that the proton and helium anomalies are sourced by local SNR can then be calculated. The p-values associated to the CREAM measurements turn out to be small, unless somewhat unrealistic propagation parameters are assumed.