BEXCO - Room A(101/102)
[CRD101] Cosmic-ray propagation in the light of the Myriad model
Abstract content -- A hardening of the proton and helium fluxes is observed above a few hundreds of GeV/nuc. The actual distribution of the 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 proton and helium anomalies. But how probable are such explanations ? To answer that question, I will discuss the Myriad model and the probabilistic nature of the predictions on primary cosmic ray fluxes. I will show that, at any given energy, these fluxes are distributed according to stable laws well-known in financial analysis. Summary -- It is amazing to realize that in the current description of Galactic propagation of cosmic rays, sources are generally not considered as point-like, at least for primary species like protons and helium nuclei. Taking into account their discreteness is more realistic, but leads to puzzling results. Actually, in the Myriad model, a naive application of the central limit theorem yields an infinite variance for the proton and helium fluxes at the Earth. Several suggestions have been made to cure this problem. One approach is to consider known local supernova remnants (SNR) and to treat the other more distant sources as a continuous jelly. The hardening of the proton and helium fluxes above a few hundreds of GeV/nuc can be explained in that framework. It is also possible to go one step further and to solve the infinite variance problem of the Myriad model by making use of the generalized central limit theorem. I will show that the probability distribution functions of primary fluxes do exist in spite of an infinite variance, and that it follows 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 and turns out to be small for most of the propagation models.