Origin of the Proton-to-Helium Ratio Anomaly in Cosmic Rays

TL;DR

The paper explains the proton-to-helium ratio anomaly in cosmic rays through a two-component model involving a nearby hydrogen-rich source, reconciling spectral differences without violating universal acceleration mechanisms.

Contribution

It introduces a two-component scenario that accounts for the p/He ratio anomaly and spectral hardening, aligning with existing data and preserving the universality of acceleration.

Findings

The p/He ratio decreases as R^{-0.08} between 40 GV and 2 TV.

Spectral hardening of proton and helium observed above 100 GV.

High-energy flattening of p/He ratio predicted at multi-TeV energies.

Abstract

Recent data on Galactic cosmic rays (CRs) revealed that the helium energy spectrum is harder than the proton spectrum. The AMS experiment has now reported that the proton-to-helium ratio as function of rigidity $R$ (momentum-to-charge ratio) falls off steadily as p/He $\sim R^\Delta$, with $\Delta\approx$-0.08 between $R\sim$40 GV and $R\sim$2 TV. Besides, the single spectra of proton and helium are found to progressively harden at $R\gtrsim$100 GV. The p/He anomaly is generally ascribed to particle-dependent acceleration mechanisms occurring in Galactic CR sources. However, this explanation poses a challenge to the known mechanisms of particle acceleration since they are believed to be "universal", composition blind, rigidity mechanisms. Using the new AMS data, we show that the p/He anomaly can be simply explained in terms of a two-component scenario where the GeV-TeV flux is ascribed to a hydrogen-rich source, possibly a nearby supernova remnant, characterized by a soft acceleration spectrum. This simple idea provides a common interpretation for the p/He ratio and for the single spectra of proton and helium: both anomalies are explained by a flux transition between two components. The "universality" of particle acceleration in sources is not violated in this model. A distinctive signature of our scenario is the high-energy flattening of the p/He ratio at multi-TeV energies, which is hinted at by existing data and will be resolutely tested by new space experiments ISS-CREAM and CALET.