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

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.
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Contributor : Emmanuelle Vernay <>
Submitted on : Tuesday, November 17, 2015 - 3:11:53 PM
Last modification on : Monday, June 25, 2018 - 3:50:03 PM

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N. Tomassetti. Origin of the Proton-to-Helium Ratio Anomaly in Cosmic Rays. The Astrophysical journal letters, Bristol : IOP Publishing, 2015, 815 (1), pp.L1. ⟨10.1088/2041-8205/815/1/L1⟩. ⟨in2p3-01229967⟩

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