Cosmic ray spectrum of protons plus helium nuclei between 6 and 158 TeV from HAWC data

Abstract

A measurement with high statistics of the differential energy spectrum of light elements in cosmic rays, in particular, of primary H plus He nuclei, is reported. The spectrum is presented in the energy range from 6 to 158 TeV per nucleus. Data was collected with the High Altitude Water Cherenkov (HAWC) Observatory between June 2015 and June 2019. The analysis was based on a Bayesian unfolding procedure, which was applied on a subsample of vertical HAWC data that was enriched to 82% of events induced by light nuclei. To achieve the mass separation, a cut on the lateral age of air shower data was set guided by predictions of CORSIKA/QGSJET-II-04 simulations. The measured spectrum is consistent with a broken power-law spectrum and shows a kneelike feature at around $E={24.0}_{\ensuremath{-}3.1}^{+3.6}\text{ }\text{ }\mathrm{TeV}$, with a spectral index $\ensuremath{\gamma}=\ensuremath{-}2.51\ifmmode\pm\else\textpm\fi{}0.02$ before the break and with $\ensuremath{\gamma}=\ensuremath{-}2.83\ifmmode\pm\else\textpm\fi{}0.02$ above it. The feature has a statistical significance of $4.1\ensuremath{\sigma}$. Within systematic uncertainties, the significance of the spectral break is $0.8\ensuremath{\sigma}$.