Direct measurement of the cosmic-ray proton spectrum from 50 GeV to 10 TeV with the calorimetric electron telescope on the international space station

Abstract

In this paper, we present the analysis and results of a direct measurement of the cosmic-ray protonspectrum with the CALET instrument onboard the International Space Station, including the detailedassessment of systematic uncertainties. The observation period used in this analysis is from October 13, 2015to August 31, 2018 (1054 days). We have achieved the very wide energy range necessary to carry outmeasurements of the spectrum from 50 GeV to 10 TeV covering, for the first time in space, with a singleinstrument the whole energy interval previously investigated in most cases in separate subranges by magneticspectrometers (BESS-TeV, PAMELA, and AMS-02) and calorimetric instruments (ATIC, CREAM, andNUCLEON). The observed spectrum is consistent with AMS-02 but extends to nearly an order of magnitudehigher energy, showing a very smooth transition of the power-law spectral index from −2.81 0.03(50?500 GeV) neglecting solar modulation effects (or −2.87 0.06 including solar modulation effects in thelower energy region) to −2.56 0.04 (1?10 TeV), thereby confirming the existence of spectral hardeningand providing evidence of a deviation from a single power law by more than 3σ.