Precision Measurement of the Helium Flux in Primary Cosmic Rays of Rigidities 1.9 GV to 3 TV with the Alpha Magnetic Spectrometer on the International Space Station

Knowledge of the precise rigidity dependence of the helium fluxis important in understanding the origin, acceleration, and propagation of cosmic rays. A precise measurement of the helium fluxin primary cosmic rayswith rigidity (momentum/charge) from 1.9 GV to 3 TV based on 50 million events is presented and compared to the proton flux. The detailed variation with rigidity of the helium flux spectral indexis presented for the first time. The spectral indexprogressively hardens at rigidities larger than 100 GV. The rigidity dependence of the helium flux spectral indexis similar to that of the proton spectral indexthough the magnitudes are different. Remarkably, the spectral indexof the proton to helium fluxratio increases with rigidity up to 45 GV and then becomes constant; the fluxratio above 45 GV is well described by a single power law.