Knee of the cosmic hydrogen and helium spectrum below 1 PeV measured by ARGO-YBJ and a Cherenkov telescope of LHAASO

The measurement of the cosmic ray energy spectrum, in particular for individual species of nuclei, is an important tool to investigate cosmic ray production and propagation mechanisms. The determination of the "knees" in the spectra of different species remains one of the main challenges in cosmic ray physics. In fact, experimental results are still conflicting. In this paper we report a measurement of the mixed proton and helium energy spectrum, obtained with the combined data of the ARGO-YBJ experiment and a wide field of view Cherenkov telescope, a prototype of the future LHAASO experiment. By means of a multiparameter technique, we have selected a high-purity proton plus helium sample. The reconstructed energy resolution is found to be about 25% throughout the investigated energy range from 100 TeV to 3 PeV, with a systematic uncertainty in the absolute energy scale of 9.7%. The found energy spectrum can be fitted with a broken power-law function, with a break at the energy E-k = 700 +/- 230(stat) +/- 70(sys) TeV, where the spectral index changes from -2.56 +/- 0.05 to -3.24 +/- 0.36. The statistical significance of the observed spectral break is 4.2 standard deviations.