Energy spectrum of cosmic protons and helium nuclei by a hybrid measurement at 4300 m a.s.l

The energy spectrum of cosmic Hydrogen and Helium nuclei has been measured below the so-called knee by using a hybrid experiment with a wide field-of-view Cherenkov telescope and the Resistive Plate Chamber(RPC) array of the ARGO-YBJ experiment at 4300 m above sea level. The Hydrogen and Helium nuclei have been well separated from other cosmic ray components by using a multi-parameter technique. A highly uniform energy resolution of about 25% is achieved throughout the whole energy range (100-700 TeV). The observed energy spectrum is compatible with a single power law with index gamma = -2.63 +- 0.06.