Cosmic ray proton spectrum determined with the imaging atmospheric Cherenkov technique - art. no. 092003

The HEGRA system of 4 imaging atmospheric Cherenkov telescopes (IACTs) has been used to determine the flux and the spectrum of cosmic ray protons over a limited energy range around 1.5 TeV. Although the IACT system is designe d for the detection of gamma-rays with energies above 500 GeV, it has also a large detection area of similar or equal to 10(6) m(2) X 3 msr for primar y protons of energies above 1 TeV and the capability to reconstruct the pri mary proton energy with a reasonable accuracy Delta E/E of 50% near this th reshold. Furthermore, the principle of stereoscopic detection of air shower s permits the effective suppression of air showers induced by heavier prima ries already on the trigger level, and in addition on the software level by analysis of the stereoscopic images. The combination of both capabilities permits a determination of the proton spectrum almost independently of the cosmic ray chemical composition. The accuracy of our estimate of the spectr al index at 1.5 TeV is limited by systematic uncertainties and is comparabl e to the accuracy achieved with recent balloon and space borne experiments. In this paper we describe in detail the analysis tools, namely the detaile d Monte Carlo simulation, the analysis procedure and the results. We determ ine the local (i.e., in the range of 1.5-3 TeV) differential spectral index to be gamma(p) = 2.72+/-0.02(stat)+/-0.15(syst) and obtain an integral flu x above 1.5 TeV of F (>1.5 TeV)=3.1+/-0.6(stat)+/-1.2(syst)X10(-2)/s sr m(2 ). [S0556-2821(99)04107-7].