No abstract

We have measured the proton-air inelastic cross section at s 1/2 =30 TeV by observing the distribution of extensive-air-shower maxima as a function of atmospheric depth. This distribution has an exponential tail whose slope is ë = 72 ± 9 g cm-2 which implies that ó tot p-air= 530 ± 66 mb. Using Glauber theory and assuming that the elastic-scattering slope parameter b is proportional to ó tot pp , we infer a value of ó tot pp =120 ± 15 mb which lies between a log 2 s and a loges extrapolation of the total pp cross section as measured at lower energies.

Protons with energies up to ~ 10 15 eV are the main component 1 of cosmic rays, but evidence for the specific locations where they could have been accelerated to these energies has been lacking 2 . Electrons are known to be accelerated to cosmic-ray energies in supernova remnants 3,4 , and the shock waves associated with such remnants, when they hit the surrounding interstellar medium, could also provide the energy to accelerate protons. The signature of such a process would be the decay of pions (π 0 ), which are generated when the protons collide with atoms and molecules in an interstellar cloud: pion decay results in γ-rays with a particular spectral-energy distribution 5,6 . Here we report the observation of cascade showers of optical photons resulting fromγ-rays at energies of ~ 10 12 eV hitting Earth's upper atmosphere, in the direction of the supernova remnant RX J1713.7-3946. The spectrum is a good match to that predicted by pion decay, and cannot be explained by other mechanisms.

No abstract

No abstract