results from gamma

Results from GAMMAAn air shower experiment on Mt. Aragats (3200 m), at the site of ANI, in Armenia; latest results

Lawrence W. Jones

University of Michigan

C2CR2007

February 28, 2007

• The energy spectrum and composition of primary cosmic rays over the energy range ~1<E<~100 PeV has been a problem for some decades. The GAMMA array on Mt. Aragats (3200 m) in Armenia has been collecting and analyzing air shower data to better understand this problem for the past several years. A search is also in progress for evidence of PeV primary gamma rays.

• This is a collaboration of Armenian, Russian French and U.S. physicists lead by Dr. Romen Martirosov of the Yerevan Physics Institute.

• The array consists of 33 scintillation counter stations on the surface, distributed in 5 concentric circles out to a radius of 100 m, and an array of 150 one m² scintillators under 2.3 Kg/cm² of rock and concrete (with ~5 GeV muon E threshold). The primary energy and composition are calculated based on the observed electron number, the “age” (deduced from the radial distribution), and the muon content. The data are interpreted based upon Monte Carlo models of the primary interaction, specifically the “QGSJET” and “SIBYLL” M.C. models.

Clearly the Monte Carlo models are imperfect, and must be improved. They are primarily based on sub-TeV fixed target data. New results from the LHC will be very valuable, in particular forward, small-angle inclusive data.

Conclusions• The GAMMA array is adding to our data

and our understanding of the primary spectrum and composition of cosmic rays above a PeV; energies inaccessible to direct balloon and satellite observation. Clearly, better inclusive accelerator data is necessary for improved M.C.s, but also cosmic ray observations of higher statistics and reduced systematic uncertainties are also highly desired.