Search for 100 TeV gamma-ray emission from the Galactic disk

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Abstract
A search for UHE (greater-than-or-equal-to 100 TeV) diffuse gamma-ray emission from the Galactic disk has been carried out, through the excess in the cosmic-ray counting rate, by means of the EAS-TOP extensive air shower array at the Gran Sasso Laboratories. Measurements are performed from angular scales /b/ almost-equal-to 2-degrees (as in 100 MeV satellite experiments, and as expected from CR interactions with the ISM) to /b/ almost-equal-to 100. The obtained upper limits (90% c.l.) to the gamma-ray excess over the cosmic-ray flux from the Galactic disk region are I(gamma)/I(p) < 0.2% for /b/ < 2-degrees, I(gamma)/I(p) < 0.04% for /b/ < 5-degrees, and I(gamma)/I(p) < 0.02% for /b/ < 12-degrees at E0 > 130 TeV, corresponding to upper limits to the flux of I(gamma) < 3.2 x 10(-13) cm-2 s-1 rad-1, I(gamma) < 2.8 x 10(-13) cm-2 s- 1 rad-1, and I(gamma) < 4 x 10(-13) cm-2 S-1 rad-1