Diffractive interaction and scaling violation in pp→π0 interaction and GeV excess in galactic diffuse gamma-ray spectrum of EGRET

We present here a new calculation of the gamma-ray spectrum from pp --> pi(0) in the Galactic ridge environment. The calculation includes the diffractive p-p interaction and incorporates the Feynman scaling violation for the first time. Galactic diffuse gamma-rays come, predominantly, from pi(0) --> gammagamma in the sub- to multiple GeV range. Hunter et al. found, however, an excess in the GeV range ("GeV excess'') in the EGRET Galactic diffuse spectrum above the prediction based on experimental pp --> pi(0) cross sections and the Feynman scaling hypothesis. We show, in this work, that the diffractive process makes the gamma-ray spectrum harder than the incident proton spectrum by similar to0.05 in the power-law index and that the scaling violation produces 30%-80% more pi(0) than the scaling model for incident proton energies above 100 GeV. A combination of the two can explain about half of the GeV excess with the local cosmic proton (power-law index similar to2.7). The excess can be fully explained if the proton spectral index in the Galactic ridge is a little harder (similar to0.2 in power-law index) than the local spectrum. Given also in the paper is that the diffractive process enhances e(+) over e(-) and the scaling violation gives 50%-100% higher (p) over bar yield than without the violation, both in the multiple GeV range.