Diffractive Interaction and Scaling Violation in pp →π 0 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 → π0 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 π0 → γγ 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 → π0 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 ~0.05 in the power-law index and that the scaling violation produces 30%-80% more π0 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 ~2.7). The excess can be fully explained if the proton spectral index in the Galactic ridge is a little harder (~0.2 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 yield than without the violation, both in the multiple GeV range.