Diffractive Interaction and Scaling Violation in pp → π 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 → π in the Galactic ridge environment. The calculation includes the diffractive pp interaction and incorporates the Feynman scaling violation for the first time. Galactic diffuse gamma-rays come, predominantly, from π → γγ in the sub-GeV to multi-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 → π 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 power-law index, and, that the scaling violation produces 30−80% more π than the scaling model for incident proton energies above 100 GeV. Combination of the two can explain about a half of the “GeV Excess” with the local cosmic proton (powerlaw 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 p̄ yield than without the violation, both in the multi-GeV range. Subject headings: gamma-rays: observation — gamma-rays: theory — diffuse radiation — cosmic rays — ISM: general Also with Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Menlo Park, CA 94025