Diffractive Interaction and Scaling Violation in<i>pp</i> →π ⁰Interaction and GeV Excess in Galactic Diffuse Gamma‐Ray Spectrum of EGRET

Abstract

We present here a new calculation of the gamma-ray spectrum from pp → π 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 π 0 → 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 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 ∼ 0.05 in power-law index, and, that the scaling violation produces 30−80 % more π 0 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 ¯p 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