Diffractive Interaction and Scaling Violation inpp →π 0Interaction and GeV Excess in Galactic Diffuse Gamma‐Ray Spectrum of EGRET

Abstract

The authors present here a new calculation of the gamma-ray spectrum from pp {yields} {pi}{sup 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}{sup 0} {yields} {gamma}{gamma} 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 {yields} {pi}{sup 0} cross-sections and the Feynman scaling hypothesis. They show, in this work, that the diffractive process makes the gamma-ray spectrum harder than the incident proton spectrum by {approx} 0.05 in power-law index, and, that the scaling violation produces 30-80% more {pi}{sup 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 (power-law index {approx} 2.7). The excess can be fully explained if the proton spectral index in the Galactic ridge is a little harder ({approx} 0.2 in power-law index) than the local spectrum. Given also in the paper is that the diffractive process enhances e{sup +} over e{supmore » -} and the scaling violation gives 50-100% higher {bar p} yield than without the violation, both in the multi-GeV range.« less