Diffractive Interaction and Scaling Violation in pp-pizero Interaction 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 pp 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% higherp yield than without the violation, both in the multi-GeV range.

Cosmic-ray propagation with DRAGON2: II. Nuclear interactions with the interstellar gas

“…From these considerations, we can clearly conclude that all the differences among leptonic cross sections models deal with the f π,k term. While semi-empirical models have been widely used in the past (e.g., [59][60][61]), these models have been showing several shortcomings in particular concerning diffractive processes or the violation of the Feynman scaling (for a detailed discussion see [62]).…”

“…Kamae2006 This model [62,63] is a combination of Monte Carlo and semi-empirical methods. In particular, it employs the parameterization taken from Blattnig et al [61] to model non-diffractive processes at low energy (T p ≤ 52.6 GeV), the Monte Carlo event generator Pythia 6.2 to account for for non-diffractive high-energy (52.6 GeV ≤ T p ≤ 512 TeV) reactions, and a specifically built MC model for diffractive processes based on [64][65][66].…”

Uncertainties in the production and propagation of cosmic rays in the Milky Way

“…As a possibility for future analysis, we would like to emphasize that it is natural to take into account models that include both of the processes, convection and reacceleration. Further measurements of antiproton and positron spectra, primary to secondary CR ratios and solar modulation, as well as the precise determination and parametrization of important nuclear cross sections (Kamae et al, 2005) seem to be crucial to determine the correct propagation model. In the framework of DC model, exotic contributions remain possible at high energies (E > 20 GeV), and not excluded at lower energies due to the relatively large uncertainties.…”