The photo-disintegration of ${}^4$He on the cosmic microwave background is less severe than earlier thought

Abstract

We thoroughly study the photo-disintegration of ${}^4$He on the cosmic microwave background using the most recent cross-section data both from the inclusive measurement observing the analog of the giant dipole resonance in ${}^4$He through the charge-exchange spin-flip ${}^4$He (${}^7$Li,${}^7$Be) reaction and from measurements of exclusive two-body and three-body processes: ${}^4\mathrm{He}(\gamma ,p){}^3\mathrm{H}$, ${}^4\mathrm{He}(\gamma ,n){}^3\mathrm{He}$, and ${}^4$He $(\gamma ,pn)$ ${}^2$H. We show that the present-day (redshift $z=0$) mean free path of ultra-relativistic (Lorentz factor $\sim 10^{10}$) helium nuclei increases by more that $15\%$ with respect to previous estimates adopted as benchmarks for Monte Carlo simulation codes of ultrahigh-energy cosmic ray propagation. This implies that the physical survival probability of ${}^4$He nuclei would be larger than predicted by existing event generators. For example, for $E\sim 10^{10.8}\mathrm{GeV}$ and a propagation distance of 3.5 Mpc, the ${}^4$He intensity would be $35\%$ larger than the output of CRPropa 3 program and $42\%$ larger than the output of SimProp v2r4 program. We provide new parametrizations for the two-body and three-body photo-disintegration cross-sections of ${}^4$He, ${}^3$He, tritium, and deuterium.

Comments: To be published in PRD