Photo-disintegration of He4 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: $^4{\rm He} \, (\gamma,p)\, ^3{\rm H}$, $^4{\rm He} \, (\gamma,n) \, ^3{\rm He}$, 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 $E \sim 10^{10.8}~{\rm GeV}$ 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.