Constraining dark matter late-time energy injection: decays and p-wave annihilations

TL;DR

This paper uses recent CMB and Lyman-alpha data to set new constraints on dark matter decay and p-wave annihilation, highlighting their effects at low redshifts and the importance of structure formation in these processes.

Contribution

It provides updated bounds on dark matter lifetime and p-wave annihilation cross sections, emphasizing the role of low-redshift signatures and structure formation effects.

Findings

Dark matter lifetime constrained to ~4×10^{25} s at 100 MeV.

P-wave annihilation effects are significantly enhanced by structure formation.

CMB constraints are still above the thermal relic cross section.

Abstract

We use the latest cosmic microwave background (CMB) observations to provide updated constraints on the dark matter lifetime as well as on p-wave suppressed annihilation cross sections in the 1 MeV to 1 TeV mass range. In contrast to scenarios with an s-wave dominated annihilation cross section, which mainly affect the CMB close to the last scattering surface, signatures associated with these scenarios essentially appear at low redshifts ($z \lesssim 50$) when structure began to form, and thus manifest at lower multipoles in the CMB power spectrum. We use data from Planck, WMAP9, SPT and ACT, as well as Lyman-$\alpha$ measurements of the matter temperature at $z \sim 4$ to set a 95 % confidence level lower bound on the dark matter lifetime of $\sim 4 \times 10^{25}$ s for $m_\chi = 100$ MeV. This bound becomes lower by an order of magnitude at $m_\chi = 1$ TeV due to inefficient energy deposition into the intergalactic medium. We also show that structure formation can enhance the effect of p--wave suppressed annihilation cross sections by many orders of magnitude with respect to the background cosmological rate, although even with this enhancement, CMB constraints are not yet strong enough to reach the thermal relic value of the cross section.