CMB anisotropy and BBN constraints on pre-recombination decay of dark matter to visible particles

TL;DR

This paper uses CMB anisotropy data to set new constraints on long-lived dark matter particles decaying before recombination, accounting for delayed energy deposition effects that extend previous bounds.

Contribution

It introduces a detailed evolution of electromagnetic cascades in the expanding universe to improve constraints on dark matter decay at high redshifts, extending prior analyses.

Findings

CMB anisotropies are sensitive to energy injection at redshifts up to 10000.

Constraints on particle lifetimes are strengthened, reaching decay times of over 10^{11} seconds.

Relaxing on-the-spot assumptions weakens constraints by a factor of a few to ten.

Abstract

Injection of high energy electromagnetic particles around the recombination epoch can modify the standard recombination history and therefore the CMB anisotropy power spectrum. Previous studies have put strong constraints on the amount of electromagnetic energy injection around the recombination era (redshifts $z\lesssim 4500$). However, energy injected in the form of energetic ($>$ keV) visible standard model particles is not deposited instantaneously. The considerable delay between the time of energy injection and the time when all energy is deposited to background baryonic gas and CMB photons, together with the extraordinary precision with which the CMB anisotropies have been measured, means that CMB anisotropies are sensitive to energy that was injected much before the epoch of recombination. We show that the CMB anisotropy power spectrum is sensitive to energy injection even at $z = 10000$, giving stronger constraints compared to big bang nucleosynthesis and CMB spectral distortions. We derive, using Planck CMB data, the constraints on long-lived unstable particles decaying at redshifts $z\lesssim 10000$ (lifetime $\tau_X\gtrsim 10^{11}$s) by explicitly evolving the electromagnetic cascades in the expanding Universe, thus extending previous constraints to lower particle lifetimes. We also revisit the BBN constraints and show that the delayed injection of energy is important for BBN constraints. We find that the constraints can be weaker by a factor of few to almost an order of magnitude, depending on the energy, when we relax the quasi-static or on-the-spot assumptions.