# Fractional Dark Matter decay: cosmological imprints and observational constraints

## TL;DR

This paper investigates how a fraction of dark matter decay affects cosmological observations, providing updated constraints on decay parameters and exploring implications for cosmological tensions and future measurements.

## Contribution

The study offers a comprehensive analysis of dark matter decay impacts on multiple cosmological observables using full Boltzmann hierarchy calculations and updates constraints with current data.

## Key findings

- Updated the fraction of decaying dark matter constrained to less than 2.73% for short-lived decay.
- No significant constraint improvement for long-lived decay from redshift space distortion data.
- Future kSZ measurements could further tighten constraints on dark matter decay.

## Abstract

If a fraction $f_{\rm dcdm}$ of the Dark Matter decays into invisible and massless particles (so-called "dark radiation") with the decay rate (or inverse lifetime) $\Gamma_{\rm dcdm}$, such decay will leave distinctive imprints on cosmological observables. With a full consideration of the Boltzmann hierarchy, we calculate the decay-induced impacts not only on the CMB but also on the redshift distortion and the kinetic Sunyaev-Zel'dovich effect, while providing detailed physical interpretations based on evaluating the evolution of gravitational potential. By using the current cosmological data with a combination of Planck 2015, Baryon Acoustic Oscillation and redshift distortion measurements which can improve the constraints, we update the $1\sigma$ bound on the fraction of decaying DM from $f_{\rm dcdm}\lesssim5.26\%$ to $f_{\rm dcdm}\lesssim2.73\%$ for the short-lived DM (assuming $\Gamma_{\rm dcdm}/H_0\gtrsim10^4$). However, no constraints are improved from RSD data ($f_{\rm dcdm}\lesssim0.94\%$) for the long-lived DM (i.e., $\Gamma_{\rm dcdm}/H_0\lesssim10^4$). We also find the fractional DM decay can only slightly reduce the $H_0$ and $\sigma_8$ tensions, which is consistent with other previous works. Furthermore, our calculations show that the kSZ effect in future would provide a further constraining power on the decaying DM.