# New Physics in the Rayleigh-Jeans Tail of the CMB

## TL;DR

This paper proposes new physics models involving dark photon decay and oscillations that could significantly modify the Rayleigh-Jeans tail of the CMB spectrum, potentially explaining the anomalous 21 cm absorption signal.

## Contribution

It introduces explicit models with dark photon decay and oscillations that can increase the photon count in the Rayleigh-Jeans tail, offering testable predictions for 21 cm observations.

## Key findings

- Dark photon decay can enhance RJ photon numbers by orders of magnitude.
- Resonant oscillations of dark photons into photons can explain the 21 cm absorption anomaly.
- Models predict observable effects in upcoming 21 cm experiments.

## Abstract

We show that despite stringent constraints on the shape of the main part of the CMB spectrum, there is considerable room for its modification within its Rayleigh-Jeans (RJ) end, $\omega \ll T_{\rm CMB}$. We construct explicit New Physics models that give an order one (or larger) increase of photon count in the RJ tail, which can be tested by existing and upcoming experiments aiming to detect the cosmological 21 cm emission/absorption signal. This class of models stipulates the decay of unstable particles to dark photons, $A'$, that have a small mass, $m_{A'} \sim 10^{-14} - 10^{-9}$ eV, non-vanishing mixing angle $\epsilon$ with electromagnetism, and energies much smaller than $T_{\rm CMB}$. The non-thermal number density of dark photons can be many orders of magnitude above the number density of CMB photons, and even a small probability of $A'\to A$ oscillations, for values of as small as $\epsilon \sim 10^{-9}$, can significantly increase the number of RJ photons. In particular, we show that resonant oscillations of dark photons into regular photons in the interval of redshifts $20 < z < 1700$ can be invoked as an explanation of the recent tentative observation of a stronger-than-expected absorption signal of 21 cm photons. We present a model that realizes this possibility, where milli-eV mass dark matter decays to dark photons, with a lifetime longer than the age of the Universe.