### Speakers

Mr
Dylan van Arneman
(University of Amsterdam)Mr
Marnix Reemst
(University of Amsterdam)

### Description

We investigate the radiative decay of the cosmic neutrino background,
and its impact on the spectrum of the cosmic microwave background (CMB) that is
known to be a nearly perfect black body.
We derive {\it exact} formulae for the decay of a heavier neutrino into
a lighter neutrino and a photon, $\nu_j \to \nu_i + \gamma$, and of
absorption as its inverse, $\nu_i + \gamma \to \nu_j$, by accounting for
the precise form of the neutrino momentum distribution.
Our calculations show that if the neutrinos are heavier than $\mathcal
O(0.1)$~eV, the exact formulae give results that differ by $\sim$50\%,
compared with approximate ones where neutrinos are assumed to be at
rest.
We also find that spectral distortion due to absorption is more
important for heavy neutrino masses (by a factor of $\sim$10 going from
a neutrino mass of 0.01~eV to 0.1~eV).
By analyzing the CMB spectral data measured with COBE-FIRAS, we obtain
lower limits on the neutrino lifetime of $\tau_{12} > 4 \times
10^{21}$~s (95\% C.L.) for the smaller mass splitting and $\tau_{13}
\sim \tau_{23} > 10^{19}$~s for the larger mass splitting.
These represent up to one order of magnitude improvement over previous
CMB constraints.
With future CMB experiments such as PIXIE, these limits will improve by
roughly 4 orders of magnitude.
This translates to a projected upper limit on the neutrino magnetic
moment (for certain neutrino masses and decay modes) of $\mu_\nu < 3
\times 10^{-11}\, \mu_B$, where $\mu_B$ is the Bohr magneton.
Such constraints would make future precision CMB measurements
competitive with lab-based constraints on neutrino magnetic moments.

### Primary authors

Dr
Bradley Kavanagh
(GRAPPA, University of Amsterdam)
Mr
Dylan van Arneman
(University of Amsterdam)
Mr
Marnix Reemst
(University of Amsterdam)
Shin'ichiro Ando
(University of Amsterdam)