Neutron lifetime and dark decay of the neutron and hydrogen

Abstract

The tension between the neutron lifetimes measured in the beam and trap experiments suggests that the neutron $n$ might have a new invisible decay channel
$n\to n' X$ into mirror neutron, its dark partner from parallel hidden sector and nearly degenerate in mass with the neutron, with $m_n-m_{n'} \simeq$ 1 MeV or so,
and $X$ being ordinary and mirror photons,  as well as more exotic massless bosons.
I discuss some phenomenological and astrophysical consequences of this scenario,
which depends on the mass range of mirror neutron $n'$.
Namely, the case $m_{n'} < m_p + m_e$ leads to a striking possibility is that the hydrogen atom $^1$H (protium),
constituting 75 per cent of the baryon mass in the Universe, could in fact be unstable:
it can decay via the electron capture into $n'$ and $\nu_e$,  with relatively short lifetime.
If instead $m_{n'} > m_p + m_e$, then the decay $n'\to pe\bar \nu_e$ is allowed and $n'$ can
represent unstable but very long living dark matter component.
Nevertheless, this decay would produce substantial diffuse gamma background.
This explanation, however, is in tension with the latest results of the experiments measuring
$\beta$-asymmetry in the neutron decay.