FASER2

Abstract:

We have searched for electrons scattered in the forward direction by neutrinos produced by dumping a 400 GeV/c proton beam on a copper target. We estimate the number of tau neutrinos produced from the decays of Ds mesons in the dump. The data limit the possible magnetic moment of tau neutrinos to be below 5.4×10-7 ωB. This rules out the suggestion that tau neutrinos of mass O(MeV) constitute the dark matter in the universe. © 1992.

Abstract:

There has recently been renewed interest in massive neutral dark-matter particle candidates with masses greater than ∼ 1 TeV which may be unstable. We re-evaluate the constraints on such particles from the possible effects of their decays on the spectrum of the microwave background-radiation and the primordially synthesised abundances of the light elements, from observations of the diffuse gamma-ray background radiation, and from searches for muons and neutrinos in nucleon-decay and cosmic-ray detectors. We find that such unstable neutral relics may well have the cosmological critical density if their lifetime exceeds ∼ 1016 yr. We illustrate our arguments by applying them to technicolour baryons and to "cryptons" in superstring-inspired models. © 1992.

Abstract:

We discuss the implications of the recent measurement of a 17 keV mass component in the electron neutrino sector. Such a heavy state must decay in order to be compatible with cosmology; this requires states additional to those of the standard model. The most likely candidates are either majorons, allowing the decay into massless weakly interacting scalars, or single neutrinos, allowing the decay via the Z into light neutrinos. We show that in the latter case there is a lower bound on the lifetime in conflict with the cosmological constraints on this decay channel and on the photon plus neutrino decay channel which is also present. The extensions of the standard model needed to reduce the lifetime are considered. We analyze the mass matrix, playing particular regard to the singlet neutrino case, and discuss how it might be extended to explain the solar neutrino deficit. © 1991.