MeV sterile neutrinos in low reheating temperature cosmological scenarios

It is commonly assumed that the cosmological and astrophysical bounds on the mixings of sterile with active neutrinos are much more stringent than those obtained from laboratory measurements. We point out that in scenarios with a very low reheating temperature T_RH << 100 MeV at the end of (the last episode of) inflation or entropy creation, the abundance of sterile neutrinos becomes largely suppressed with respect to that obtained within the standard framework. Thus, in this case cosmological bounds become much less stringent than usually assumed, allowing sterile neutrinos to be ``visible'' in future experiments. Here, we concentrate on massive (mostly sterile) neutrinos heavier than 1 MeV.Comment: 14 pp, 7 fig

The search for heavy Majorana neutrinos

The Majorana nature nature of neutrinos can be experimentally verified only via lepton-number violating processes involving charged leptons. We study 36 lepton-number violating (LV) processes from the decays of tau leptons and pseudoscalar mesons. These decays are absent in the Standard Model but, in presence of Majorana neutrinos in the mass range similar to 100 MeV to 5 GeV, the rates for these processes would be enhanced due to their resonant contribution. We calculate the transition rates and branching fractions and compare them to the current bounds from direct experimental searches for Delta L = 2 tau and rare meson decays. The experimental non-observation of such LV processes places stringent bounds on the Majorana neutrino mass and mixing and we summarize the existing limits. We also extend the search to hadron collider experiments. We find that, at the Tevatron with 8fb(-1) integrated luminosity, there could be 2 sigma (5 sigma) sensitivity for resonant production of a Major an a neutrino in the mu(+/-)mu(+/-) modes in the mass range of similar to 10 - 180 GeV (10 - 120 GeV). This reach can be extended to similar to 10 - 375 GeV (10 - 250 GeV) at the LHC of 14 TeV with 100 fb(-1). The production cross section at the LHC of 10 TeV is also presented for comparison. We study the mu(+/-)e(+/-) modes as well and find that the signal could be large enough even taking in to account the current bound from neutrinoless double-beta decay. The signal from the gauge boson fusion channel W+W+-&gt; l(1)(+)l(2)(+) at the LHC is found to be very weak given the rather small mixing parameters. We comment on the search strategy when at lepton is involved in the final state