Restrictions on the lifetime of sterile neutrinos from primordial nucleosynthesis

We analyze the influence of decaying sterile neutrinos with the masses in the range 1-140 MeV on the primordial Helium-4 abundance, explicitly solving the Boltzmann equations for all particle species, taking into account neutrino flavour oscillations, and paying special attention to systematic uncertainties. We show that the Helium abundance depends only on the sterile neutrino lifetime and not on the way the active-sterile mixing is distributed between flavours, and derive an upper bound on the lifetime. We also demonstrate that the recent results of Izotov & Thuan [arXiv:1001.4440], who find 2sigma higher than predicted by the standard primordial nucleosynthesis value of Helium-4 abundance, are consistent with the presence in the plasma of sterile neutrinos with the lifetime 0.01-2 seconds. The decay of these particles perturbs the spectra of (decoupled) neutrinos and heats photons, changing the ratio of neutrino to photon energy density, that can be interpreted as extra neutrino species at the recombination epoch.Comment: 17 pp. + Appendices. Analysis of deuterium bounds and more accurate account of CMB bounds on Helium-4 is added. Final version to appear in JCA

Experimental bounds on sterile neutrino mixing angles

We derive bounds on the mixing between the left-chiral ("active") and the right-chiral ("sterile") neutrinos, provided from the combination of neutrino oscillation data and direct experimental searches for sterile neutrinos. We demonstrate that the mixing of sterile neutrinos with any flavour can be significantly suppressed, provided that the angle theta_13 is non-zero. This means that the lower bounds on sterile neutrino lifetime, coming from the negative results of direct experimental searches can be relaxed (by as much as the order of magnitude at some masses). We also demonstrate that the results of the negative searches of sterile neutrinos with PS191 and CHARM experiments are not applicable directly to the see-saw models. The reinterpretation of these results provides up to the order of magnitude stronger bounds on sterile neutrino lifetime than previously discussed in the literature. We discuss the implications of our results for the Neutrino Minimal Standard Model (the NuMSM).Comment: 18 pages + Appendices. Journal version with updated figure

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

The standard model of elementary particle physics has provided a consistent description of Nature's fundamental constituents and their interactions. Its predictions have been tested and confirmed by numerous experiments. The Large Hadron Collider's runs at 7 and 8 TeV culminated in the discovery of a Higgs boson-like particle with the mass of about 126 GeV—the last critical standard model component [1–5]. Thus for the first time we are in the situation when all the particles, needed to explain the results of all previous accelerator experiments have been found. At the same time, no significant deviations from the standard model were found in direct or in indirect searches for new physics (see e.g. the summary of the recent search results in [6–25] and most up-to-date information at [26–29]). For this particular value of the Higgs mass it is possible that the standard model remains mathematically consistent and valid as an effective field theory up to a very high energy scale, possibly all the way to the scale of quantum gravity, the Planck scale [30–32]