Proton Decay into Charged Leptons.

We discuss proton and neutron decays involving three leptons in the final state. Some of these modes could constitute the dominant decay channel because they conserve lepton-flavor symmetries that are broken in all usually considered channels. This includes the particularly interesting and rarely discussed p→e^{+}e^{+}µ^{-} and p→µ^{+}µ^{+}e^{-} modes. As the relevant effective operators arise at dimension 9 or 10, observation of a three-lepton mode would probe energy scales of order 100 TeV. This allows us to connect proton decay to other probes such as rare meson decays or collider physics. UV completions of this scenario involving leptoquarks unavoidably violate lepton-flavor universality and could provide an explanation to the recent b→sµµ anomalies observed in B-meson decays.

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

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

Higgs Doublet Decay as the Origin of the Baryon Asymmetry.

We consider a question that curiously had not been properly considered thus far: in the standard seesaw model, what is the minimum value the mass of a right-handed (RH) neutrino must have for allowing successful leptogenesis via CP-violating decays? Answering this question requires us to take into account a number of thermal effects. We show that, for low RH neutrino masses, and thanks to these effects, leptogenesis turns out to proceed efficiently from the decay of the standard model scalar doublet components into a RH neutrino and a lepton. Such decays produce the asymmetry at low temperatures, slightly before sphaleron decoupling. If the RH neutrino has thermalized prior to producing the asymmetry, this mechanism turns out to lead to the bound m_{N}>2 GeV. If, instead, the RH neutrinos have not thermalized, leptogenesis from these decays is enhanced further and can be easily successful, even at lower scales. This Higgs-decay leptogenesis new mechanism works without requiring an interplay of flavor effects and/or cancellations of large Yukawa couplings in the neutrino mass matrix. Last but not least, such a scenario turns out to be testable, from direct production of the RH neutrino(s).

Low-scale leptogenesis and soft supersymmetry breaking.

We investigate the possibility of low-scale leptogenesis in the minimal supersymmetric standard model extended with right-handed (s)neutrinos. We demonstrate that successful leptogenesis can be easily achieved at a scale as low as approximately TeV where lepton number and CP violation comes from soft supersymmetry breaking terms. The scenario is shown to be compatible with neutrino masses data.