The Majorana 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 ∼ 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 ∆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 8 fb −1 integrated luminosity, there could be 2σ (5σ) sensitivity for resonant production of a Majorana neutrino in the µ ± µ ± modes in the mass range of ∼ 10 − 180 GeV (10 − 120 GeV). This reach can be extended 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 µ ± e ± modes as well and find that the signal could be large enough even taking into account the current bound from neutrinoless double-beta decay. The signal from the gauge boson fusion channel W + W + → 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 a τ lepton is involved in the final state.

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Abstract. We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved -cosmology, astrophysics, nuclear, and particle physics -in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

It was commonly thought that the observation of low energy leptonic CP-violating phases would not automatically imply the existence of a baryon asymmetry in the leptogenesis scenario. This conclusion does not generically hold when the issue of flavor is relevant and properly taken into account in leptogenesis. We illustrate this point with various examples studying the correlation between the baryon asymmetry and the CP-violating asymmetry in neutrino oscillations and the effective Majorana mass in neutrinoless double beta decay. DOI: 10.1103/PhysRevD.75.083511 PACS numbers: 98.80.Cq, 11.30.Er, 11.30.Fs, 14.60.Pq Leptogenesis [1] is a simple mechanism to explain the baryon number asymmetry (per entropy density) of the Universe Y B 0:87 0:02 10 ÿ10 [2]. A lepton asymmetry is dynamically generated and then converted into a baryon asymmetry due to (B L)-violating sphaleron interactions [3,4] which exist in the standard model (SM). A simple model in which this mechanism can be implemented is the ''seesaw''(type I) [5], consisting of the SM plus three right-handed (RH) Majorana neutrinos. In thermal leptogenesis [6] the heavy RH neutrinos are produced by thermal scatterings after inflation and subsequently decay out-of-equilibrium in a lepton number and CP-violating way, thus satisfying Sakharov's constraints [4]. At the same time the smallness of neutrino masses suggested by oscillation experiments [7] can be ascribed to the seesaw mechanism where integrating out heavy RH Majorana neutrinos generates mass terms for the lefthanded flavor neutrinos which are inversely proportional to the mass of the RH ones.Establishing a connection between the CP-violation in low energy neutrino physics and the CP-violation at high energy necessary for leptogenesis has received much attention in recent years [8] and is the subject of the present paper. In the case of three neutrino mixing, CP-violation at low energy is parameterized by the phases in the Pontecorvo-Maki-Nagakawa-Sakata (PMNS) [9] lepton mixing matrix U. It contains the Dirac phase and, if neutrinos are Majorana particles, two Majorana phases 21 and 31 [10]. The Dirac phase enters in the probability of neutrino oscillations. The corresponding CP-asymmetry is given by the difference between the oscillation probability for neutrino and antineutrinos, P P ! e ÿ and m 2 are the mass square differences which drive the solar and the atmospheric neutrino oscillations, respectively, and m i i 1; 2; 3 are the light neutrino masses. One Majorana phase can, in principle, be observed although this represents a challenge. For a detailed discussion see Refs. [13,14].It was commonly accepted that the future observation of leptonic low energy CP-violation would not automatically imply a nonvanishing baryon asymmetry through leptogenesis. This conclusion, however, was shown in [15][16][17] not to hold universally. The reason is based on a new ingredient recently accounted for in the leptogenesis scenario, lepton flavor [15][16][17][18]. The dynamics of leptogenesis is usually addr...