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.

We study the impact of an additional U (1) gauge symmetry with flavour-dependent charges for quarks and leptons on the LHC flavour anomalies observed inand h → µτ . In its minimal version with two scalar doublets, the resulting model naturally explains the deviations from the Standard Model observed in B → K * µ + µ − and R(K). The CMS access in h → µτ can be explained by introducing a third scalar doublet, which gives rise to a prediction for τ → 3µ. We investigate constraints from flavour observables and direct LHC searches for pp → Z → µ + µ − . Our model successfully generates the measured fermionmixing matrices and does not require vector-like fermions, unlike previous attempts to explain these anomalies.

The LHC has observed, so far, 3 deviations from the Standard Model (SM) predictions in flavor observables: LHCb reported anomalies in B→K*μ(+)μ(-) and R(K)=B→Kμ(+)μ(-)/B→Ke(+)e(-), while CMS found an excess in h→μτ. We show, for the first time, how these deviations from the SM can be explained within a single well-motivated model: a two-Higgs-doublet model with gauged L(μ)-L(τ) symmetry. We find that, despite the constraints from τ→μμμ and B(s)-B¯(s) mixing, one can explain h→μτ, B→K*μ(+)μ(-) and R(K) simultaneously, obtaining interesting correlations among the observables.

The difference between baryon number B and lepton number L is the only anomaly-free global symmetry of the Standard Model, easily promoted to a local symmetry by introducing three right-handed neutrinos, which automatically make neutrinos massive. The non-observation of any (B-L)-violating processes leads us to scrutinize the case of unbroken gauged B-L; besides Dirac neutrinos, the model contains only three parameters, the gauge coupling strength g , the Stückelberg mass M Z , and the kinetic mixing angle χ .The new force could manifest itself at any scale, and we collect and derive bounds on g over the entire testable range M Z = 0-10 13 eV, also of interest for the more popular case of spontaneously broken B-L or other new light forces. We show in particular that successful Big Bang nucleosynthesis provides strong bounds for masses 10 eV < M Z < 10 GeV due to resonant enhancement of the rate f f ↔ ν R ν R . The strongest limits typically arise from astrophysics and colliders, probing scales M Z /g from TeV up to 10 10 GeV.

The extension of the standard model by a spontaneously broken abelian gauge group based on the L À L lepton number can resolve the long-standing discrepancy between experimental and theoretical values for the magnetic moment of the muon. It furthermore naturally generatessymmetric lepton mixing, introduces neutrino nonstandard interactions, and the associated gauge boson Z 0 serves as a mediator to the right-handed neutrino sector. A detailed fit to electroweak data is performed to identify the allowed values for the mass of Z 0 and its mixing with the standard model Z. An economical new scalar sector is constructed that spontaneously breaks L À L and leads to experimental consequences such as lepton flavor violation and collider signatures. Furthermore we discuss the nonabelian extension to an SUð2Þ 0 , particularly the neutrino sector.

BABAR, Belle, and LHCb Collaborations report evidence for new physics in B → Dτν and B → D Ã τν of approximately 3.8σ. There is also the long lasting discrepancy of about 3σ in the anomalous magnetic moment of the muon, and the branching ratio for τ → μνν is 1.8σ (2.4σ) above the standard model expectation using the HFAG (PDG) values. Furthermore, CMS Collaboration finds hints for a nonzero decay rate of h → μτ. Interestingly, all these observations can be explained by introducing new scalars. In this Letter we consider these processes within a lepton-specific two-Higgs doublet model (i.e., of type X) with additional nonstandard Yukawa couplings. It is found that one can accommodate τ → μνν with modified Higgs-τ couplings. The anomalous magnetic moment of the muon can be explained if the additional neutral CP-even Higgs boson H is light (below 100 GeV). Also RðDÞ and RðD Ã Þ can be easily explained by additional t-c-Higgs couplings. Combining these t-c couplings with a light H the decay rate for t → Hc can be in a testable range for the LHC. Effects in h → μτ are also possible, but in this case a simultaneous explanation of the anomalous magnetic moment of the muon is difficult due to the unavoidable τ → μγ decay. Introduction.-In addition to direct searches for new physics (NP) beyond the standard model (SM) performed at high energies at the LHC, low-energy precision flavor observables provide a complementary window to NP. The anomalous magnetic moment of the muon ðg − 2Þ μ is a prime example as it is very sensitive to physics beyond the SM entering via quantum corrections. Also, tauonic B meson decays and τ → μνν are excellent probes of NP: they test lepton flavor universality and are sensitive to new degrees of freedom that couple proportional to the mass of the involved particles (e.g., Higgs bosons [1]) because of the heavy τ lepton involved. The observation of flavorviolating decays of the SM Higgs boson, e.g., h → μτ, would prove the existence of NP.Let us briefly review the current experimental and theoretical situation in these processes. The world average of the measurement of a μ ≡ ðg − 2Þ μ =2 is completely dominated by the Brookhaven experiment E821 [2] and is given by [3]

Models with spontaneously broken global lepton number can lead to a pseudoGoldstone boson as a long-lived dark matter candidate. Here we revisit the case of singlet majoron dark matter and discuss multiple constraints. For masses above MeV, this model could lead to a detectable flux of monochromatic mass-eigenstate neutrinos, which have flavor ratios that depend strongly on the neutrino mass hierarchy. We provide a convenient parametrization for the loop-induced majoron couplings to charged fermions that allows us to discuss three-generation effects such as lepton flavor violation. These couplings are independent of the low-energy neutrino parameters but can be constrained by the majoron decays into charged fermions.

We show that left-right symmetric models can easily accommodate stable TeV-scale dark matter particles without the need for an ad-hoc stabilizing symmetry. The stability of a newly introduced multiplet arises either accidentally as in the Minimal Dark Matter framework or comes courtesy of the remaining unbroken Z2 subgroup of B − L. Only one new parameter is introduced: the mass of the new multiplet. As minimal examples we study left-right fermion triplets and quintuplets and show that they can form viable two-component dark matter. This approach is in particular valid for SU (2) × SU (2) × U (1) models that explain the recent diboson excess at ATLAS in terms of a new charged gauge boson of mass 2 TeV.