We consider the hypothesis that dark matter (DM) has tree-level interactions only with leptons. Such a framework, where DM recoils against electrons bound in atoms, has been proposed as an explanation for the annually modulated scintillation signal in DAMA/LIBRA data versus the absence of a signal for nuclear recoils in experiments like CDMS or XENON10. However, even in such a leptophilic DM scenario there are loop-induced DM-hadron interactions, where photons emitted from virtual leptons couple to the charge of a nucleus. Using a general effective field theory approach we show that, if such an interaction is induced at one-or two-loop level, then DM-nucleus scattering dominates over DM-electron scattering. This is because the latter is suppressed by the bound state wave function. One obtains a situation similar to standard DM-nucleus scattering analyses with considerable tension between the results of DAMA and CDMS/XENON10. This conclusion does not apply in the case of pseudoscalar or axialvector coupling between DM and leptons, where the loop diagrams vanish. In this case the explanation of the DAMA signal in terms of DM-electron scattering is strongly disfavored by the spectral shape of the signal. Furthermore, if DM can annihilate into neutrinos or tau leptons, the required cross sections are excluded by many orders of magnitude using the Super-Kamiokande bound on neutrinos from DM annihilations in the Sun.

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 propose a new production mechanism for keV sterile neutrino Dark Matter. In our setting, we assume the existence of a scalar singlet particle which never entered thermal equilibrium in the early Universe, since it only couples to the Standard Model fields by a really small Higgs portal interaction. For suitable values of this coupling, the scalar can undergo the so-called freeze-in process, and in this way be efficiently produced in the early Universe. These scalars can then decay into keV sterile neutrinos and produce the correct Dark Matter abundance. While similar settings in which the scalar does enter thermal equilibrium and then freezes out have been studied previously, the mechanism proposed here is new and represents a versatile extension of the known case. We perform a detailed numerical calculation of the DM production using a set of coupled Boltzmann equations, and we illustrate the successful regions in the parameter space. Our production mechanism notably can even work in models where active-sterile mixing is completely absent. * no primordial lepton asymmetry is present in the early Universe [17,18]. Indeed, a large enough primordial lepton asymmetry could lead to a resonant transition -the so-called Shi-Fuller mechanism [46]-producing a considerable amount of sterile neutrinos with a cooler non-thermal spectrum, in addition to the ones produced by the DW mechanism. In this way, some bounds could be evaded. On the other hand, in frameworks where the SM gauge group is extended, the sterile neutrinos could be charged non-trivially under the full gauge group and be sterile only with respect to SM interactions. In this case, although this is not compulsory [47], thermal production of keV neutrinos could be revived [48,49]. However, this mechanism would generically produce too much DM and by this overclose the Universe, thus requiring some dilution by the production of additional entropy [50]. Moreover, it could get into trouble with bounds from Big Bang nucleosynthesis [51].Probably the most versatile production mechanism from a particle physics point of view is the non-thermal production of keV sterile neutrinos by the decays of particles [52,53,54,55,56], in particular of singlet scalars. Examples of this production mechanism exists for the scalar being an inflaton [57,58] or a more general equilibrated scalar singlet particle [59,60]. This case is particularly interesting because it tends to lead to smaller bounds on the mass of the keV neutrino, a desirable feature, since keV-neutrinos with too large masses could be in danger with X-ray bound. For a recent collections of observational bounds from the non-observation of the decay into a light neutrino and a photon, N 1 → νγ, see Refs. [17,18,61] and references therein. 1 The aim of this paper is to study a variant of the scalar decay production mechanism discussed in Refs. [59,60]. The decisive point is that the scalar σ, which decays into the keV neutrinos, σ → N 1 N 1 , has to be efficiently produced in the early Universe, as otherwise it would ...

We present cosmological upper limits on the sum of active neutrino masses using large-scale power spectrum data from the WiggleZ Dark Energy Survey and from the Sloan Digital Sky Survey -Data Release 7 (SDSS-DR7) sample of Luminous Red Galaxies (LRG). Combining measurements on the Cosmic Microwave Background temperature and polarisation anisotropies by the Planck satellite together with WiggleZ power spectrum results in a neutrino mass bound of 0.37 eV at 95% C.L., while replacing WiggleZ by the SDSS-DR7 LRG power spectrum, the 95% C.L. bound on the sum of neutrino masses is 0.38 eV. Adding Baryon Acoustic Oscillation (BAO) distance scale measurements, the neutrino mass upper limits greatly improve, since BAO data break degeneracies in parameter space. Within a ΛCDM model, we find an upper limit of 0.13 eV (0.14 eV) at 95% C.L., when using SDSS-DR7 LRG (WiggleZ) together with BAO and Planck. The addition of BAO data makes the neutrino mass upper limit robust, showing only a weak dependence on the power spectrum used. We also quantify the dependence of neutrino mass limit reported here on the CMB lensing information. The tighter upper limit (0.13 eV) obtained with SDSS-DR7 LRG is very close to that recently obtained using Lyman-alpha clustering data, yet uses a completely different probe and redshift range, further supporting the robustness of the constraint. This constraint puts under some pressure the inverted mass hierarchy and favours the normal hierarchy.