We report a weak line at 3.52 AE 0.02 keV in x-ray spectra of the Andromeda galaxy and the Perseus galaxy cluster observed by the metal-oxide-silicon (MOS) and p-n (PN) CCD cameras of the XMMNewton telescope. This line is not known as an atomic line in the spectra of galaxies or clusters. It becomes stronger towards the centers of the objects; is stronger for Perseus than for M31; is absent in the spectrum of a deep "blank sky" data set. Although for each object it is hard to exclude that the feature is due to an instrumental effect or an atomic line, it is consistent with the behavior of a dark matter decay line. Future (non-)detections of this line in multiple objects may help to reveal its nature. DOI: 10.1103/PhysRevLett.113.251301 PACS numbers: 95.35.+d, 13.35.Hb, 14.60.St, 95.85.Nv The nature of dark matter (DM) is a question of crucial importance for both cosmology and for fundamental physics. As neutrinos-the only known particles that could be DM candidates-are too light to be consistent with various observations [1][2][3][4], it is widely anticipated that new particles should exist. Although many candidates have been put forward (see, e.g., Ref.[5]), little is known experimentally about the properties of DM particles: their masses, lifetimes, and interaction types remain largely unconstrained. A priori, a given DM candidate can possess a decay channel if its lifetime exceeds the age of the Universe. Therefore, the search for a DM decay signal provides an important test to constrain the properties of DM in a model-independent way. For fermionic particles, one should search above the Tremaine-Gunn limit [1] (≳300 eV). If the mass is below 2m e c 2 , such a fermion can decay to neutrinos and photons with energy E γ ¼ 1 2 m dm [6]. One can search for such particles in x rays [7,8] (see Ref.[9] for a review of previous searches). For each particular model, the particle's parameters are related by the requirement to provide the correct DM abundance. For example, for one very interesting DM candidate-the right-handed neutrino-this requirement restricts the mass range to 0.5-100 keV [9,10]. A large part of the available parameter space for sterile neutrinos is consistent with all astrophysical and cosmological bounds [11], and it is important to probe it further.The DM decay line is much narrower than the spectral resolution of the existing x-ray telescopes and, as previous searches have shown, should be rather weak. The x-ray spectra of astrophysical objects are crowded with weak atomic and instrumental lines, not all of which may be known. Therefore, even if the exposure of available observations continues to increase, it is hard to exclude an astrophysical or instrumental origin of any weak line found in the spectrum of an individual object. However, if the same feature is present in the spectra of many different objects, and its surface brightness and relative normalization between objects are consistent with the expected behavior of the DM signal, this can provide much more convincing evidence about it...

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.

We present a comprehensive overview of an extension of the Standard Model that contains three right-handed (sterile) neutrinos with masses below the electroweak scale [the Neutrino Minimal Standard Model, (νMSM)]. We consider the history of the Universe from the inflationary era through today and demonstrate that most of the observed phenomena beyond the Standard Model can be explained within the framework of this model. We review the mechanism of baryon asymmetry of the Universe in the νMSM and discuss a dark matter candidate that can be warm or cold and satisfies all existing constraints. From the viewpoint of particle physics the model provides an explanation for neutrino flavor oscillations. Verification of the νMSM is possible with existing experimental techniques.

We revisit Lyman-α bounds on the dark matter mass in Λ Warm Dark Matter (ΛWDM) models, and derive new bounds in the case of mixed Cold plus Warm models (ΛCWDM), using a set up which is a good approximation for several theoretically well-motivated dark matter models. We combine WMAP5 results with two different Lyman-α data sets, including observations from the Sloan Digital Sky Survey. We pay a special attention to systematics, test various possible sources of error, and compare the results of different statistical approaches. Expressed in terms of the mass of a non-resonantly produced sterile neutrino, our bounds read m nrp ≥ 8 keV (frequentist 99.7% confidence limit) or m nrp ≥ 12.1 keV (Bayesian 95% credible interval) in the pure ΛWDM limit. For the mixed model, we obtain limits on the mass as a function of the warm dark matter fraction F wdm . Within the mass range studied here (5 keV < m nrp < ∞), we find that any mass value is allowed when F wdm < 0.6 (frequentist 99.7% confidence limit); similarly, the Bayesian joint probability on (F wdm , 1/m nrp ) allows any value of the mass at the 95% confidence level, provided that F wdm < 0.35. 1 3 particles are suppressed for scales below their free-streaming length, 1 which affects structure formation on those scales. If the maximum free-streaming length roughly coincides with galaxy scales, the particles are called Warm Dark Matter (WDM), see e.g. [48].Currently, the best way to distinguish between WDM and CDM models is the analysis of Lyman-α (Ly-α) forest data (for an introduction see e.g. [49,50,51]). This method requires particular care. Part of the physics entering into the Ly-α analysis is not fully understood (for a recent review see [52]), and can be significantly influenced by DM particles (see e.g. [53,54,55,56,57,58,59]). The results can also be affected by approximations related to computational difficulties. Indeed, at the redshifts probed by Ly-α data, structures are already undergoing non-linear evolution. In order to relate the measured flux spectrum with the parameters of each cosmological model, one would have to perform a prohibitively large number of numerical simulations. Therefore, various simplifying approximations have to be realized. Different approaches are presented in [60,61,62].Previous works analyzing Ly-α constrains on WDM [63,65,64,66,67,68] assumed the simplest ΛWDM model with a cut-off in the linear power spectrum (PS) of matter density fluctuations. Such a PS arises when WDM particles are thermal relics. However, in many super-weakly interacting DM models, the PS has a complicated non-universal form due to the non-thermal primordial velocity distribution (see e.g. [69,70,71,72]). For example, in gravitino models with broken R-parity, gravitino production occurs in two stages: thermally at high temperatures (see e.g. [73,74,75]), and then non-thermally through the late decay of the next-tolightest supersymmetric particle (see e.g. [76,69]). Therefore the primordial velocity distribution results from the superimposition of a colder...

High resolution N-body simulations of galactic cold dark matter haloes indicate that we should expect to find a few satellite galaxies around the Milky Way whose haloes have a maximum circular velocity in excess of 40 kms. Yet, with the exception of the Magellanic Clouds and the Sagittarius dwarf, which likely reside in subhaloes with significantly larger velocities than this, the bright satellites of the Milky Way all appear to reside in subhaloes with maximum circular velocities below 40 kms. As recently highlighted by Boylan-Kolchin et al., this discrepancy implies that the majority of the most massive subhaloes within a cold dark matter galactic halo are much too concentrated to be consistent with the kinematic data for the bright Milky Way satellites. Here we show that no such discrepancy exists if haloes are made of warm, rather than cold dark matter because these haloes are less concentrated on account of their typically later formation epochs. Warm dark matter is one of several possible explanations for the observed kinematics of the satellites.Comment: 8 pages, 7 figures, accepted by MNRAS. Text and figures update

We show that the evolution of magnetic fields in a primordial plasma, filled with standard model particles at temperatures T * 10 MeV, is strongly affected by the chiral anomaly-an effect previously neglected. Although reactions, equilibrating left and right electrons, are in thermal equilibrium for T & 80 TeV, a left-right asymmetry develops in the presence of strong magnetic fields. This results in magnetic helicity transfer from shorter to longer scales and lepton asymmetry present in the plasma until T $ 10 MeV, which may strongly affect many processes in the early Universe. DOI: 10.1103/PhysRevLett.108.031301 PACS numbers: 98.62.En, 98.80.Cq Magnetic fields are expected to play an important role in the early Universe. Recent observational indications of the presence of magnetic fields in the intergalactic medium [1][2][3] suggest that cosmological magnetic fields (CMFs) may survive even until the present epoch. Thus, they could have played the role of seeds for the formation of galactic magnetic fields. A number of mechanisms for the creation of CMFs at very high temperatures have been proposed (see, e.g., [4][5][6], and references therein).In this Letter, we concentrate, however, on a different problem: We assume that strong CMFs were already generated at a temperature * 100 GeV, and we study the subsequent evolution of such fields. Usually, this evolution is described by the system of Maxwell plus Navier-Stokes equations (for a detailed review, see [5,7]). Here we will argue that, for temperatures T * 10 MeV, this system of MHD equations should be extended to include a new effective degree of freedom, even if all particles and reactions are described by just the standard model of particle physics. This significantly affects the evolution of CMFs and the state of the primordial plasma.At such temperatures, rates of all perturbative processes related to the electron's finite mass are suppressed as ðm e =TÞ 2 . Ignoring these corrections for a moment, the number of left-and right-chiral electrons [8] is conserved independently [9]. That is, apart from the vector current j ¼ " c c describing conservation of electric charge (n L þ n R ), the average number density of the left-(right-) chiral electrons n L;R ¼ 1 2V R d 3 xc y ð1 AE 5 Þc does not change with time. This is true on time scales smaller than the chirality-flipping scale À À1 f . Although the chiralityflipping rate is suppressed as compared to the rate of chirality-preserving weak and electromagnetic processes, it is faster than the Hubble expansion rate HðTÞ for temperatures below 80 TeV [10], and chirality-flipping processes are in thermodynamic equilibrium. Yet, on time scales À À1 EM;weak < t < À À1 f , one should introduce independent chemical potentials L and R for two approximately conserved number densities, with n L;R ¼ L;R 6 T 2 . In the presence of external classical fields, the conservation of the axial current is spoiled, however, by the chiral anomaly [11]-a quantum effect leading to a change of n L À n R :where ¼ e 2 4 is the fine-...

We propose a strategy for how to look for dark matter particles possessing a radiative decay channel and derive constraints on their parameters from observations of x rays from our own Galaxy and its dwarf satellites. When applied to sterile neutrinos in the keV mass range this approach gives a significant improvement to restrictions on neutrino parameters compared with previous works. DOI: 10.1103/PhysRevLett.97.261302 PACS numbers: 95.35.+d, 14.60.Pq, 95.85.Nv Introduction.-It was noticed long ago that a sterile neutrino with mass in the keV range appears to be a viable dark matter (DM) candidate [1]. Moreover, being warm DM, a sterile neutrino eases the tension between observations and predictions of the cold DM model on small scales. The interest in this scenario has been revived since the discovery of neutrino oscillations (see, e.g., [2] for a review). Indeed, one of the simplest ways to explain these data is to add to the standard model (SM) several gauge singlet fermions-right-handed, or sterile, neutrinos. It has been demonstrated recently [3] that a simple extension of the SM by three singlet fermions with masses smaller than the electroweak scale, dubbed the MSM in [3], allows one to describe all confirmed data on neutrino oscillations, provides a DM particle candidate in the form of a sterile neutrino, and allows one to explain the baryon asymmetry of the Universe. The simplicity of the model, the similarity of its quark and lepton right-handed sectors, together with a considerable number of other phenomena it can simultaneously describe, forces us to take this model seriously and thus provides additional motivation for the study of keV mass range sterile neutrinos as a DM candidate.The sterile neutrino has a radiative decay channel, emitting a photon with energy E m s =2 (m s being the mass of the sterile neutrino). Parametrically, the decay width is proportional to m 5 s sin 2 2 [4], where is the mixing angle between active and sterile neutrino.If such a neutrino is a main ingredient of the DM, it is potentially detectable in various x-ray observations. The most obvious candidates are diffuse extragalactic x-ray background (XRB) [5][6][7][8], clusters of galaxies [6,9,10], and galaxies [6], including our own.The aim of the present Letter is to discuss the best strategy to search for a DM sterile neutrino and to derive the constraints on its properties. Although we concentrate on the sterile neutrino, the constraints we get can be applied to any DM candidate with a radiative two-body decay channel in the keV range. We analyze various types of astrophysical objects and show that the strongest constraints on sterile neutrino come from neutrino decays in the Milky Way halo and, in particular, in the halo dwarf

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