Alexey Boyarsky

The Role of Sterile Neutrinos in Cosmology and Astrophysics

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 that 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.

Constraints on sterile neutrinos as dark matter candidates from the diffuse X-ray background

Sterile neutrinos with masses in the keV range are viable candidates for the warm dark matter. We analyze existing data for the extragalactic diffuse X-ray background for signatures of sterile neutrino decay. The absence of detectable signal within current uncertainties of background measurements puts model-independent constraints on allowed values of sterile neutrino mass and mixing angle, which we present in this work.

Strategy for searching for a dark matter sterile neutrino

We propose a strategy of how to look for dark matter (DM) 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 the sterile neutrinos in keV mass range, it allows a significant improvement of restrictions to its parameters, as compared with previous works.

Constraints on decaying dark matter from XMM-Newton observations of M31

We derive constraints on the parameters of the radiatively decaying dark matter (DM) particle, using the XMM-Newton EPIC spectra of the Andromeda galaxy (M31). Using the observations of the outer (5-13 arcmin) parts of M31, we improve the existing constraints. For the case of sterile neutrino DM, combining our constraints with the latest computation of abundances of sterile neutrinos in the Dodelson-Widrow (DW) scenario, we obtain the lower mass limit m(s) 5.6 keV), we argue that the scenario in which all the DM is produced via the DW mechanism is ruled out. We discuss, however, other production mechanisms and note that the sterile neutrino remains a viable candidate for DM, either warm or cold.

Constraints on the parameters of radiatively decaying dark matter from the dark matter halos of the Milky Way and Ursa Minor

Aims. We improve the earlier restrictions on parameters of the dark matter (DM) in the form of a sterile neutrino. Methods. The results were obtained from non-observing the DM decay line in the X-ray spectrum of the Milky Way (using the recent XMM-NewtonPN blank sky data). We also present a similar constraint coming from the recent XMM-Newtonobservation of Ursa Minor ‐ dark, X-ray quiet dwarf spheroidal galaxy. Results. The new Milky way data improve on (by as much as the order of magnitude at masses∼ 3.5 keV) existing constraints. Although the observation of Ursa Minor has relatively poor statistics, the constraints are comparable to those recentl y obtained using observations of the Large Magellanic Cloud or M31. This confirms a recent proposal that dwarf satellites of the MW ar e very interesting candidates for the DM search and dedicated studies should be made to this purpose.

The Masses of active neutrinos in the nuMSM from X-ray astronomy

In an extension of the Standard Model by three relatively light right-handed neutrinos (the νMSM model) the role of the dark matter particle is played by the lightest sterile neutrino. We demonstrate that the observations of the extragalactic x-ray background allow us to put a strong upper bound on the mass of the lightest active neutrino and predict the absolute values of the mass of the two heavier active neutrinos in the νMSM provided that the mass of the dark matter sterile neutrino is larger than 1.8 keV.

Constraining dark matter properties with SPI

Using the high-resolution spectrometer SPI on board the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) , we search for a spectral line produced by a dark matter (DM) particle with a mass in the range40keV < M D M < 14M eV , decaying in the DM halo of the Milky Way. To distinguish the DM decay line from numerous instrumental lines found in the SPI background spectrum, we study the dependence of the intensity of the line signal on the offset of the SPI pointing from the direction toward th e Galactic Centre. After a critical analysis of the uncertainties of the DM density profile in the inner Galaxy, we find that the intensity of the DM decay line should decrease by at least a factor of 3 when the offset from the Galactic Centre increases from 0 � to 180 � . We find that such a pronounced variation of the line flux across the sky is not observed for any line, detec ted with a significance higher than 3� in the SPI background spectrum. Possible DM decay origin is not ruled out only for the unidentified spectral lines, having low ( � 3�) significance or coinciding in position with the instrumental ones. In the energy interval from 20 keV to 7 MeV, we derive restrictions on the DM decay line flux, implied by the (non-)detection of the D M decay line. For a particular DM candidate, the sterile neutrino of mass M D M , we derive a bound on the mixing angle.

Constraints on Parameters of Radiatively Decaying Dark Matter from the Galaxy Cluster 1E 0657–56

We derived constraints on parameters of a radiatively decaying warm dark matter particle, e.g., the mass and mixing angle for a sterile neutrino, using Chandra X-ray spectra of galaxy cluster 1E 0657?56 (the bullet cluster). The constraints are based on nondetection of the sterile neutrino decay emission line. This cluster exhibits spatial separation between the hot intergalactic gas and the dark matter, which helps to disentangle their X-ray signals. It also has a very long X-ray observation and a total mass measured via gravitational lensing. This makes the resulting constraints on a sterile neutrino complementary to earlier results that used different cluster mass estimates. Our limits are comparable to the existing cluster constraints (although they are weaker than the best constraints derived from M31 and the Milky Way halo).

Anomaly induced effects in a magnetic field

We consider a modification of electrodynamics by an additional light massive vector field, interacting with the photon via Chern-Simons-like coupling. This theory predicts observable effects for the experiments studying the propagation of light in an external magnetic field, very similar to those, predicted by theories of axion and axion-like particles. We discuss a possible microscopic origin of this theory from a theory with non-trivial gauge anomaly cancellation between massive and light particles (including, for example, millicharged fermions). Due to the conservation of the gauge current, the production of the new vector field is suppressed at high energies. As a result, this theory can avoid both stellar bounds (which exist for axions) and the bounds from CMB considered recently, allowing for positive results in experiments like ALPS, LIPPS, OSQAR, PVLAS-2, BMV, QA, etc

THE PROBLEM OF CLASSICAL LIMIT IN QUANTUM COSMOLOGY: THE EFFECTIVE ACTION LANGUAGE

The tool of functional averaging over some “large” diffeomorphisms is used to describe quantum systems with constraints, in particular quantum cosmology, in the language of quantum Effective. Action. Simple toy models demonstrate a supposedly general phenomenon: the presence of a constraint results in “quantum repel” from the classical mass shell.