arXiv subjects
Publications and source records attributed to Mikhail Shaposhnikov.
We demonstrate that the combination of the ideas of unimodular gravity, scale invariance, and the existence of an exactly massless dilaton leads to the evolution of the universe supported by present observations: inflation in the past, followed by the radiation and matter dominated stages and accelerated expansion at present. All mass scales in this type of theories come from one and the same source.
A number of observed phenomena in high energy physics and cosmology lack their resolution within the Standard Model of particle physics. These puzzles include neutrino oscillations, baryon asymmetry of the universe and existence of dark matter. We discuss the suggestion that all these problems can be solved by new physics which exists only below the electroweak scale. The dedicated experiments that can confirm or rule out this possibility are discussed.
We study in detail the mechanism of baryon and lepton asymmetry generation in the framework of the $ν$MSM (an extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale). We elucidate the issue of CP-violation in the model and define the phase relevant for baryogenesis. We clarify the question of quantum-mechanical coherence, essential for the lepton asymmetry generation in singlet fermion oscillations and compute the relevant damping rates. The range of masses and couplings of singlet leptons which can lead to successful baryogenesis is determined. The conditions which ensure survival of primordial (existing above the electroweak temperatures) asymmetries in different leptonic numbers are analysed. We address the question whether CP-violating reactions with lepton number non-conservation can produce leptonic asymmetry {\em below} the sphaleron freeze-out temperature. This asymmetry, if created, leads to resonant production of dark matter sterile neutrinos. We show that the requirement that a significant lepton asymmetry be produced puts stringent constraints on the properties of a pair of nearly degenerate singlet fermions, which can be tested in accelerator experiments. In this region of parameters the $ν$MSM provides a common mechanism for production of baryonic matter and dark matter in the universe. We analyse different fine-tunings of the model and discuss possible symmetries of the $ν$MSM Lagrangian that can lead to them.
An extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale allows to explain simultaneously neutrino oscillations, dark matter and baryon asymmetry of the Universe. We discuss the properties of neutral leptons in this model and the ways they can be searched for in particle physics experiments. We establish, in particular, a lower and an upper bound on the strength of interaction of neutral leptons coming from cosmological considerations and from the data on neutrino oscillations. We analyse the production of neutral leptons in the decays of different mesons and in $pp$ collisions. We study in detail decays of neutral leptons and establish a lower bound on their mass coming from existing experimental data and Big Bang Nucleosynthesis. We argue that the search for a specific missing energy signal in kaon decays would allow to strengthen considerably the bounds on neutral fermion couplings and to find or definitely exclude them below the kaon threshold. To enter into cosmologically interesting parameter range for masses above kaon mass the dedicated searches similar to CERN PS191 experiment would be needed with the use of intensive proton beams. We argue that the use of CNGS, NuMI, T2K or NuTeV beams could allow to search for singlet leptons below charm in a large portion of the parameter space of the nuMSM. The search of singlet fermions in the mass interval 2-5 GeV would require a considerable increase of the intensity of proton accelerators or the detailed analysis of kinematics of more than 10^{10} B-meson decays.
We argue that there may be no intermediate particle physics energy scale between the Planck mass $M_{Pl}\sim 10^{19}$ GeV and the electroweak scale $M_W \sim 100$ GeV. At the same time, the number of problems of the Standard Model (neutrino masses and oscillations, dark matter, baryon asymmetry of the Universe, strong CP-problem, gauge coupling unification, inflation) could find their solution at $M_{Pl}$ or $M_W$. The crucial experimental predictions of this point of view are outlined.
The chiral Abelian Higgs model contains an interesting class of solitons found by Rubakov and Tavkhelidze. These objects carry non-zero fermion number $N_F$ (or Chern-Simons number $N_{CS}$, what is the same because of the chiral anomaly) and are stable for sufficiently large $N_F$. In this paper we study the properties of these anomalous solitons. We find that their energy-versus-fermion-number ratio is given by $E\sim N_{CS}^{3/4}$ or $E\sim N_{CS}^{2/3}$ depending on the structure of the scalar potential. For the former case we demonstrate that there is a lower bound on the soliton energy, which reads $E\geq c N_{CS}^{3/4}$, where $c$ is some parameter expressed through the masses and coupling constants of the theory. We construct the anomalous solitons numerically accounting both for Higgs and gauge dynamics and show that they are not spherically symmetric. The thin wall approximation valid for macroscopic solutions with $N_{CS} \gg 1$ is discussed as well.
We describe an extention of the Standard Model (the $ν$MSM) by three light singlet Majorana fermions -- sterile neutrinos, which allows to address simultaneously the problem of neutrino oscillations and the problems of dark matter and baryon asymmetry of the Universe. We discuss the ways these new particles can be searched for in astrophysical, laboratory, and accelerator experiments.
An extension of the Standard Model by three right-handed neutrinos with masses smaller than the electroweak scale (the $ν$MSM) can explain simultaneously dark matter and baryon asymmetry of the Universe, being consistent with the data on neutrino oscillations. A dark matter candidate in this theory is the sterile neutrino with the mass in keV range. We discuss the constraints on the properties of this particle and mechanisms of their cosmological production. Baryon asymmetry generation in this model is reviewed. Crucial experiments that can confirm or rule out the $ν$MSM are briefly discussed.
If the dark matter of the Universe is made of sterile neutrinos with the mass in keV region they can be searched for with the help of X-ray satellites. We discuss the prospects of laboratory experiments that can be competitive and complimentary to Space missions. We argue that the detailed study of beta decays of tritium and other nuclei with the help of Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS) can potentially enter into interesting parameter range and even supersede the current astronomical bounds on the properties of dark matter sterile neutrino.
To explain the dark matter and the baryon asymmetry of the Universe, the parameters of the $ν$MSM (an extension of the Minimal Standard Model by three singlet neutrinos with masses smaller than the electroweak scale) must be fine-tuned: one of the masses should be in the ${\cal O} (10)$ keV region to provide a candidate for the dark-matter particle, while two other masses must be almost the same to enhance the CP-violating effects in the sterile neutrino oscillations leading to the baryon asymmetry. We show that a specifically defined global lepton-number symmetry, broken on the level of ${\cal O} (10^{-4})$ leads to the required pattern of sterile neutrino masses being consistent with the data on neutrino oscillations. Moreover, the existence of this symmetry allows to fix the flavour structure of couplings of singlet fermions to the particles of the Standard Model and indicates that their masses are likely to be smaller than ${\cal O} (1)$ GeV, opening a possibility of their search in decays of charmed, beauty and even $K$ or $π$-mesons.
We show how to enlarge the $ν$MSM (the minimal extension of the standard model by three right-handed neutrinos) to incorporate inflation and provide a common source for electroweak symmetry breaking and for right-handed neutrino masses. In addition to inflation, the resulting theory can explain simultaneously dark matter and the baryon asymmetry of the Universe; it is consistent with experiments on neutrino oscillations and with all astrophysical and cosmological constraints on sterile neutrino as a dark matter candidate. The mass of inflaton can be much smaller than the electroweak scale.
Sterile neutrinos with masses in the keV range are considered to be a viable candidate for warm dark matter. The rate of their production through active-sterile neutrino transitions peaks, however, at temperatures of the order of the QCD scale, which makes it difficult to estimate their relic abundance quantitatively, even if the mass of the sterile neutrino and its mixing angle were known. We derive here a relation, valid to all orders in the strong coupling constant, which expresses the production rate in terms of the spectral function associated with active neutrinos. The latter can in turn be expressed as a certain convolution of the spectral functions related to various mesonic current-current correlation functions, which are being actively studied in other physics contexts. In the naive weak coupling limit, the appropriate Boltzmann equations can be derived from our general formulae.
We explore the range of parameters for dark-matter sterile neutrinos in an extention of the Minimal Standard Model by three singlet fermions with masses below the electroweak scale (the $ν$MSM). This simple model can explain a wide range of phenomena, including neutrino oscillations, baryogenesis, the pulsar velocities, and the early reionization. The presence of two heavier sterile neutrinos and the possibility of entropy production in their decays broadens the allowed range of parameters for the dark-matter sterile neutrinos (or other types of dark matter, for example, the gravitino). In addition, the primordial production of dark matter sterile neutrinos allows to escape most of the constraints.
We find restrictions on the mass and mixing angle of the dark matter sterile neutrinos using X-ray observations of Coma and Virgo galaxy clusters.
The anomaly cancellation condition of the Standard Model may be unnatural in theories with extra dimensions as an anomaly of a low-energy 4-dimensional theory can be canceled by an inflow from a bulk. This inflow may give rise to an observable effect at low energies. We analyze several physical models in which this effect exists and estimate constraints on its value, imposed by the modern experimental data. We show that the effect can be large enough to be observed even when these constraints are satisfied. Positive result of such an experiment would be a low-energy signature of the existence of extra dimensions.
In theories with chiral couplings, one of the important consistency requirements is that of the cancellation of a gauge anomaly. In particular, this is one of the conditions imposed on the hypercharges in the Standard Model. However, anomaly cancellation condition of the Standard Model looks unnatural from the perspective of a theory with extra dimensions. Indeed, if our world were embedded into an odd-dimensional space, then the full theory would be automatically anomaly free. In this paper we discuss the physical consequences of anomaly non-cancellation for effective 4-dimensional field theory. We demonstrate that in such a theory parallel electric and magnetic fields get modified. In particular, this happens for any particle possessing both electric charge and magnetic moment. This effect, if observed, can serve as a low energy signature of extra dimensions. On the other hand, if such an effect is absent or is very small, then from the point of view of any theory with extra dimensions it is just another fine-tuning and should acquire theoretical explanation.
Stable baryonic Q-balls, which appear in supersymmetric extensions of the Standard Model, could form at the end of cosmological inflation from fragmentation of the Affleck -- Dine condensate. We reconsider astrophysical constraints on such Q-balls as dark matter candidates. Baryonic Q-balls interact with matter by absorbing the baryon number and, effectively, leading to a rapid baryon number non-conservation. We have recently shown that this process can occur at a much faster rate than that used in previous calculations. As a consequence, stability of neutron stars imposes a stringent constraint on the types of Q-balls that can be dark matter. Only the Q-balls that correspond to baryonic flat directions lifted by baryon-number violating operators are allowed as dark-matter candidates.
Supersymmetric extensions of the Standard Model contain non-topological solitons, Q-balls, which can be stable and can be a form of cosmological dark matter. Understanding the interaction of SUSY Q-balls with matter fermions is important for both astrophysical limits and laboratory searches for these dark matter candidates. We show that a baryon scattering off a baryonic SUSY Q-ball can convert into its antiparticle with a high probability, while the baryon number of the Q-ball is increased by two units. For a SUSY Q-ball interacting with matter, this process dominates over those previously discussed in the literature.