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, (nuMSM)]. 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 nuMSM 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 nuMSM is possible with existing experimental techniques.
Deep Dive into 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, (nuMSM)]. 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 nuMSM 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 nuMSM is possible with existing experimental techniques.
The Standard Model (SM) of elementary particles [1,2,3], defined as a renormalizable field theory, based on the SU(3)×SU( 2)×U(1) gauge group, and containing three fermionic families -left-handed particles, SU(2) doublets, right-handed particles, SU(2) singlets (no right-handed neutrinos) and one Higgs doublet -has successfully predicted a number of particles and their properties. However, there is no doubt that the SM is not a final theory. Indeed, over the past several decades it has become increasingly clear that it fails to explain a number of observed phenomena in particle physics, astrophysics, and cosmology. These phenomena beyond the SM (BSM) are (a) neutrino oscillations (transition between neutrinos of different flavors), (b) baryon asymmetry (excess of matter over anti-matter in the Universe), (c) dark matter (about 80% of all matter in the Universe consisting of unknown particles), (d) inflation (a period of rapid accelerated expansion in the early Universe), and (e) dark energy (late-time accelerated expansion of the Universe) This list of well-established observational drawbacks of the SM is considered complete at present. All the other BSM problems -for, instance, the gauge hierarchy and strong-CP problems -require theoretical fine-tuning.
At what energies should the SM be superseded by some other, more fundamental theory? The existence of gravity with the coupling related to the Planck scale -M P l = G -1/2 N = 1.2 × 10 19 GeV, where G N is the Newtonian gravitational constant -implies that the cut-off is at least below the Planck scale. If the cutoff is identified with M P l , the low-energy Lagrangian can contain all sorts of higherdimensional SU(3)×SU( 2)×U(1)-invariant operators that are suppressed by the Planck scale,
where L SM is the Lagrangian of the SM. These operators lead to a number of physical effects that cannot be described by the SM, such as neutrino masses and mixings, proton decay, etc. However, as we will discuss below, even the theory shown in Eq. (1) does not survive when confronted with different experiments and observations. Alternatively, one can place a cut-off Λ ≪ M P l in Eq. (1), which would imply that new physics (and new particles) appears well below the Planck scale at energies E ∼ Λ. If Λ ≫ M W , where M W is the mass of the weak W boson, the resulting theory suffers from the so-called gauge hierarchy problem, that is, the problem of quantum stability of the mass of the Higgs boson against quantum corrections from heavy particles.
Most of the research in BSM physics carried out during the past few decades was devoted to solving the gauge hierarchy problem. Many different suggestions were proposed concerning how to achieve the “naturalness” of electroweak symmetry breaking. These propositions are based on supersymmetry, technicolor, and large extra dimensions, among other ideas. Finding a solution to the gauge hierarchy problem, coupled with the need to solve observational and other fine-tuning problems of the SM, is extremely challenging. Most of the approaches postulate the existence of new particles with masses above the electroweak scale (ranging from 10 2 GeV to 10 15 -10 16 GeV). As a result, the proposed theories contain a plethora of (not yet observed) new particles and parameters.
In this review, we describe a conceptually different scenario for BSM physics and its consequences for astrophysics and cosmology in an attempt to address the BSM problems named above without introducing new energy scales (that is, in addition to the electroweak and the Planck scales). In such an approach, the hierarchy problem is shifted to the Planck scale, and there is no reason to believe that the field theoretical logic is still applicable to it.
Below we show (following Refs. [4,5] and a number of subsequent works) that this goal may be achieved with a very simple extension of the SM. The only new particles, added to the SM Lagrangian are three gauge-singlet fermions (i.e., sterile neutrinos) with masses below the electroweak scale. Right-handed neutrinos are strongly motivated by the observation of neutrino flavor oscillations. In Section 2 we review neutrino oscillations and introduce the corresponding Lagrangian. We summarize the choice of parameters of the Neutrino Minimal Standard Model (νMSM) in Section 3. In Section 4, we present a νMSM cosmology. We discuss the restrictions from astrophysics, cosmology, and particle physics experiments, as well as future searches in Section 5. In Section 6, we conclude with a discussion of possible extensions of the νMSM and potential astrophysical applications of sterile neutrinos.
When the SM was conceived, neutrinos were thought to be massless and different lepton numbers were believed to be conserved (which would not be the case if right-handed neutrinos were present). This was a reason for not introducing righthanded neutrinos decades ago.
Transitions between neutrinos of different flavors have been observed. These n
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
