It has been proposed that two resonances could coincide in the early universe at temperatures $T \sim 0.2 ... 0.5$ GeV: one between two nearly degenerate GeV-scale sterile neutrinos, producing a large lepton asymmetry through freeze-out and decays; another between medium-modified active neutrinos and keV-scale sterile neutrinos, converting the lepton asymmetry into dark matter. Making use of a framework which tracks three sterile neutrinos of both helicities as well as three separate lepton asymmetries, and scanning the parameter space of the GeV-scale species, we establish the degree of fine-tuning that is needed for realizing this scenario.
We derive the neutrino oscillation probabilities including sterile neutrinos by using the Dirac equation. If neutrinos have both the Dirac and the Majorana mass terms, left-handed neutrino $ν_L$ and right-handed anti-neutrino $ν_R^c$ enter the same multiplet and can transfer each other. Sterile neutrinos have been introduced by hand as the fourth generation neutrino in many papers, however, we can understand sterile neutrinos naturally in the framework of three generations. We also point out that the oscillations into sterile neutrinos strongly suggest the existence of both the Dirac and the M…
We review probes of heavy sterile neutrinos, focusing on direct experimental searches and neutrinoless double beta decay. Working in a phenomenological parametrization, we emphasize the importance of the nature of sterile neutrinos in interpreting neutrinoless double beta decay searches. While current constraints on the active-sterile neutrino mixing are already stringent, we highlight planned future efforts that will probe regimes motivated by the lightness of active neutrinos.
Searches for light sterile neutrinos are motivated by the unexpected observation of electron neutrino appearance in short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and the Mini Booster Neutrino Experiment (MiniBooNE). In light of these unexpected results, a campaign using natural and anthropogenic sources to find light -- mass-squared-difference around 1 eV$^{2}$ -- sterile neutrinos is underway. Among the natural sources, atmospheric neutrinos provide a unique gateway to search for sterile neutrinos due to the broad range of baseline-to-energy ratios, $L/E…
We address the phenomenology of light sterile neutrinos, with emphasis on short-baseline neutrino oscillations. After a review of the observed short-baseline neutrino oscillation anomalies, we discuss the global fit of the data and the current appearance-disappearance tension. We also review briefly the effects of light sterile neutrinos in $β$ decay, neutrinoless double-$β$ decay and cosmology. Finally, we discuss the future perspective in the search for the effects of eV-scale sterile neutrinos.
Experiments measuring the parameters of active neutrino oscillations can also search for the sterile neutrinos in a part of sterile neutrino parameter space. In this paper, we analyze the prospects for the sterile neutrino search in the upcoming experiment DUNE for the sterile neutrinos with masses at GeV scale. As it relies on the still-undecided design of the Near Detector, we provide the expected number of the sterile neutrino decays in the Near Detector volume. Our most optimistic predictions show that the corresponding limit on mixing can be approximately of the same order as the previous…
Light sterile neutrinos contributions both for active neutrinos oscillations and neutrinoless double beta decay characteristics are estimated in the framework of the phenomenological model with three active and three sterile neutrinos assuming the Majorana nature of neutrino. Appearance and survival probabilities for active neutrinos with contributions of eV-sterile neutrinos in application to the short-baseline anomalies in neutrino data are obtained at the same test values of the model parameters. Modified graphical dependences for the survival probability of electron neutrinos/antineutrinos…
This is a pedagogical introduction to the main concepts of the sterile neutrino - a hypothetical particle, coined to resolve some anomalies in neutrino data and retain consistency with observed widths of the $W$ and $Z$ bosons. We briefly review existing anomalies and the oscillation parameters that best describe these data. We discuss in more detail how sterile neutrino can be observed, as well as the consequences of its possible existence. In particular, we pay attention to a possible loss of coherence in a model of neutrino oscillations with sterile neutrinos, where this effect might be of …
We consider a simple extension of the Standard Model (SM) by a complex scalar doublet and a singlet along with three sterile neutrinos. The sterile neutrinos mix with the SM neutrinos to produce three light neutrino states consistent with the oscillation data and three heavy sterile states. The lightest sterile neutrino has lifetime longer than the age of the Universe and can provide correct dark matter relic abundance. Utilizing tree-level flavor changing interactions of a light scalar with mass~$\sim\mathcal{O}(100)$~MeV along with sterile neutrinos, we can explain the anomalous magnetic mom…
In this paper, we study the effect of active-neutrino-sterile-neutrino mixing in the expected high-energy astrophysical neutrino flavor content. Non-unitarity in the measurement of the three active neutrinos can be due to the existence of sterile neutrino states. We introduce the concept of the four-flavor tetrahedron in order to visualize the lack of unitarity in the astrophysical neutrino three-flavor triangle. We demonstrate that active-sterile neutrino mixings modify the allowed region of the astrophysical flavor ratio from the standard case. However, a projection of the four-flavor tetrah…
We analyse the sensitivity of the Deep Underground Neutrino Experiment (DUNE) to a sterile neutrino, combining information from both the Near Detector (ND) and the Far Detector (FD). DUNE's sterile exclusion reach is affected by taking into account the information on the neutrino production point, in contrast to assuming a point-like neutrino source. Visible differences remain after taking into account energy bin-to-bin uncorrelated systematics.
A phenomenological model with active and sterile neutrinos is used for calculations of neutrino oscillation characteristics at the normal mass hierarchy of active neutrinos. Taking into account the contributions of sterile neutrinos, appearance and survival probabilities for active neutrinos are calculated. Modified graphical dependencies for the probability of appearance of electron neutrinos/antineutrinos in muon neutrino/antineutrino beams as a function of the ratio of the distance to the neutrino energy and other model parameters are obtained. It is shown that in the case of a certain type…
Light sterile neutrinos with a mass around 1 eV have been studied for many years as a possible explanation of the so called short-baseline neutrino oscillation anomalies. Recently, several neutrino oscillation experiments reported preferences for non-zero values of the mixing angles and squared mass differences for active-sterile mixing, which however are not always in agreement. I will review our current knowledge on the light sterile neutrino in the 3+1 model, starting with a separate discussion on the status of the most relevant searches and then analyzing the problems that arise when combi…
We explore the possibility of a single generation of $keV$ scale sterile neutrino ($m_S$) as a dark matter candidate within the minimal extended seesaw (MES) framework and it's influence in neutrinoless double beta decay ($0νββ$) study. Three hierarchical right-handed neutrinos were considered to explain neutrino mass. We also address baryogenesis via the mechanism of thermal leptogenesis considering the decay of the lightest RH neutrino to a lepton and Higgs doublet. A generic model based on $A_4\times Z_4\times Z_3$ flavor symmetry is constructed to explain both normal and inverted hierarchy…
The resonant production of keV sterile-neutrino dark matter mainly takes place during the QCD epoch of the early universe. It has been argued that it could be strongly affected by the opacities (or damping rates) of active neutrinos, which receive non-perturbative QCD-contributions. We find that for lepton asymmetries $ n_{ L_α} /s $ below $ 10 ^ { -6 } $ the opacities significantly affect the sterile-neutrino yield, but that for larger asymmetries, which are necessary for producing a significant fraction of the dark matter, the yield is insensitive to changes of the opacities. Thus non-pertur…
We review the status of light sterile neutrino searches, motivated by the original Short BaseLine (SBL) anomalies. Here, we discuss how sterile neutrino properties can be constrained by different types of neutrino oscillation experiments (considering appearance or disappearance probes in different oscillation channels) and non-oscillation measurements. These latter include experiments aiming at obtaining a value for the absolute scale of neutrino masses ($β$ decay probes) and the indirect constraints that we could obtain from cosmological observations.
We make a comparative study of the neutrinoless double beta decay constraints on heavy sterile neutrinos versus other direct and indirect constraints from both lepton number conserving and violating processes, as a sensitive probe of the extent of lepton number violation and possible interference effects in the sterile sector. We introduce a phenomenological parametrisation of the simplified one-generation seesaw model with one active and two sterile neutrino states in terms of experimentally measurable quantities, such as active-sterile neutrino mixing angles, CP phases, masses and mass split…
