We discuss the effects of one additional sterile neutrino at the Neutrino Factory. Compared to earlier analyses, which have been motivated by LSND results, we do not impose any constraint on the additional mass squared splitting. This means that the additional mass eigenstate could, with small mixings, be located among the known ones, as it is suggested by the recent analysis of cosmological data. We use a self-consistent framework at the Neutrino Factory without any constraints on the new parameters. We demonstrate for a combined short and long baseline setup that near detectors can provide the expected sensitivity at the LSND-motivated Δm_{41}^2-range, while some sensitivity can also be obtained in the region of the atmospheric mass splitting from the long baselines. We point out that limits on such very light sterile neutrinos may also be obtained from a re-analysis of atmospheric and solar neutrino oscillation data, as well as from supernova neutrino observations. In the second part of the analysis, we compare our sensitivity with the existing literature using additional assumptions, such as |Δm_{41}^2| \gg |Δm_{31}^2| leading to averaging of the fast oscillations in the far detectors. We demonstrate that while the Neutrino Factory has excellent sensitivity compared to existing studies using similar assumptions, one has to be very careful interpreting these results for a combined short and long baseline setup where oscillations could occur in the near detectors. We also test the impact of additional ν_τdetectors at the short and long baselines, and we do not find a substantial improvement of the sensitivities.
We point out that the excess of electron-like neutrino events recently observed by the T2K collaboration may have a common origin with the similar excess events previously reported by the LSND and MiniBooNE experiments and interpreted as a signal from the radiative decays of a sterile neutrino ν_h with the mass around 50 MeV produced in muon neutrino neutral current (NC) interactions. In this work we assumed that the ν_h can also be produced in tau neutrino NC reactions.
We first study how sterile neutrinos can fit the 5sigma bar-nu_mu --> bar-nu_e LSND anomaly: 2+2 solutions are strongly disfavoured by solar and atmospheric data, while 3+1 solutions can still give a poor fit (for a specific range of oscillation parameters, to be tested by MiniBooNE). If instead MiniBoone will find no nu_mu --> nu_e signal, we will have a hint for CPT violation. Already now, unlike sterile neutrinos, CPT-violating neutrino masses can nicely accomodate all safe and unsafe data. We study how much CPT must be conserved according to atmospheric and K2K data and list which CPT-viol…
We examine the compatibility between existing experimental data and a recently proposed explanation of the LSND and MiniBooNE anomalies, given in terms of a sterile neutrino $N$ whose decay is dominated by a radiative mode. We find that current experimental data on $τ\rightarrow μννγ$ decays are compatible with the sterile neutrino parameters required for the explanation of the anomalies, but $K\rightarrow μνγ$ shows a marginal tension with those parameters. We also propose experimental cuts on radiative $K$ decays that could test the sterile neutrino hypothesis better. Finally, we study the c…
We develop the consequences of introducing a purely leptonic, lepton number violating non-standard interaction (NSI) and standard model neutrino mixing with a fourth, sterile neutrino in the analysis of short-baseline, neutrino experiments. We focus on the muon decay at rest (DAR) result from the Liquid Scintillation Neutrino Experiment (LSND) and the Karlsruhe and Rutherford Medium Energy Neutrino Experiment (KARMEN). We make a comprehensive analysis of lepton number violating, NSI effective operators and find nine that affect muon decay relevant to LSND results. Two of these preserve the sta…
Although the hypothetic sterile neutrino $ν_s$ is probably not involved significantly in the deficits of solar $ν_e$'s and atmospheric $ν_μ$'s, it may cause the possible LSND effect. In fact, we face such a situation, when the popular nearly bimaximal texture of active neutrinos $ν_e$, $ν_μ$, $ν_τ$ is perturbed through a small rotation in the 14 plane, where $ν_4$ is the extra neutrino mass state induced by the sterile neutrino $ν_s$. Then, with $m^2_1\simeq m^2_2$ we predict in the simplest case of $s_{13}\to0$ that $\sin^22θ_{\rm LSND}=s^4_{14}/2$ and $Δm^2_{\rm LSND} = |Δm^2_{41}|$. However…
We show that a double seesaw framework for neutrino masses with $μ-τ$ exchange symmetry can lead to one of the righthanded seesaw partners of the light neutrinos being massless. This can play the role of a light sterile neutrino, giving a $3+1$ model that explains the LSND results. We get a very economical scheme, which makes it possible to predict the full $4\times 4$ neutrino mass matrix if CP is conserved. Once CP violation is included, effect of the LSND mass range sterile neutrino is to eliminate the lower bound on neutrinoless double beta decay rate which exists for the three neutrino ca…
We propose a new explanation of the intriguing LSND evidence for electron antineutrino appearance in terms of heavy (mostly sterile) neutrino decay via a coupling with a light scalar and light (mostly active) neutrinos. We perform a fit to the LSND data, as well as all relevant null-result experiments, taking into account the distortion of the spectrum due to decay. By requiring a coupling g ~ 10^{-5}, a heavy neutrino mass m_4 ~ 100 keV and a mixing with muon neutrinos |U_{mu 4}|^2 ~ 10^{-2}, we show that this model explains all existing data evading constraints that disfavor standard (3+1) n…
Results from the LSND and MiniBooNE experiments have been interpreted as evidence for a sterile neutrino with a mass near the electronvolt scale. Here we propose to test such a scenario by measuring the coherent elastic scattering rate of neutrinos from a pulsed spallation source. Coherent scattering is universal across all active neutrino flavors, and thus can provide a measurement of the total Standard Model neutrino flux. By performing measurements over different baselines and making use of timing information, it is possible to significantly reduce the systematic uncertainties and to indepe…
Standard active--sterile neutrino oscillations do not provide a satisfactory description of the LSND evidence for neutrino oscillations together with the constraints from MiniBooNE and other null-result short-baseline oscillation experiments. However, if the mass or the mixing of the sterile neutrino depends in an exotic way on its energy all data become consistent. I explore the phenomenological consequences of the assumption that either the mass or the mixing scales with the neutrino energy as $1/E_ν^r (r > 0)$. Since the neutrino energy in LSND is about 40 MeV, whereas MiniBooNE operates at…
The accurate prediction of the neutrino beam produced in muon decays and the absence of opposite helicity contamination for a particular neutrino flavor make a future neutrino factory the ideal place to look for the lepton flavor violating (LFV) decays of the kind $μ^+\ra e^+\nuebar\numu$ and lepton number violating (LNV) processes like $μ^-\ra e^-\nue\numu$. Excellent sensitivities can be achieved using a detector capable of muon and/or electron identification with charge discrimination. This would allow to set experimental limits that improve current ones by more than two orders of magnitude…
The results of LSND have yet to be confirmed at the $5 σ$-level. An experiment is proposed utilizing a 3 GeV muon storage ring that would allow for both disappearance and appearance channels to be explored at short-baselines. The appearance channel could provide well over $5 σ$ confirmation or rejection of the LSND result. Other physics could also be performed at such a facility such as the measurement of electron-neutrino cross sections. The sensitivity of experiments at a Very Low Energy Neutrino Factory (VLENF) to neutrinos at the eV-scale is presented.
A particular form of mixing matrix for three active and one sterile neutrinos is proposed. Its $3\times 3 $ part describing three active neutrinos arises from the popular bimaximal mixing matrix that works satisfactorily in solar and atmospheric experiments if the LSND effect is ignored. Then, the sterile neutrino, effective in the fourth row and fourth column of the proposed mixing matrix, is responsible for the possible LSND effect by inducing one extra neutrino mass state to exist actively. The LSND effect, if it exists, turns out to reveal its {\it perturbative} nature related to small mix…
As a contribution to the hypothesis of mixing of three active neutrinos with, at least, one sterile neutrino, we report on a simple $4\times4$ texture whose $3\times3$ part arises from the popular bimaximal texture for three active neutrinos $ν_e,ν_μ,ν_τ$, where $c_{12}=1/\sqrt{2} = s_{12}$, $c_{23} = 1/\sqrt{2} = s_{23}$ and $s_{13} = 0$. Such a $3\times 3$ bimaximal texture is perturbed through a rotation in the 14 plane, where $ν_4$ is the extra neutrino mass state induced by the sterile neutrino $ν_s$ which becomes responsible for the LSND effect. Then, with $m^2_1 \simeq m^2_2$ we predict…
The simplest 3+2 neutrino model is described, where two light sterile neutrinos mix very weakly with three active neutrinos and mutually do not mix at all, while the mass-squared scale of the possible LSND effect is provided by m_5^2 - m^2_4 (ν_4 and ν_5 being two additional mass neutrinos connected with the existence of two sterile neutrinos ν_s and ν_{s'}. This 3+2 model is not better for explaining the LSND effect than the simplest 3+1 neutrino model, where one light sterile neutrino mixes very weakly with three active neutrinos, while the mass-squared scale of the possible LSND effect is g…
Considering the hypothesis of mixing of three active neutrinos with, at least, one sterile neutrino, we report on a simple $4\times 4$ texture whose $ 3 \times 3 $ part arises from the popular bimaximal texture for three active neutrinos $ ν_e, ν_μ, ν_τ$, where $ c_{12} = 1/\sqrt{2} = s_{12}$, $ c_{23} = 1/\sqrt{2} = s_{23}$ and $ s_{13} = 0$. Such a $3\times 3 $ bimaximal texture is perturbed through a rotation in the 14 plane, where $ν_4 $ is the extra neutrino mass state induced by the sterile neutrino $ν_s $ which becomes responsible for the LSND effect. Then, with $m^2_1\simeq m^2_2$ we p…
We explore the possibility to explain the LSND result in the context of extra-dimensional theories. If sterile neutrinos take shortcuts through extra dimensions, this results in altered neutrino dispersion relations. Active-sterile neutrino oscillations thus are modified and new types of resonances occur.
We propose an explanation of the LSND evidence for electron antineutrino appearance based on neutrino decay. We introduce a heavy neutrino, which is produced in pion and muon decays because of a small mixing with muon neutrinos, and then decays into a scalar particle and a light neutrino, predominantly of the electron type. We require values of $g m_4\sim$ few eV, $g$ being the neutrino--scalar coupling and $m_4$ the heavy neutrino mass, e.g. $m_4$ in the range from 1 keV to 1 MeV and $g \sim 10^{-6} - 10^{-3}$. Performing a fit to the LSND data as well as all relevant null-result experiments,…
This paper reviews the results of the LSND and MiniBooNE experiments. The primary goal of each experiment was to effect sensitive searches for neutrino oscillations in the mass region with $Δm^2 \sim 1$ eV$^2$. The two experiments are complementary, and so the comparison of results can bring additional information with respect to models with sterile neutrinos. Both experiments obtained evidence for $\bar ν_μ\rightarrow \bar ν_e$ oscillations, and MiniBooNE also observed a $ν_μ\rightarrow ν_e$ excess. In this paper, we review the design, analysis, and results from these experiments. We then con…
We study oscillations of the high energy atmospheric neutrinos in the Earth into sterile neutrinos with the eV-scale mass. The MSW resonance and parametric enhancement of the $\barν_μ\to\barν_s$ oscillations lead to distortion of the zenith angle distribution of the muon-track events which can be observed by IceCube. Due to matter effect, the IceCube signal depends not only on the mixing element $U_{μ4}$ relevant for LSND and MiniBooNE but also on $U_{τ4}$ and the CP-violating phase $δ_{24}$. We show that the case with $U_{τ4} = δ_{24} = 0$ leads to the weakest IceCube signal and therefore sho…
The current case for atmospheric $ν_μ$ oscillations into active or sterile neutrinos is reviewed. It is argued that neither the study of neutral current events at Super-Kamiokande, nor the information obtained from future long baseline experiments might be sufficient to unambigously decide between these two scenarios. However, a combination of these results with the results from future short or intermediate baseline $τ$ appearance experiments would clearly resolve most of the remaining ambiguities. This conclusion does not strongly depend on whether the results from LSND will be confirmed or n…
We explore the hypothesis that the unexplained data from LSND and MiniBooNE are evidence for a new, heavy neutrino mass-eigenstate that mixes with the muon-type neutrino and decays into an electron-type neutrino and a new, very light scalar particle. We consider two different decay scenarios, one with Majorana neutrinos, one with Dirac neutrinos; both fit the data equally well. We find a reasonable, albeit not excellent, fit to the data of MiniBooNE and LSND. The decaying-sterile-neutrino hypothesis, however, cleanly evades constraints from disappearance searches and precision measurements of …
If active to active neutrino transitions are dominant modes of the atmospheric ($ν_μ \to ν_τ$) and the solar neutrino oscillations ($ν_{e}\to ν_μ/ν_τ$), as is indicated by recent data, the favoured scheme which accommodates the LSND result - the so called (2+2)-scheme - should be discarded. We introduce the parameters $η_s^{atm}$ and $η_s^{sun}$ which quantify an involvement of the sterile component in the solar and atmospheric neutrino oscillations. The (2+2)-scheme predicts $η_s^{atm} + η_s^{sun} = 1$ and the experimental proof of deviation from this equality will discriminate the scheme. In…
Using the most recent atmospheric neutrino data, as well as short-baseline, long-baseline and tritium $β$-decay data we show that the joint interpretation of the LSND, solar and atmospheric neutrino anomalies in (3+1) sterile neutrino schemes is severely disfavored, in contrast to the theoretically favored (2+2) schemes.
A simple unifying mass matrix is presented for the three active and one sterile neutrinos $ν_e$, $ν_μ$, $ν_τ$ and $ν_s$, using an extension of the radiative mechanism proposed some time ago by Zee. The total neutrino-oscillation data are explained by the scheme $ν_e \leftrightarrow ν_s$ (solar), $ν_μ\leftrightarrow ν_τ$ (atmospheric) and $ν_e \leftrightarrow ν_μ$ (LSND). We obtain the interesting approximate relationship $(Δm^2)_{atm} \simeq 2[(Δm^2)_{solar} (Δm^2)_{LSND}]^{1/2}$ which is well satisfied by the data.
We discuss short-baseline electron and muon neutrino disappearance searches into sterile neutrinos at a Very Low Energy Neutrino Factory (VLENF) with a muon energy between about two and four GeV. A lesson learned from reactor experiments, such as Double Chooz and Daya Bay, is to use near and far detectors with identical technologies to reduce the systematical errors. We therefore derive the physics results from a combined near-far detector fit and illustrate that uncertainties on cross sections x efficiencies can be eliminated in a self-consistent way. We also include the geometry of the setup…
Sensitivity of a neutrino factory to various mixing angles in a scheme with one sterile neutrino is studied using $ν_e\toν_μ$, $ν_μ\toν_μ$, $ν_e\toν_τ$ and $ν_μ\toν_τ$. While the "discovery-channel" $ν_μ\toν_τ$ is neither useful in the standard three flavor scheme nor very powerful in the sensitivity study of sterile neutrino mixings, this channel is important to check unitarity and to probe the new CP phase in the scheme beyond the standard neutrino mixing framework.
Models with low-scale breaking of global symmetries in the neutrino sector provide an alternative to the seesaw mechanism for understanding why neutrinos are light. Such models can easily incorporate light sterile neutrinos required by the LSND experiment. Furthermore, the constraints on the sterile neutrino properties from nucleosynthesis and large scale structure can be removed due to the non-conventional cosmological evolution of neutrino masses and densities. We present explicit, fully realistic supersymmetric models, and discuss the characteristic signatures predicted in the angular distr…
Since the 3+1 neutrino models with one light sterile neutrino turn out to be not very effective, at least two light sterile neutrinos may be needed to reconcile the solar and atmospheric neutrino experiments with the LSND result, {\it if} this is confirmed by the ongoing MiniBooNE experiment (and when the CPT invariance is assumed to hold for neutrino oscillations). We present an attractive 3+2 neutrino model, where two light sterile neutrinos mix maximally with each other, in analogy to the observed maximal mixing of muon and tauon active neutrinos. But, while the mixing of $ν_e$ and $(ν_μ-ν_…
A future high-luminosity $Z$-factory will offer the possibility to study rare $Z$ decays, as those leading to lepton flavour violating final states. Processes such as $Z \to \ell_1^\mp \ell_2^\pm$ are potentially complementary to low-energy (high-intensity) observables of lepton flavour violation. In this work we address the impact of new sterile fermions on lepton flavour violating $Z$ decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints on the sterile states. We consider a minimal extension of the Standard Model by one st…
We analyze the energy and zenith angle distributions of the latest 2-year IceCube dataset of upward going atmospheric neutrinos to constrain sterile neutrinos at the eV scale in the 3+1 scenario. We find that the parameters favored by a combination of LSND and MiniBooNE data are excluded at more than the 99% C.L. We explore the impact of nonstandard matter interactions on this exclusion, and find that the exclusion holds for nonstandard interactions (NSI) that are within the stringent model-dependent bounds set by collider and neutrino scattering experiments. However, for large NSI parameters …
A future high-luminosity $Z$-factory has the potential to investigate lepton flavour violation. Rare decays such as $Z \to \ell_1^\mp \ell_2^\pm$ can be complementary to low-energy (high-intensity) observables of lepton flavour violation. Here we consider two extensions of the Standard Model which add to its particle content one or more sterile neutrinos. We address the impact of the sterile fermions on lepton flavour violating $Z$ decays, focusing on potential searches at FCC-ee (TLEP), and taking into account experimental and observational constraints. We show that sterile neutrinos can give…
I give a brief overview over various attempts to reconcile the LSND evidence for oscillations with all other global neutrino data, including the results from MiniBooNE. I discuss the status of oscillation schemes with one or more sterile neutrinos and comment on various exotic proposals.
It has been very recently reported (McKeen, Pospelov, arXive:1011.3046) that the parameter region suggested for an explanation of the neutrino oscillation results from the LSND, KARMEN and MiniBooNE experiments in terms of the production and radiative decay of a heavy neutrino ($ν_h$) can be ruled out based on the measurements of the radiative muon capture (RMC) on hydrogen. We calculate limits on mixing strength between the $ν_h$ and $ν_μ$ by using results of this experiment, and find out that they essentially disagree with the reported bounds. For the $ν_h$ with mass 60 MeV our limit is wors…
We present a supersymmetric axion model in which the fermionic superpartner of axion, i.e. the axino, corresponds to a sterile neutrino which would accommodate the LSND data with atmospheric and solar neutrino oscillations.
It is shown on the ground of a simple 6\times 6 neutrino mixing model that one of three conventional sterile (righthanded) neutrinos, if light enough, may be consistently used for explaining a {\it small} LSND effect. Then, it is still considerably heavier than the three active (lefhanded) neutrinos, so that a kind of {\it soft} seesaw mechanism can work. The usual condition that the Majorana lefthanded component of the overall 6\times 6 neutrino mass matrix ought to vanish, {\it implies} the smallness of active-neutrino masses versus sterile-neutrino masses, when three mixing angles between b…
We propose that a hierarchical spectrum of sterile neutrinos (eV, keV, $10^{13-15}$ GeV) is considered to as the explanations for MiniBooNE and LSND oscillation anomalies, dark matter, and baryon asymmetry of the universe (BAU) respectively. The scenario can also realize the smallness of active neutrino masses by seesaw mechanism.
We discuss the possibility to explain the anomalies in short-baseline neutrino oscillation experiments in terms of sterile neutrinos. We work in a 3+1 framework and pay special attention to recent new data from reactor experiments, IceCube and MINOS+. We find that results from the DANSS and NEOS reactor experiments support the sterile neutrino explanation of the reactor anomaly, based on an analysis that relies solely on the relative comparison of measured reactor spectra. Global data from the $ν_e$ disappearance channel favour sterile neutrino oscillations at the $3σ$ level with $Δm^2_{41} \a…
It has been recently shown that excess events observed by the LSND and MiniBooNE neutrino experiments could be interpreted as a signal from the radiative decay of a heavy sterile neutrino ν_h produced in ν_μneutral current-like interactions. If the ν_h exist, it would be also produced by the ν_μbeam from the CERN SPS in the neutrino beam line shielding. The ν_h's would penetrate the shielding and be observed through the decay ν_h -> νγ, followed by the photon conversion into e+e- pair in the active target of the NOMAD detector. The ν_h's could be also produced in the iron of the magnetic spect…
The recent Super-Kamiokande data now admit only one type of mass hierarchy in a framework with three active and one sterile neutrinos. We show that neutrino masses and mixings generated by R-parity-violating couplings, with values within their experimental upper limits, are capable of reproducing this hierarchy, explaining all neutrino data particularly after including the LSND results.
Based on the "democratic" universal seesaw model, where mass matrices $M_f$ of quarks and leptons f_i (f=u,d,ν,e; i=1,2,3) are given by a seesaw form M_f\simeq -m_L M_F^{-1} m_R, and m_L and m_R are universal for all the fermion sectors, and the mass matrices M_F of hypothetical heavy fermions F_i have a democratic structure, a possible neutrino mass matrix is investigated. In the model, there are three sterile neutrinos ν_{iR} which mix with the active neutrinos ν_{iL} with θ\sim 10^{-2} and which are harmless for constraint from the bib bang nucleosynthesis. The atmospheric, solar and LSND n…
We discuss a possible new resonance in active-sterile neutrino oscillations arising in theories with large extra dimensions. Fluctuations in the brane effectively increase the path-length of active neutrinos relative to the path-length of sterile neutrinos through the extra-dimensional bulk. Well below the resonance, the standard oscillation formulas apply. Well above the resonance, active-sterile oscillations are suppressed. We show that a resonance energy in the range of 30-400 MeV allows an explanation of all neutrino oscillation data, including LSND data, in a consistent four-neutrino mode…
Oscillation interpretation of the results from the LSND, MiniBooNE and some other experiments requires existence of sterile neutrino with mass $\sim 1$ eV and mixing with the active neutrinos $|U_{μ0}|^2 \sim (0.02 - 0.04)$. It has been realized some time ago that existence of such a neutrino affects significantly the fluxes of atmospheric neutrinos in the TeV range which can be tested by the IceCube Neutrino Observatory. In view of the first IceCube data release we have revisited the oscillations of high energy atmospheric neutrinos in the presence of one sterile neutrino. Properties of the o…
If sterile neutrinos have a neutral coupling to standard model fermions, matter effect resonant transitions to sterile neutrinos and excess neutral-current events could manifest at long baseline experiments. Assuming a single sterile neutrino with a neutral coupling to fermionic matter, we re-examine bounds on sterile neutrino production at long baselines from the MINOS result $P_{ν_μ \rightarrow ν_s} < 0.22$ (90% CL). We demonstrate that sterile neutrinos with a neutral vector coupling to fermionic matter could evade the MINOS limit, allowing a higher fraction of active to sterile neutrino co…
Neutrinos from STORed Muons (nuSTORM; formerly VLENF) is a proposed experiment to use 3.8 GeV/c muon decay to produce a well-understood beam of electron and muon neutrinos that can be used for short baseline physics. A magnetized far detector allows for the wrong-sign muon appearance physics of electron-neutrino to muon-neutrino. This channel is the CPT of the LSND anomaly and provides 10 sigma sensitivity to the anomaly. This note contains a detailed explanation of the nSTORM appearance analysis that was included in the Letter of Intent.
We propose an explanation of the LSND signal via quantum-decoherence of the mass states, which leads to damping of the interference terms in the oscillation probabilities. The decoherence parameters as well as their energy dependence are chosen in such a way that the damping affects only oscillations with the large (atmospheric) $Δm^2$ and rapidly decreases with the neutrino energy. This allows us to reconcile the positive LSND signal with MiniBooNE and other null-result experiments. The standard explanations of solar, atmospheric, KamLAND and MINOS data are not affected. No new particles, and…
We point out that neutrino events observed at Kamiokande and IMB from SN1987A disfavor the neutrino oscillation parameters preferred by the LSND experiment. For $Δm^2 > 0$ (the light side), the electron neutrinos from the neutronization burst would be lost, while the first event at Kamiokande is quite likely to be due to an electron neutrino. For $Δm^2 < 0$ (the dark side), the average energy of the dominantly $\barν_e$ events is already lower than the theoretical expectations, which would get aggravated by a complete conversion from $\barν_μ$ to $\barν_e$. If taken seriously, the LSND data ar…
A four-neutrino effective texture is described, where a sterile neutrino mixes nearly maximally with the electron neutrino and so, is responsible for the deficit of solar $ν_e $'s (according to the large-angle MSW solution or vacuum solution, of which the former is selected a posteriori). But, while maximal mixing of muon neutrino with tauon neutrino causes the deficit of atmospheric $ν_μ$'s, the original magnitude of LSND effect is reduced by as much as four orders, becoming unobservable.
The recently observed event excess in MiniBooNE anti-neutrino data is in agreement with the LSND evidence for electron anti-neutrino appearance. We propose an explanation of these data in terms of a (3+1) scheme with a sterile neutrino including non-standard neutrino interactions (NSI) at neutrino production and detection. The interference between oscillations and NSI provides a source for CP violation which we use to reconcile different results from neutrino and anti-neutrino data. Our best fit results imply NSI at the level of a few percent relative to the standard weak interaction, in agree…
We investigate a minimal extension of the standard model in which the only new ingredient is the sterile neutrinos. We do not introduce extra Higgs multiplets or high dimensional effective operators to induce mass terms for the active neutrinos, and the model is renormalizable in itself. We show for arbitrary numbers of generations and sterile neutrinos that the independent physical parameters in the leptonic sector are much less than previously anticipated. For instance, with three active and two sterile neutrinos, there are four mixing angles and three CP phases in addition to four non-vanis…
It has been recently shown that puzzling excess events observed by the LSND and MiniBooNE neutrino experiments could be interpreted as a signal from the radiative decay of a heavy sterile neutrino ($ν_h$) of the mass from 40 to 80 MeV, with a muonic mixing strength $\mix \simeq 10^{-3} - 10^{-2}$, and the lifetime $10^{-11} < τ_h < 10^{-9}$ s. We discuss bounds on $\mix$ obtained from the recent precision measurements with muons and show that they are consistent with the above limits. If the $ν_h$ exists its admixture in the ordinary muon or kaon decay would result in the decay chain $\muhd$ o…
We investigate adding two sterile neutrinos to resolve the apparent tension existing between short-baseline neutrino oscillation results and CPT-conserving, four-neutrino oscillation models. For both (3+1) and (3+2) models, the level of statistical compatibility between the combined dataset from the null short-baseline experiments Bugey, CHOOZ, CCFR84, CDHS, KARMEN, and NOMAD, on the one hand; and the LSND dataset, on the other, is computed. A combined analysis of all seven short-baseline experiments, including LSND, is also performed, to obtain the favored regions in neutrino mass and mixing …
The possibility of the validation of the 3+1 neutrino model is considered in the context of the new result of the Neutrino-4 experiment, the direct observation of the oscillation effect at parameter region $Δm_{14}^2=(7.3 \pm 0.13_{stat} \pm 1.16_{syst})\text{eV}^2$ and $\sin^2(2θ_{14}) = 0.36 \pm 0.12_{stat} (2.9σ)$, also using LSND anomaly, MiniBooNE anomaly, reactor antineutrino anomaly, and gallium anomaly observed in experiments with radioactive sources GALLEX/GNO and SAGE. We analyze agreement of the neutrino parameter values obtained in the Neutrino-4 experiment with the results of othe…
We explore the possibility that the 3 active (doublet) neutrinos have nearly degenerate masses which are split only by the usual seesaw mechanism from 3 sterile (singlet) neutrinos in the presence of a softly broken $A_4$ symmetry. We take the unconventional view that the sterile neutrinos may be light, i.e. less than 1 keV, and discuss some very interesting and novel phenomenology, including a connection between the LSND neutrino data and solar neutrino oscillations.
We investigate resonant active-sterile neutrino oscillations emergent from the interplay between matter, CPT- and Lorentz-violating coefficients with different energy dependences. Novel and experimentally accessible resonant neutrino oscillation phenomenologies are predicted and possible implications for observed, but yet unexplained, neutrino oscillation anomalies such as LSND and MiniBooNE are discussed.
The MiniBooNE collaboration recently released a report claiming have observed an excess of electron and anti-electron neutrino with significance of $4.8 \, σ$ C.L. corroborating, in this way, the long-standing LSND anomaly. Combined LSND and MiniBooNE analysis reach a significance of $6.0\, σ$ C.L. Such a result, if confirmed by future experiments, will cause considerable impact on particle physics since that such anomalies, when interpreted in terms of neutrino oscillation, require the existence of at least one light sterile neutrino. It happens that, on according to standard scenarios, such …
Although the framework for oscillations of the three neutrino flavors in the Standard Model has been convincingly established, indications persist that it may be incomplete. Challenges are coming from the LSND and MiniBooNe short-baseline experiments, from the neutrino sources used in the Gallex and Sage solar neutrino experiments and, more recently,from an a-posteriori analysis of reactor neutrino experiments. One way to accommodate the reported "anomalies", if real, is to introduce one or more sterile neutrinos in the mass range $δm^2 \sim 1 eV^2$. TeV atmospheric neutrinos propagating throu…
Post LSND results, sterile neutrinos have drawn attention and motivated the high energy physics, astronomy and cosmology to probe physics beyond the standard model considering minimal 3+1 (3 active and 1 sterile) to 3+N neutrino schemes. The analytical equations for neutrino conversion probabilities are developed in this work for 3+1 neutrino scheme. Here, we have tried to explore the possible signals of T and CPT violations with four flavor neutrino scheme at neutrino factory. Values of sterile parameters considered in this analysis are taken from two different types of neutrino experiments v…
In light of recent Super-Kamiokande data and global fits that seem to exclude both pure ν_μ\to ν_s oscillations of atmospheric neutrinos and pure ν_e \to ν_s oscillations of solar neutrinos (where ν_s is a sterile neutrino), we reconsider four-neutrino models to explain the LSND, atmospheric, and solar neutrino oscillation indications. We argue that the solar data, with the exception of the ^{37}Cl results, are suggestive of ν_e \to ν_s oscillations that average to a probability of approximately 1/2. In this interpretation, with two pairs of nearly degenerate mass eigenstates separated by orde…
We propose an explanation for the LSND anomaly based on quantum decoherence, postulating an exponential behavior for the decoherence parameters as a function of the neutrino energy. Within this ansatz decoherence effects are suppressed for neutrino energies above 200 MeV as well as around and below few MeV, restricting deviations from standard three-flavour oscillations only to the LSND energy range of 20--50 MeV. The scenario is consistent with the global data on neutrino oscillations, alleviates the tension between LSND and KARMEN, and predicts a null-result for MiniBooNE. No sterile neutrin…
The recent recomputation of the neutrino fluxes from nuclear reactors relaxes the tension between the LSND and MiniBooNE anomalies and disappearance data when interpreted in terms of sterile neutrino oscillations. The simplest extension of the Standard Model with such fermion singlets is the addition of right-handed sterile neutrinos with small Majorana masses. Even when introducing three right-handed neutrinos, this scenario has less free parameters than the 3+2 scenarios studied in the literature. This begs the question whether the best fit regions obtained can be reproduced by this simplest…
