Νe Oscillations Research Articles

Decaying sterile neutrinos at short baselines

Long-standing anomalous experimental results from short-baseline neutrino experiments have persisted for decades. These results, when interpreted with one or more light sterile neutrinos, are inconsistent with numerous null results experimentally. However, if the sterile neutrino decays en route to the detector, this can mimic νμ→νe oscillation signals while avoiding many of these external constraints. We revisit this solution to the MiniBooNE and LSND puzzles in view of new data from the MicroBooNE experiment at Fermilab. Using MicroBooNE’s liquid-argon time-projection chamber search for an excess of νe in the Booster beam, we derive new limits in two models’ parameter spaces of interest; where the sterile neutrino decays (I) via mixing with the active neutrinos, or (II) via higher-dimensional operators. We also provide an updated, comprehensive fit to the MiniBooNE neutrino- and antineutrino-beam data, including appearance (νe) and disappearance (νμ) channels. Despite alleviating the tension with muon neutrino disappearance experiments, we find that the latest MicroBooNE analysis rules out the decaying sterile neutrino solution in a large portion of the parameter space at more than 99% CL. Published by the American Physical Society 2024

Non-Unitary Neutrino Mixing in the NOνA Near Detector Data

The νμ→νe oscillation probability over a short baseline (≲1 km) would be negligible for the case when the mixing matrix for three active neutrinos is unitary. However, in the case of a non-unitary mixing of three neutrinos, this probability would be non-negligible due to the so-called “zero distance” effect. Hence, the near detector of accelerator experiments such as NOνA can provide strong constraints on the parameters of the non-unitary mixing with very large statistics. By analyzing the NOνA near-detector data, we find that the non-unitary mixing does not improve fits to the νe or νμ events over the standard unitary mixing. This leads to constraints on the non-unitary parameters: α00>0.911, |α10|<0.020, and α11>0.952 at 90% C.L. A combined analysis with the near- and far-detector data does not change these constraints significantly.

A search for muon neutrino to electron neutrino oscillation mediated by sterile neutrinos in MINOS+

The MINOS experiment made precision measurements of the neutrino oscillation parameters that are governed by the atmospheric mass-squared splitting. These measurements were made with data that were collected while the NuMI muon neutrino beam operated in a low energy mode that peaks around 3 GeV. Today the NuMI beam is running with a higher energy mode that produces a neutrino energy spectrum that peaks around 7 GeV, allowing the MINOS+ experiment to probe neutrino oscillation phenomena that could potentially be governed by a fourth mass-squared splitting. If observed, the presence of a fourth mass-squared splitting would be compelling evidence for a sterile neutrino state. In this analysis, we will present the results of a search for νµ → νe oscillation mediated by sterile neutrinos in MINOS+. The results will be contrasted against the measurements made by the LSND experiment.

νμ → ν e oscillations search in the OPERA experiment

The tracking capabilities of the OPERA detector allow to reconstruct τ-leptons and electrons. It gives a possibility to observe νμ → ντ oscillations in the appearance mode and to study νμ → νe oscillations in the νμ CNGS beam. Current results on νμ → νe channel in the three-flavour mixing model are presented. The same data allow to constrain the presence of additional sterile neutrino states. The analysis of the full 2008–2012 OPERA data set and work on its improvement are going on. Details of the achievements are presented.

First neutrino oscillation measurements in NOvA

The NOvA experiment uses the Fermilab NuMI neutrino beam and a newly constructed 14 kt detector to address several open questions in neutrino oscillations including the neutrino mass hierarchy, the precise value of the angle θ23, and the CP-violating phase δCP. The experiment has been running since 2014 and has recently released its first results from an equivalent exposure of 2.74×1020 protons-on-target equal to 8% of the eventual data set. Measurements of νμ→νμ oscillations find Δm322=(2.52−0.18+0.20)×10−3 eV2 and 0.38<sin2⁡θ23<0.65 for the normal neutrino mass hierarchy. The experiment has observed νμ→νe oscillations at 3.3 σ C.L. in this early data and disfavors the inverted neutrino mass hierarchy in the range 0.1π<δCP<0.5π at the 90% C.L.

Search for νμ → νe oscillations with the OPERA experiment in the CNGS beam

The OPERA hybrid detector, designed to prove neutrino oscillations in the νμ→ντ channel, was exposed to the CNGS νμ beam at a distance of 730 km from the neutrino source. Profiting of the tracking capabilities of its Emulsion Cloud Chamber system, OPERA can perform also a search for νμ→νe oscillations. Current results are compatible with the non-oscillation hypothesis in the three flavour mixing model. The same data allow to constrain the non-standard oscillation parameters θnew and Δmnew2 indicated by the LSND and MiniBooNE experiments.

Highlights from Super-Kamiokande

Recent results from Super-Kamiokande experiment are reviewed in this paper; Neutrino mass hierarchy and CP violation in the lepton sector are investigated via νμ → νe oscillation of the atmospheric neutrinos. The event rate, correlation with solar activity, energy spectrum of the solar neutrinos are measured via electron elastic scattering interactions. Neutrino emission from the WIMP annihilation at the center of the Sun are searched in the GeV energy regions. New project, SK-Gd project, to enhance anti-neutrino identification capability, has been approved inside the collaboration group.

Non-Standard Neutrino Interactions in T2K and Daya Bay

We show that the relatively large best fit value of sin2⁡2θ13=0.14(0.17) measured in the T2K experiment for fixed values of i) the Dirac CP violation phase δ=0, and ii) the atmospheric neutrino mixing parameters θ23=π/4, |Δm322|=2.4×10−3 eV2, can be reconciled with the Daya Bay result sin2⁡2θ13=0.090±0.009 if the effects of non-standard neutrino interactions (NSI) in the relevant ν‾e→ν‾e and νμ→νe oscillation probabilities are taken into account.

Results from the OPERA experiment

The OPERA neutrino experiment was designed to perform a unique appearance measurement in the CNGS beam to confirm the oscillation mechanism in the atmospheric sector. Runs were successfully carried out from 2008 to 2012. The detection of τ leptons produced in ντ CC interactions and of their decays is accomplished exploiting the high spatial resolution of nuclear emulsions. Furthermore OPERA has good capabilities in detecting electron neutrino interactions, setting limits on the νμ→νe oscillation channel. The status of the analysis is reported together with updated results on both oscillation channels.

Development and operational experience of magnetic horn system for T2K experiment

A magnetic horn system to be operated at a pulsed current of 320kA and to survive high-power proton beam operation at 750kW was developed for the T2K experiment. The first set of T2K magnetic horns was operated for over 12 million pulses during the four years of operation from 2010 to 2013, under a maximum beam power of 230kW, and 6.63×1020 protons were exposed to the production target. No significant damage was observed throughout this period. This successful operation of the T2K magnetic horns led to the discovery of the νμ→νe oscillation phenomenon in 2013 by the T2K experiment. In this paper, details of the design, construction, and operation experience of the T2K magnetic horns are described.

Neutrino Physics: Status and Open questions

Overview of the status of the present knowledge of neutrino physics, including measurements of νμ→νe oscillation and θ13, an initial test of CP violation, the neutrino-less double beta decay, high energy astrophysical neutrinos, and sterile neutrinos, is given.

Library Event Matching event classification algorithm for electron neutrino interactions in the NOνA detectors

We describe the Library Event Matching classification algorithm implemented for use in the NOνA νμ→νe oscillation measurement. Library Event Matching, developed in a different form by the earlier MINOS experiment, is a powerful approach in which input trial events are compared to a large library of simulated events to find those that best match the input event. A key feature of the algorithm is that the comparisons are based on all the information available in the event, as opposed to higher-level derived quantities. The final event classifier is formed by examining the details of the best-matched library events. We discuss the concept, definition, optimization, and broader applications of the algorithm as implemented here. Library Event Matching is well-suited to the monolithic, segmented detectors of NOνA and thus provides a powerful technique for event discrimination.

Recent Results from the OPERA Experiment

The OPERA experiment has been designed to perform the first detection of neutrino oscillations in the νμ → ντ channel in direct appearance mode, through the event by event detection of the tau lepton produced in ντ charged current interactions. OPERA is a hybrid detector, made of emulsion/lead target elements and of electronic detectors, placed in the CNGS muon neutrino beam from CERN to Gran Sasso, 730km away from the source. Neutrino interactions from the CNGS neutrino runs were recorded from 2008 until the end of 2012. We report on the data sample analysed so far and give the latest OPERA results on νμ → ντ and νμ → νe oscillation searches.

Review of electron neutrino appearance oscillation experiments

A review of νe appearance oscillation experiments is presented. The measurement of the appearance signal is of particular interest because this mode is sensitive to both θ13, δ CP and other mixing parameters. Using the value of θ13 measured by reactor neutrino experiments (disappearance studies) present and next generation appearance experiments can explore CP violation.

The Daya Bay and T2K results on [formula omitted] and non-standard neutrino interactions

We show that the relatively large best fit value of sin2⁡2θ13=0.14(0.17) measured in the T2K experiment for fixed values of i) the Dirac CP violation phase δ=0, and ii) the atmospheric neutrino mixing parameters θ23=π/4, |Δm322|=2.4×10−3 eV2, can be reconciled with the Daya Bay result sin2⁡2θ13=0.090±0.009 if the effects of non-standard neutrino interactions (NSI) in the relevant ν¯e→ν¯e and νμ→νe oscillation probabilities are taken into account.

OPERA: Status and prospects

The Oscillation Project with Emulsion-tRacking Apparatus (OPERA) neutrino experiment was designed to perform a unique appearance measurement in the CNGS beam to confirm the oscillation mechanism in the atmospheric sector. Runs were successfully carried out from 2008 to 2012. The status of the analysis is reported and the topology and the kinematics of the first three ντ candidate events are described. A first result of the νμ→νe oscillation search is also presented.

NOvA experiment in light of largeθ13

NOvA is an off-axis long baseline neutrino experiment searching for νμ → νe oscillations using an upgraded NuMI neutrino beam from Fermilab, Batavia, IL. The main physics goal is a measurement of the CP violation and establishing the neutrino masses hierarchy. A large 14 kton Far detector, comprised of liquid scintillator contained in extruded PVC cells, will also provide an opportunity for other non-accelerator physics searches. While civil construction at the far detector is underway, a smaller prototype near detector has been assembled at Fermilab and is being studied.

Recent Results from the T2K Experiment

The Tokai to Kamioka (T2K) experiment studies neutrino oscillations using a beam of muon neutrinos produced by an accelerator. The neutrinos travel from J-PARC on the east coast of Japan and are detected 295 kilometers further away in the Super-Kamiokande detector. A complex of near detectors located 280 meters away from the neutrino production target is used to better characterize the neutrino beam and reduce systematic uncertainties. The experiment aims at measuring electronic neutrino appearance (νμ→νe oscillation) to measure the neutrino mixing angle θ13, and muon neutrino disappearance to measure the neutrino mixing angle θ23 and mass splitting |Δm322|. We report here electron neutrino appearance results using three years of data, recorded until the 2012 summer, as well as muon neutrino disappearance results based on the data coming from the first two years of the experiment.

Review of electron neutrino appearance oscillation experiments

A review of νe appearance oscillation experiments is presented. The measurement of the appearance signal is of particular interest because this mode is sensitive to both θ13, δCP and other mixing parameters. Using the value of θ13 measured by reactor neutrino experiments (disappearance studies) present and next generation appearance experiments can explore CP violation.

Evidence of electron neutrino appearance in a muon neutrino beam

The T2K Collaboration reports evidence for electron neutrino appearance at the atmospheric mass splitting, |Δm322|≈2.4×10−3 eV2. An excess of electron neutrino interactions over background is observed from a muon neutrino beam with a peak energy of 0.6 GeV at the Super-Kamiokande (SK) detector 295 km from the beam’s origin. Signal and background predictions are constrained by data from near detectors located 280 m from the neutrino production target. We observe 11 electron neutrino candidate events at the SK detector when a background of 3.3±0.4(syst) events is expected. The background-only hypothesis is rejected with a p value of 0.0009 (3.1σ), and a fit assuming νμ→νe oscillations with sin22θ23=1, δCP=0 and |Δm322|=2.4×10−3 eV2 yields sin22θ13=0.088−0.039+0.049(stat+syst).35 MoreReceived 4 April 2013DOI:https://doi.org/10.1103/PhysRevD.88.032002© 2013 American Physical Society