Electron Neutrino Appearance Research Articles

3-flavor and 4-flavor implications of the latest T2K and NO ν A electron (anti-)neutrino appearance results

The two long-baseline experiments T2K and NO$\nu$A have recently presented new findings. T2K has shown the first $\bar \nu_e$ appearance data while NO$\nu$A has released the first $\nu_e$ appearance results. These data are of particular importance because they allow us to probe for the first time in a direct (or manifest) way the leptonic CP-violation. In fact, it is the first time that a hint of CP-violation arises from the comparison of the observations of neutrinos and antineutrinos. We consider the implications of such new results both for the standard 3-flavor framework and for the non-standard 3+1 scheme involving one sterile neutrino species. The 3-flavor analysis shows a consolidation of the previous trends, namely a slight preference for $\sin \delta <0$, disfavoring CP conservation ($\delta =0, \pi$) with a statistical significance close to $90\%$ C.L., and a mild preference (at more than 68\% C.L.) for the normal hierarchy. In a 3+1 framework, the data constrain two CP-phases ($\delta_{13} \equiv \delta$ and $\delta_{14}$), which exhibit a slight preference for the common value $\delta_{13} \simeq \delta_{14} \simeq -\pi/2 $. Interestingly, in the enlarged four neutrino scheme the preference for the normal hierarchy found within the 3-flavor framework completely disappears. This indicates that light sterile neutrinos may constitute a potential source of fragility in the capability of the two LBL experiments of discriminating the neutrino mass hierarchy.

Highlights from T2K

T2K is a long-baseline neutrino experiment designed to measure neutrino oscillations parameters. A high-intensity beam of muon neutrinos produced at the J-PARC accelerator complex is sent towards the near detector facility (the ND280 and INGRID detectors, located 280 m away from the neutrino source) and the far detector, Super-Kamiokande (295 km away). The change in the measured intensity and the composition of the neutrino beam between the near and far detectors are used to provide information on the oscillation parameters. T2K has delivered the world’s best measurement of the θ23 angle by observing muon neutrino disappearance. It was also the first experiment to observe electron neutrino appearance (2013) with a significance of 7.3 σ, to measure the associated θ13 mixing angle, and to provide the first hint of a non-zero δCP phase. The first running of anti-neutrinos in the T2K experiment shows a clear dip below 1 GeV, as expected for an oscillation signal. The T2K experiment is also capable of providing information on neutrino-nucleon cross sections at energies around 1 GeV, thanks to a large amount of target material present in the near detector facility.

Current results of the NOvA experiment

NOvA is a long baseline neutrino oscillation experiment at Fermi National Accelerator Laboratory, USA. It is one of the so-called “off-beam-axis” experiments with two highly active detectors separated by 810 km of Earth’s crust. Both detectors are identical and consist of PVC cells filled with a liquid scintillator. The NOvA collaboration has already published its first results for muon neutrino disappearance [1] and electron neutrino appearance [2] mode with data collected between July 2013 and March 2015. This talk is devoted to a discussion on NOvA’s first results.

T2K and Beyond

This paper presents thestate of the artof the T2K experiment and the measurements prospects for the incoming years. After a brief description of the experiment, the most recent results will be illustrated. The observation of the electron neutrino appearance in a muon neutrino beam and the new high-precision measurements of the mixing angleθ13by the reactor experiments have led to a reevaluation of the expected sensitivity to the oscillation parameters, relative to what was given in the original T2K proposal. For this reason the new physics potential of T2K for7.8×1021p.o.t. and for data exposure 3 times larger than that expected to be reachable with accelerator and beam line upgrades in 2026 before the start of operation of the next generation of long-baseline neutrino oscillation experiments will also be described in the text. In particular the last challenging scenario opens the door to the possibility of obtaining, under some conditions, a 3σmeasurement excludingsin⁡(δCP)=0.

Implementation of an upward-going muon trigger for indirect dark matter searches at the NOvA far detector

The NOvA collaboration has constructed a 14,000 ton, fine-grained, low-Z, total absorption tracking calorimeter at an off-axis angle to an upgraded NuMI neutrino beam. This detector, with its excellent granularity and energy resolution and relatively low-energy neutrino thresholds, was designed to observe electron neutrino appearance in a muon neutrino beam, but it also has unique capabilities suitable for more exotic efforts. In fact, if an efficient upward- going muon trigger with sufficient cosmic ray background rejection can be demonstrated, NOvA will be capable of a competitive indirect dark matter search for low-mass WIMPs. The cosmic ray muon rate at the NOvA far detector is about 100 kHz and provides the primary challenge for triggering and optimizing such a search analysis. The status of the NOvA upward-going muon trigger is presented.

Study of neutrino interactions at the T2K near detector

The Tokai to Kamioka experiment (T2K) is a long baseline neutrino accelerator operating in Japan and designed to measure electron neutrino appearance and muon neutrino disappearance. Three gaseous TPCs compose its near detector complex. The latest T2K analysis combines the results from the appearance and disappearance channels to extract simultaneously four neutrino oscillation parameters. Improvements to the TPC field distortion correction are under development to reduce systematic uncertainties for future oscillation analyses. In parallel a cross section measurement of neutrino-gas interactions happening in the T2K TPCs is in preparation in particular to extract unique proton multiplicity and kinematic information.

Summary of the Latest Results and Future Prospects from the T2K Experiment

The T2K long-baseline experiment is located in Japan and is designed to study oscillations of muon neutrinos. T2K obtains a beam of muon neutrinos peaked at 0.6 GeV that are produced at J-PARC accelerator complex by converting a beam of 30-GeV protons hitting a graphite target. Upon traveling 295 km, neutrinos are detected by the Super-Kamiokande (SK) water Cherenkov detector. Located at 280 m from the target, the near detector complex (ND280) provides information about un-oscillated neutrino flux, beam stability and interaction cross-sections. The T2K experiment observed electron neutrino appearance at SK with the significance of 7.3σ and measured the associated oscillation parameter θ13 for both normal and inverted mass hierarchies. In addition, by looking at muon neutrino disappearance T2K provided improved measurements of the θ23 and Δm232 parameters. The results of these measurements are presented as well as a brief summary of the selected neutrino cross-section measurements. Future prospects of the T2K experiment are discussed.

Measurements of neutrino oscillation in appearance and disappearance channels by the T2K experiment with6.6×1020protons on target

We report on measurements of neutrino oscillation using data from the T2K long-baseline neutrino experiment collected between 2010 and 2013. In an analysis of muon neutrino disappearance alone, we find the following estimates and 68% confidence intervals for the two possible mass hierarchies: Normal Hierarchy: $\sin^2\theta_{23}=0.514^{+0.055}_{-0.056}$ and $\Delta m^2_{32}=(2.51\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ Inverted Hierarchy: $\sin^2\theta_{23}=0.511\pm0.055$ and $\Delta m^2_{13}=(2.48\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ The analysis accounts for multi-nucleon mechanisms in neutrino interactions which were found to introduce negligible bias. We describe our first analyses that combine measurements of muon neutrino disappearance and electron neutrino appearance to estimate four oscillation parameters and the mass hierarchy. Frequentist and Bayesian intervals are presented for combinations of these parameters, with and without including recent reactor measurements. At 90% confidence level and including reactor measurements, we exclude the region: $\delta_{CP}=[0.15,0.83]\pi$ for normal hierarchy and $\delta_{CP}=[-0.08,1.09]\pi$ for inverted hierarchy. The T2K and reactor data weakly favor the normal hierarchy with a Bayes Factor of 2.2. The most probable values and 68% 1D credible intervals for the other oscillation parameters, when reactor data are included, are: $\sin^2\theta_{23}=0.528^{+0.055}_{-0.038}$ and $|\Delta m^2_{32}|=(2.51\pm0.11)\times 10^{-3}$ eV$^2$/c$^4$.

Neutrino oscillation physics potential of the T2K experiment

The observation of the recent electron neutrino appearance in a muon neutrino beam and the high-precision measurement of the mixing angle $\theta_{13}$ have led to a re-evaluation of the physics potential of the T2K long-baseline neutrino oscillation experiment. Sensitivities are explored for CP violation in neutrinos, non-maximal $\sin^22\theta_{23}$, the octant of $\theta_{23}$, and the mass hierarchy, in addition to the measurements of $\delta_{CP}$, $\sin^2\theta_{23}$, and $\Delta m^2_{32}$, for various combinations of $\nu$-mode and \(\bar{\nu}\)-mode data-taking. With an exposure of $7.8\times10^{21}$~protons-on-target, T2K can achieve 1-$\sigma$ resolution of 0.050(0.054) on $\sin^2\theta_{23}$ and $0.040(0.045)\times10^{-3}~\rm{eV}^2$ on $\Delta m^2_{32}$ for 100\%(50\%) neutrino beam mode running assuming $\sin^2\theta_{23}=0.5$ and $\Delta m^2_{32} = 2.4\times10^{-3}$ eV$^2$. T2K will have sensitivity to the CP-violating phase $\delta_{\rm{CP}}$ at 90\% C.L. or better over a significant range. For example, if $\sin^22\theta_{23}$ is maximal (i.e $\theta_{23}$=$45^\circ$) the range is $-115^\circ<\delta_{\rm{CP}}<-60^\circ$ for normal hierarchy and $+50^\circ<\delta_{\rm{CP}}<+130^\circ$ for inverted hierarchy. When T2K data is combined with data from the NO$\nu$A experiment, the region of oscillation parameter space where there is sensitivity to observe a non-zero $\delta_{CP}$ is substantially increased compared to if each experiment is analyzed alone.

Baseline optimization for the measurement ofCPviolation, mass hierarchy, andθ23octant in a long-baseline neutrino oscillation experiment

Next-generation long-baseline electron neutrino appearance experiments will seek to discover CP violation, determine the mass hierarchy and resolve the $\theta_{23}$ octant. In light of the recent precision measurements of $\theta_{13}$, we consider the sensitivity of these measurements in a study to determine the optimal baseline, including practical considerations regarding beam and detector performance. We conclude that a detector at a baseline of at least 1000 km in a wide-band muon neutrino beam is the optimal configuration.

Estimating matter-inducedCPTviolation in long-baseline neutrino experiments

We examine matter-induced $CPT$ violation effects in long-baseline electron neutrino appearance experiments in a low energy neutrino factory setup. Assuming $CPT$ invariance in vacuum, the magnitude of $CPT$ violating asymmetry in matter has been estimated using the exact expressions for the transition probabilities. The dependence of the asymmetry on the oscillation parameters like mixing angles, mass squared differences, as well as on the Dirac $CP$ violating phase has been investigated.

T2K on-axis detector results: cross section measurements

The T2K experiment brings the world leading constraints on the atmospheric parameters, sin2 θ 23 = 0.514 ± 0.082 and |Δm 32 2 | = (2.44−0.15 +0.17 ) × 10−3 eV2 .c−4 , along with first observation of the electron neutrino appearance in the muon neutrino beam. In the upcoming years, a higher precision measurement of the PMNS matrix parameters with possible evidence of a non zero CP violation phase requires a reduction in the main systematics which come from the cross section model. The INGRID detector is the onaxis detector of the long-baseline T2K experiment. Though its main goal is to monitor the neutrino flux intensity and off-axis angle, this proceeding presents the νμ cross section measurements that have been performed to reduce the uncertainty on the neutrino cross section models.

T2K on-axis detector results: cross section measurements

The T2K experiment brings the world leading constraints on the atmospheric parameters, sin2θ23 = 0.514 ± 0.082 and |Δm322| = (2.44−0.15+0.17) × 10−3eV2.c−4, along with first observation of the electron neutrino appearance in the muon neutrino beam. In the upcoming years, a higher precision measurement of the PMNS matrix parameters with possible evidence of a non zero CP violation phase requires a reduction in the main systematics which come from the cross section model. The INGRID detector is the onaxis detector of the long-baseline T2K experiment. Though its main goal is to monitor the neutrino flux intensity and off-axis angle, this proceeding presents the νμ cross section measurements that have been performed to reduce the uncertainty on the neutrino cross section models.

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.

New results from the T2K experiment

Since its discovery, neutrino oscillation has been the subject of intense research. The θ13 parameter of the neutrino mixing matrix is of particular importance. A non-zero θ13 opens the possibility to study CP violation in the neutrino sector which could help explain the matter anti-matter asymmetry in the Universe. The Tokai-To-Kamioka (T2K) experiment was designed to measure electron neutrino appearance and muon neutrino disappearance using a muon neutrino beam produced at the J-PARC facility in Tokai, Japan. The Super-Kamiokande detector, located 295 km from the neutrino source, acts as the far detector. An improved analysis and all the data taken through June 9, 2012 provide 11 electron neutrino candidates in the far detector corresponding to 3.1 standard deviations from the θ13=0 hypothesis. This excess is currently the strongest indication of electron neutrino appearance and it yields for normal hierarchy and δCP=0, 0.053<sin2⁡2θ13<0.141 at 68% confidence level.

Measurement of the intrinsic electron neutrino component in the T2K neutrino beam with the ND280 detector

The T2K experiment has reported the first observation of the appearance of electron neutrinos in a muon neutrino beam. The main and irreducible background to the appearance signal comes from the presence in the neutrino beam of a small intrinsic component of electron neutrinos originating from muon and kaon decays. In T2K, this component is expected to represent 1.2% of the total neutrino flux. A measurement of this component using the near detector (ND280), located 280 m from the target, is presented. The charged current interactions of electron neutrinos are selected by combining the particle identification capabilities of both the time projection chambers and electromagnetic calorimeters of ND280. The measured ratio between the observed electron neutrino beam component and the prediction is 1.01+-0.10 providing a direct confirmation of the neutrino fluxes and neutrino cross section modeling used for T2K neutrino oscillation analyses. Electron neutrinos coming from muons and kaons decay are also separately measured, resulting in a ratio with respect to the prediction of 0.68+-0.30 and 1.10+-0.14, respectively.

Observation of Electron Neutrino Appearance in a Muon Neutrino Beam

The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of 7.3σ when compared to 4.92±0.55 expected background events. In the Pontecorvo-Maki-Nakagawa-Sakata mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles θ12, θ23, θ13, a mass difference Δm(32)(2) and a CP violating phase δ(CP). In this neutrino oscillation scenario, assuming |Δm(32)(2)|=2.4×10(-3) eV(2), sin(2)θ(23)=0.5, and Δm322>0 (Δm(32)(2)<0), a best-fit value of sin(2)2θ(13)=0.140(-0.032)(+0.038) (0.170(-0.037)(+0.045)) is obtained at δ(CP)=0. When combining the result with the current best knowledge of oscillation parameters including the world average value of θ(13) from reactor experiments, some values of δ(CP) are disfavored at the 90% C.L.

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.

Highlights from T2K

T2K is a long baseline neutrino oscillation experiment. A high intensity proton beam at J-PARC produces a narrow-band muon-neutrino beam with a peak energy of 0.6 GeV at the far detector, Super-Kamiokande, 295km from the production point. The T2K experiment has observed electron neutrino appearance in a muon neutrino beam and measured sin22θ13 = 0.140−0.032+0.038 (0.170−0.037+0.045) assuming the normal (inverted) hierarchy. In addition the latest measurements of θ23 and ∆m322 are reported, along with the latest T2K neutrino interaction cross section measurements of the inclusive CC, CCQE and NCE channels.