T2K Experiment Research Articles

Pion production in the T2K experiment

Background: Pion production gives information on the axial form factors of nucleon resonances. It also introduces a noticeable background to quasi-elastic measurements on nuclear targets and thus has a significant impact on precision studies of neutrino oscillation parameters. Purpose: To clarify neutrino-induced pion production on nucleons and nuclei. Method: The Giessen Boltzmann--Uehling--Uhlenbeck (GiBUU) model is used for the description of neutrino-nucleus reactions. Results: Theoretical results for differential cross sections for the T2K neutrino flux at the ND280 detector and integrated cross sections as a function of neutrino energy are given. Two sets of pion production data on elementary targets are used as inputs to obtain limits for pion production in neutrino-nucleus reactions. Conclusions: Pion production in the T2K ND280 detector can help to narrow down the uncertainties in the elementary pion production cross sections. It can also give valuable information on the nucleon-Delta axial form factor.

Physics potential of neutrino oscillation experiment with a far detector in Oki Island along the T2K baseline

Oki Island is located between Japan and Korea along the Tokai-To-Kamioka (T2K) baseline. The distance from J-PARC to Oki Island is about 653km, which is twice that of the T2K experiment (L=295km). When the off-axis angle of the neutrino beam from J-PARC is 3.0^\circ (2.0^\circ) at Super-Kamiokande (SK), the off-axis beam (OAB) with 1.4^\circ (0.6^\circ) reaches at Oki Island. We examine physics case of placing a far detector in Oki Island during the T2K experimental period. We estimate the matter density profile along the Tokai-to-Oki baseline by using recent seismological measurements. For a detector of 100kton fiducial volume and 2.5x10^21 POT (protons on target) exposure for both \nu_\mu and \bar{\nu}_\mu beams, we find that the mass hierarchy pattern can be distinguished at 3\sigma level if sin^22theta_rct \equiv 4|U_{e3}|^2(1-|U_{e3}|^2) \gsim 0.09, by observing the electron-like CCQE (Charged-Current Quasi Elastic) events. The CP phase in the Maki-Nakagawa-Sakata lepton flavor mixing matrix, \delta_mns, can be constrained with \pm 20^\circ. As a reference, we repeat the same analysis by placing the same detector in Korea at L=1000km and OAB=0.5^\circ (T2KK) and also by placing it at the SK site (T2K_122). The Tokai-to-Kamioka-OKI (T2KO) sensitivity to the mass hierarchy is about 1/3 (in \Delta \chi^2_min) of T2KK, while the sensitivity to the phase \delta_mns is similar between T2KO and T2KK. The T2K_122 option has almost no sensitivity to the mass hierarchy, and cannot measure the CP phase except when \delta_mns \sim -90^\circ (90^\circ) for the normal (inverted) hierarchy.

Octant sensitivity for large θ 13 in atmospheric and long-baseline neutrino experiments

One of the unknown parameters in neutrino oscillations is the octant of the mixing angle theta_{23}. In this paper, we discuss the possibility of determining the octant of theta_{23} in the long baseline experiments T2K and NOvA in conjunction with future atmospheric neutrino detectors, in light of non-zero value of theta_{13} measured by reactor experiments. We consider two detector technologies for atmospheric neutrinos - magnetized iron calorimeter and non-magnetized Liquid Argon Time Projection Chamber. We present the octant sensitivity for T2K/NOvA and atmospheric neutrino experiments separately as well as combined. For the long baseline experiments, a precise measurement of theta_{13}, which can exclude degenerate solutions in the wrong octant, increases the sensitivity drastically. For theta_{23} = 39^o and sin^2 2 theta_{13} = 0.1, at least ~2 sigma sensitivity can be achieved by T2K+NOvA for all values of delta_{CP} for both normal and inverted hierarchy. For atmospheric neutrinos, the moderately large value of theta_{13} measured in the reactor experiments is conducive to octant sensitivity because of enhanced matter effects. A magnetized iron detector can give a 2 sigma octant sensitivity for 500 kT yr exposure for theta_{23} = 39^o, delta_{CP} = 0 and normal hierarchy. This increases to 3 sigma for both hierarchies by combining with T2K+NOvA. This is due to a preference of different theta_{23} values at the minimum chi^2 by T2K/NOvA and atmospheric neutrino experiments. A Liquid Argon detector for atmospheric neutrinos with the same exposure can give higher octant sensitivity, due to the interplay of muon and electron contributions and superior resolutions. We obtain a ~3 sigma sensitivity for theta_{23} = 39^o for normal hierarchy. This increases to > ~4 sigma for all values of delta_{CP} if combined with T2K+NOvA. For inverted hierarchy the combined sensitivity is ~3 sigma.

Detection methods at reactor neutrino experiments

In 2011 and 2012, the smallest neutrino mixing angle θ13 was determined to be unexpected large by reactor neutrino experiments Daya Bay, Double Chooz, and RENO, and accelerator experiments T2K and MINOS. The most precise measurement is sin22θ13=0.089±0.010(stat.)±0.005(syst.), provided by Daya Bay. The measurement of θ13 opened the gateway to the mass hierarchy and CP phase measurements. It also marked the beginning of precision measurements in neutrino studies. With near-far relative measurement and improvements in detector design, the relative precision of neutrino detectors reached 0.2%. Detection methods for reactor neutrinos are reviewed. The highlighted techniques include gadolinium-doped liquid scintillator, three-layer detectors, functionally identical detectors, reflective panel, background shielding, etc. The next generation reactor neutrino experiment Daya Bay II and its technical challenges are briefly described.

Determining the neutrino mass hierarchy with INO, T2K, NOvA and reactor experiments

The relatively large measured value of $\theta_{13}$ has opened up the possibility of determining the neutrino mass hierarchy through earth matter effects. Amongst the current accelerator-based experiments only NOvA has a long enough baseline to observe earth matter effects. However, NOvA is plagued with uncertainty on the knowledge of the true value of $\delta_{CP}$, and this could drastically reduce its sensitivity to the neutrino mass hierarchy. The earth matter effect on atmospheric neutrinos on the other hand is almost independent of $\delta_{CP}$. The 50 kton magnetized Iron CALorimeter at the India-based Neutrino Observatory (ICAL@INO) will be observing atmospheric neutrinos. The charge identification capability of this detector gives it an edge over others for mass hierarchy determination through observation of earth matter effects. We study in detail the neutrino mass hierarchy sensitivity of the data from this experiment simulated using the Nuance based generator developed for ICAL@INO and folded with the detector resolutions and efficiencies obtained by the INO collaboration from a full Geant4-based detector simulation. The data from ICAL@INO is then combined with simulated data from T2K, NOvA, Double Chooz, RENO and Daya Bay experiments and a combined sensitivity study to the mass hierarchy is performed. With 10 years of ICAL@INO data combined with T2K, NOvA and reactor data, one could get about $2.3\sigma-5.7\sigma$ discovery of the neutrino mass hierarchy, depending on the true value of $\sin^2\theta_{23}$ [0.4 -- 0.6], $\sin^22\theta_{13}$ [0.08 -- 0.12] and $\delta_{CP}$ [0 -- 2$\pi$].

Hadron production measurement from NA61/SHINE

An overview of the recent NA61/SHINE results on determination of charged hadron yields in proton-carbon interactions is presented. The results aim to improve predictions of the neutrino flux in the T2K experiment. The data were recorded using a secondary-proton beam of 31 GeV/c momentum from CERN SPS scattered off a graphite target. To determine the inclusive production cross-section for charged pions, kaons and protons the thin (0.04 λI) target was exploited. Results of this measurement are used in the T2K beam simulation program to reweight hadron yields in the primary interaction vertex and allow to constrain directly about 60% of the neutrino flux. At the same time, NA61/SHINE results obtained with the T2K replica target (1.9 λI) would allow to constrain up to 90% of the neutrinos, thus reducing significantly a model dependence of the neutrino beam prediction.The proposal to collect hadroproduction data on thin and NuMI (Neutrinos at the Main Injector in Fermilab) replica targets exposed to a 120 GeV/c proton beam is reviewed. These data will be used further in neutrino beam calculations for the MINERvA, MINOS(+) and LBNE experiments.

Heavy neutrino search in accelerator-based experiments

We explore the feasibility of detecting heavy neutrinos by the existing facilities of neutrino experiments. A heavy neutrino in the mass range 1 MeV ≲ M N ≲ 500 MeV is produced by pion or kaon decay, and decays to charged particles which leave signals in neutrino detectors. Taking the T2K experiment as a typical example, we estimate the heavy neutrino flux produced in the neutrino beam line. Due to massive nature of the heavy neutrino, the spectrum of the heavy neutrino is significantly different from that of the ordinary neutrinos. While the ordinary neutrinos are emitted to various directions in the laboratory frame due to their tiny masses, the heavy neutrinos tend to be emitted to the forward directions and frequently hit the detector. The sensitivity for the mixing parameters is studied by evaluating the number of signal events in the near detector ND280. For the electron-type mixing, the sensitivity of T2K at 1021 POT is found to be better than that of the previous experiment PS191, which has placed the most stringent bounds on the mixing parameters of the heavy neutrinos for 140 MeV ≲ M N ≲ 500 MeV.

T2K results and future plans

We present the νμ → νe appearance and the νμ disappearance results, using a total of 1.43 × 1020 protons on target collected with the T2K experiment. T2K is long baseline neutrino experiment in Japan with detectors located at J-PARC, Tokai, and at Kamioka in the Gifu Prefecture, situated 295 km away from J-PARC. The muon neutrino beam is produced and measured at the near detectors at J-PARC whilst the neutrino rates after oscillation are measured with the Super-Kamiokande detector, at Kamioka. A total of six events pass all the selection criteria for νμ → νe oscillations at the far detector Super-Kamiokande, leading to 0.03(0.04) < sin2 2θ13 < 0.28(0.34) for δCP = 0 and normal (inverted) hierarchy at 90% C.L. The νμ disappearance analysis excludes no oscillations at 4.3σ. At 90% C.L., the best fit values are sin2 2θ23 > 0.84 and 2.1 × 10−3 < Δm223(eV2) < 3.1 × 10−3. Finally, we present an overview of the T2K plans from 2011 onwards.

Predicting neutrino flux for the T2K experiment

The T2K experiment is a long-baseline neutrino oscillation experiment. One of its main goals is the discovery of the νμ to νe oscillations governed by the last unknown mixing angle θ13. Additionally T2K aims to make precision measurements of Δm223 and θ23 oscillation parameters. The experiment uses the two detector approach where a near detector measures the neutrino flux close to the ν source, while a far detector measures the oscillated flux. To determine the oscillation parameters the extrapolation of the measurements at the near detector to the far detector has to be done. Such extrapolation relies on the predictions of the neutrino fluxes for the two detectors. In this paper the estimation of the neutrino flux predictions for the T2K experiment is briefly described.

Mass hierarchy determination using current and upcoming neutrino experiments

Determination of mass hierarchy is a crucial open ended question in neutrino physics today. In the wake of recent T2K [1], MINOS [2] and DChooz [3] results which support a high best-fit to θ13, it is worthwhile to explore the potential of the upcoming and already running neutrino experiments for determining mass hierarchy. We consider the long baseline experiments NOνA and T2K and the reactor neutrino experiments Daya Bay, DChooz and RENO. In our analysis we find out that NOνA by itself is insufficient and would require another long baseline experiment such as T2K. This work assumes that by the time reactor neutrino experiments are over, we will have a precise θ13 information. Final results are presented as hierarchy exclusion plots in sin2 2θ13 − δCP plane.

Hadron production experiments

The HARP and NA61/SHINE hadroproduction experiments as well as their implications for neutrino physics are discussed. HARP measurements have already been used for predictions of neutrino beams in K2K and MiniBooNE/SciBooNE experiments and are also being used to improve the atmospheric neutrino flux predictions and to help in the optimization of neutrino factory and super-beam designs. First measurements released recently by the NA61/SHINE experiment are of significant importance for a precise prediction of the J-PARC neutrino beam used for the T2K experiment. Both HARP and NA61/SHINE experiments provide also a large amount of input for validation and tuning of hadron production models in Monte-Carlo generators.

New results from the T2K neutrino oscillation experiment

The T2K experiment searches for the appearance of electron neutrinos in a muon neutrino beam. The rate of this process is sensitive to the neutrino mixing parameter θ13. Recent measurements that \(\theta_{13} \ne 0\) imply that νμ → νe oscillations should be observable. Using all data through May 15, 2012 the T2K experiment has detected 10 candidate νe events, with an expected background for θ13 = 0 of 2.73±0.37 events. This 3.2σ excess of νe events is the strongest indication to date for appearance of electron neutrinos in a neutrino oscillation experiment, and for normal mass hierarchy and δCP = 0 yields \(0.059 < \sin^2 2\theta_{13} < 0.164\) at the 68 % C.L.

Results from the T2K Experiment

Results from the T2K Experiment

Implications of nonzero θ13 for the neutrino mass hierarchy

The Daya Bay, RENO, and Double Chooz experiments have discovered a large non-zero value for θ13. We present a global analysis that includes these three experiments, Chooz, the Super-K atmospheric data, and the νμ → νe T2K and MINOS experiments that are sensitive to the hierarchy and the sign of θ13. We report preliminary results in which we fix the mixing parameters other than θ13 to those from a recent global analysis. Given there is no evidence for a non-zero CP violation, we assume δ = 0. T2K and MINOS lie in a region of L/E where there is a hierarchy degeneracy in the limit of θ13 → 0 and no matter interaction. For nonzero θ13, the symmetry is partially broken, but a degeneracy under the simultaneous exchange of both hierarchy and the sign of θ13 remains. Matter effects break this symmetry such that the positions of the peaks in the oscillation probabilities maintain the two-fold symmetry, while the magnitude of the oscillations is sensitive to the hierarchy. This renders T2K and NOvA, with different baselines and different matter effects, better able in combination to distinguish the hierarchy and the sign of θ13. The present T2K and MINOS data do not distinguish between hierarchies or the sign of θ13, but the large value of θ13 yields effects from atmospheric data that do. We find for normal hierarchy, positive θ13, sin2 2θ13 = 0.090 ± 0.020 and is 0.2% probable it is the correct combination; for normal hierarchy, negative θ13, sin2 2θ13 = 0.108 ± 0.023 and is 2.2% probable; for inverse hierarchy, positive θ13, sin2 2θ13 = 0.110±0.022 and is 7.1% probable; for inverse hierarchy, negative θ13, sin2 2θ13 = 0.113 ± 0.022 and is 90.5% probable, results that are inconsistent with two similar analyses.

Neutrino energy reconstruction in quasielastic-like scattering in the MiniBooNE and T2K experiments

Neutrino oscillation probabilities, which are being measured in long-baseline experiments, depend on neutrino energy. The energy in a neutrino beam, however, is broadly smeared so that the neutrino energy in a particular event is not directly known, but must be reconstructed from final state properties. In this paper we investigate the contributions from different reaction mechanisms on the energy-reconstruction method widely used in long-baseline neutrino experiments. Difference between the true-QE and QE-like cross sections in MiniBooNE experiment is investigated in details. It is shown, that fake QE-like events lead to significant distortions in neutrino energy reconstruction. Flux-folded and unfolded cross sections for QE-like scattering are calculated as functions of both true and reconstructed energies. Flux-folded momentum transfer distributions are calculated as functions of both true and reconstructed momentum transfer. Distributions versus reconstructed values are compared with the experimental data. Also presented are the conditional probability densities of finding a true energy for a given reconstructed energy. It is shown, how the energy reconstruction procedure influences the measurement of oscillation parameters in T2K experiment. For the reconstruction procedure based on quasielastic (QE) kinematics, all other reaction channels beside true-QE scattering show a shift of the reconstructed energy towards lower values as compared to the true energy. On average in the MiniBooNE and T2K experiments the shift is 100 - 200 MeV and depends on energy. The oscillation signals are similarly affected. These uncertainties may limit the extraction of a CP violating phase from an oscillation result.

Optical transition radiation monitor for the T2K experiment

An optical transition radiation monitor has been developed for the proton beam-line of the T2K long baseline neutrino oscillation experiment. The monitor operates in the highly radioactive environment in proximity to the T2K target. It uses optical transition radiation, the light emitted from a thin metallic foil when the charged beam passes through it, to form a two-dimensional image of the 30GeV proton beam profile in the transverse plane. One of its key features is an optical system capable of transporting the light over a large distance out of the harsh environment near the target to a lower radiation area where it is possible to operate a camera to capture this light. The monitor measures the proton beam position and width with an accuracy better than 0.5mm, meeting the physics requirements of the T2K experiment.

Nonstandard interactions in neutrino oscillations and the recent Daya Bay and T2K experiments

We study the possible constraints on nonstandard interaction (NSIs) in a model-independent way by considering the recent results from the T2K and Daya Bay neutrino oscillations experiments. Using the perturbation method we present generic formulas (suitable for T2K baseline and for large ${\ensuremath{\theta}}_{13}$ as evident from Daya Bay) for the probability of oscillation for ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\ensuremath{\nu}}_{e}$, taking into account NSIs at the source (${ϵ}^{s}$), the detector (${ϵ}^{d}$), and during propagation (${ϵ}^{m}$) of neutrinos through matter. Two separate cases of perturbation with small (slightly large) NSI [${ϵ}_{\ensuremath{\alpha}\ensuremath{\beta}}^{m}\ensuremath{\sim}0.03(0.18)$] are discussed in detail. Using various possible presently allowed NSI values we reanalyze numerically the ${\ensuremath{\theta}}_{13}\ensuremath{-}\ensuremath{\delta}$ allowed region given by recent T2K experimental data. We obtain model-independent constraints on NSIs in the $\ensuremath{\delta}\ensuremath{-}{ϵ}_{\ensuremath{\alpha}\ensuremath{\beta}}^{m}$ plane using the ${\ensuremath{\theta}}_{13}$ value as measured by Daya Bay, where $\ensuremath{\delta}$ is the $CP$ violating phase. Depending on $\ensuremath{\delta}$ values, significant constraints on ${ϵ}_{e\ensuremath{\tau}}$ and ${ϵ}_{\ensuremath{\tau}\ensuremath{\tau}}$, in particular, are possible for both hierarchies of neutrino masses. Corresponding to T2K's 66% confidence level result, the constraints on ${ϵ}_{\ensuremath{\tau}\ensuremath{\tau}}$ are shown to be independent of any $\ensuremath{\delta}$ value.

The T2K Side Muon Range Detector (SMRD)

The T2K experiment is a long baseline neutrino oscillation experiment aiming to observe the appearance of νe in a νμ beam. The νμ beam is produced at the Japan Proton Accelerator Research Complex (J-PARC), observed with the 295km distant Super-Kamiokande Detector and monitored by a suite of near detectors at 280m from the proton target. The near detectors include a magnetized off-axis detector (ND280) which measures the unoscillated neutrino flux and neutrino cross-sections. The present paper describes the outermost component of ND280 which is a Side Muon Range Detector (SMRD) composed of scintillation counters with embedded wavelength shifting fibers and Multi-Pixel Photon Counter readout. The components, performance and response of the SMRD are presented.

Particle production cross-sections by 30 GeV protons on Carbon at the NA61/SHINE experiment

One of the physics goals of the NA61/SHINE experiment [N. Antoniou et al. [NA61/SHINE Collaboration], CERN-SPSC-2007- 019, (2007)] is to perform new hadron production measurements in proton-Carbon interactions at 30 GeV. This data will be used to constrain the prediction of neutrino fluxes for the T2K experiment at J-PARC [Y. Hayato et al. [T2K Collaboration], T2K at J-PARC, Nucl. Phys. Proc. Suppl. 143, (2005)]. For precise predictions of the T2K neutrino beam parameters the measurements were performed on two di!erent types of targets: thin carbon target and the replica of the actual target of the T2K experiment [L.S. Esposito, [NA61/SHINE Collaboration], NA61 data for T2K flux calculations, poster at Neutrino 2010.]. The preliminary results on π+ and π− inelastic cross sections at thin carbon target from 2007 pilot run are presented.

NA61 data for T2K flux calculations

The approved T2K neutrino oscillation physics program requires a 5% neutrino flux determination and the ability to extrapolate the neutrino flux from near to far detector with a precision of 3%. The NA61 experiment at the CERN North Area measures hadron production over all the phase space needed by the T2K experiment and aims at producing a measurement of particle yields with a precision of 5% or better for pions and 10% for Kaons.