Long-baseline Neutrino Oscillation Experiment Research Articles

The T2K fine-grained detectors

T2K is a long-baseline neutrino oscillation experiment searching for νe appearance in a νμ beam. The beam is produced at the J-PARC accelerator complex in Tokai, Japan, and the neutrinos are detected by the Super-Kamiokande detector located 295km away in Kamioka. A suite of near detectors (ND280) located 280 m downstream of the production target is used to characterize the components of the beam before they have had a chance to oscillate and to better understand various neutrino interactions on several nuclei. This paper describes the design and construction of two massive fine-grained detectors (FGDs) that serve as active targets in the ND280 tracker. One FGD is composed solely of scintillator bars while the other is partly scintillator and partly water. Each element of the FGDs is described, including the wavelength shifting fiber and Multi-Pixel Photon Counter used to collect the light signals, the readout electronics, and the calibration system. Initial tests and in situ results of the FGDs' performance are also presented.

Muon neutrino to electron neutrino oscillations in the MINOS experiment

MINOS is a long-baseline neutrino oscillation experiment situated along Fermilabʼs high-intensity NuMI neutrino beam. MINOS has completed an updated search for muon neutrino to electron neutrino transitions, observation of which would indicate a non-zero value for the neutrino mixing angle θ13. The present 7×1020 protons-on-target data set represents more than double the exposure used in the previous analysis. The new results are presented.

Status of NOνA

NOνA is a long-baseline neutrino oscillation experiment looking for the appearance of electron neutrinos and anti-neutrinos in the NuMI neutrino beam. It is comprised of a near detector located on-site at Fermilab, approximately 1 km from the neutrino source and a far detector located 810 km from the source in northern Minnesota. Both detectors are positioned 14 mrad off the beam axis to observe a narrow range of neutrino energies peaked at 2.2 GeV. Construction of the NOνA experiment has begun and the details are outlined below.

An incremental approach to unravel the neutrino mass hierarchy and CP violation with a long-baseline superbeam for large θ 13

Recent data from long-baseline neutrino oscillation experiments have provided new information on \theta_{13}, hinting that 0.01\lesssim sin^2 2\theta_{13} \lesssim 0.1 at 2 sigma C.L. Confirmation of this result with high significance will have a crucial impact on the optimization of the future long-baseline oscillation experiments designed to probe the neutrino mass ordering and leptonic CP violation. In this context, we expound in detail the physics reach of an experimental setup where neutrinos produced in a conventional wide-band beam facility at CERN are observed in a proposed Giant Liquid Argon detector at the Pyh\"asalmi mine, at a distance of 2290 km. This particular setup would have unprecedented sensitivity to the mass ordering and CP violation in the light of large \theta_{13}. With a 10 to 20 kt `pilot' detector and just a few years of neutrino beam running, the mass hierarchy could be determined, irrespective of the true values of \delta_{CP} and the mass hierarchy, at 3 sigma (5 sigma) C.L. if sin^2 2\theta_{13}(true) = 0.05 (0.1). With the same exposure, we start to have 3 sigma sensitivity to CP violation if sin^2 2\theta_{13}(true) > 0.05, in particular testing maximally CP-violating scenarios at a high C.L. After optimizing the neutrino and anti-neutrino running fractions, we study the performance of the setup as a function of the exposure, identifying three milestones to have roughly 30%, 50% and 70% coverage in \delta_{CP}(true) for 3 sigma CP violation discovery. For comparison, we also study the CERN to Slanic baseline of 1540 km. This work demonstrates that an incremental program, staged in terms of the exposure, can achieve the desired physics goals within a realistically feasible timescale, and produce significant new results at each stage.

First muon-neutrino disappearance study with an off-axis beam

We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to 1.43 10**20 protons on target, we observe 31 fully-contained single muon-like ring events in Super-Kamiokande, compared with an expectation of 104 +- 14 (syst) events without neutrino oscillations. The best-fit point for two-flavor nu_mu -> nu_tau oscillations is sin**2(2 theta_23) = 0.98 and |\Delta m**2_32| = 2.65 10**-3 eV**2. The boundary of the 90 % confidence region includes the points (sin**2(2 theta_23),|\Delta m**2_32|) = (1.0, 3.1 10**-3 eV**2), (0.84, 2.65 10**-3 eV**2) and (1.0, 2.2 10**-3 eV**2).

Current status of the T2K experiment

The T2K long-baseline neutrino-oscillation experiment was started in January 2010 for the purpose of physics data-taking. Until the massive earthquake on March 11, 2011 in Japan, 1.43 × 1020 pot data were accumulated. In this data, 6 possible νe appearance candidates are found for expected background of 1.5±0.3(syst.) if sin2 2θ13 = 0 is assumed. The probability that 6 or more events can be observed for 1.5±0.3 expected events is 7×10−3, equivalent to 2.5σ significance. This is the first indication of a νe appearance or, in other words, the first indication of a non-zero sin2 2θ13.

Oscillation Analysis with and without Solar Term Using Super-K-I and Super-K-II Atmospheric Neutrino data

As part of the European LAGUNA design study on a next-generation neutrino detector, we propose the liquid-scintillator detector LENA (Low Energy Neutrino Astronomy) as a multipurpose neutrino observatory. The outstanding successes of the Borexino and KamLAND experiments demonstrate the large potential of liquid-scintillator detectors in low-energy neutrino physics. Low energy threshold, good energy resolution and efficient background discrimination are inherent to the liquid-scintillator technique. A target mass of 50 kt will offer a substantial increase in detection sensitivity.At low energies, the variety of detection channels available in liquid scintillator will allow for an energy – and flavor-resolved analysis of the neutrino burst emitted by a galactic Supernova. Due to target mass and background conditions, LENA will also be sensitive to the faint signal of the Diffuse Supernova Neutrino Background. Solar metallicity, time-variation in the solar neutrino flux and deviations from MSW–LMA survival probabilities can be investigated based on unprecedented statistics. Low background conditions allow to search for dark matter by observing rare annihilation neutrinos. The large number of events expected for geoneutrinos will give valuable information on the abundances of Uranium and Thorium and their relative ratio in the Earth’s crust and mantle. Reactor neutrinos enable a high-precision measurement of solar mixing parameters. A strong radioactive or pion decay-at-rest neutrino source can be placed close to the detector to investigate neutrino oscillations for short distances and sub-MeV to MeV energies.At high energies, LENA will provide a new lifetime limit for the SUSY-favored proton decay mode into kaon and antineutrino, surpassing current experimental limits by about one order of magnitude. Recent studies have demonstrated that a reconstruction of momentum and energy of GeV particles is well feasible in liquid scintillator. Monte Carlo studies on the reconstruction of the complex event topologies found for neutrino interactions at multi-GeV energies have shown promising results. If this is confirmed, LENA might serve as far detector in a long-baseline neutrino oscillation experiment currently investigated in LAGUNA-LBNO.

Precision Neutrino Oscillation Physics with MINOS

MINOS is a long-baseline neutrino oscillation experiment utilising Fermilabʼs NuMI beam and two functionally identical steel/scintillator calorimeter detectors. Here we describe the short and long term physics goals of MINOS and we give a brief overview of the beam and detector technology used to achieve them.

The T2K experiment

The T2K experiment is a long-baseline neutrino oscillation experiment. Its main goal is to measure the last unknown lepton sector mixing angle {\theta}_{13} by observing {\nu}_e appearance in a {\nu}_{\mu} beam. It also aims to make a precision measurement of the known oscillation parameters, {\Delta}m^{2}_{23} and sin^{2} 2{\theta}_{23}, via {\nu}_{\mu} disappearance studies. Other goals of the experiment include various neutrino cross section measurements and sterile neutrino searches. The experiment uses an intense proton beam generated by the J-PARC accelerator in Tokai, Japan, and is composed of a neutrino beamline, a near detector complex (ND280), and a far detector (Super-Kamiokande) located 295 km away from J-PARC. This paper provides a comprehensive review of the instrumentation aspect of the T2K experiment and a summary of the vital information for each subsystem.

A new approach to anti-neutrino running in long-baseline neutrino oscillation experiments

We study the possibility to replace the anti-neutrino run of a long baseline neutrino oscillation experiment, with anti-neutrinos from muon decay at rest. The low energy of these neutrinos allows the use of inverse beta decay for detection in a Gadolinium-doped water Cerenkov detector. We show that this approach yields a factor of five times larger anti-neutrino event sample. The resulting discovery reaches in theta_13, the mass hierarchy and leptonic CP violation are compared with those from a conventional superbeam experiment with combined neutrino and anti-neutrino running. We find that this approach yields a greatly improved reach for CP violation and theta_13 while leaving the ability to measure the mass hierarchy intact.

Towards seeking θ13: Status and initial data from T2K

T2K is a long-baseline neutrino oscillation experiment with the primary aim of measuring the smallest and most elusive neutrino mixing angle, θ13. 2010 saw the beginning of the T2K data-taking phase. This paper gives an overview of the T2K experiment, its goals and current status, and will discuss some of the initial data. All parts of the experiment: beamline, near detectors and far detector, are functioning well. In the far detector, 23 events have been selected from the first T2K dataset. Particle identification in the far detector also is discussed in this paper. Detailed analysis has not yet been completed.

New MINOS Oscillation Results

The MINOS experiment (Main Injector Neutrino Oscillation Search) is a two detector long-baseline neutrino oscillation experiment. Oscillation parameters are determined by comparing the spectrum and composition of the neutrino beam measured at the far detector with that measured at the near detector. This paper summarises the four main beam neutrino oscillation results: muon neutrino and muon anti-neutrino disappearance, electron neutrino appearance and oscillations to sterile neutrinos, derived from the data collected during MINOS' first four years of operation.

A generic diagonalization of the [formula omitted] neutrino mass matrix and its implications on the [formula omitted] flavor symmetry and maximal CP violation

In the flavor basis where the mass eigenstates of three charged leptons are identified with their flavor eigenstates, one may diagonalize a 3×3 Majorana neutrino mass matrix Mν by means of the standard parametrization of the 3×3 neutrino mixing matrix V. In this treatment the unphysical phases of Mν have to be carefully factored out, unless a special phase convention for neutrino fields is chosen so as to simplify Mν to Mν′ without any unphysical phases. We choose this special flavor basis and establish some exact analytical relations between the matrix elements of Mν′Mν′† and seven physical parameters — three neutrino masses (m1, m2, m3), three flavor mixing angles (θ12, θ13, θ23) and the Dirac CP-violating phase (δ). Such results allow us to derive the conditions for the μ–τ flavor symmetry with θ23=π/4 and maximal CP violation with δ=±π/2, which should be useful for discussing specific neutrino mass models. In particular, we show that θ23=π/4 and δ=±π/2 keep unchanged when constant matter effects are taken into account for a long-baseline neutrino oscillation experiment.

The T2K Experiment

Over the last dozen years neutrino physics has crossed a threshold in its basic understanding of the fundamental nature of neutrinos. The first compelling demonstration that neutrinos have mass came when the Super-Kamiokande experiment showed a zenith-angle dependent suppression of muon neutrinos arising from cosmic ray interactions with the atmosphere. That had already been suspected, because initial indications of an anomaly in the flux of neutrinos from the sun from the pioneering experiments of Ray Davis and his collaborators and from the Kamiokande experiment, supported by measurements of the low-energy solar neutrino fluxes by the SAGE and GALLEX experiments, were hard to explain without invoking new neutrino physics. The SNO experiment then demonstrated that the shortfall of solar neutrino was caused by neutrino coming from the sun changing flavor, further pointing to the existence of neutrino oscillations. This was confirmed by the observation (or rather non-observation!) of the disappearance of reactor neutrinos by the KamLAND experiment and of accelerator neutrinos by the K2K long-baseline neutrino oscillation experiment (subsequently confirmed by the MINOS experiment). The combination of all these experimental results clearly demonstrates the existence of neutrino oscillations (the first confirmed physics beyond the Standard Model of particle physics).

Adsorption Behavior of Radionuclides on Ion-Exchange Resin from Cooling Water for the K2K Target and Magnetic Horns

The beam power in a new project of a long-baseline neutrino oscillation experiment from Tokai to Kamioka (the T2K experiment) will be approximately 100 times higher than that in a substantial long-baseline neutrino oscillation experiment from the High Energy Accelerator Research Organization (KEK) to Kamioka (the K2K experiment). In the T2K experiment, radionuclides at serious activity levels will be produced in a target, magnetic horns, and partially dissolved into the cooling water. Radionuclides at serious activity levels will also be produced in the cooling water. Therefore, we measured the total activity and distribution of 7Be, 22Na, 54Mn, 57Co, 60Co, 88Y, 101gRh, 102mRh, and 110mAg collected on a demineralizer in the K2K water cooling system, in order to consider reducing both exposure to personnel from the demineralizer and radioactivity released by draining the cooling water at regular intervals in the T2K experiment. The total activity of the individual nuclides was estimated to range from 0.9 MBq to 0.7 GBq at the end of the 2-yr K2K operation. When the results are projected to the T2K experiment, 70 GBq of 7Be and 6 GBq of 22Na are particularly high, and shielding from the radiation must be provided for the entire water system. Half of the demineralizer was saturated with the Al, Cu, and Ag ions dissolved from the target and magnetic horns. When the entire column is saturated with the ions, all high activity of 22Na located at the bottom of the demineralizer will be released into the cooling water immediately. Although 88Y, 101gRh, and 102mRh cannot be collected completely because of weak retention by hydrolysis and/or association with colloid, a reduction in the surface area of the Ag metal is possible for new magnetic horns and will result in a decrease of the activities in the cooling water.

Relating the neutrino mixing angles to a lepton mass hierarchy

We propose two phenomenological scenarios of lepton mass matrices and show that either of them can exactly give rise to tan2θ13=me/(me+2mμ), tan2θ23=mμ/(me+mμ) and tan2θ12=(mem2+2mμm1)/(mem1+2mμm2) in the standard parametrization of lepton flavor mixing. The third relation, together with current experimental data, predicts a normal but weak hierarchy for the neutrino mass spectrum. We also obtain θ13≈2.8° for the smallest neutrino mixing angle and J≈1.1% for the Jarlskog invariant of leptonic CP violation, which will soon be tested in the long-baseline reactor and accelerator neutrino oscillation experiments. A seesaw realization of both scenarios is briefly discussed.

Long-Baseline Neutrino Oscillation Experiments in North America

This contribution to the proceedings of the 2008 NOW Workshop summarizes current and future long-baseline neutrino oscillation experiments in the United States.

Future possibilities with Fermilab neutrino beams

We will start with a brief overview of neutrino oscillation physics with emphasis on the remaining unanswered questions. Next, after mentioning near future reactor and accelerator experiments searching for a non zero θ13, we will introduce the plans for the next generation of long-baseline accelerator neutrino oscillation experiments. We will focus on experiments utilizing powerful (0.7 − 2.1 MW) Fermilab neutrino beams, either existing or in the design phase.

TeV-SCALE TYPE-II SEESAW MODELS AND POSSIBLE COLLIDER SIGNATURES

A natural extension of the standard model to accommodate massive neutrinos is to introduce one Higgs triplet and three right-handed Majorana neutrinos, leading to a 6×6 neutrino mass matrix. We show that three light Majorana neutrinos (i.e., the mass eigenstates of νe, νμ and ντ) are exactly massless, if and only if [Formula: see text] exactly holds in this seesaw model. We propose three simple Type-II seesaw scenarios with broken A4 × U(1)x flavor symmetry to interpret the observed neutrino mass spectrum and neutrino mixing pattern. Such a TeV-scale neutrino model can be tested in two complementary ways: (1) searching for possible collider signatures of lepton number violation induced by the right-handed Majorana neutrinos and doubly-charged Higgs particles; and (2) searching for possible consequences of unitarity violation of the 3 × 3 neutrino mixing matrix in the future long-baseline neutrino oscillation experiments.

Compromise between neutrino masses and collider signatures in the type-II seesaw model

A natural extension of the standard $\mathrm{SU}(2{)}_{\mathrm{L}}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1{)}_{\mathrm{Y}}$ gauge model to accommodate massive neutrinos is to introduce one Higgs triplet and three right-handed Majorana neutrinos, leading to a $6\ifmmode\times\else\texttimes\fi{}6$ neutrino mass matrix which contains three $3\ifmmode\times\else\texttimes\fi{}3$ submatrices, ${M}_{\mathrm{L}}$, ${M}_{\mathrm{D}}$ and ${M}_{\mathrm{R}}$. We show that three light Majorana neutrinos (i.e., the mass eigenstates of ${\ensuremath{\nu}}_{e}$, ${\ensuremath{\nu}}_{\ensuremath{\mu}}$, and ${\ensuremath{\nu}}_{\ensuremath{\tau}}$) are exactly massless in this model, if and only if ${M}_{\mathrm{L}}={M}_{\mathrm{D}}{M}_{\mathrm{R}}^{\ensuremath{-}1}{M}_{\mathrm{D}}^{T}$ exactly holds. This no-go theorem implies that small but nonvanishing neutrino masses may result from a significant but incomplete cancellation between ${M}_{\mathrm{L}}$ and ${M}_{\mathrm{D}}{M}_{\mathrm{R}}^{\ensuremath{-}1}{M}_{\mathrm{D}}^{T}$ terms in the Type-II seesaw formula, provided three right-handed Majorana neutrinos are of $\mathcal{O}(1)\text{ }\text{ }\mathrm{TeV}$ and experimentally detectable at the LHC. We propose three simple Type-II seesaw scenarios with the ${A}_{4}\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1{)}_{\mathrm{X}}$ flavor symmetry and its explicit breaking to interpret the observed neutrino mass spectrum and neutrino mixing pattern. Such a TeV-scale neutrino model can be tested in two complementary ways: (1) searching for possible collider signatures of lepton number violation induced by the right-handed Majorana neutrinos and doubly-charged Higgs particles; and (2) searching for possible consequences of unitarity violation of the $3\ifmmode\times\else\texttimes\fi{}3$ neutrino mixing matrix in the future long-baseline neutrino oscillation experiments.