Evidence forν¯μ→ν¯eOscillation

Abstract

A search forνµ →νe oscillations has been conducted at the Los Alamos Meson Physics Facility by usingνµ from µ + decay at rest. Theνe are detected via the reactionνe p → e + n, correlated with a γ from np → dγ (2.2 MeV). The use of tight cuts to identify e + events with correlated γ rays yields 22 events with e + energy between 36 and 60 MeV and only 4.6 ± 0.6 background events. A fit to the e + events between 20 and 60 MeV yields a total excess of 51.8 +18.7 −16.9 ± 8.0 events. If attributed toνµ →νe oscillations, this corresponds to an oscillation probability of (0.31 +0.11 −0.10 ± 0.05)%. 14.60. Pq, 13.15.+g We present the results from a search for neutrino oscillations using the Liquid Scintillator Neutrino Detector (LSND) apparatus described in reference [1]. The existence of neutrino oscillations would imply that neutrinos have mass and that there is mixing among the different flavors of neutrinos. Candidate events in a search for the transformationν µ →ν e from neutrino oscillations with the LSND detector have previously been reported [2] for data taken in 1993 and 1994. Data taken in 1995 have been included in this paper, and the analysis has been made more efficient.Protons are accelerated by the LAMPF linac to 800 MeV kinetic energy and pass through a series of targets, culminating with the A6 beam stop. The primary neutrino flux comes from π + produced in a 30-cm-long water target in the A6 beam stop [1]. The total charge delivered to the beam stop while the detector recorded data was 1787 C in 1993, 5904 C in 1994, and 7081 C in 1995. Most of the π + come to rest and decay through the sequence π + → µ + ν µ , followed by µ + → e + ν eνµ , supplyingν µ with a maximum energy of 52.8 MeV. The energy dependence of theν µ flux from decay at rest (DAR) is very well known, and the absolute value is known to 7% [1,3]. The open space around the target is short compared to the pion decay length, so only 3% of the π + decay in flight (DIF). A much smaller fraction (approximately 0.001%) of the muons DIF, due to the difference in lifetimes and that a π + must first DIF. The totalν µ flux averaged over the detector volume, including contributions from upstream targets and all elements of the beam stop, was 7.6 × 10 −10ν µ /cm 2 /proton. Aν e component in the beam comes from the symmetrical decay chain starting with a π − . This background is suppressed by three factors in this experiment. First, π + production is about eight times the π − production in the beam stop. Second, 95% of π − will come to rest and are absorbed before decay in the beam stop. Third, 88% of µ − from π − DIF are captured from atomic orbit, a process which does not give aν e . Thus, the relative yield, compared to the positive channel, is estimated to be ∼ (1/8) × 0.05 × 0.12 = 7.5 × 10 −4 . A detailed Monte Carlo simulation [3], gives a value of 7.8 × 10 −4 for the flux ratio ofν e toν µ .The detector is a tank filled with 167 metric tons of dilute liquid scintillator, located about 30 m from the neutrino source, and surrounded on all s...

On CPT Symmetry: Cosmological, Quantum-Gravitational and Other Possible Violations and Their Phenomenology

“…Such modifications, as we discussed in previous sections, may lead to CPTV and LIV effects of order g s E 2 /M s , where E is a typical neutrino energy at the frame of observation. Such terms seem to be much smaller than the suggested neutrino-antineutrino squared mass difference of O(0.1 − 1 eV 2 ) to explain the LSND anomaly [47,50].…”

“…If one associates oscillations with Dirac mass terms for neutrinos, then the LSND result [47], if correct, points towards CPT violation in the sense of an antineutrino Dirac mass being much higher than a neutrino one, the mass difference between neutrino and antineutrino being of the order of 1 eV. This idea was put forward first in [50] in a purely phenomenological setting (c.f.…”

“…The reason why we chose to discuss this topic in a separate section is the relatively recent claims from the LSND experiment [47] on evidence for observation of oscillations in the antineutrino sector ν e ←→ ν µ , but the absence of such oscillation in the corresponding neutrino sector 6 . The situation will be clarified experimentally, when experiments looking directly for ν µ → ν e , will be in operation, like MiniBoonNE [48] 7 .…”

Detecting the (quasi-) two-body decays of leptons in short-baseline neutrino oscillation experiments

“…A large deficit of muon neutrinos, as observed in atmospheric showers [2], suggests that they transform to another flavor with ∆m 2 on the order of 10 −2 -10 −3 eV 2 and with an effective mixing, defined as sin 2 (2θ) = 4U αi U βi U αj U βj , between 0.5 and 1.0. On the other hand, an accelerator experiment using neutrinos from π + and µ + decays at rest [3] has presented evidence for theν µ →ν e transition with a much larger mass difference of ∆m 2 ∼ 1 eV 2 and with a much smaller effective mixing of sin 2 (2θ) ∼ 6 × 10 −3 .…”

“…Therefore, if the bigger ∆m 2 ij is on an order of 1 eV 2 as suggested by the data of LSND [3], one should look for the corresponding ν µ → ν τ transition in the short-baseline (L ∼ 1 km) and medium-baseline (L ∼ 10 − 20 km) experiments at accelerators. Quite importantly, the accelerators like SPS at CERN and Main Injector at Fermilab offer intense and almost pure beams of muon neutrinos with mean energies well above the threshold for τ production, as required by experiments with ν τ appearance.…”