Evidence for the Appearance of Atmospheric Tau Neutrinos in Super-Kamiokande

Abstract

Super-Kamiokande atmospheric neutrino data were fit with an un-binned maximum likelihood method to search for the appearance of tau leptons resulting from the interactions of oscillation-generated tau neutrinos in the detector. Relative to the expectation of unity, the tau normalization is found to be 1. 42 ± 0.35 (stat) +0.14 −0.12 (sys) excluding the no-tau-appearance hypothesis, for which the normalization would be zero, at the 3.8σ level. We estimate that 180.1 ± 44.3 (stat) +17.8 −15.2 (sys) tau leptons were produced in the 22.5 kton fiducial volume of the detector by tau neutrinos during the 2806 day running period. In future analyses, this large sample of selected tau events will allow the study of charged current tau neutrino interaction physics with oscillation produced tau neutrinos. RENO [16] experiments reported the first precision measurements of the θ 13 mixing angle which drives three-flavor oscillation.Definitive proof of flavor oscillation requires unambiguous appearance of the charged current interaction of a neutrino not in the original source. In the dominant oscillation for ν µ at GeV energies, ν µ → ν τ oscillations, observing the resulting τ lepton is quite difficult. This is because producing a tau lepton requires a neutrino of energy greater than a threshold of 3.5 GeV. Long-baseline experiments tuned to the neutrino oscillation maximum for their distances tend to have the bulk of their neutrinos below this energy. Furthermore, the tau lepton immediately decays to final states with an electron, muon or mesons plus a tau neutrino so the tau lepton itself cannot be easily seen. Nevertheless, the OPERA collaboration was recently able to show evidence for a single reconstructed event in their emulsion consistent with tau appearance [17]. The Super-K collaboration first published a search for tau appearance in atmospheric neutrinos in 2006 [18]. Since the atmospheric neutrino flux extends to energies well above 10 GeV, and spans a wide range of baselines, we expect to see tau leptons produced in the Super-K detector. However, these events must be distinguished from other high-energy atmospheric neutrino interactions. Further comparisons of these techniques can be found in [19] and prospects for future detectors can be found in [20].This letter reports a result from a new search utilizing the Super-Kamiokande experiment. This analysis addresses the question of whether the atmospheric data is consistent with lack of oscillation-generated ν τ , or whether they are necessary to explain the observations. Super-Kamiokande (Super-K) is a 50,000 ton water Cherenkov detector[21] with 22.5 ktons of fiducial volume. It consists of two concentric detectors: a inner-detector with 11,129 inward-looking 20 inch photodetectors, and an outer-detector with 1885 outward-facing 8 inch photo-detectors which acts as a veto. Its large target mass makes it well suited to look for the rare appearance of tau neutrinos from oscillations. The typical energy of atmospheric neutrinos is about 1 GeV. Due to the previ...

Atmospheric Neutrino Results from Super-Kamiokande

“…Super-K performed a search for charged-current tau neutrino events with a neural network algorithm. Details of the neural network are described in [5]. The analysis performed a 2D unbinned maximum likelihood fit of data against the PDFs built from MC,…”

Research in High Energy Physics at Duke University

“…Approximately 1.4 times the expected number of events were seen and the no-oscillation hypothesis was excluded at the 3.8σ level. This result has now been published in PRL [9]. With the new reduction in systematic errors, Walter also began working to further explore precision three-flavor oscillation physics using this sample.…”

Intensity Frontier Instrumentation

“…Direct detection of tau neutrinos by DONUT (17) and OPERA (18) used nuclear emulsions to provide the necessary tracking resolution. The Super-K detector was also able to provide evidence of ν τ appearance in a statistical manner (19). Further advances in direct detection remain challenging, although liquid argon detectors hold promise.…”