A search for sterile neutrinos at the MINOS experiment

Abstract

MINOS is a long baseline neutrino oscillation experiment based at the Fermi National Accelerator Laboratory in Illinois, USA. The experiment was designed to study neutrino oscillation phenomena. The ν µ beam produced by the NuMI beam facility at FNAL is used along with two functionally identical detectors. The Near Detector at FNAL and a Far Detector 735 km away in the Soudan Underground Laboratory in northern Minnesota. Comparison of the observed spectra of neutrinos at the two detectors provides the evidence for neutrino oscillations.This thesis presents work on the postulated phenomena of sterile neutrinos. Oscillations between active and sterile neutrinos will lead to a deficit in the expected rate of measured Neutral Current interactions at the Far Detector. A technique for selecting Neutral Current events utilising an Artificial Neural Network is presented with resulting overall efficiency of 91.1% and purity of 66.0%. A method of predicting the expected Charged and Neutral Current energy spectra at the Far Detector given the data recorded at the Near Detector is presented. A model to search for oscillations between sterile and active neutrinos is developed. Sources of systematic uncertainty that can effect the results of the analysis are discussed.The analysis developed is applied to a Standard Model 3 flavour oscillation model as a cross check under the scenarios with and without ν e appearance. The oscillation parameters measured by this model are ∆m −0.11 for the no ν e appearance result. An analysis of the resulting prediction reveals no evidence for active neutrino disappearance.The analysis is then performed using the 4 flavour neutrino oscillation model developed. Again this is done under the 2 scenarios of ν e appearance and no ν e appearance. The results of this analysis are ∆m −0.19 and θ 34 = 0.00 +0.60 for ν e appearance. This is consistent with no oscillations between active and sterile neutrinos.

A Sterile-Neutrino Search with the MINOS Experiment

“…The analysis presented in this thesis uses the event separation method described in [106] to provide separation between NC and CC events. The method uses a set of cuts to preclassify a subset of events as NC or CC, and passes the remaining events to an artificial neural network (ANN) trained on MINOS Monte Carlo, which gives an output value which tends towards 1 for events that are CC-like, and towards 0 for events which are NC-like.…”

“…Although an NC/CC separation cut is not used per se in this thesis, it is sometimes necessary to define subsets of events that are NC or CC enriched. To this end, the cut value of 0.675 from [106] is used. This value was obtained by maximizing the figure of merit εp/(2 − p).…”

“…Previous analyses of the MINOS neutral current data [109,106] have used two spectra, one selected to be mostly neutral current, and one mostly charged current, to fit neutrino oscillation parameters. The NC selected spectrum provides sensitivity to sterile neutrino oscillation parameters, while the CC spectrum constrains ∆m 2 and f µ .…”

Sterile neutrino oscillations in MINOS and hadron production in pC collisions

“…Any change in any of these three components correspond to a change in the detector response. The electronics is known to be fairly stable (studied in [105] at the Far Detector) but it is affected by hardware swaps. When a hardware swap is made, the electronics is recalibrated to minimally change the overall detector response.…”

“…Given the MINOS detector characteristics, there is not much shape information on the hadronic shower development that can be exploited to select neutral current events. Alternative methods to the one presented in this thesis have been studied [142,141,143,144]: the most powerful variables to select neutral current events are the ones which try to identify and reject the muon tracks. The inelasticity y is defined as the fraction of the neutrino energy E ν that is converted into hadronic energy E hadr :…”