A study of atmospheric neutrino oscillations in the MINOS far detector

Abstract

In recent years, neutrino experiments have begun to challenge the Standard Model assumption that neutrinos are massless. There is now firm evidence that neutrinos undergo quantum mechanical oscillations between flavors. This would imply that neutrinos possess mass and that neutrino flavors are mixed by the weak interaction. Atmospheric neutrinos, produced by the interactions of cosmic rays in the earth's atmosphere, can be used to study these oscillations. The MINOS Far Detector has been collecting atmospheric neutrino data since 1st August 2003 using a 5.4 kT steel-scintillator sampling calorimeter located 700 m underground (2100 m water-equivalent) at the Soudan Underground Laboratory, Minnesota. The Far Detector is the first massive underground detector to possess a magnetic field. This makes the separation of atmospheric {nu}{sub {mu}} and {bar {nu}}{sub {mu}} charged current interactions possible for the first time. This thesis presents a study of atmospheric neutrino oscillations in the Far Detector, based on a total detector exposure of 316 days (3.3 kT-Yrs fiducial exposure). The separation of atmospheric neutrinos from the high background of cosmic muons is outlined. A total of 82 candidate events are observed, with an expectation of 109.9 {+-} 21.4 events in the absence of oscillations. Of the selected events,more » 40 events have a clearly identified charge, with 27 events tagged as neutrinos and 13 events tagged at anti-neutrinos. This represents the first direct observation of atmospheric {nu}{mu} and {bar {nu}}{sub {mu}} charged current interactions. A maximum likelihood analysis us used to determine the allowed region for the oscillation parameters {Delta}m{sub 23}{sup 2} and sin{sup 2}2{theta}{sub 23}. This disfavors the null oscillation hypothesis at the 79% confidence level. With the current low statistics, the sensitivity of the analysis is limited. The expected future sensitivity of the atmospheric neutrino analysis is discussed.« less