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Soudan 2 atmospheric neutrino oscillation studies

A 5.1 fiducial-kiloton year exposure of the Soudan 2 detector has been analyzed for evidence of neutrino oscillations. The flavor-ratio is R = (νµ/νe) data (νµ/νe)MC = 0.68 ± 0.11(stat) ± 0.06(syst.). A number of data sets, with different sensitivities to neutrino oscillations, are presented and their use in the analysis described. Under the hypothesis of ν µ → ν τ oscillations the data likelihoods are determined and fits to oscillation parameters are carried out using the Feldman-Cousins method. The Soudan 2 detector is a 963 ton fine-grained tracking calorimeter located at a depth of 2100 meters-water-equivalent in the Soudan Mine State Park in Soudan, Minnesota. The data analyzed for this report comprise 5.1 fiducial-kiloton years of the total detector exposure. The detector itself is a modular, slow drift calorimeter. The drift elements are 1 m long hytrel tubes, embedded in a stack of thin corrugated steel plates which form a honeycomb-like structure. The entire detector is made of 224 1m x 1m x 2.7m modules which weigh 4.3 tons, and is surrounded by a nearly hermetic veto shield of proportional tubes. Events are classified in Soudan 2 on the basis of their topology, which is determined through a physicist scan. Events are placed in one of three categories: tracks, showers, or multiprongs, which are meant to encompass quasi-elastic ν µ , quasi-elastic ν e , and inelastic events, respectively. All tracks and showers in each event are identified by hand using a graphical display program which interfaces to the experiment reconstruction code. The flavor assignment for multiprong events is based on a leading prong algorithm: an event with a leading, non-scattering track with ionization consistent with a muon is a candidate ν µ charged current (CC) event, an event having a prompt energetic shower is a candidate ν e CC event.