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Persistent URL http://purl.org/net/epubs/work/29676
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Record Id 29676
Title A Study of the Atmospheric Neutrino Flavour Content using the Soudan 2 Detector
Abstract The flavour content [nu(mu)/nu(e)] of neutrinos generated in the upper atmosphere by cosmic ray interactions is studied. This is a quantity of interest because it is sensitive to the open question of neutrino mass, which, at the present time, is assumed to be zero in the Standard Model of particle physics. Other experiments have reported measurements on the flavour content of atmospheric neutrinos that are inconsistent with the Standard Model. The data presented in this thesis has been collected with the Soudan 2 detector and is analysed by means of a new automated technique. The fiducial exposure of the data-period analysed is 2.68 kTyears. The event selection is conducted with a series of software cuts optimised to the rejection of cosmic-ray muon induced events and the selection of atmospheric neutrino interactions. The acceptance is 22.3% for neutrinos interacting in the Soudan 2 detector, as determined by a Monte Carlo study. The physics analysis of the data is conducted on the complete sample of contained events using estimators sensitive to flavour and neutrino L/E(nu) phase. Particular attention is given to the question of background contamination of the data. The numbers of nu(e)-like and nu(mu)-like data events after applying a background cut are 111.5+/-15.7 and 74.9+/-13.5 respectively. A very large statistics Monte Carlo simulation of the experiment predicted 137.6 nu(e)-like and 150.0 nu(mu)-like events when subjected to identical processing. The flavour Ratio of Ratios, [nu(mu)/nu(e)](data)/[nu(mu)/nu(e)](expectation) is found to be 0.62+/-0.14(stat)+/-0.05(syst), where consistency with unity would support Standard Model Physics. It is demonstrated that the anomaly is not due to background contamination of the data sample. An alternative multi-variable analysis that fits the data to a sum of MC and background (using a log-likelihood approach) concludes that the probability that the data is in agreement with Standard Model Physics is 0.5%. The data is fitted to three models of neutrino mixing, the 2-generation nu(mu)<-->nu(tau) and nu(mu)<-->nu(e) and 3-generation maximal neutrino mixing, with different masses and appropriate parameters. The best fit probabilities for the three mixing modes are 30.4%, 38.1% and 18.2% respectively. For the 2-generation mixing modes (Delta)m(2)>5*10(-3)eV(2) for sin(2)2theta~1 is allowed at a 90% confidence level. Similarly, (Delta)m(2)>5*10(-3)eV(2) at 90% c.l. for the 3-generation hypothesis, which allows the HPS solution.
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Language English (EN)
Type Details URI(s) Local file(s) Year
Thesis RAL Theses RAL-TH-1998-006. 1998. 1998