Atmospheric electron neutrinos in the MINOS far detector.

摘要

Neutrinos produced as a result of cosmic-ray interactions in the earth's atmosphere offer a powerful probe into the nature of this three-membered family of low-mass, weakly-interacting particles. Ten years ago, the Super-Kamiokande Experiment has confirmed [1] earlier indications that neutrinos undergo lepton-flavor oscillations during propagation, proving that they are massive contrary to the previous Standard Model assumptions. The Soudan Underground Laboratory, located in northern Minnesota, was host to the Soudan2 Experiment, which has made important contributions to atmospheric neutrino research. This same lab has more recently been host to the MINOS far detector, a neutrino detector which serves as the downstream element of an accelerator-based long-baseline neutrino-oscillation experiment.; This thesis has examined 418.5 live days of atmospheric neutrino data (fiducial exposure of 4.18 kton-years) collected in the MINOS far detector prior to the activation of the NuMI neutrino beam, with a specific emphasis on the investigation of electron-type neutrino interactions. Atmospheric neutrino interaction candidates have been selected and separated into showering or track-like events. The showering sample consists of 89 observed events, while the track-like sample consists of 112 observed events. Based on the Bartol atmospheric neutrino flux model of Barr et al. [2] plus a Monte Carlo (MC) simulation of interactions in the MINOS detector, the expected yields of showering and track-like events in the absence of neutrino oscillations are 88.0 +/- 1.0 and 149.1 +/- 1.0 respectively (where the uncertainties reflect only the limited MC statistics). Major systematic uncertainties, especially those associated with the flux model, are cancelled by forming a double ratio of these observed and expected yields: Rdatatrk/shw/RMCtrk /shw=0.74+0.12-0.10 stat.+/-0.04syst. This double ratio should be equal to unity in the absence of oscillations, and the value above disfavors null oscillation with 96.0% confidence. In addition, the showering sample can be used to measure the atmospheric neutrino flux. Based on the analysis presented in this thesis, the Bartol flux should be scaled by a factor of: Satm=1.08+/-0.12stat. +/-0.08syst. This is larger than, but consistent with, a measurement at the same location by the Soudan2 Experiment [3] of Satm = 0:91 +/- 0:07.