University of Minnesota Physics Jon Urheim

May 2007

Abstract:

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 or events. The sample consists of 89 observed events, while the 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 and events in the absence of neutrino oscillations are 88.0 $\pm$ 1.0 and 149.1 $\pm$ 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:

$${R^{data}_{trk/shw}} / {R^{MC}_{trk/shw}} = 0.74 \; ^{+0.12}_{-0.10} (stat.) \; \pm 0.04 (syst.)$$

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 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:

$$S_{atm} = 1.08 \pm 0.12(stat.) \pm 0.08(syst.)$$

This is larger than, but consistent with, a measurement at the same location by the Soudan2 Experiment [3] of $S_{atm} = 0.91 \pm 0.07$.

331 There are many people that have earned my gratitude for their contribution to my time in graduate school. To my family, many thanks to my parents and siblings for enduring my childhood and your continuing support over the years. To my incoming classmates, thanks to Chris Stepaniak, Dave Engebretson, and many other for those wonderful first years.

Thanks to all of the physics and astronomy professors at the University of Minnesota, especially to Roger Jones, Earl Peterson, and Benjamin Bayman and Ken Heller whose masterful teaching inspired and cultivated my love of physics. Also thanks the Peter Litchfield and Keith Ruddick for their significant editing contribution to this thesis.

Thanks all of the personnel at the Soudan Underground Lab for their amazing job in building and maintaining the detector. Bill Miller, Jerry Meier, and many others are owed an immense debt of gratitude from myself and the entire MINOS collaboration.

Thanks to my office mates and fellow MINOS graduate students Jeremy, Dipu, Sujeewa, Erik, Eric, Emily, and Bernie for lightening (and in some cases occupying a significant fraction of) my time in graduate school. Thanks to the Minnesota MINOS postdocs Hugh, Jeff, Leon, Sue, David, and Kregg for helping to understand everything about our experiment.

Thanks to the Cambridge group for their hospitality and for everything they have done with the atmospheric neutrino analysis, especially Pat Ward who laid the foundation for showering neutrino analysis. I can't imagine ever having graduated without the unbelievable efforts of John Chapman, Andy Blake, Caius Howcroft, Pat Ward, and Mark Thomson.

Thanks to my loving wife Siri and her wonderfully supportive family. Thank you Siri for enduring the countless times that I had just "one more year left" of graduate school, for help editing this thesis. Thanks are also due to Brad, Dot, and Hamilton for maintaining my sanity.

Finally thanks to my advisor Jon Urheim for his continuing support through my overly long and storied graduate career. I have no doubt that one day there will be songs written about his herculean patience. To my caring wife, and our dogs.

Tiger got to hunt, Bird got to fly;

Man got to sit and wonder, "Why, why, why?"

Tiger got to sleep, Bird got to land;

Man got to tell himself he understand

Kurt Vonnegut - Cat's Cradle