A study of muon neutrino to electron neutrino oscillations in the MINOS experiment

Thesis/Dissertation · Sun Mar 01 00:00:00 EST 2009

DOI:https://doi.org/10.2172/950512· OSTI ID:950512

Yang, Tingjun ^[1]

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1. Stanford Univ., CA (United States)

The observation of neutrino oscillations (neutrino changing from one flavor to another) has provided compelling evidence that the neutrinos have non-zero masses and that leptons mix, which is not part of the original Standard Model of particle physics. The theoretical framework that describes neutrino oscillation involves two mass scales (Δm_atm² and Δm_sol²), three mixing angles (θ₁₂, θ₂₃, and θ₁₃) and one CP violating phase (δ_CP). Both mass scales and two of the mixing angles (θ₁₂ and θ₂₃) have been measured by many neutrino experiments. The mixing angle θ₁₃, which is believed to be very small, remains unknown. The current best limit on θ13 comes from the CHOOZ experiment: θ₁₃ < 11° at 90% C.L. at the atmospheric mass scale. δ_CP is also unknown today. MINOS, the Main Injector Neutrino Oscillation Search, is a long baseline neutrino experiment based at Fermi National Accelerator Laboratory. The experiment uses a muon neutrino beam, which is measured 1 km downstream from its origin in the Near Detector at Fermilab and then 735 km later in the Far Detector at the Soudan mine. By comparing these two measurements, MINOS can obtain parameters in the atmospheric sector of neutrino oscillations. MINOS has published results on the precise measurement of Δm_atm² and θ₂₃ through the disappearance of muon neutrinos in the Far Detector and on a search for sterile neutrinos by looking for a deficit in the number of neutral current interactions seen in the Far Detector. MINOS also has the potential to improve the limit on the neutrino mixing angle θ₁₃ or make the first measurement of its value by searching for an electron neutrino appearance signal in the Far Detector. This is the focus of the study presented in this thesis. We developed a neural network based algorithm to distinguish the electron neutrino signal from background. The most important part of this measurement is the background estimation, which is done through extrapolation. The number of background events is measured at the Near Detector, then extrapolated to the Far Detector. Since different background sources extrapolate differently, some knowledge about the relative contribution from different background sources is necessary. We developed a method that can be used to obtain relative contributions of various background sources from comparison of background rates in the horn-on and horn-off configurations. We also described our effort to improve two aspects of the Monte Carlo simulation which are very important for the v_e appearance analysis: one is the hadronization model in the neutrino-nucleon interactions, the other is the modeling of PMT crosstalk. We performed a blind analysis and examined several sidebands before looking at the signal region. After we opened the box, we observed a 1.4 σ excess of v_e-like events in the Far Detector compared with the number of predicted background events. The excess is well within the statistical fluctuation of the background events. If we interpret the excess as a v_e signal from v_μ → v_e oscillation, the best fit sin² 2θ₁₃ value is consistent with the CHOOZ limit. However we want to emphasize that our result is consistent with θ₁₃ = 0 at 90% C.L..

• Research Organization: Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-07CH11359
• OSTI ID: 950512
• Report Number(s): FERMILAB-THESIS-2009-04; TRN: US0902005
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
ALGORITHMS
ELECTRON NEUTRINOS
EXTRAPOLATION
FERMILAB
FERMILAB ACCELERATOR
FLUCTUATIONS
LEPTONS
MUON NEUTRINOS
NEURAL NETWORKS
NEUTRAL-CURRENT INTERACTIONS
NEUTRINO OSCILLATION
NEUTRINO-NUCLEON INTERACTIONS
NEUTRINOS
ORIGIN
OSCILLATIONS
PHYSICS
STANDARD MODEL
Experiment-HEP