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Title: Atmospheric electron neutrinos in the MINOS far detector

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 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. 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: R$$data\atop{trk/shw}$$/R$$MC\atop{trk/shw}$$ = 0.74$$+0.12\atop{-1.0}$$(stat.) ± 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 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: S atm = 1.08 ± 0.12(stat.) ± 0.08(syst.) This is larger than, but consistent with, a measurement at the same location by the Soudan2 Experiment of S atm = 0.91 ± 0.07.

Authors:
 [1]
  1. Univ. of Minnesota, Minneapolis, MN (United States)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
919094
Report Number(s):
FERMILAB-THESIS-2007-26
TRN: US0806310
DOE Contract Number:
AC02-07CH11359
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ELECTRON NEUTRINOS; MINNESOTA; NEUTRINO BEAMS; NEUTRINO OSCILLATION; PROBES; SIMULATION; STANDARD MODEL; STATISTICS; Experiment-HEP

Citation Formats

Speakman, Benjamin Phillip. Atmospheric electron neutrinos in the MINOS far detector. United States: N. p., 2007. Web. doi:10.2172/919094.
Speakman, Benjamin Phillip. Atmospheric electron neutrinos in the MINOS far detector. United States. doi:10.2172/919094.
Speakman, Benjamin Phillip. Mon . "Atmospheric electron neutrinos in the MINOS far detector". United States. doi:10.2172/919094. https://www.osti.gov/servlets/purl/919094.
@article{osti_919094,
title = {Atmospheric electron neutrinos in the MINOS far detector},
author = {Speakman, Benjamin Phillip},
abstractNote = {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 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. 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: R$data\atop{trk/shw}$/R$MC\atop{trk/shw}$ = 0.74$+0.12\atop{-1.0}$(stat.) ± 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 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 ± 0.12(stat.) ± 0.08(syst.) This is larger than, but consistent with, a measurement at the same location by the Soudan2 Experiment of Satm = 0.91 ± 0.07.},
doi = {10.2172/919094},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}

Thesis/Dissertation:
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  • The phenomenon of flavour oscillations of neutrinos created in the atmosphere was first reported by the Super-Kamiokande collaboration in 1998 and since then has been confirmed by Soudan 2 and MACRO. The MINOS Far Detector is the first magnetized neutrino detector able to study atmospheric neutrino oscillations. Although it was designed to detect neutrinos from the NuMI beam, it provides a unique opportunity to measure the oscillation parameters for neutrinos and anti-neutrinos independently. The MINOS Far Detector was completed in August 2003 and since then has collected 2.52 kton-years of atmospheric data. Atmospheric neutrino interactions contained within the volume of the detector are separated from the dominant background from cosmic ray muons. Thirty seven events are selected with an estimated background contamination of less than 10%. Using the detector's magnetic field, 17 neutrino events and 6 anti-neutrino events are identified, 14 events have ambiguous charge. The neutrino oscillation parameters for v μ andmore » $$\bar{v}$$ μ are studied using a maximum likelihood analysis. The measurement does not place constraining limits on the neutrino oscillation parameters due to the limited statistics of the data set analysed. However, this thesis represents the first observation of charge separated atmospheric neutrino interactions. It also details the techniques developed to perform atmospheric neutrino analyses in the MINOS Far Detector.« less
  • The MINOS Far Detector is a 5400 ton iron calorimeter located at the Soudan state park in Soudan Minnesota. The MINOS far detector can observe atmospheric neutrinos and separate charge current ν μ andmore » $$\bar{v}$$ μ interactions by using a 1.4 T magnetic field to identify the charge of the produced muon. The CPT theorem requires that neutrinos and anti-neutrinos oscillate in the same way. In a fiducial exposure of 5.0 kilo-ton years a total of 41 candidate neutrino events are observed with an expectation of 53.1 ± 7.6(system.) ± 7.2(stat.) unoscillated events or 31.6 ± 4.7(system.) ± 5.6(stat.) events with Δm 2 = 2.4 x 10 -3 eV 2, sin 2(2θ) = 1.0 as oscillation parameters. These include 28 events which can have there charge identified with high confidence. These 28 events consist of 18 events consistent with being produced by ν μ and 10 events being consistent with being produced by $$\bar{v}$$ μ. No evidence of CPT violation is observed.« less
  • In recent years, neutrino experiments have begun to challenge the Standard Model assumption that neutrinos are massless. There is now firm evidence that neutrinos undergo quantum mechanical oscillations between flavors. This would imply that neutrinos possess mass and that neutrino flavors are mixed by the weak interaction. Atmospheric neutrinos, produced by the interactions of cosmic rays in the earth's atmosphere, can be used to study these oscillations. The MINOS Far Detector has been collecting atmospheric neutrino data since 1st August 2003 using a 5.4 kT steel-scintillator sampling calorimeter located 700 m underground (2100 m water-equivalent) at the Soudan Underground Laboratory, Minnesota. The Far Detector is the first massive underground detector to possess a magnetic field. This makes the separation of atmospheric v μ andmore » $$\bar{v}$$ μ charged current interactions possible for the first time. This thesis presents a study of atmospheric neutrino oscillations in the Far Detector, based on a total detector exposure of 316 days (3.3 kT-Yrs fiducial exposure). The separation of atmospheric neutrinos from the high background of cosmic muons is outlined. A total of 82 candidate events are observed, with an expectation of 109.9 ± 21.4 events in the absence of oscillations. Of the selected events, 40 events have a clearly identified charge, with 27 events tagged as neutrinos and 13 events tagged at anti-neutrinos. This represents the first direct observation of atmospheric v μ and $$\bar{v}$$ μ charged current interactions. A maximum likelihood analysis us used to determine the allowed region for the oscillation parameters Δm$$2\atop{23}$$ and sin 223. This disfavors the null oscillation hypothesis at the 79% confidence level. With the current low statistics, the sensitivity of the analysis is limited. The expected future sensitivity of the atmospheric neutrino analysis is discussed.« less
  • The Main Injector Neutrino Oscillation Search (MINOS) is a long baseline neutrino oscillation experiment. The MINOS Far Detector, located in the Soudan Underground Laboratory in Soudan MN, has been collecting data since August 2003. The scope of this dissertation involves identifying the atmospheric neutrino induced muons that are created by the neutrinos interacting with the rock surrounding the detector cavern, performing a neutrino oscillation search by measuring the oscillation parameter values of Δmmore » $$2\atop{23}$$ and sin 223, and searching for CPT violation by measuring the charge ratio for the atmospheric neutrino induced muons. A series of selection cuts are applied to the data set in order to extract the neutrino induced muons. As a result, a total of 148 candidate events are selected. The oscillation search is performed by measuring the low to high muon momentum ratio in the data sample and comparing it to the same ratio in the Monte Carlo simulation in the absence of neutrino oscillation. The measured double ratios for the ''all events'' (A) and high resolution (HR) samples are R A = R$$data\atop{low/high}$$/R$$MC\atop{low/high}$$ = 0.60$$+0.11\atop{-0.10}$$(stat) ± 0.08(syst) and R HR = R$$data\atop{low/high}$$/R$$MC\atop{low/high}$$ = 0.58$$+0.14\atop{-0.11}$$(stat) ± 0.05(syst), respectively. Both event samples show a significant deviation from unity giving a strong indication of neutrino oscillation. A combined momentum and zenith angle oscillation fit is performed using the method of maximum log-likelihood with a grid search in the parameter space of Δm 2 and sin 2 2θ. The best fit point for both event samples occurs at Δm$$2\atop{23}$$ = 1.3 x 10 -3 eV 2, and sin 223 = 1. This result is compatible with previous measurements from the Super Kamiokande experiment and Soudan 2 experiments. The MINOS Far Detector is the first underground neutrino detector to be able to distinguish the charge of the muons. The measured charge is used to test the rate of the neutrino to the anti-neutrino oscillations by measuring the neutrino induced muon charge ratio. Using the high resolution sample, the μ + to μ - double charge ratio has been determined to be R CPT = R$$data\atop{μ-/μ+}$$/R$$MC\atop{μ-/μ+}$$ = 0.90$$+0.24\atop{-0.18}$$(stat) ± 0.09(syst). With the uncertainties added in quadrature, the CPT double ratio is consistent with unity showing no indication for CPT violation.« less
  • This thesis presents the results of atmospheric neutrino observations from a 12.23 ktyr exposure of the 5.42 kt MINOS Far Detector between 1st August 2003 until 1st March 2006. The separation of atmospheric neutrino events from the large background of cosmic muon events is discussed. A total of 277 candidate contained vertex v/more » $$\bar{v}$$ μ CC data events are observed, with an expectation of 354.4±47.4 events in the absence of neutrino oscillations. A total of 182 events have clearly identified directions, 77 data events are identified as upward going, 105 data events are identified as downward going. The ratio between the measured and expected up/down ratio is: R$$data\atop{u/d}$$/R$$MC\atop{u/d}$$ = 0.72$$+0.13\atop{-0.11}$$(stat.)± 0.04 (sys.). This is 2.1σ away from the expectation for no oscillations. A total of 167 data events have clearly identified charge, 112 are identified as v μ events, 55 are identified as $$\bar{v}$$ μ events. This is the largest sample of charge-separated contained-vertex atmospheric neutrino interactions so far observed. The ratio between the measured and expected $$\bar{v}$$ μ/v μ ratio is: R$$data\atop{$$\bar{v}$v}$/ R$$MC\atop{$$\bar{v}$v}$ = 0.93 $$+0.19\atop{-0.15}$$ (stat.) ± 0.12 (sys.). This is consistent with v μ and $$\bar{v}$$ μ having the same oscillation parameters. Bayesian methods were used to generate a log(L/E) value for each event. A maximum likelihood analysis is used to determine the allowed regions for the oscillation parameters Δm$$2\atop{32}$$ and sin 223. The likelihood function uses the uncertainty in log(L/E) to bin events in order to extract as much information from the data as possible. This fit rejects the null oscillations hypothesis at the 98% confidence level. A fit to independent v μ and $$\bar{v}$$ μ oscillation assuming maximal mixing for both is also performed. The projected sensitivity after an exposure of 25 ktyr is also discussed.« less