MINOS Document 3674-v1

Atmospheric Neutrino Observations In The MINOS Far Detector

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Mark A Thomson
Updated by:
Mark A Thomson
Document Created:
29 Sep 2007, 05:05
Contents Revised:
29 Sep 2007, 05:05
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16 Jan 2009, 16:41
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This thesis presents the results of atmospheric neutrino observations from a 12.23 ktyr exposure of the 5.42 kton 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 numu/numubar 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:
0.72^{+0.13}_{-0.11} (stat.) \pm 0.04 (sys.).
This is 2.1 sigma away from the expectation for no oscillations. A total of 167 data events have clearly identified charge, 112 are identified as numu events, 55 are identified as numubar events. This is the largest sample of charge-separated contained-vertex atmospheric neutrino interactions so far observed. The ratio between the measured and expected numubar/numu ratio is:
0.93^{+0.19}_{-0.15} (stat.) \pm 0.12 (sys.)$.
This is consistent with numu and numubar having the same oscillation parameters. Bayesian methods were used to generate a log(L/E) PDF for each selected event with a clearly identified direction. This information was used to give a measure of the uncertainty in the reconstructed log(L/E) value for each event. A maximum likelihood analysis is used to determined the allowed regions for the oscillation parameters Deltamsq and SinSq2Theta. 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 numu and numubar oscillation assuming maximal mixing for both is also performed. The projected sensitivity after an exposure of 25 ktyr is also discussed.
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Referenced by:
  • MINOS-doc-4771: Archive of Old Timing Calibration Talks
  • MINOS-doc-4926: Far Detector Timing Calibration
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