Atmospheric neutrinos in Soudan 2. Page: 4 of 6
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Data: gold 105 159
Background corrected "v" 83.6 119.7
MC 847 805
Table 1: Classifications for the contained events before corrections.
multiprong classification. Results are shown in Table 2. Events with (without) shield hits are labeled
"rock" ("gold") events. The quality of the flavour assignment was measured using the Monte Carlo
data. We found that 87% of events assigned as tracks have muon flavour and 96% of showers electron
The majority of the events classified as contained are due to the interactions of neutral particles
(neutrons or photons) produced by muon interactions in the rock around the detector. Calculations
show that only a few percent of such events will not have an accompanying charged track traversing
our shield, which was placed as close to the cavern wall and as far away from the detector as possible
to maximize the probability of detecting the accompanying charged particles. The efficiency of the
shield has been measured using cosmic ray muons detected in the main detector. It ranges from 81%
during the early data runs before the geometrical coverage was complete to 93% at the end of this
data period. Also, 8.9% of Monte Carlo events had a random shield coincidence.
Our large sample of rock events enables us to investigate this potential background by studying
the depth distribution of the events in the detector. This allows us to simultaneously measure any
backgrounds due to either shield inefficiency or contained events due to neutral particles entering
the detector without being accompanied by charged particles in the shield. Neutrino events will be
distributed uniformly throughout the detector, while background events will be attenuated towards
the center. We define a measure of the proximity of the event to the detector exterior by calculating
the minimum perpendicular distance from the event vertex to the detector edge.
In using the depth distribution of the rock events to correct for background, we note that the
measured flavor ratio as a function of shield multiplicity is observed to be a constant value of 0.76
0.07. We then correct the track to shower ratio in the data by fitting the track and shower depth
distribution to a sum of those in the rock events and Monte Carlo, constraining the flavor ratio of
the rock events to its observed value. The result of the fit gives the corrected neutrino induced rate
of 83.6 tracks and 119.7 showers. From this we calculate R = 0.66 0.11, where the error includes
the statistical error on the data, the statistical error on the Monte Carlo, and the error on the fit.
3 The High Resolution Sample
If the low value of R is the result of neutrino oscillations, the L/E distribution will be sensitive to
Am2. The ability to identify an oscillation signature in an L/E distribution is mainly limited by the
measurement of the incident neutrino direction. The neutrino directional measurement is smeared by
detector resolution, target Fermi motion, and the failure to image all final state particles. We have
found that by placing energy cuts on the data, we can obtain a subsample of events which have the
potential for good directional measurement, and hence better sensitivity to oscillation parameters.
In Soudan 2, the identification of a recoil proton greatly enhances the ability to identify the neutrino
direction. The cuts that isolate this sample are:
" Tracks and Showers
plept > 150MeV/c if a recoil is present
plept > 600MeV/c if no recoil is present
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Goodman, M. C. & Collaboration, The Soudan 2. Atmospheric neutrinos in Soudan 2., article, June 22, 1999; Illinois. (digital.library.unt.edu/ark:/67531/metadc628462/m1/4/: accessed December 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.