True Transition Probability
POT 20 Events / 10
6.1 Selection
Chapter 6
Antineutrinos in an Antineutrino Beam
The sensitivity of the antineutrino oscillation analysis with the neutrino-mode beam, presented in the previous chapter, is fundamentally limited for two reasons: the small size of the antineutrino component of the neutrino-mode beam, and the high average energy of the antineutrino events.
These two effects limit the sensitivity to oscillations in the parameter region where oscillations have already been measured for neutrinos. The solution to both of these shortcomings is the same: a focused antineutrino beam. Such a beam can be produced by reversing the direction of the current in the NuMI horns, focusing negative mesons instead of positive ones, giving an antineutrino focusing peak. MINOS accumulated 1.7×1020 POT with NuMI in antineutrino mode thanks to a proposal from the Caltech neutrino group.
106 Antineutrinos in an Antineutrino Beam
Figure 6.1: q/pdistribution of selected events before charge sign selection in the Near Detector. The red curve represents MC expectation with the flux uncertainty and black dots represent data. Antineutrino-like events are on the right (positive) and neutrino-like events are on the left (negative).
or neutral current interactions of any neutrino species which have a shower element that fakes a muon track (NC’s),
νx+N →νx+ fake muon + hadrons. (6.3)
The first selection step is to apply the same preselection cuts used in the neutrino-mode analysis:
• The beam and detector must have been in good operating condition.
• The event must occur during the beam spill, eliminating atmospheric neutrinos and cosmic rays.
• The event vertex must occur in a fiducial volume separated from the edges of the detector, ensuring that all the energy of the event is contained in the detector and can be measured.
• The event must have a reconstructed track, eliminating most neutral current events.
The first piece of the selection proper is a cut on the reconstructed charge of the muon track to eliminate the bulk of the CCνµ’s in the sample. In practice, the quantity measured in the detector is the track’s curvature, which is proportional to the ratio of charge to momentum (q/p). The distribution of this quantity in the Near Detector is shown in Figure 6.1. Only tracks with a positive reconstructed charge (q/p >0) are accepted.
The second piece of the selection is a cut on a multivariate CC/NC separation parameter called RoID. It is the output of a 4-parameter k nearest neighbors (kNN) algorithm [152]. A kNN uses a multi-dimensional space populated with simulated events, both signal and background. A metric
based on these 4 variables defines the distance between two events. The separation parameter for a given input event is then constructed based on what fraction of the k nearest neighbors (k events with the smallest distance from the input event) are CC events.
RoID uses 4 variables (seen in Figure 6.2) which can distinguish between charged current and neutral current interactions. The first two variables described below are topological and the second two describe the energy deposition along the track.
Number of active planes in the track
True muon tracks tend to be longer (i.e. cross more planes) than the tracks of particles from the hadronic shower.
Transverse profile parameter
Also called fraction of pulse height in the track, this variable looks at the amount of energy deposited in the transverse vicinity of the track, away from the shower at the vertex. True muon tracks typically leave only a single hit on a scintillator plane while non-muon tracks typically have other unconnected hits sitting near the reconstructed track. So, the larger the fraction of energy in the track as opposed to surrounding it, the more likely that the track is a muon.
Average pulse height per plane in the track
The average deposited energy (pulse height) per plane of the track away from the event vertex. It is related to dE/dx and distinguishes muons, which are typically minimum-ionizing, from the more energetic interactions of hadronic shower parti- cles.
Ratio of mean low pulse height to mean high pulse height
This variable compares the mean energies of the lowest and highest energy strips in the track. Muon energy loss is relatively uniform, typically occurring through ionization where large energy losses are rare. Hadronic shower energy loss happens through completely different physical processes with much larger fluctuations in deposited energy. The closer in energy the low mean and the high mean are, the more likely the event is a muon.
The final output parameter from RoID in the Near Detector is shown in Figure 6.3. While this variable primarily distinguishes charged current interactions from neutral current interactions, it also eliminates some wrong sign background: CCνµ’s have a higher averagey-distribution than CC¯νµ’s, meaningνµ’s tend to look a little more NC-like.
The efficiency and purity performance of the selection in the Far Detector without oscillations are shown in Figure 6.4. The overall reconstruction and selection efficiency, with a CC/NC cut at
108 Antineutrinos in an Antineutrino Beam
Figure 6.2: Distribution of the 4 kNN input variables before the CC/NC selection cut is applied. The red histogram represents the Monte Carlo expectation with the flux error, the blue histogram represents the total (charged and neutral current) background with the background uncertainty. Black points represent data. Each shows some separation between the background and the bulk of the sample.
Figure 6.3: CC/NC separation parameter on a semi-log scale. The red histogram represents the simulation with the flux error, the blue histogram represents the total background with the background uncertainty.
Black points represent data. Events with a CC/NC separation parameter less than 0.3 (30% CC’s in the nearest neighbors) are cut. The peak in the background at high-PID contains CCνµ’s whose charge-sign has been mis-identified.
Reconstructed Energy (GeV)
0 5 10 15 20 25
Selector Performance
0 0.2 0.4 0.6 0.8 1
MINOS Preliminary Simulated Far Detector
NC Contamination NC After Selection
Contamination νµ
After Selection νµ
Selection Efficiency
Figure 6.4: Performance of the antineutrino selection (RoID >0.3, q/p > 0) in the Far Detector. The dashed lines show the contamination before selection and the solid show efficiency and contamination after selection. The CCνµcontamination rises at higher energies because these tracks do not curve as much and so are more difficult to reliably assign a charge to. The background is reduced to nearly nothing in the 1−6 GeV region most sensitive to oscillations.
0.3, is 93%1 and the sample is 92% pure antineutrinos. Below 6 GeV, the region most sensitive to oscillations, the purity is 98% with the same efficiency. The wrong sign background is largest where the curvature of the muon track is difficult to measure: at low energies (<2 GeV) where the track is too short and at high energies (>10 GeV) where the track is too straight. NC’s are only significant at low energies where a shower element might be mistaken for a short muon track.