Selection

78 Antineutrinos in a Neutrino Beam

Reconstructed Energy (GeV) Fraction

µ

ν Non-

0 0.2 0.4 0.6 0.8 1

Total

Charge Current νµ

Mis-ID

Neutral Current

MINOS Preliminary Monte Carlo

Near Detector, Low Energy Beam

0 5 10 15 20 30 40 50

Figure 5.3: Contamination in the positive sample after preselection only. The black line represents the total contamination (1−purity), the red line represents the fraction of events that are mis-identifiedνµCC (wrong sign) and the blue line represents the fraction of events that are neutral currents. After preselection the sample is still mostly background, especially at low energies.

(q/p) (q/p)/m

0 5 10 15 20

Events3 10

0 2 4 6 8

10 MINOS Preliminary 2.9×10²⁰ POT Low Energy Beam

Near Detector Data

MC w/ flux error MC background

(q/p) (q/p)/m

0 5 10 15 20

Events3 10

0 2 4 6 8 10

/) abs(Relative Angle -

0 1 2 3

Events3 10

0 5 10 15 20

25 MINOS Preliminary

20 POT 10

× 2.9

Low Energy Beam Near Detector

Data MC w/ flux error MC background

/) abs(Relative Angle -

0 1 2 3

0 5 10 15 20 25

Figure 5.4: Charge-sign selection variables (q/p)/ σ_q/p and |Relative Angle−π| are shown for the Near Detector in data (black points) and simulation (total in red, background in cyan). The flux uncertainty on the Monte Carlo is represented by the shaded red bars. In each plot, all other selection cuts have been applied. These selectors keep only well-measured positive tracks.

• The event must have a reconstructed track, eliminating most neutral current events.

The next step in selecting antineutrinos is to keep only events with positive reconstructed charge (the PQ sample). However, the antineutrino component is so small that even this sample is domi- nated by backgrounds (see Figure 5.3). The backgrounds are split evenly between neutral currents and wrong-sign neutrinos. The positive sample is at least half background at all energies, and the contamination gets worse below 7 GeV.

An additional selection step is applied for each of these backgrounds. Two cuts are made to address the charge sign, and a third cut is made on a likelihood-based CC/NC separator. The two

Track Length (Planes)

0 100 200 300

Events3 10

0 20 40 60 80 100

120 MINOS Preliminary

20 POT 10

× 2.9

Low Energy Beam

Near Detector Data MC w/ flux error MC background

Track Length (Planes)

0 100 200 300

Events3 10

0 20 40 60 80 100 120

Track pulse height fraction

0 0.2 0.4 0.6 0.8 1

Events3 10

0 5 10

15 MINOS Preliminary

20 POT 10

× 2.9

Low Energy Beam

Near Detector Data

MC w/ flux error MC background

Track pulse height fraction

0 0.2 0.4 0.6 0.8 1

Events3 10

0 5 10 15

Track Mean Pulse Height per Plane

500 1000 1500

Events3 10

0 10 20

30 MINOS Preliminary 2.9×10²⁰ POT Low Energy Beam Near Detector

Data MC w/ flux error MC background

Track Mean Pulse Height per Plane

500 1000 1500

Events3 10

0 10 20 30

Figure 5.5: The three variables that form the PDFs in theDpID CC/NC separator: the track length, the fraction of the event energy in the track, and the mean energy deposited per plane are shown for the Near Detector in data (black points) and simulation (total in red, background in cyan). The flux uncertainty on the Monte Carlo is represented by the shaded red bars. Each shows some separation between the background and the bulk of the sample.

charge-sign selection variables are the ratio of the track’s curvature (q/p) to the uncertainty on that curvature (σq/p) and the relative angle between the straight-line projections of the first few hits and the last few hits of the track. The distributions of these two variables are shown in Figure 5.4.

The CC/NC separation parameter, called DpID, is built up from 1-dimensional PDFs of three variables that each have some power to distinguish charged current events from neutral currents.

Track length

True muon tracks tend to be longer (i.e. cross more planes) than the tracks of particles coming from the hadronic shower.

Track energy fraction

The fraction of the event energy that is in the track (lepton) as opposed to the shower (hadrons). It is related to the kinematicy (inelasticity).

Track energy per plane

The amount of energy deposited per plane of the track. It is related todE/dxwhich can distinguish true muons (typically minimum-ionizing) from the tracks formed by hadronic shower components.

These three variables are shown in Near Detector data and simulation in Figure 5.5. The distribution

80 Antineutrinos in a Neutrino Beam

CC/NC separation parameter

-2 -1 0 1

Events

3

10

0 10 20 30

40

2.9 ×10²⁰ POT MINOS Preliminary

Low Energy Beam Near Detector

Data

MC w/ flux error MC background

CC/NC separation parameter

-2 -1 0 1

Events

3

10

0 10 20 30 40

NC component

µ CC ν Mis-ID

CC/NC separation parameter

-2 -1 0 1

Events

3

10

0 10 20 30 40

Figure 5.6: CC/NC separation parameter (DpID) is shown for the Near Detector in data (black points) and total Monte Carlo with flux uncertainty (red line and shaded bars). Also shown is the neutral current background (magenta), mis-identifiedνµ background (green), and total background (blue). The CC/NC separator has some power to reject the wrong-sign background in addition to the neutral current background due to the higher average inelasticity (y) of neutrinos compared to antineutrinos, which is one of the input variables. All other cuts have been applied.

Energy (GeV) i µ

Efficiency, Contamination ₀

0.2 0.4 0.6 0.8 1

Contamination after Preselection Contamination after Full Selection Full Selection Efficiency

MINOS Preliminary Monte Carlo Far Detector, Low Energy Beam

0 5 10 15 20 30 40 50

Figure 5.7: The contamination in the sample of events passing preselection (dashed blue, same as the black in Figure 5.3) as well as the contamination after the full selection has bene applied (solid blue).

This contamination is made up of approximately equal parts neutral current interactions and mis-identified negative charged current interactions. Also shown is the selection efficiency (red). By comparing the dashed and solid blue lines, it is clear that the selection dramatically increased the purity of the antineutrino sample.

of the separation parameter, again in Near Detector data and simulation, is shown in Figure 5.6.

This figure also shows the two individual background components. In addition to removing neutral current events,DpID is also effective at removing many wrong-sign events because one of its input PDFs is related to inelasticity (kinematicy) and neutrino interactions typically have a higherythan antineutrinos.

Figure 5.7 shows the performance of the antineutrino selection in purity and efficiency³ as well as the purity in the positive sample before selection (as shown in Figure 5.3). The CC/NC separator and the two selection cuts on charge-sign dramatically improve the purity of the sample from 34%

to 97% while keeping the overall efficiency at 82% in the Far Detector.