ν µ Charged-Current Inclusive Selection in FGDs

1717

The selection used in this analysis is identical to theνµCC inclusive selection developed

1718

for the oscillation analyses ([142, 150]), except for a small change in thefiducial volume

1719

of the FGDs. The goal of the selection criteria is to identify a sample of neutrino

1720

interactions which originate in the FGD1 or FGD2 detector and contain a reconstructed

1721

muon track of negative charge crossing the following TPC.

1722

Thefiducial volume used in References [142, 150] is slightly different between FGD1

1723

and FGD2. However, for this analysis the same fiducial volume has been applied for

1724

both FGDs, in order to ensure the same acceptance for the two selections. In the

1725

coordinates orthogonal to the beam direction (x and y) the fiducial volume begins

1726

72.17 mm inward from the edges of the FGDs. In the coordinate parallel to the beam

1727

direction (z) thefiducial volume begins 10.125 mm inward from the edges of the FGDs,

1728

which corresponds in discarding thefirst and the last scintillator layers.

1729

FGD1 TPC2 FGD2 TPC3

Size in X [mm] 1864.34 2300 1864.34 2300 Size in Y [mm] 1864.34 2400 1864.34 2400 Size in Z [mm] 331.75 974 333.75 974

Table 5.2: FGDs and TPCs positions in the ND280 coordinate system. Very small asymmetries: FGD2 is 2 mm larger than FGD1 in Z; the FGD2-TPC3 gap is 1 mm smaller than the FGD1-TPC2 gap (25.625 mm and 26.625 mm respectively); in X the FGDs are exactly centred with respect to the TPCs, but in Y they are 25 mm off.

As described in Section 2.1, the T2K beam spill is constituted of eight bunches,

1730

separated by 0.6 µs. The selection is performed over the tracks grouped together in

1731

bunches according to their timing, i.e. occurring within the time windows of the beam

1732

bunches. The selection criteria allow to select only one event per bunch, either in FGD1

1733

or in FGD2. The probability of having more than one event per bunch is very low,

1734

anyhow a pile-up systematic uncertainties is evaluated to account for it (Section 6.2.4.3).

1735

The νµ CC-inclusive selection criteria are as follows.

1736

Figure 5.1: FGDs and TPCs relative positions in the yz plane (drawing to scale). The centres of the FGDs and of the TPCs are almost aligned (only 25 mm off); in thexz plane instead they are exactly aligned. The dashed line shows thefiducial volume.

1. Data qualityflag. The full spill must have a good global ND280 data qualityflag.

1737

2. Muon candidate identification. The muon candidate is chosen as the highest

1738

momentum track (if any) among those satisfying the following criteria:

1739

(a) start position inside the FGDfiducial volume (FV);

1740

(b) negatively charged (according to its curvature in the magneticfield);

1741

(c) have more than 18 clusters in the TPC (“TPC track quality” requirement

1742

to reject short tracks for which the reconstruction is less reliable).

1743

3. External veto. Some reconstruction failures can lead to a muon candidate track

1744

starting in the FGDfiducial volume even if the real muon started far upstream.

1745

For example a muon originating in the PØD and undergoing a large scatter in

1746

FGD1 may be reconstructed as two tracks (one PØD-TPC1-FGD1, and the other

1747

FGD1-TPC2). In order to exclude such events, if there is a TPC track with higher

1748

momentum than the muon candidate and starting more than 150 mm upstream

1749

(outside the FV) the event is rejected. Additionally, for FGD2 selection, the event

1750

is vetoed if there is a potential muon candidate in FGD1fiducial volume.

1751

4. Broken track veto. A TPC-FGD track isfirst reconstructed in the TPC and then

1752

projected to the FGD to match its hits incrementally. Matching failures are more

1753

likely to happen in the first matched hits, resulting in a broken track starting

1754

at the end of the FGD and crossing the TPC, which might be taken as muon

1755

candidate even though the other part of the broken track was starting outside

1756

thefiducial volume. To avoid this, the broken track veto rejects events with the

1757

muon candidate starting in the last XY module of the FGD and with another

1758

FGD track starting outside thefiducial volume (and not reaching the TPC).

1759

5. Muon PID cut. The particle identification procedure (PID) is applied to the

1760

muon candidate based on the dE/dx distribution measured in the TPC. The

1761

energy deposit in the TPC is compared with the energy deposit expected under

1762

the assumption of four particle hypothesis: muon, pion, electron and proton.

1763

Based on that, a discrimination function is applied.

1764

The dE/dx is estimated as a truncated mean of the energy released in the

1765

TPC.Pulls are calculated as:

1766

Pull_i= (dE/dx_measured−dE/dx_expected,i)

σ_{(dE/dxmeasured−dE/dxexpected,i)} (5.1) wheredE/dx_expected,iis the value of the truncated mean for the particle hypothesis

1767

Electrons, which are not minimum ionising particles (MIP), are rejected by

applied only for tracks withp <500 MeV/c. A further cut removes protons and

1771

Number of MC events

0

Number of MC events

0

18000 Integral 9.134e+04

mu-

Figure 5.2: Distributions ofLMIP (Eq. (5.3)) andLµ(Eq. (5.2)). The red lines show the cut value decided to enhance the muon candidate purity of the sample.

Fig. 5.2 shows the distributions ofL_MIPandL_µ. The red lines show the cut value

1773

decided to enhance the muon candidate purity of the sample.

1774

Events passing these criteria define theν_µCC-inclusive selection either in FGD1 or

1775

in FGD2.

1776

5.2.1 Data-MC Comparison

1777

Fig. 5.3 shows the data-MC comparison for the νµ CC inclusive selections in FGD1

1778

and in FGD2, as a function of the reconstructed energy evaluated with the

kine-1779

matic formula of Eq. (4.6): for both selections the MC simulation well agrees with

1780

the data; the mean reconstructed energy is well reproduced by the MC at the level of

1781

1.5±3.1/650 MeV. The momentum is measured in the TPC and extrapolated at the

1782

beginning of the track, correcting for the energy lost in the FGD. For a muon

origi-1783

nated in the water of FGD2 the track length between the vertex and the first hit in

1784

the nearest scintillating bar, is not taken into account. This might be the reason, or at

1785

least part of it, why the average reconstructed energy for the FGD2 selection is slightly

1786

lower than the FGD1 selection, as can be seen in Fig. 5.3. Anyway, considering that

1787

the energy loss for a minimum ionising particle in water is about 2 MeV per cm, and

1788

that the water modules have a width of only 2.5 cm, the correction would be very small.

1789

1200 FGD1 DATA

CC

Figure 5.3: Data-Monte Carlo comparison of the reconstructed energy distribution for both the FGD1 and the FGD2 selections. Red and blue circles (with statistical error bars) are the data points for FGD1 and FGD2 respectively. The coloured area is the MC distribution for FGD1, broken down by the predicted NEUT reactions, whilst the blue line is the FGD2 MC.

5.2.2 Efficiency, Purity and Background

1790

The efficiency is defined as:

1791

�= N_{selected|generated}

N_generated (5.5)

whereNgeneratedis the number of interactions generated by the MC andNselected — generated

1792

represents how many of them were reconstructed and selected.

1793

Note that with this definition, the efficiency includes also the selection acceptance.

1794

Fig. 5.4 shows the efficiency evaluated at each step of the selection described in

1795

Section 5.2, for both selections, in FGD1 (red) and in FGD2 (blue). For the number

1796

of generated interactions, all theν_µCC interactions predicted by NEUT in thefiducial

1797

volume are considered (cf. Section 7.3). Thefinal efficiency predicted by NEUT, after

1798

the last cut, is 53.66 % for the selection in FGD1 and 53.85 % for the selection in FGD2.

1799

quality+fiducial veto External FGD1 muon PID

0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85

muon candidate External veto Broken track veto muon PID

FGD1 FGD2

Figure 5.4: Efficiency evaluated at each step of the selection described in Section 5.2 for both selections, in FGD1 (red) and in FGD2 (blue).

Fig. 5.5 shows the efficiency as a function of the true muon direction, in terms

1800

of the θ angle respect to the neutrino direction (the lepton produced by the neutrino

1801

interaction associated to the selected muon candidate). The requirement of crossing

1802

a TPC (cf. Section 5.2) significantly limits the efficiency at high angles. Timing

1803

information of tracks crossing both FGDs can tell whether the particle is going from

1804

FGD1 towards FGD2 or vice versa. This helps the reconstruction of backward-going

1805

tracks originating in FGD2, and explains the better efficiency of the selection in FGD2

1806

for negativecosθ. Nevertheless the fraction of reconstructed events with a

backward-1807

going muon, shown as well in Fig. 5.5, is quite negligible.

1808

Dans le document Measurement of the water to scintillator charged-current cross-section ratio for muon neutrinos at the T2K near detector (Page 128-134)