Neutral Pion Production in the T2K ND280

Introduction

Water Cherenkov detectors like Super-Kamiokande cannot distinguish between electrons and photons: both create an electromagnetic shower with many nearly collinear electrons and positrons above Cherenkov threshold, producing the characteristic fuzzy ring. Therefore, any process which generates photons in the far detector is a potential background for νe appearance measurements. Neutral pions decay into two photons, one of which may not be reconstructed if it is soft or too nearly collinear with the other photon, so they are a significant potential background. The cross-section for π0 production in neutrino-nucleus interactions is not well known, so the uncertainty in this background—a systematic error in the appearance measurement—is large. Better measurements of π0 production cross-sections are therefore an important goal of the ND280 physics programme. This is a major part of the Sheffield group's physics analysis.

ND280: the T2K off-axis near detector

Exploded diagram of the T2K off-axis near detector, ND280

The T2K near detector complex consists of two main parts: the on-axis detector INGRID, which is designed to monitor the beam profile and neutrino interaction rate, and the off-axis ND280, which is intended to characterise the unoscillated beam at the same off-axis angle as Super-K and to measure neutrino interaction cross-sections.

ND280 is built around the former UA1 dipole magnet, which in this configuration produces a field of 0.2 T. It consists of two principal subdetectors:

  • the Pi-0 Detector or P0D, a tracking calorimeter consisting of plastic scintillator bars interleaved with metal foil and water bags (the latter intended to provide a water target for direct comparison with Super-K);
  • the Tracker, consisting of three time-projection chambers (TPCs) for precision tracking of charged particles, interleaved with two fine-grained detectors (FGDs), constructed of plastic scintillator bars (and water layers in the case of FGD2) and providing an active target for neutrino interactions, and surrounded on five sides by the ECal, a sampling calorimeter of plastic scintillator bars and lead sheets.

In addition, the magnet yoke is instrumented with plastic scintillator panels to act as muon detectors, forming the Side Muon Range Detector (SMRD).

In principle, as its name suggests, the P0D is designed for π0 reconstruction while the Tracker is intended to analyse charged-particle final states. However, the combination of charged-particle tracking and electromagnetic calorimetry does provide the necessary functionality for π0 reconstruction, providing a cross-check of the P0D results with different systematic errors.

Neutral pion production in neutrino interactions

Neutral pions can be produced in neutrino-nucleus interactions through either W exchange (charged current, in which the neutrino converts to the equivalent charged lepton) or Z exchange (neutral current, in which the neutrino transfers energy and momentum but remains a neutrino).  The π0 may be produced singly (CC1π0, e.g. νμ + n → μ+ + p + π0, or NC1π0, e.g. νμ + p → νμ + p + π0) or in conjunction with other mesons.

As regards background to νe appearance in Super-K, the NC1π0 reaction is the most important, since this produces the final state that is most easily mistaken for an electron.  In CC reactions, the final-state muon would produce a Cherenkov ring which would prevent misidentification; in cases where more than one meson is present in the final state, it is unlikely that the observed event in Super-K would consist only of a single electron-like ring.  However, the NC1π0 reaction is quite difficult to study in ND280, since the frequent lack of a charged track (there may be a soft proton, but this usually does not travel far and may not be reconstructed) makes it hard to locate the event vertex.  For this reason, we also study the closely related CC1π0 process.

Charged current π0 (CC1π0) production

In this analysis, the muon was identified using cuts developed by the νμ working group and the π0 was reconstructed from two isolated ECal showers. The event vertex is required to lie in one of the FGDs, and the ECal showers may be detected in the barrel or downstream ECals. This yields six different event topologies (three ECal combinations, DsDs, DsBrl and BrlBrl, for each FGD) which have different kinematic acceptance and were analysed individually.

Example output of the BDT for the CC1pi0 analysis

After the muon selection, events with two or three isolated ECal clusters were selected as π0 candidates. The principal selection tool was a boosted decision tree (BDT), a form of multivariate analysis. BDTs, neural networks and similar multivariate analyses are valuable because (1) they can take correlations between variables into account and (2) they are optimised by "training" on samples where the desired outcome is known (generally simulations), so that there is no need for the user to know exactly what the discriminating features are.  The disadvantage is that, precisely because the user does not know exactly what the discriminating features are, the method is susceptible to systematic errors caused by discrepancies between simulation and data, and these systematic errors can be difficult to evaluate.  In this particular case, this is a price worth paying, because the differences between signal and background are quite subtle and a fully cuts-based analysis would be very difficult to implement.

Example pi0 mass plot (FGD2DsDs topology) for the CC1pi0 analysisAcross all topologies, an overall selection efficiency of 10% and purity of 12% were achieved.  As can be seen in the example invariant mass plot for the selected events shown on the left, the principal background is CCπ0+, i.e. events which do indeed include a π0, but accompanied by additional final-state particles.  The selection in fact produces a purity of 28% for correctly reconstructing a π0 from any final state (more than 28% of the events come from a reaction which included a π0, but not all the π0s in such events were correctly reconstructed.)

For more information about this analysis, please see Matt Lawe's PhD thesis.

Neutral current π0 (NC1π0) production

The final state in an NC1π0 event consists of two photons from the π0 decay and some number of nucleons ejected from the struck nucleus.  Because the T2K beam is low energy, the ejected nucleons are very soft and frequently not reconstructed (in the worst-case scenario from a reconstruction point of view, the ejected nucleon may be a single neutron, but even soft protons will stop quickly and may not leave a reconstructable track).  Furthermore, the two photons from π0 decay are of quite low energy, and will therefore produce few hits in the ND280 electromagnetic calorimeter (which was optimized for detecting electrons from νe-induced CC events—an essential check on the νe content of the J-PARC neutrino beam).  The efficiency for reconstructing the softer of the two photons in the ND280 ECal is therefore quite low, making selection of NC1π0 events very challenging.  In addition, if no charged tracks are present it becomes very difficult to locate the event vertex, as the low-energy ECal clusters produced by the soft π0 photons are small and therefore do not have very well-defined directions.  This is a serious problem, since misreconstructing the vertex both distorts the invariant mass calculation (since the directions of the photons are incorrect) and makes it impossible to define the fiducial volume accurately (with obvious implications for cross-section calculations).

Accuracy of vertex reconstruction in NC1pi0 events.To achieve the highest possible precision in locating the event vertex, a sequential algorithm is used:

  1. If there is at least one TPC track, the vertex is defined to be the start point of the highest momentum TPC track.
  2. If there are no TPC tracks, but there is at least one FGD-only track, the vertex is defined to be the start point of the highest momentum FGD track.
  3. If there are no reconstructed tracks, but there are hits in one or both FGDs which have not been successfully combined into a track, the charge weighted mean position of the hits in the FGD with the higher number of hits is adopted as the vertex.
  4. If there is no FGD activity, the thrust axis of the higher-energy ECal cluster is extrapolated back to the central plane of the appropriate FGD and this is adopted as the vertex.  If the extrapolation is outside the FGD, the procedure is repeated for the lower-energy cluster.

As can be seen from the plot on the right, this sequence ensures that the most accurate possible method is used for each event.  Despite this, out-of-fiducial volume events remain the highest background in the selection, in contrast to the CC1π0 analysis where this background is very small.

Invariant mass plot for NC1pi0 analysisApart from the vertexing, the selection proceeds along similar lines to the CC1π0 analysis, requiring two isolated ECal clusters as the photon candidates (events with more than two clusters are rejected to avoid issues with combinatorics) and using a boosted decision tree to reject background.

The overall efficiency of the NC1π0 selection is 34%, with a purity of 23%, calculated with respect to those NC1π0 events where both the decay photons really did convert in the ECal.  Calculated with respect to the entire NC1π0 sample, including events where at least one photon converted before reaching the ECal, the efficiency goes down to 19% (unsurprising as the selection requires exactly two isolated ECal clusters), but the purity increases to 29% (i.e., some of the "background" is genuine NC1π0 events which should have been reconstructed as containing an e+e pair from a photon conversion before the ECal, but were not—perhaps because the conversion occurred too far out to yield a reconstructable track).  As can be seen from the invariant mass plot on the left, the data sample is significantly smaller than the Monte Carlo expectation, although the shapes of distributions are similar.  As the calculated cross section is in disagreement with an analysis by MiniBooNE, this finding requires confirmation before it can be accepted as a real effect.  However, it does demonstrate the potential importance of measuring this cross section as an input to the νe appearance analysis.

For more information about this study, please see Leon Pickard's PhD thesis.