How to add a sample in NUISANCE

This is an informal step-by-step guide on how to add samples into the NUISANCE framework.

For this tutorial I'll be adding the T2K CC1pi+ H2O data. The data comes from the ​T2K site and ​arxiv. The data is supplied both in a ROOT file and a number of .csv files: I'll be using the ROOT file here.

Versions: NUISANCE v1r0, NEUT 5.3.6, arxiv 1605.07964v2.pdf

Examining the data and choosing distributions

Finding the neutrino flux and generating events

The first issue at hand is to find the flux for the experiment. If we don't have this (or its very difficult to find), we cant make a generator prediction---unless the measurement is a total cross-section without any phase space cuts.

A quick search through the document finds it is documented in reference [12]. It is also in our flux list.

I then generate events in NEUT 5.3.6 with a suitable card-file, see our NEUT guide on how to do this. We need the correct target (water) and flux, and perform the model selections we want (e.g. Rein-Sehgal or Berger-Sehgal coherent model). The procedure is very similar for other generators too.

Aim to generate around 1M events with all interaction modes turned on. This way we make sure to get all interaction modes into the topologically defined cross-section. We get a small amount of CCQE events which excite a pion from the outgoing nucleon interacting in the nucleus to kick out a pion, leading to a CCQE+1pi ~ CC1pi+ final state, which is signal for this particular sample.

Choosing a distribution

The T2K CC1pi+ H2O data release contains various distributions in FIG 4. In this tutorial I'll look at adding one kinematic distribution and one derived distribution: p_mu and E^rec_nu respectively.

In NUISANCE we try to add all available distributions from a publication. However, some distributions will have detector effects "folded" into them (i.e. they will be raw detector-level data). We can only use these if there is some method which bring detector-level variables (e.g. p_mu seen in the detector) to truth-level variable (e.g. p_mu seen after correcting for the detector effects).

Implementing a sample

#include "T2K_CC1pip_H2O_XSec_1Dpmu_nu.h"

// The muon momentum

//******************************************************************** 
T2K_CC1pip_H2O_XSec_1Dpmu_nu::T2K_CC1pip_H2O_XSec_1Dpmu_nu(std::string inputfile, FitWeight *rw, std::string  type, std::string fakeDataFile){
//******************************************************************** 

  fName = "T2K_CC1pip_H2O_XSec_1Dpmu_nu";
  fPlotTitles = "; p_{#mu} (GeV/c); d#sigma/dp_{#mu} (cm^{2}/(GeV/c)/nucleon)";
  EnuMin = 0.;
  EnuMax = 10.;
  Measurement1D::SetupMeasurement(inputfile, type, rw, fakeDataFile);

  // Data comes in ROOT file
  // hResultTot is cross-section with all errors
  // hResultStat is cross-section with stats-only errors
  // hTruthNEUT is the NEUT cross-section given by experimenter
  // hTruthGENIE is the GENIE cross-section given by experimenter
  SetDataFromFile(GeneralUtils::GetTopLevelDir()+"/data/T2K/CC1pip/H2O/nd280data-numu-cc1pi-xs-on-h2o-2015.root","MuMom/hResultTot");
  SetCovarFromDataFile(GeneralUtils::GetTopLevelDir()+"/data/T2K/CC1pip/H2O/nd280data-numu-cc1pi-xs-on-h2o-2015.root", "MuMom/TotalCovariance", true);

  SetupDefaultHist();

  fScaleFactor = (fEventHist->Integral("width")*1E-38)/double(fNEvents)/TotalIntegratedFlux("width");
};

//******************************************************************** 
// Find the momentum of the muon
void T2K_CC1pip_H2O_XSec_1Dpmu_nu::FillEventVariables(FitEvent *event) {
//******************************************************************** 

  // Need to make sure there's a muon
  if (event->NumFSParticle(13) == 0) return;

  // Get the muon
  TLorentzVector Pmu  = event->GetHMFSParticle(13)->fP;

  double p_mu = FitUtils::p(Pmu);

  fXVar = p_mu;

  return;
};

//******************************************************************** 
// Beware: The H2O analysis has different signal definition to the CH analysis!
bool T2K_CC1pip_H2O_XSec_1Dpmu_nu::isSignal(FitEvent *event) {
//******************************************************************** 
  return SignalDef::isCC1pip_T2K_H2O(event, EnuMin, EnuMax);
}