Custom histogrammer

This is probably the most complex excercise, involving creation of a custom app, modifying the config, and adding a custom HistogramsFiller. The task is the following: Produce a histogram of Δφ between each pair of electron-muon in events from backgrounds_dy.root file.

Create and fill in a custom HistogramsFiller

Using create.py script (see create.py script for details) to create a new HistogramsFiller class:

python tea/create.py --type HistogramsFiller --name TTZHistogramsFiller

Open the .cpp file produced. The only method we’re interested in is Fill(...), which… fills histograms. It receives an event and has access to HistogramsHandler (as well as EventProcessor and ConfigManager, but we don’t need that now). You also don’t need to define histograms here, but simply use histogramsHandler->Fill(name, value) to fill the histogram. For simplicity, let’s assume Δφ = |φμ - φe|.

The example code would look like this:

void TTZHistogramsFiller::Fill(const std::shared_ptr<Event> event) {
  auto muons = event->GetCollection("Muon");
  auto electrons = event->GetCollection("Electron");

  for(auto muon : *muons) {
    float muonPhi = muon->Get("phi");

    for(auto electron : *electrons) {
      float electronPhi = electron->Get("phi");
      float deltaPhi = fabs(muonPhi - electronPhi);
      histogramsHandler->Fill("electron_muon_delta_phi", deltaPhi);
    }
  }
}

Create custom histogrammer app

In order to call our TTZHistogramsFiller, we will need to also create a custom histogrammer app. Use create.py script (see create.py script for details):

python tea/create.py --type histogrammer --name ttZ_analysis_histogrammer

Open the .cpp file and create an instance of TTZHistogramsFiller. In the event loop, call the Fill(…) method of our TTZHistogramsFiller. The example (skipping typical parts of an app which were auto-generated) would look like this:

#include "TTZHistogramsFiller.hpp"
// ...
int main(int argc, char **argv) {
  // ...
  auto ttZhistogramsFiller = make_unique<TTZHistogramsFiller>(histogramsHandler);
  // ...
  for (int iEvent = 0; iEvent < eventReader->GetNevents(); iEvent++) {
    auto event = eventReader->GetEvent(iEvent);
    ttZhistogramsFiller->Fill(event);
  }
  // ...
  histogramsHandler->SaveHistograms();
  return 0;
}

Prepare the config

In the config file, apart from defining input/output files and other typical values, we should define a custom histogram:

histParams = (
    ("electron_muon_delta_phi",      100,  0,      7,     ""),
)

Build & run

Finally, build the project and run the app, specifying the corresponding config file:

source /tea/build.sh
cd bin
./ttZ_analysis_histogrammer ttZ_analysis_histogrammer_config.py

Have a look at the output file - you should see histogram of Δφ between electrons and muons.