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= Student Project BND - Spa September 2019 =

== Pre-requisites ==

To successfully run this tutorial the following prerequisite packages should be installed:

- gfortran/gcc/tcl:

For linux users gcc/tcl should be already installed. For Mac users you should install XCode.

- ROOT:

can be downloaded from https://root.cern.ch/downloading-root
Go on latest release, and download a version under "Binary distributions".

- MadGraph5

a version of MG5 can be downloaded here: 
https://launchpad.net/mg5amcnlo/2.0/2.5.x/+download/MG5_aMC_v2.5.5.tar.gz

- Delphes

start from here:
https://cp3.irmp.ucl.ac.be/projects/delphes/wiki/WorkBook/QuickTour

- Pythia8:

following instructions from here (or using the Pythia8 installation in !MadGraph):

https://cp3.irmp.ucl.ac.be/projects/delphes/wiki/WorkBook/Pythia8


== 0) Download data sample ==

From the Delphes main directory, download the pseudo-data file {{{data.root}}} :
{{{
wget -O data.root https://cernbox.cern.ch/index.php/s/9BmyMB66JkALj2t/download
}}}

== 1) Access information in the ROOT file

Open the ROOT file (ignore the various warnings) and visualize its content via the TBrowser.

{{{
root -l data.root
TBrowser t;
}}} 

Open the Delphes Tree and then Event branch (by double-clicking). How many events are in this dataset?
Likewise explore the Tree content by looking at other branches. 

Non trivial information can also be plotted from the command line. 

{{{
TFile *f = TFile::Open("data.root");
TBrowser browser;
f->Get("Delphes")->Draw("Muon.PT");
}}}

Some information is not directly accessible from the browser interface but can nevertheless be access via command line. Every Delphes object has a 4-vector (called P4()) member of type TLorentzVector (see here for documentation: https://root.cern.ch/doc/master/classTLorentzVector.html)

So for example to plot the magnitude of the 3-momenta of the jets, or the jet invariant mass:

{{{
f->Get("Delphes")->Draw("Electron.P4().P()");
f->Get("Delphes")->Draw("Jet.P4().M()");
}}}


In order to perform an event-loop with a more sophisticated event selection you will have to write a small python macro. This macro takes as input the {{{data.root}}} and produce an output file that contains a couple of histograms. Copy paste inside a file called {{{macro.py}}}, and run it via the command {{{python example.py histograms.root}}}.
The {{{histograms.root}}} file can then be browsed as done before with the data file. 

{{{
# example event loop macro
#!/usr/bin/env python

import sys
import ROOT

if len(sys.argv) < 2:
  print " Usage: python macro.py data.root histograms.root"
  sys.exit(1)

ROOT.gSystem.Load("libDelphes")

try:
  ROOT.gInterpreter.Declare('#include "classes/SortableObject.h"')
  ROOT.gInterpreter.Declare('#include "classes/DelphesClasses.h"')
  ROOT.gInterpreter.Declare('#include "external/ExRootAnalysis/ExRootTreeReader.h"')
except:
  pass

inputFile = sys.argv[1]
outputFile = sys.argv[2]

# Create chain of root trees
chain = ROOT.TChain("Delphes")
chain.Add(inputFile)

# Create object of class ExRootTreeReader
treeReader = ROOT.ExRootTreeReader(chain)
numberOfEntries = treeReader.GetEntries()

# Get pointers to branches used in this analysis
branchMuon = treeReader.UseBranch("Muon")
branchJet = treeReader.UseBranch("Jet")

# Book histograms
histMuonMultiplicity = ROOT.TH1F("histMuonMultiplicity", "M_{recoil} [GeV]", 60, 100.0, 160.0)

ptot = ROOT.TLorentzVector(0.,0.,0.,240.)
# Loop over all events
for entry in range(0, numberOfEntries):
  # Load selected branches with data from specified event
  treeReader.ReadEntry(entry)

  # If event contains at least 2 muons
  if branchMuon.GetEntries() > 1:

    mu1 = branchMuon.At(0)
    mu2 = branchMuon.At(1)

    pll = mu1.P4() + mu2.P4()
    ph = ptot - pll

    histMass.Fill(ph.M())

out_root = ROOT.TFile(outputFile,"RECREATE")
histMass.Write()
}}}