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Student Project BND - Spa September 2019


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 Go on latest release, and download a version under "Binary distributions".

  • MadGraph5

a version of MG5 can be downloaded here:

  • Delphes

start from here:

  • Pythia8:

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

0) Download data sample

From the Delphes main directory, download the pseudo-data file data.root :

wget -O data.root

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;

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:

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


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. Download the file called, and run it via the command:

python data.root hist_data.root

This will create a file called hist_data.root that can be browsed as done earlier with data.root.

2) Characterisation of a BSM excess

  • The provided data corresponds to LHC events that contains at least two isolated leptons (lepton=electron/muon) with pT > 15. Prove it!
  • The data contains the equivalent of an integrated luminosity of 60 pb-1. I presents an excess corresponding to a new physical state. Find the observable that makes this excess explicit.
  • What is the new particle's mass?
  • What does it decay into?
  • For each decay channel, express the cross section times branching ratio as a function of the number of observed events (Nobs) the number of background events (Nb), the integrated luminosity (L) and the event selection efficiency.
  • How would you measure the background yield from this data sample?

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