In PyROOT, how can one plot the invariant mass of Z using 4-vector component variables in a tree?

ROOT
1

Let’s say I have a ROOT file containing a tree of variables that have been selected for the purposes of creating a Z invariant mass plot (i.e. leptons of opposite sign charge etc.). The variables in the tree are el_pt, el_eta, el_phi, el_e and so on (for the electron case).

For each event, one could create a TLorentzVector for each electron, add them and then get the invariant mass using the M2 function of the resultant TLorentzVector. These invariant mass values could be appended to a list and then plotted in a histogram.

In PyROOT, this approach could be slow and memory intensive. So, is there a more efficient way to do something like this? It appears appealing to use the machinery behind the tree “project” functionality (root.cern.ch/root/html/TTree.html#TTree:Project), but it is not obvious to me how something like an invariant mass calculation could be specified for it. Do you have any suggestions for this or another approach?

2

Hi,

one of the advantages of using pyROOT is the possibility to easily loop over trees.
You can start from this example, cooked starting from the information of your post:

def calcInvMass(px1,py1,pz1,e1,px2,py2,pz2,e2):
[...]
myFile = ROOT.TFile("myFileName.root")
myTree = myFile.myTreeName
for event in myTree:
    invMass = calcInvMas(event.el1_px, event.el1_py, event.el1_pz, event.el1_e,
                                    event.el2_px, event.el2_py, event.el2_pz, event.el2_e)
     myHistogram.Fill(invMass)
[...]

As for the performance, I would not be worried as the time will be almost entirely spent in IO operations which are performed by routines in the ROOT libraries which are written in C++ and compiled.

Cheers,
Danilo