Save rho for each event in output ntuple

[Jim Hirschauer]

Is it possible to save the rho energy density per event?
We are already calculating this with the FastJetFinder, but are unsure
how to save it. Thanks.

[Michele Selvaggi]

Yes it is possible.

By default the rho variable is computed in the FastJetFinder module as a candidate with 4-momentum vector with entries (rho,0,0,rho).
The default name for the rho output array is Rho/rho.

To store it in the output file, you can add in the TreeWriter section of your delphes card the following line (using the processScalarHT method of the TreeWriter class):

add Branch Rho/rho Rho ScalarHT

Indeed, if rho is computed by another instance of the FastJetFinder X and has an array name Y, you should write:

add Branch X/Y Rho ScalarHT

[Pavel Demin]

It's also possible to add a class for Rho. Something like

class Rho: public TObject
{
public:
  Float_t Rho; // rho energy density

  ClassDef(Rho, 1)
};

[Pavel Demin]

Here is a version with the Rho class and branch:

​http://cp3.irmp.ucl.ac.be/downloads/Delphes-3.0.8.pre3.tar.gz

[Pavel Demin]

The Rho class and branch are in Delphes 3.0.8 now.