How to define a jet using different jet algorithm or condition?

[Li Huang]

Hi,

I see that in fastjet we can define jet by some different jet algorithm and using some different condition for example in anti-kt algorithm we can choose different deltaR. So I wonder how to define a jet in delphes using the delphes output file .

Like:

----------------------------------------------------------------------
/ \file
/ \page Example02 02 - changing the jet definition
/
/ fastjet basic example program:
/ illustration of the usage of how to change the jet definition
/ used for the clustering (see also fastjet::JetDefinition)
/
/ run it with : ./02-jetdef < data/single-event.dat
/
/ Source code: 02-jetdef.cc
----------------------------------------------------------------------

STARTHEADER
$Id: 02-jetdef.cc 3663 2014-09-05 07:23:19Z soyez $

Copyright (c) 2005-2011, Matteo Cacciari, Gavin P. Salam and Gregory Soyez

----------------------------------------------------------------------
This file is part of FastJet.

FastJet is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

The algorithms that underlie FastJet have required considerable
development and are described in hep-ph/0512210. If you use
FastJet as part of work towards a scientific publication, please
include a citation to the FastJet paper.

FastJet is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with FastJet. If not, see <​http://www.gnu.org/licenses/>.
----------------------------------------------------------------------
ENDHEADER

#include "fastjet/ClusterSequence.hh"
#include <iostream> needed for io
#include <cstdio> needed for io

using namespace std;

/ an example program showing how to use fastjet
int main(){

read in input particles
----------------------------------------------------------
vector<fastjet::PseudoJet> input_particles;

double px, py , pz, E;
while (cin >> px >> py >> pz >> E) {

create a fastjet::PseudoJet with these components and put it onto
back of the input_particles vector
input_particles.push_back(fastjet::PseudoJet(px,py,pz,E));

}

create a jet definition:
a jet algorithm with a given radius parameter
----------------------------------------------------------

select a jet algorithm to use

this could be one of
{kt_algorithm, cambridge_algorithm, antikt_algorithm,
genkt_algorithm, ee_kt_algorithm, ee_genkt_algorithm}
see example 03-plugin.cc for extra options using plugins
instead of naive algorithms)
fastjet::JetAlgorithm jet_alg = fastjet::kt_algorithm;

when appropriate select a radius to use

this would not be mandatory for e^{+ e}- algorithms (see 05-eplus_eminus.cc)
double R = 0.6;

select an optional recombination scheme

this could be one of
{E_scheme, pt_scheme, pt2_scheme, Et_scheme, Et2_scheme, BIpt_scheme,
BIpt2_scheme, WTA_pt_scheme, WTA_E_scheme, WTA_modp_scheme,
external_sheme}

Notes:
- for the usage of a user-defined recombination scheme
(external_scheme), see 11-boosted_higgs.cc
- WTA_E_scheme, WTA_modp_scheme are meant for e+e- clusterings

By default, the E_scheme is used
fastjet::RecombinationScheme recomb_scheme=fastjet::E_scheme;

select an optional strategy

this could be chosen among
{N2MinHeapTiled, N2Tiled, N2PoorTiled, N2Plain, N3Dumb,
Best,
NlnN, NlnN3pi, NlnN4pi, NlnNCam4pi, NlnNCam2pi2R, NlnNCam}

By default, the Best strategy is chosen and we advise to keep
that default unless you are targeting a very specific usage. Note
also that the N log (N) strategies for algorithms other than
Cambridge/Aachen need CGAL support.
fastjet::Strategy strategy = fastjet::Best;

create the JetDefinition from the above information
fastjet::JetDefinition jet_def(jet_alg, R, recomb_scheme, strategy);

run the jet clustering with the above jet definition
----------------------------------------------------------
fastjet::ClusterSequence clust_seq(input_particles, jet_def);

get the resulting jets ordered in pt
----------------------------------------------------------
double ptmin = 5.0;
vector<fastjet::PseudoJet> inclusive_jets = sorted_by_pt(clust_seq.inclusive_jets(ptmin));

tell the user what was done
- the description of the algorithm used
- extract the inclusive jets with pt > 5 GeV
show the output as
{index, rap, phi, pt}
----------------------------------------------------------
cout << "Ran " << jet_def.description() << endl;

label the columns
printf("%5s %15s %15s %15s\n","jet #", "rapidity", "phi", "pt");

print out the details for each jet
for (unsigned int i = 0; i < inclusive_jets.size(); i++) {

printf("%5u %15.8f %15.8f %15.8f\n",

i, inclusive_jets[i].rap(), inclusive_jets[i].phi(),
inclusive_jets[i].perp());

}

return 0;

}

Are there any similar code or way to do that in the Delphes?

Best,
Li

[Michele Selvaggi]

Hi Li,

as long as you are simply choosing some standard jet algorithm (anti kt, kt, C/A..) and changing the basic parameters you can do it in Delphes via the FastJetFinder. The default cards are already set so that this module is configured. Take the CMS card for instance:

​https://github.com/delphes/delphes/blob/master/cards/delphes_card_CMS.tcl

At line 480, the FastJetFinder is configured to run with antikT, with R=0.5.

and Jet collection in the output ROOT file contains such jets. Note that you can define various jet collections with different parameters and store all of them in the output. To do this you'll have to add in the ExecutionPath a new call of the FastJetFinder (but call it with a different name), configure it with new parameters as in line 480, and add the relevant collection in the TreeWriter section.

Nevertheless, if you want to use some arbitrary FastJet code, and have full flexibility, I suggest you have a look at:

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

that explains how to do this.

Cheers,
Michele

[Li Huang]

Hi,

Thanks for your help , it solved my problem!

Best,
Li