Do I need to disable QED radition when interfacing with Pythia

Asked by Veit Scharf

Dear Madgraph Team,

I generated the W + 2 photon process with MadGraph and would now like to interface this with Pythia8. Looking at the diagrams the process contains photons from quartic and triple gauge couplings in combination with ISR and FSR. Do I need to disable the QED showering in Pythia in order to avoid double counting, or is this done automatically?

Below is my proc card for completeness.

Thanks for your help!

Cheers,
Veit

#************************************************************
#* MadGraph 5 *
#* *
#* * * *
#* * * * * *
#* * * * * 5 * * * * *
#* * * * * *
#* * * *
#* *
#* *
#* VERSION 1.4.8.2 2012-07-30 *
#* *
#* The MadGraph Development Team - Please visit us at *
#* https://server06.fynu.ucl.ac.be/projects/madgraph *
#* *
#************************************************************
#* *
#* Command File for MadGraph 5 *
#* *
#* run as ./bin/mg5 filename *
#* *
#************************************************************

import model sm
* *

import model sm
define p = g u c d s u~ c~ d~ s~
define j = p
define l+ = e+ mu+
define l- = e- mu-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
generate p p > W- > l- vl~ a a QED=99 QCD=99
a QED=99 QCD=99
output madevent -f

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Johan Alwall Edit question
Solved by:
Johan Alwall
Solved:
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi Veit,

Since you have only one multiplicity of photon in the final state, I don't think that they are any problem with double counting.
Johan could you confirm?

Cheers,

Olivier

Revision history for this message
Best Johan Alwall (johan-alwall) said :
#2

Hello Veit,

Olivier is right. The additional photons generated by Pythia are legitimate. You would have double counting only in the cases where:
1. You want to include both (e.g.) 1-photon and 2-photon samples in the same analysis, or
2. The events where Pythia generates a harder photon than the softest photon from the matrix element (where "hard" means both E or pt and distance from the closest jet/lepton). However, due to the smallness of \alpha_EM, this should happen in maximum 1% of the events, so any bias from this should be absolutely negligible. If you still worry about this, you can easily implement a simple veto procedure to remove such events.

All the best,
Johan

Revision history for this message
Veit Scharf (veit-scharf) said :
#3

Thanks Johan Alwall, that solved my question.

Revision history for this message
Veit Scharf (veit-scharf) said :
#4

I will check the fraction of events for case 2. Thanks for the fast answer.

Cheers,
Veit