# MAXJETS

Asked by Matthew Low on 2013-11-20

Hi,

I've been trying to figure out exactly what MAXJETS does (as mentioned in https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Matching). In the example they mention p p > t b~ j j where for one diagram one jet is from a QCD vertex and in another diagram two jets are from QCD vertices. It says the highest multiplicity is 1 jet and to set MAXJETS=1. What does highest multiplicity mean here? Is it the highest number of jets from non-QCD vertices?

I know this was addressed in a question someone else posted some time ago, but for the process p p > w+ w- j j, would I choose MAXJETS=2 since there are diagrams where both jets come from non-QCD vertices (and there are diagrams with 1 and 2 jets from QCD vertices)? What should MINJETS be set to and what role does it play?

Also, do you know what MAXJETS actually does in terms of what Pythia does with it?

Thanks,
- Matthew

## Question information

Language:
English Edit question
Status:
Open
For:
Assignee:
de Visscher Edit question
Last query:
2013-11-20
2013-11-20
 Olivier Mattelaer (olivier-mattelaer) said on 2013-11-20: #1

Hi,

> I've been trying to figure out exactly what MAXJETS does (as mentioned in https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Matching).

MAXJET is a tag for pythia, telling him that he doesn't have to apply the veto on hard radiation if they are that number of jets (or more) in the final state.
This allow pythia to radiate high energy jet from that sample and therefore allow to generate sample with a high number of high energy jet.

In 99% of the case, you don't have to touch this parameter but if you generate each multiplicity separately and not using the "add process".
The single top is an example where it didn't works correctly since you have some important diagram (which can't be seen as radiation) when you add the first jet.

> I know this was addressed in a question someone else posted some time ago, but for the process p p > w+ w- j j, would I choose MAXJETS=2 since there are diagrams where both jets come from non-QCD vertices (and there are diagrams with 1 and 2 jets from QCD vertices)? What should MINJETS be set to and what role does it play?

I don't think that you are in such special case. But in order to have meaningful matching, you have to forbid the additional jet to come from QED radiation:
so a correct syntax is
generate p p > w+ w-
add process p p > w+ w- j
add process p p > w+ w- j j
In that case indeed MAXJETS should be set on 2 (and will be automatically in fact)

> What should MINJETS be set to and what role does it play?

That's the exact opposite of MAXJETS, it tells if you include in your analysis all the sample below a given multiplicity. I don't know all the detail about this one,
But this also indicate to pythia that it can some special behavior in that multiplicity since it doesn't have double counting with sample below that multiplicity.

Cheers,

Olivier

On Nov 20, 2013, at 7:21 AM, Matthew Low <email address hidden> wrote:

> New question #239511 on MadGraph5:
>
> Hi,
>
> I've been trying to figure out exactly what MAXJETS does (as mentioned in https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Matching). In the example they mention p p > t b~ j j where for one diagram one jet is from a QCD vertex and in another diagram two jets are from QCD vertices. It says the highest multiplicity is 1 jet and to set MAXJETS=1. What does highest multiplicity mean here? Is it the highest number of jets from non-QCD vertices?
>
> I know this was addressed in a question someone else posted some time ago, but for the process p p > w+ w- j j, would I choose MAXJETS=2 since there are diagrams where both jets come from non-QCD vertices (and there are diagrams with 1 and 2 jets from QCD vertices)? What should MINJETS be set to and what role does it play?
>
> Also, do you know what MAXJETS actually does in terms of what Pythia does with it?
>
> Thanks,
> - Matthew
>
> --
> You received this question notification because you are a member of

 Matthew Low (mattlow) said on 2013-11-20: #2

Hi Olivier,

Thanks for the useful response! Is this correct: you need MAXJETS when adding a jet opens up a new diagram so that pythia won't try to replace the jet with a softer parton shower jet? If so, why does this not affect the diagrams where the jet is radiated and you want pythia to replace the jet with one from the parton shower?

In particular I'm interested in:
generate p p > w+ w- j j
add process p p > w+ w- j j j

In this case, only diagrams from radiation appear for the 3 jet process, so I should leave MAXJETS at it's default of -1? Also, if I were to include "add process p p > w+ w- j", is that where I would want to use MINJETS=2 so that only events with at least one jet from the parton shower would be in the sample?

Thanks,
- Matthew

 Matthew Low (mattlow) said on 2013-11-26: #3

Hi,

Actually, if anyone knows what PTCLUS in the pythia code does that might be helpful also.

Thanks,
- Matthew