# A proper way to choose a minimum DeltaR cut in case of parton showering

I want to use MadGraph events as an input for a parton shower event generator.
And my question is how to choose properly "distance between jets" cut in MG run card?

For example I produce parton events for a process p p > 4 jests where jets consist of massless quarks.
If I understand it correctly I have to imposed dR_min cut, otherwise I may run into numerical problems related to collinear IR divergences.
Now after performing parton showering I need to run a jet reconstruction algorithm, so I have to choose a jet radius R
(as an input parameter for a FastJet reconstruction algorithm)

Now I am getting confuse because if I understand it properly the dR_min parameter in MG run card has nothing to do with jet radius R. Moreover a total MG cross section depends on the chose of dR_min so it may affect my jet distributions.

So my questions are:
1) Is it true that dR_min in MG run card regularizes collinear IR divergences?
2) What is the best strategy to chose dR_min in MG run card and jet radius R in FastJet reconstruction algorithm?

Thank you,
with best regards,
Oleh

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2018-05-08
 Olivier Mattelaer (olivier-mattelaer) said on 2018-05-08: #1

Hi,

> 1) Is it true that dR_min in MG run card regularizes collinear IR divergences?

yes

> 2) What is the best strategy to chose dR_min in MG run card and jet radius R in FastJet reconstruction algorithm?

> Now after performing parton showering I need to run a jet reconstruction algorithm, so I have to choose a jet radius R

Do you mean after hadronization or did you bypass it?

In any case, you should setup your fastjet cut in a way which is close to your experimental cut.

At madgraph level you should put some quite loose cut.
The optimal value for the cut at madgraph level being the hardest cut that you can put at madgraph level such that you do not have ANY impact on your observable after fastjet (whatever level that one is).
Obviously finding such optimal value is not trivial and typically no one try to find such optimal value.

Cheers,

Olivier

> On 8 May 2018, at 17:12, oleh <email address hidden> wrote:
>
> New question #668743 on MadGraph5_aMC@NLO:
>
>
> I want to use MadGraph events as an input for a parton shower event generator.
> And my question is how to choose properly "distance between jets" cut in MG run card?
>
> For example I produce parton events for a process p p > 4 jests where jets consist of massless quarks.
> If I understand it correctly I have to imposed dR_min cut, otherwise I may run into numerical problems related to collinear IR divergences.
> Now after performing parton showering I need to run a jet reconstruction algorithm, so I have to choose a jet radius R
> (as an input parameter for a FastJet reconstruction algorithm)
>
> Now I am getting confuse because if I understand it properly the dR_min parameter in MG run card has nothing to do with jet radius R. Moreover a total MG cross section depends on the chose of dR_min so it may affect my jet distributions.
>
> So my questions are:
> 1) Is it true that dR_min in MG run card regularizes collinear IR divergences?
> 2) What is the best strategy to chose dR_min in MG run card and jet radius R in FastJet reconstruction algorithm?
>
>
>
> Thank you,
> with best regards,
> Oleh
>
>
> --

 oleh (fedkevych) said on 2018-05-08: #2

Dear Oliver,

>Do you mean after hadronization or did you bypass it?
In fact I switch it of since I am interested only in parton showering effects.

>Obviously finding such optimal value is not trivial and typically no one try to find such optimal value.
Does it make sense to perform a variation with respect to dR_min and check how it affects the differential distributions profiles?

Am II right that following the same logic one has also to set up the lower pT cut to a certain value in order to avoid singular cross section?

Thank you!

With best regards,
Oleh

 Olivier Mattelaer (olivier-mattelaer) said on 2018-05-08: #3

Hi,

> Does it make sense to perform a variation with respect to dR_min and check how it affects the differential distributions profiles?

If you observe a difference, it means that you screw up your generation.
But in order to check that you are indeed insensitive to your madgraph cut it does make sense.

> Am II right that following the same logic one has also to set up the
> lower pT cut to a certain value in order to avoid singular cross
> section?

Yes. (or anyother set of cuts which remove the singularity obviously)

Cheers,

Olivier

> On 8 May 2018, at 18:27, oleh <email address hidden> wrote:
>
> Question #668743 on MadGraph5_aMC@NLO changed:
>
>
> oleh is still having a problem:
> Dear Oliver,
>
>
> questions:
>
>> Do you mean after hadronization or did you bypass it?
> In fact I switch it of since I am interested only in parton showering effects.
>
>> Obviously finding such optimal value is not trivial and typically no one try to find such optimal value.
> Does it make sense to perform a variation with respect to dR_min and check how it affects the differential distributions profiles?
>
> Am II right that following the same logic one has also to set up the
> lower pT cut to a certain value in order to avoid singular cross
> section?
>
> Thank you!
>
> With best regards,
> Oleh
>
> --

 oleh (fedkevych) said on 2018-05-08: #4

Dear Oliver,

thank you very much for clarifying this issue to me.