Matching: how to chose the xqcut value ?

Hi Swasti,

> I read your matching document where it says : "This value should be related to the hard scale (e.g. mass of produced particle, HT cut, or similar) in the process, and set to ~(1/6-1/3 x hard scale)" .
> Does this mean that I can choose any value in the range of 1/6(mx) - 1/3(mx), where mx is the mass of the particle x (for eg. 1- 100 GeV) ?

No it means that we have no clue in general which value to choose. You have to test in a case by case fashion which value works.
This is a guideline which works most of the time. It will not always work, for example, I doubt that the guideline will work if you have a particle of 1 GeV.

> Also, I don’t completely understand that in my RUN_CARD, if I put a parton level cut on my leading jet PT say of 80 GeV or a minimum PT cut of 80 GeV for jets, how will matching with an xqcut (say of 30 GeV) work in this case?

If you want to put a cut on the leading jet, then you need
1) to have at least one jet in each sample. (i.e. do not include the 0 jet sample in the matching/merging procedure)
2) that you use the cut in the run_card ptj1min forcing the value of the leading jet
3) that you choose a value of Qcut which is LOWER than your cut on the leading jet. (otherwise do not make any sense)
4) the hard scale of your process that you should use for determining your qcut value need to include the cut on the first jet (i.e. in this case your hard scale is ~ 80 => we suggest to look for value between 15 and 40)

Note that
a) ptj is not use by default in matching computation since you have auto_pt_mjj =True. In any case you can not have ptj larger than xqcut.
b) I never run such kind of computation, but the above rule seems the minimal in order to have a coherent treatment of the jets.

> Apart from choosing an xqcut value, I understand that I should use the auto_ptj_mjj and change my Pythia parameters.

Using auto_ptj_mjj is strongly advise, since it’s allow to use the most optimal value for ptj and mjj for the generation.
The only case where this is not advise is in presence of QED jet (coming from W/Z decay) where you typically do not want such cut.

For Pythia, you typically have to specify the qcut value. (for kt-mlm not that you have to put QCUT > xqcut) a difference of 10 GeV is advise as a minimum difference.

Cheers,

Olivier

PS: I would advise you to study the following slides:
https://cp3.irmp.ucl.ac.be/projects/madgraph/attachment/wiki/FHEP/Mattelaer_parton_shower_matching_merging.pdf
Which are covering all what you need to know about parton-shower, matching,merging

On 27 Mar 2015, at 18:06, Swasti Belwal <email address hidden> wrote:

> New question #264266 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/264266
>
> Hi ,
>
> I want to generate a matched sample for a process say p p > x x~ j. For this I add another process p p > x x~ to my PROC_CARD.
>
> I want to know how should I chose the right xqcut value for MLM matching in my case.
>
> I read your matching document where it says : "This value should be related to the hard scale (e.g. mass of produced particle, HT cut, or similar) in the process, and set to ~(1/6-1/3 x hard scale)" .
> Does this mean that I can choose any value in the range of 1/6(mx) - 1/3(mx), where mx is the mass of the particle x (for eg. 1- 100 GeV) ?
>
> Also, I don't completely understand that in my RUN_CARD, if I put a parton level cut on my leading jet PT say of 80 GeV or a minimum PT cut of 80 GeV for jets, how will matching with an xqcut (say of 30 GeV) work in this case?
>
> Apart from choosing an xqcut value, I understand that I should use the auto_ptj_mjj and change my Pythia parameters.
>
> Please guide me on the right way to do the matching here.
>
> Thanks in adavance!
> Swasti
>
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