Matching for top-quark FCNC decay

Asked by Salvatore La Cagnina

We are currently studying potential interference effects in top-quark FCNC processes and are facing a problem regarding matching. We are producing samples at LO using the TopFCNC UFO model. Specifically we are generating

a) the "FCNC production process" with "generate p p > t z $$ t~" and "add process p p > t z j $$ t~" (+ antiparticle processes)
including the extra jet in the final state leads to possible interference with the decay process

b) the "FCNC decay process" with "generate p p > t t~ > w+ b u~ z" (+ antiparticle process)

these two are compared with

c) a sample containing both processes with "generate p p > t z" and "add process p p > t z j" (+ antiparticle processes)

The samples a) and c) have matching turned on.
Sample b) shouldn't need matching but we switched matching on and off to see the effect.
As it turns out there are quite some differences in the kinematic properties.
The spectrum of the pT of the Z Boson and the leading Jet pT for instance are softer in case matching is turned on.

The main question is weather matching should be turned on or off in case of the decay process when it is not necessary but shows to have a visible impact. Or if maybe there is additional information to be considered.

The setting for the matching used in the run_card.dat:

#*********************************************************************
# Matching parameter (MLM only)
#*********************************************************************
 1 = ickkw ! 0 no matching, 1 MLM
 1.0 = alpsfact ! scale factor for QCD emission vx
 False = chcluster ! cluster only according to channel diag
 5 = asrwgtflavor ! highest quark flavor for a_s reweight
 False = auto_ptj_mjj ! Automatic setting of ptj and mjj if xqcut >0
                                   ! (turn off for VBF and single top processes)
 30.0 = xqcut ! minimum kt jet measure between partons
#*********************************************************************
#
#*********************************************************************

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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

You have two impact of this:

1) you have the xqcut variable which will put a cut in your phase-space that is not present otherwise (I guess).

2) you are modifying the factorisation and renormalisation scale to pass to a vertex by vertex scale choice
(instead of a global scale). Also the dependence in the PDF is now quite complex since each ISR vertex is called with an additional ratio of pdf.

The first of those two points can actually lead to bias/..., while the second should be within your theoretical uncertainty (and if not it means that your theoretical uncertainties are too small)

Cheers,

Olivier

> On 15 Mar 2019, at 00:32, Salvatore La Cagnina <email address hidden> wrote:
>
> New question #679210 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/679210
>
> We are currently studying potential interference effects in top-quark FCNC processes and are facing a problem regarding matching. We are producing samples at LO using the TopFCNC UFO model. Specifically we are generating
>
> a) the "FCNC production process" with "generate p p > t z $$ t~" and "add process p p > t z j $$ t~" (+ antiparticle processes)
> including the extra jet in the final state leads to possible interference with the decay process
>
> b) the "FCNC decay process" with "generate p p > t t~ > w+ b u~ z" (+ antiparticle process)
>
> these two are compared with
>
> c) a sample containing both processes with "generate p p > t z" and "add process p p > t z j" (+ antiparticle processes)
>
> The samples a) and c) have matching turned on.
> Sample b) shouldn't need matching but we switched matching on and off to see the effect.
> As it turns out there are quite some differences in the kinematic properties.
> The spectrum of the pT of the Z Boson and the leading Jet pT for instance are softer in case matching is turned on.
>
> The main question is weather matching should be turned on or off in case of the decay process when it is not necessary but shows to have a visible impact. Or if maybe there is additional information to be considered.
>
> The setting for the matching used in the run_card.dat:
>
> #*********************************************************************
> # Matching parameter (MLM only)
> #*********************************************************************
> 1 = ickkw ! 0 no matching, 1 MLM
> 1.0 = alpsfact ! scale factor for QCD emission vx
> False = chcluster ! cluster only according to channel diag
> 5 = asrwgtflavor ! highest quark flavor for a_s reweight
> False = auto_ptj_mjj ! Automatic setting of ptj and mjj if xqcut >0
> ! (turn off for VBF and single top processes)
> 30.0 = xqcut ! minimum kt jet measure between partons
> #*********************************************************************
> #
> #*********************************************************************
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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