MadGraph5_aMC@NLO

MadSpin branching ratio

Asked by Sihyun Jeon

Hi Olivier and all MG team,

I have one question regarding the branching ratio calculated using MadSpin along with MadGraph.

I am using model card in [1] with the run config card like below :

import model EffLRSM_NLO
generate p p > n2 n2 QCD=0 [QCD]
output PairProduction_MuMu_ZR1000_N300_WR5000_NLO
launch
shower=OFF
madspin=ON
0
set param_card mn2 300
set param_card mzr 1000
set param_card mwr 5000
set param_card mn1 1.000000e+14
compute_widths zr
compute_widths n2
compute_widths wr+
compute_widths wr-
set nevents 20000
set run_card pdlabel lhapdf
set run_card lhaid 306000
set run_card reweight_scale False
set no_parton_cut
0

(I run it after saving the lines above as "run_config.dat" and "./bin/mg5_aMC run_config.dat")

with MadSpin default card set as below (in the Template/Common/Cards/madspin_card_default.dat) :

set Nevents_for_max_weigth 250 # number of events for the estimate of the max. weight
set max_weight_ps_point 400 # number of PS to estimate the maximum for each event
set max_running_process 100
set spinmode none
define q = j t t~ b b~
decay n2 > mu q q
launch

With the settings like above, I expect the process where zr is produced and then decay into 2 pair of n2 and this n2 both decays into muons with jets (from wr).
The problem is that n2 > mu j j branching ratio is theoretically 1.0 and so I expected that the
crosssection(p p > n2 n2) = crosssection(p p > n2 n2) * BR(n2 > mu j j ) * BR(n2 > mu j j)

But looking at the results I have, the BR are somewhat different from 1., sometimes even bigger than 1. which does not physically make sense.

Is this again the use of nwa causing this problem?

We would prefer to take crosssection(p p > n2 n2) as our signal cross sections rather than BR multiplied ones but still we think it's better if we have understanding on this issue.

Thanks in advance,
Sihyun Jeon.

[1] http://feynrules.irmp.ucl.ac.be/wiki/EffLRSM

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Solved by:
Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

This should be related to some numerical inaccuracy.
Multiple sources are possible but the more likely is that the BR is in this case computed
from a ratio of the width used to generate decay event compare to the total width written in the param_card
(in your case the width evaluated via the Auto method)

Those two numbers are evaluated with different seed and will therefore not agree even if they are theoretically identical.
And this can indeed sometimes lead to BR>1.

So in your case it make perfectly sense to simply use BR=1 in this case.

Cheers,

Olivier

> On 10 Mar 2018, at 05:32, Sihyun Jeon <email address hidden> wrote:
>
> New question #665443 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/665443
>
> Hi Olivier and all MG team,
>
> I have one question regarding the branching ratio calculated using MadSpin along with MadGraph.
>
> I am using model card in [1] with the run config card like below :
>
> import model EffLRSM_NLO
> generate p p > n2 n2 QCD=0 [QCD]
> output PairProduction_MuMu_ZR1000_N300_WR5000_NLO
> launch
> shower=OFF
> madspin=ON
> 0
> set param_card mn2 300
> set param_card mzr 1000
> set param_card mwr 5000
> set param_card mn1 1.000000e+14
> compute_widths zr
> compute_widths n2
> compute_widths wr+
> compute_widths wr-
> set nevents 20000
> set run_card pdlabel lhapdf
> set run_card lhaid 306000
> set run_card reweight_scale False
> set no_parton_cut
> 0
>
> (I run it after saving the lines above as "run_config.dat" and "./bin/mg5_aMC run_config.dat")
>
> with MadSpin default card set as below (in the Template/Common/Cards/madspin_card_default.dat) :
>
> set Nevents_for_max_weigth 250 # number of events for the estimate of the max. weight
> set max_weight_ps_point 400 # number of PS to estimate the maximum for each event
> set max_running_process 100
> set spinmode none
> define q = j t t~ b b~
> decay n2 > mu q q
> launch
>
>
> With the settings like above, I expect the process where zr is produced and then decay into 2 pair of n2 and this n2 both decays into muons with jets (from wr).
> The problem is that n2 > mu j j branching ratio is theoretically 1.0 and so I expected that the
> crosssection(p p > n2 n2) = crosssection(p p > n2 n2) * BR(n2 > mu j j ) * BR(n2 > mu j j)
>
> But looking at the results I have, the BR are somewhat different from 1., sometimes even bigger than 1. which does not physically make sense.
>
> Is this again the use of nwa causing this problem?
>
> We would prefer to take crosssection(p p > n2 n2) as our signal cross sections rather than BR multiplied ones but still we think it's better if we have understanding on this issue.
>
>
>
> Thanks in advance,
> Sihyun Jeon.
>
>
>
> [1] http://feynrules.irmp.ucl.ac.be/wiki/EffLRSM
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Sihyun Jeon (shjeon) said : 		#2

Thanks Olivier Mattelaer, that solved my question.