How to get the total cross section (i.e. xsec_SM+xsec_BSM)

Asked by S


I'm trying to calculate the cross section of a certain process, including an EFT (BSM) model. Before loading the model, I calculate the SM cross section,

generate [process]

then I calculate the BSM contributions (linear and quadratic) after loading the model,

generate [process] [operator]^2==1
generate [process] [operator]==1

But when I try to calculate the total cross section after loading the model,

generate [process]
(tried generate [process] [operator]<=1)

 it doesn't add up to the sum of xsec_SM and xsec_BSM

am I doing something wrong when calculating the total cross section?

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Olivier Mattelaer (olivier-mattelaer) said :


Note that
1) default run_card depend of the process so you need to be sure that you use the same cuts
2) You need to use a simple dynamical scale (i.e. not our default -1,since -1 does not make sense for pure interference).
If you use -1 agreement should be checked within scale uncertainty.



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S (askings) said :

Dear Olivier, many thanks!

So here is an example of how I run it:

import model SM_Ltotal_Ind5v2020v2_UFO
generate a a > e+ e- mu+ mu-
set lpp1 2
set lpp2 2
set fixed_fac_scale True
set anoinputs fm7 1.e-8
set anoinputs fs0 0
set anoinputs fs1 0
set anoinputs fs2 0
set anoinputs ft0 0
set anoinputs ft1 0
set anoinputs ft2 0
set anoinputs ft3 0
set anoinputs ft4 0
set anoinputs ft5 0
set anoinputs ft6 0
set anoinputs ft7 0
set anoinputs ft8 0
set anoinputs ft9 0
set anoinputs fm0 0
set anoinputs fm1 0
set anoinputs fm2 0
set anoinputs fm3 0
set anoinputs fm4 0
set anoinputs fm5 0
set anoinputs fm6 0

What would you change here to calculate the total cross section including the BSM contribution? Because when I run this, the value obtained is not in agreement with the sum of xsec_SM and xsec_BSM. My apologies for naivety, I'm fairly new to this

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Olivier Mattelaer (olivier-mattelaer) said :

First for the second, third run I would pass the path to the run_card of the first run such that you are sure to use the same run_card.dat
Then I would either add
set fixed_ren_scale True
set dynamical_scale_choice 3 # 3 means Ht/2

Now, I do not know that model and the associated convention. So I do not know what is included when you do not specify any coupling order in the process definition.

Now you should also be aware that phase-space integration for interference term are very difficult and that I have actually no clue how to do those efficiently. So it is also possible that such contribution is biased for your process. I would advise to check that both integration method (sde_strategy=1 and sde_strategy=2) are consistent for the interference term. This might be your issue, since elastic photon on their own are already quite hard to integrate (so adding one more difficulty can be a problem): see 2102.00773

Finally, I would advise to use a restriction model instead of setting a lot of parameters to zero. This can also help the phase-space integrator:

Can you help with this problem?

Provide an answer of your own, or ask S for more information if necessary.

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