CalcHEP

Can't generate events for the process e+e- -> e+e-e+e-

Asked by Gabriele Martelli

Hello everyone,

I am trying to generate a high number of events (one million) for the process e+e- -> e+e-e+e- with center of mass energy equal to 1 GeV (I want to simulate a collider so each particle of the initial state has energy equal to 500 MeV).
The model I am using is the Standard Model SM where I have personally defined the electron mass equal to 0.511 MeV and the electromagnetic alpha constant for processes that do not include the Z boson, alfa_em = 3.0286E-01.
During the MonteCarlo simulation, when I run "Start Integration", CalcHEP is able to calculate cross section values ​​but they are very different from each other and have a very high error.
Once the integration is over, I run the generation of events but this does not seem to work, as the system is unable to produce events. There are no CalcHEP errors, simply the generation of events seems to be incredibly slow. I have tried to generate even a very small number of events (around a thousand events) but the problem remains the same.
I have tried to generate this process both on my local computer and on the Cern computers with the latest updated version of CalcHEP without getting results.

Can anyone help me out?

Thank you for your time,

Best regards,
Dr. Gabriele Martelli

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Alexander Belyaev (alexander.belyaev) said : 		#1

You need to achieve first a high accuracy of the integration(below 1%)
before you generate events, otherwise it will take forever -- i.e.
if the accuracy if bad
the efficiency of the integration/generation is too low.

You should try to make regularisations (see manual) -- this would allow you
to achieve better accuracy and then to generate events

You can use HEPMDB (hepmdb.soton.ac.uk) cluster to rub long job on HPC computer
with a lot of callc-- I think you need of the order ot 1M Vegas calls

Best
Alexander

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Alexander Belyaev (alexander.belyaev) said : 		#2

Hello Gabriele,

I did not hear any reaction from your side -- hrere is an update for you

To generate events for this proess you need:

1. choose quadruple precision in calchep and compile it with this precision

in folder "include"
edit nType.h and comment
//#define _LONG_

and uncomment
#define _QUADGCC_

2. Use regualrisation
#Regularization
*** Table ***
 Regularization
 Momentum |> Mass <|> Width <| Power|
13 |0 |0 |2
14 |0 |0 |2
25 |0 |0 |2
26 |0 |0 |2
35 |1 |0 |2
35 |1 |0 |2
45 |1 |0 |2
46 |1 |0 |2
=========================================

3. Request 1M calls for Vegas session and run it for ~2 days

Then you will get results

Regards,
Alexander

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