Crossection value is coming but no event is generating for a particular process

Asked by Atanu Guha on 2020-09-17

I am trying to generate 10000 events for a process, but after running for more than 6 hours, no events has been generated. Calchep did not show any error though, it is returning the correct crosssection... Please find my batchfile below.

###################################################
Model: SM
Model changed: False
Gauge: Feynman

Process: e%,E%->e,E,A

Kinematics : 12 -> 34 , 5
Kinematics : 34 -> 3 , 4

pdf1: ISR & Beamstrahlung
pdf2: ISR & Beamstrahlung

Bunch x+y sizes (nm) : 560.0
Bunch length (mm) : 0.4
Number of particles : 2.0E+10

Polarization1:0.4
Polarization2:0.15

p1: 500
p2: 500

Cut parameter: T(A)
Cut invert: False
Cut min: 0
Cut max: 500

Cut parameter: E(A)
Cut invert: False
Cut min: 8
Cut max: 500

Cut parameter: N(A)
Cut invert: False
Cut min: -2.99448
Cut max: +2.99448

Dist parameter: M(e,E)
Dist min: 0
Dist max: 600
Dist n bins: 100
Dist title: e,E->e,E,A
Dist x-title: M(e,E) (GeV)

Dist parameter: N(A)
Dist min: -5
Dist max: 5
Dist n bins: 300
Dist title: e,E->e,E,A
Dist x-title: N(A)

Dist parameter: T(A)
Dist min: 0
Dist max: 600
Dist n bins: 100
Dist title: e,E->e,E,A
Dist x-title: T(A) (GeV)

Dist parameter: E(A)
Dist min: 0
Dist max: 600
Dist n bins: 100
Dist title: e,E->e,E,A
Dist x-title: E(A) (GeV)

Dist parameter: Y(A)
Dist min: -4.0
Dist max: +4.0
Dist n bins: 100
Dist title: e,E->e,E,A
Dist x-title: Y(A)

Dist parameter: J(e,E)
Dist min: 0
Dist max: 7
Dist n bins: 300
Dist title: e,E->e,E,A
Dist x-title: J(e,E)

Number of events (per run step): 10000
Filename: ee_eeA

NTuple: False
Cleanup: False
Max number of cpus: 8

nSess_1: 5
nCalls_1: 100000
nSess_2: 5
nCalls_2: 100000

###############################

I don't understand what might be gone wrong.

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Solved by:
Alexander Belyaev
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Alexander Belyaev (alexander.belyaev) said :
#1

Dear Atanu,

when you do such calculations,
you need to also check the accuracy of your result
as well as covergency of this accuracy.

The accuracy of your result is 20-30% from your batch and it does not converge.
This means that you can not really trust the cross section you are getting.
Moreover, if the accuracy is bad, then efficiency of the event generation is very low and
the process of event generation could take forever. This what you have.

So solve the problem you need to make sure first of all that your cross section converges and the accuracy i good ( ~ 1%).

In your particular case you have collinear divergence which you did not solve:
you have the cut only on the photon energy(which only eliminates "soft" divergence) , but not angle.

You need to make sure that your cross section converges first.

Regards,
Alexander

Revision history for this message
Atanu Guha (atanu-guha) said :
#2

Right. Thank you. Now I got the physical sense what was going wrong. But still I have doubt about the solution...

I mean... I already set up the following cut

######################
Cut parameter: N(A)
Cut invert: False
Cut min: -2.99448
Cut max: +2.99448
######################

Do I need to specify another cut directly on cos theta to remove the divergence?

Revision history for this message
Best Alexander Belyaev (alexander.belyaev) said :
#3

If final state electron radiates collinear photon,

there is divergence

On 2020-09-28 14:41, Atanu Guha wrote:
> Question #692972 on CalcHEP changed:
> https://answers.launchpad.net/calchep/+question/692972
>
> Status: Answered => Open
>
> Atanu Guha is still having a problem:
> Right. Thank you. Now I got the physical sense what was going wrong. But
> still I have doubt about the solution...
>
> I mean... I already set up the following cut
>
> ######################
> Cut parameter: N(A)
> Cut invert: False
> Cut min: -2.99448
> Cut max: +2.99448
> ######################
>
> Do I need to specify another cut directly on cos theta to remove the
> divergence?
>
--
______________________________________________________________________
Prof. Alexander S Belyaev (<email address hidden>)
https://www.hep.phys.soton.ac.uk/content/alexander-belyaev

School of Physics & Astronomy, University of Southampton
Office 5047, SO17 1BJ, TEL: +44 23805 98509, FAX: +44 23805 93910
.....................................................................
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.....................................................................
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______________________________________________________________________

Revision history for this message
Atanu Guha (atanu-guha) said :
#4

Thanks a lot. Yeah I got it now...

Revision history for this message
Atanu Guha (atanu-guha) said :
#5

Thanks Alexander Belyaev, that solved my question.

Revision history for this message
SAUMYEN KUNDU (saumyen.k) said :
#6

Hi Alexander,
I am also facing similar problem for this process. In my system I'm getting even getting different cross-sections for different number of generated events.
Can you suggest something to remove/deal with this collinear divergence in this case? I mean, is there a way out?

Regards,
Saumyen

Revision history for this message
Alexander Belyaev (alexander.belyaev) said :
#7

Dear Saumyen

1. which version of CalcHEP you are using?
2. Please send me prt_ and session .dat from your calculation
to <email address hidden>

Next time please open new question.

Regards
Alexander