Different cross section calculated in two versions

Asked by Westley Watt

Dear all,

accidentally, I have found a big difference in the cross section of some interactions calculated with two versions of the CalcHEP code. For example, the total cross section for the interaction u,ne->u,u,U,ne is calculated as ~145 and ~6.8 pb at p=100 GeV in the code version 3.7 (Apr 28, 2018) and 3.7.5 (Apr 23, 2019), respectively. Interestingly, the number of Feynman and squared diagrams is the same in both versions. The same initial conditions, operating system and computer were used in both calculations.

I would be interested to learn what code change causes this difference and what version of the code gives correct result for this type of interactions.

Best regards,
Westley

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Alexander Pukhov (pukhov) said :
#1

If you set cuts

C(u) < 0.9

M(u,U) > 1

then you'll get the same answer in both versions. About 0.195pb.

The problem is caused by lost of precision in case of  forward
u-scattering.  May be there is a finite answer without cuts.

Best

    Alexander Pukhov

On 04.12.2019 19:17, Westley Watt wrote:
> New question #686661 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/686661
>
> Dear all,
>
> accidentally, I have found a big difference in the cross section of some interactions calculated with two versions of the CalcHEP code. For example, the total cross section for the interaction u,ne->u,u,U,ne is calculated as ~145 and ~6.8 pb at p=100 GeV in the code version 3.7 (Apr 28, 2018) and 3.7.5 (Apr 23, 2019), respectively. Interestingly, the number of Feynman and squared diagrams is the same in both versions. The same initial conditions, operating system and computer were used in both calculations.
>
> I would be interested to learn what code change causes this difference and what version of the code gives correct result for this type of interactions.
>
> Best regards,
> Westley
>

Revision history for this message
Alexander Belyaev (alexander.belyaev) said :
#2

Dear Westley,
did this solve your problem?
Alexander

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

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