e+e- into gamma gamma

Dear Federica,

indeed for M^2 CalcHEP provides non-symmetrised analytical expression,

while as you have noticed numerically it takes into account u-channel
diagram.

To get the correct analytical expression and take into account u-channel
diagram, you should use

  sum_22.m file

located in $CALCHEP/utile

folder

see $CALCHEP/utile/README and CalcHEP's manual

Regards,

Alexander

On 12/02/2020 15:02, FEDERICA giacchino wrote:
> New question #688703 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/688703
>
> Dear all,
> I am computing e+e- -> gamma gamma and looking to the squared amplitude calchep consider only t-channel^2 and u-channel*t-channel... u-channel^2 is not included. there is any way to include it? indeed comparing the differential cross-section from calchep and analytically there is a difference that I blame on that diagram.
>
> Thanks
>
--
______________________________________________________________________
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
.....................................................................
Particle Physics Department, Rutherford Appleton Laboratory
Didcot, OX11 0QX, TEL: +44 12354 45562, FAX: +44 12354 46733
.....................................................................
CERN, CH-1211 Geneva 23, Switzerland
Office 40/1-B20, Mailbox: E27910, TEL: +41 2276 71642
______________________________________________________________________

Dear Alexander,

thanks for the answer.

In my analytical computations I included u-channel: I understand the difference asking to "set distribution" the differential cross-section in angle and plotting with my analytical computation. But I need that in Calchep is included, because I have to make MC simulations with all diagrams. Is it possible?

Federica

Dear Federica,

as I said, the numerical calculation as well as plots from numerical session

include u-channel -- i.e. CalcHEP produces the correct result in the
numerical session.

So it is already there!

Regards,

Alexander

On 12/02/2020 15:28, F.g. wrote:
> Question #688703 on CalcHEP changed:
> https://answers.launchpad.net/calchep/+question/688703
>
> Status: Answered => Open
>
> F.g. is still having a problem:
> Dear Alexander,
>
> thanks for the answer.
>
> In my analytical computations I included u-channel: I understand the
> difference asking to "set distribution" the differential cross-section
> in angle and plotting with my analytical computation. But I need that in
> Calchep is included, because I have to make MC simulations with all
> diagrams. Is it possible?
>
> Federica
>
--
______________________________________________________________________
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
.....................................................................
Particle Physics Department, Rutherford Appleton Laboratory
Didcot, OX11 0QX, TEL: +44 12354 45562, FAX: +44 12354 46733
.....................................................................
CERN, CH-1211 Geneva 23, Switzerland
Office 40/1-B20, Mailbox: E27910, TEL: +41 2276 71642
______________________________________________________________________

Ok, thanks.