User-defined width

On 16.11.2020 17:45, Michał Iglicki wrote:
> New question #694041 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/694041
>
> Dear calcHEP Team,
>
> I am working on a model containing some additional particles. I am investigating a resonant region, where widths in the propagators play important role. Since some corrections to the widths are present, I would like to modify the widths by hand.
> My questions are:
>
> (1) How can I provide a value of particle's width by hand instead of letting calcHEP calculate it automatically? I tried to remove the "!" mark before the width's name in prtcls.mdl (e.g. "!wX" -> "wX") and define the width as a parameter in vars.mdl, but it didn't work and the width is still calculated automatically.

Change model  using CalcHEP GUI interface.  Say, add and remove one
symbol in one file model  and save correction.

Alternatively  you can remove directory

     results/aux/

>
> (2) Is there a way to define by hand a momentum-dependent width of a given particle? I saw one of our answers suggesting that edition of sqme.c file is required, however it wasn't clear to me how to do it.

    Which dependence  you need? For small  virtual momentum width has to
go to zero. For large momentum width has to increase. Here we have the
following problem.  Large width breaks gauge invarience.

   So, I have in mind to implement  running width somehow.

   I look at code and write you how we can implement running widths.

Best

    Alexander Pukhov

>
> With best regards
> Michał Iglicki
>

Dear Prof. Pukhov,

thank you for your answer! Should removing the "aux" directory change the result of width calculation in the GUI or only affect the propagator during a numerical session? When I click "Numerical evaluation"->"Masses Widths, Branch." and I check the given particle, the width is still calculated even though I provided a number via model files.

Here:

  https://www.fuw.edu.pl/~miglicki/mdl/mdl.tar

you can find the model files, the particle of my interest is called "psi-" and denoted as "~~Fm" (the tildas are here due to micrOMEGAs conventions). Could you please verify if the width is calculated as 0 (automatic value) or 1 (provided value) on your machine? Maybe it is a problem with configuration of micrOMEGAs.

Best
Michał Iglicki

PS. Just checked that the cross section calculated during a numerical session changes if I introduce a non-zero width manually, so probably only the width calculated in "Numerical evaluation" menu of a symbolic session is not affected.

Yes, it is right.  In  "Numerical evaluation" we calculate branching
and, thus, have to recalculate width.

Here also there is a way to  avoid width calculation. You can load (via
func#.mdl)  an slha file with widths&branchings.

Then  instead of width evaluation,  CalcHEP will use widths from this  file.

Best

     Alexander Pukhov

On 16.11.2020 21:25, Michał Iglicki wrote:
> Question #694041 on CalcHEP changed:
> https://answers.launchpad.net/calchep/+question/694041
>
> Michał Iglicki gave more information on the question:
> PS. Just checked that the cross section calculated during a numerical
> session changes if I introduce a non-zero width manually, so probably
> only the width calculated in "Numerical evaluation" menu of a symbolic
> session is not affected.
>

Thank you! So, (1) is solved.

Best,
Michał Iglicki

Dear Mihal,
at the moment the only solution to (2) is the following:

if the process you study involves only one resonant diagram,
you can program
./utile/usrFF.c

(user form factor file and define FF as new_propagator/old_propagator )
where new propagator includes expression with momentum dependnet width
and old propagor is the one which CalcHEP is using with the fixed width

Regards,
Alexander

Dear Alexander,

thank you for the answer!

Best,
Michał

Dear Michal,

just one remark the form factor is applied at the level of the Squared matrix element, so the correction will be

(new_propagator/old_propagator )^2

Regards,
Alexander