CalcHEP

Fixed-target process: electroproduction of a muon pair

Asked by Andrea Celentano

Dear colleagues,
I am trying to simulate the process e- N -> e- mu- mu+ N with CalcHEP, where "N" is a Tungsten nuclei. This is the order of the particles as entered in CalcHEP
To do so, I modified the SM model as follows:

* I introduced "N" as a scalar particle in CalcHEP, with appropriate mass.
* I introduced the gamma-N-N vertex
* I set the electron mass and the muon mass to their physical values
* I introduced a custom form factor for the gamma-N-N vertex, following what is reported in arXiv: 0906.0580, appendix A. I did so by using the usrFF.c function

By doing so, I am able to write the process in CalcHEP and start the numerical session (I excluded diagrams with the Z). I set the momenta of the initial state particles to 4 GeV and 0 GeV, respectively, to simulate a fixed-target collision.

I modified the "kinematics" session as follows:

in = 12 -> out1 = 36 out2 = 45
in = 36 -> out1 = 3 out2 = 6
in = 45 -> out1 = 4 out2= 5

I have not entered any regularization, neither any kinematic cut.

If I start the integration, the results are really unstable, with the cross section fluctuating between positive and negative values, a large number of negative points (50% - 60%), and a "bad precision" message reported.

I created a tar file of the folder where I am executing CalcHEP model, and also placed the usrFF.c file there. This is available here: http://www.ge.infn.it/~celentan/testMupMum.tar

I thank you in advance for your support.
Bests,

Andrea Celentano

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Revision history for this message
Alexander Pukhov (pukhov) said : 		#1

I repeated  calculation of your processes with tau lepton instead of
Tungsten.  Then I set  tau mass Ml=177.7   and launched calculations again.

It both cases I see a good MC  convergence  and absence of warnings.

Could you repeat this example and confirm that CalcHEP works fine for
calculations of  processes of such type?

Best

    Alexander Pukhov

On 6/20/23 10:35, Andrea Celentano wrote:
> New question #707044 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/707044
>
> Dear colleagues,
> I am trying to simulate the process e- N -> e- mu- mu+ N with CalcHEP, where "N" is a Tungsten nuclei. This is the order of the particles as entered in CalcHEP
> To do so, I modified the SM model as follows:
>
> * I introduced "N" as a scalar particle in CalcHEP, with appropriate mass.
> * I introduced the gamma-N-N vertex
> * I set the electron mass and the muon mass to their physical values
> * I introduced a custom form factor for the gamma-N-N vertex, following what is reported in arXiv: 0906.0580, appendix A. I did so by using the usrFF.c function
>
>
> By doing so, I am able to write the process in CalcHEP and start the numerical session (I excluded diagrams with the Z). I set the momenta of the initial state particles to 4 GeV and 0 GeV, respectively, to simulate a fixed-target collision.
>
> I modified the "kinematics" session as follows:
>
> in = 12 -> out1 = 36 out2 = 45
> in = 36 -> out1 = 3 out2 = 6
> in = 45 -> out1 = 4 out2= 5
>
> I have not entered any regularization, neither any kinematic cut.
>
> If I start the integration, the results are really unstable, with the cross section fluctuating between positive and negative values, a large number of negative points (50% - 60%), and a "bad precision" message reported.
>
> I created a tar file of the folder where I am executing CalcHEP model, and also placed the usrFF.c file there. This is available here: http://www.ge.infn.it/~celentan/testMupMum.tar
>
> I thank you in advance for your support.
> Bests,
>
> Andrea Celentano
>
>
>

Revision history for this message
Andrea Celentano (celentan) said : 		#2

Dear Alexander,
thanks for your quick reply. I tried your suggestion, and it works. Since the role of the target nucleus is almost negligible in this reaction, I'll try to proceed with this solution and check the obtained results.

Thanks,
Andrea