Photon from Electron

Asked by on 2020-05-01

We have a question regarding the probability distribution of photons coming from electron as can be generated by the switches of "0" and "3" in the Madgraph runCard.dat.
Could you please let us know the functions that are used there?

Apparently we cannot match the distributions by both of these tools for electron, photon collisions where the photon comes from electrons.

We find some details for the CalcHEP approximation and the values differ for the choices of Q, which are explained below. If you can help us in understanding the Madgraph calculation that would be of great help.

In CalcHEP, they have used Q in Weizsaecker-Williams approximation

f(x) = (q 2 α/(2π))(log((1 − x)/(x 2 δ))(1 + (1 − x) 2 )/x − 2(1 − x − δ x2 )/x),

, δ = (m/Qmax) 2 . Qmax sets out the region of photon virtuality (P 2 > −Q2 max) w

 "V. M. Budnev, I. F. Ginzburg, G. V. Meledin, V. G. Serbo, The Two photon particle production mechanism. Physical problems. Applications. Equivalent photon approximation, Phys. Rept. 15 (1975) 181–281. doi:10.1016/0370-1573(75)90009-5."

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"0" means no PDF (i.e. fixed energy)
"3" means "Weizsaecker-Williams approximation"
If you want to check the formula, you will find it in Source/PDF/PhotonFlux.f

you also have a parameter Qmax that you can setup.
Such setup should be quite clear if you use the latest version of the code.
This parameter can be setup via the "dsqrt_q2fact1" (or "dsqrt_q2fact2")
of the run_card.dat.

If you are not using the latest version of the code, the code behaves slightly differently but let's not discuss it.



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