e+ e- > e+ X diverges for large etal
Hi,
1. how does MadGraph compute the processes e+ e- > e+ X, when the final e+ becomes collinear?
2. also, how is the pdf of a photon from electron implemented in MadGraph?
We found something explicit mentioned for the pdf of a photon from a proton here (on the Manual-March-2007), but nothing regarding photon from electron.
The reasons of the above questions is the following problem.
We are interested in a process like
e+ e- > e+ vl w-
with e+ undetected.
Therefore we’d like to integrate the above cross section for all the final e+ that do not enter the detector, so say with \theta < mrad (eta >~ 5).
The cross section computed with the photon pdf should in principle provide an upper bound to the result, cause it is obtained by summing on the available final e+ phase space. Correct?
MadGraph indeed gives a finite result for this calculation, as expected.
We find that the cross section for e+ e- > vl e+ w- diverges as we increase etal at the generator level, while it should converge to the value obtained using the photon pdf.
Is there anything we don’t understand in the way this is implemented in MadGraph?
We find these results using v2_5_5 and v2_4_3.
Thank you in advance
Dario, Filippo and Andrea
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