disagreement between theoretical and MadGraph2.9 results for t-channel processes
Hi,
I notice that recently you have released MadGraph2.9, which can efficiently deal with the phase space integrals especially for t-channel processes. Congratulations!
I have then transferred from MadGraph2.6.7 to MadGraph2.9.1.2 for the neutrino trident production project. The process I am considering is a muon neutrino scatters off an oxygen target. However, MadGraph gives very different cross sections compared with the theoretical ones. For example, for the "vm nf- > vm mu+ mu- nf-" process (with nf- representing the oxygen target), when the incident neutrino energy is 100GeV, Madgraph gives "3.448e-06 +- 3.737e-09 pb" with 100k events and using the dipole form factor, while the theoretical prediction (using an equivalent photon approximation and a dipole form factor for the target) is 1.10917e-4. Alternatively, using CompHEP with also a dipole form factor, I got 1.1857e-4 (uncertainty is within 1%).
In addition, when the incident neutrino energy is around 500GeV, the resulting cross section is of O(10) when I turn off the form factor. In comparison, the theoretical prediction is of O(1e-3) with the form factor either on or off. This seems to me that a divergent result is returned from MadGraph or the sampling is taken only for large momentum transfer where the contributions are expected to be negligible. ---- To produce the above numbers by MadGraph, I have removed all cuts applied to the leptons in the run_card.
Do you have a clue about why the difference for the cross sections? My UFO file, run_card, proc_card will be attached below for debugging.
To provide more information: Since all the processes are t-channel ones, while using CompHEP, I specify the kinematics as well as the regularization to tell CompHEP how to deal with the situation where the virtual propagators become on-shell. Given the difference on the cross sections above, I am thinking that if MadGraph could also allow us to set the kinematics and/or the regularization similar to CompHEP so the dangerous regime of the phase space integral can be avoided. Thanks.
Best,
Yong
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