# W + jets at NLO

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I would like to calculate $pp \rightarrow W (\rightarrow \ell \nu)$ + n-jets (n=2, maybe also 3) at NLO, with some cuts and plot some distributions.

I used MadGraph extensively for LO processes and I was wondering how reliable it is at NLO? Does anyone know any reference comparing multiple NLO codes? I have tried MCFM and I am not very experienced with it but the calculation seems to take a lot of time (obviously since it's NLO). Is there a way to make it run in parallel? I know that you can separate the real emission and virtual parts and then combine them later, but that doesn't seem to give a huge speed increase. Any help is appreciated. Thanks.

This post imported from StackExchange Physics at 2014-04-23 15:14 (UCT), posted by SE-user henry mcfly
You'd probably be better off asking selected colleagues who know these tools rather than Physics SE. The difficulty being the level of specialization ... I'm an experimental particle physicist and I've never heard of either tool. (Because I've never done collider physics ... it's been JLAB and neutrinos for me).

This post imported from StackExchange Physics at 2014-04-23 15:14 (UCT), posted by SE-user dmckee
Note also that parallelization of a code is often a non-trivial exercise.

This post imported from StackExchange Physics at 2014-04-23 15:14 (UCT), posted by SE-user Kyle Kanos
@dmckee: yes, I will ask my experimental colleagues as well but as far as I understand there are groups in the experimental collaborations who only deal with generating data, and I don't know anyone from that group and the others don't seem to know much about it.

This post imported from StackExchange Physics at 2014-04-23 15:14 (UCT), posted by SE-user henry mcfly
@KyleKanos: For this case it is more or less trivial, since each Feynman diagram and interference terms can be calculated separately (at least for the LO case), but divergences at NLO order might make things a little bit more complicated.

This post imported from StackExchange Physics at 2014-04-23 15:14 (UCT), posted by SE-user henry mcfly
@dmckee: As far as I know Madgraph is the most used software for simulations at the LHC. I don't know enough about it to answer the question, but I think there is a chance somebody might be able to offer some insight.

This post imported from StackExchange Physics at 2014-04-23 15:14 (UCT), posted by SE-user JeffDror

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A search of the CERN document server for W+jets at NLO gives this reference as most recent and relevant:

In these proceedings we present results from a recent calculation for the production of a W boson in conjunction with five jets at next-to-leading order in perturbative QCD. We also use results at lower multiplicities to extrapolate the cross section to the same process with six jets.

In the PDF they refer to the programs used. You might contact one of the authors and get some help.

We use SHERPA [13] in association with the virtual matrix elements from BLACKHAT[14] to obtain our NLO predictions.

This post imported from StackExchange Physics at 2014-04-23 15:14 (UCT), posted by SE-user anna v
answered Apr 23, 2014 by (2,005 points)

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