Cross sections for processes with mixed orderEW/orderQCD diagrams

[Frank Siegert]

When generating processes without fixing the Order_EW or Order_QCD of the diagrams, Sherpa will wrongly enhance the interference between diagrams of different Order_EW/Order_QCD. This is not a problem in any of our default setups, because there we restrict the Order_EW, but if one includes all (e.g. because of cuts which enhance the EW contributions such that they are needed), one will run into this bug.

Example: Sherpa's cross section for pp -> P b bar around the Z->b bbar peak doesn't reproduce the one in Madgraph if the following cuts are applied:

• pT(photon) > 200 GeV
• pT(b) > 50 GeV
• pT(bbar) > 50 GeV
• |eta(photon)| < 2.5
• |eta(b)| < 2.5
• |eta(bbar)| < 2.5
• dR(photon, b) > 0.6
• dR(photon, bbar) > 0.6
• 89<m_{b bbar}<93

Using cteq6l1 and a fixed fac/ren scale=mZ, Madgraph gives 0.007710 pb, while Sherpa gives 0.020742 pb. If I select the resonant diagrams only, i.e. pp -> P Z[b bbar] then Sherpa (0.005917 pb) and Madgraph (0.005869 pb) agree well.

[Frank Siegert]

Hmmm, I just notice that the Madgraph integration results seem to have huge errors, ~100%. So the numbers above can't be trusted. I'll try to find out how to improve that. Also I have to change the ren/fac to actually be fixed (.true. instead of F) and maybe just to be safe set the bwcutoff higher, like 150. Will post back with new results.

[Frank Siegert]

Even if I integrate to lower errors (and with actually fixed scales), I still see the same discrepancy. I've simplified it even further, by looking at only one subprocess, namely u ubar -> gamma b bbar (in Madgraph that's uu~>bb~a and u~u>bb~a). Furthermore I've tried to identify whether it's the cuts that may be wrong by removing them successively. But whatever I do, in all cases Madgraph's cross section is only ~40% of Sherpa's. Are there any other ideas where this difference could come from?

[Frank Siegert]

This is fixed on Sherpa SVN but will need to be thoroughly tested before it can be released.