Matching Details

Asked by Matthew Low on 2014-01-06


I wanted to understand the Madgraph matching behaviour a little better. In particular I wanted to clarify some of the statements in If we consider two processes:
 (A) p p > ve ve~ j, p p > ve ve~ j j
 (B) p p > t b~ j, p p > t b~ j j

In both cases the process contains a mix of different diagrams with different highest jet multiplicities. For (A) p p > ve ve~ j j contains diagrams with an s-channel Z which has two radiated jets, and diagrams with an s-channel quark with one radiated jet. For (B) p p > t b~ j j contains diagrams with an s-channel W which has two radiated jets, and diagrams with a t-channel W with one radiated jet.

My first question is, suppose I was also matching p p > ve ve~ j j j for (A). Then matching is being done separately for the s-channel Z diagrams, and the s-channel quark diagrams. The DJR distributions are sums of these results? If so, couldn't the sum of the distributions end up not looking smooth but still be matched fine?

Secondly, applying MAXJETS=1 to (A) behaves as I would guess. With MAXJETS=1 applied more events are rejected (than with MAXJETS=-1). For (B), however when applying MAXJETS=1 fewer events are rejected. I thought that MAXJETS told MadEvent to also reject events from the 2-jet bin? I realize the difference between (A) and (B) is that for (A) the higher multiplicity sample (s-channel quark diagrams) dominates the cross-section but for (B) it is either the lower multiplicity sample (t-channel w diagrams) or the interference terms and that (B) is labelled as a special case in the Matching instructions. Why is it that Pythia will not treat this process correctly unless the s-channel w diagrams with 2 radiated jets are rejected? And why is (A) fine?

Thanks and sorry for the long question,
- Matthew

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