Eta asymmetry in W production, H7

[Peter Richardson]

Dear Herwig experts,
I would like to ask you for a help with understanding of an output of herwig, when generating
very simple single W production with muon decay. (in file attached)

I plot muon pseudorapidity (after pT cut 5 GeV, |eta| < 5.0 ) for two sets of muons:
1) Matrix Element (ME) outgoing muons:
(using: event->primarySubProcess()->outgoing(); )
2) Final state muons - muons at the end of the loop over children of the previous muon - having no further children.

I observe asymmetry, where muons slightly tend to go into the negative side, mean eta = -0.072
It is visible by eye in the plot - red line. Statistics is 10x1M events, all samples have similar asym.
Strange thing is that muons coming directly from ME show much smaller asymmetry, mean =-0.0025 (greem line)

Please, do you see an obvious reason (or a bug/missing thing in the in file) for that behavior?

I tried to turn off the QED radiation by:
set /Herwig/Shower?/Evolver:Interactions QCDOnly
but the result did not really changed (blue and black lines)

I run H7.0.2 now, but I remember seeing this also in older H++ versions.

Thank you!

All the best,
Miroslav

[Peter Richardson]

input file

[Peter Richardson]

plot

[Peter Richardson]

This is a problem with the handling of events with the quark in the negative z direction in the hard matrix element correction, fixed in rfe6900a23531