[Rivet] Jet fractions

[Peter Skands]

Hi Riveteers,

I am wondering if there could be a normalization problem in the LEP jet-fraction 
analysis. For a while, we have not been able to make sense of the results from 
it on mcplots, and therefore cannot include it on the public site. It would be 
really nice to be able to fix it. The clearest example is the one-jet fraction 
at 200 GeV,

http://mcplots-dev.cern.ch/?query=plots,ee,zhad,rate-1jet#ee200

For ycut = 1 (ln(y)=0), the data goes to unity. I.e., all events have at least 
one jet. However, all the generator curves only go to 10%, leaving me wondering 
what the 90% other LEP events are.

The observable is sigma(1jet)/sigma(tot). Is there something in the way that 
ratio is calculated that could explain it? Note that we are seeing problems with 
all the jet fractions, not just the one-jet one. I just took the simplest example.

Peter