[Rivet] Rivet bug in LEP Jet Resolutions Y12 ?

[Peter Skands]

Hi all,

I believe there is a bug in the code that generates the following analysis in 
Rivet, ALEPH_2004_S5765862, see the plot here
  http://mcplots.cern.ch/?query=plots,ee,zhad,Y2-ch,Main

As you can see, all the generators seem to have a very wide tail whereas the 
data hits a shoulder and falls off more steeply. There is a residual effect of 
opposite sign also in Y23.

This has nothing to do with ISR on/off or anything like that. It's worse. There 
is one particular thing which causes exactly this effect: whether neutrinos are, 
or are not, included in the final state. I believe Rivet is trying to do the 
"right" thing by throwing away the neutrinos. However, one obtains precisely 
that shoulder which the data exhibits if one *does not* throw away the neutrinos.

In the paper, they state (p.461 of the EPJC journal article):
"The data are corrected for acceptance, detector resolution, undetected 
particles such as neutrinos, particle masses, final state photon radiation and 
the residual effects of ISR by means of multiplicative factors."

This would seem consistent with the supposition that they are actually putting 
the neutrinos *back*. Otherwise there would be no need to "correct" for them - 
they would simply not be there, neither in the numerator nor in the denominator 
of y_ij. I think, therefore, this analysis has to include the neutrinos in the 
jet clustering, although I completely agree that this is a horrid idea and the 
responsibles ought to be publicly flogged. One should certainly add a remark 
about it to the manual entry, to ensure that people don't start flogging *us* 
for it.

Peter