[Rivet] CDF Run-1 pTZ

[Andy Buckley]

Emily Nurse wrote:
> We can double check with Gavin but I think this is the case (ie/ from
> the statement  "We use PHOTOS [15] to simulate the effects of 
> final state photon radiation.").  I also asked the author of the analysis
> a while ago and I think that's what he said.
> 
> I'll bet the Run I analysis did the same but we should check.

Ok, worth checking.

> So it seems to be a pattern that all the analyses so far have done this,
> so we need to cluster back all the photons that we think Photos would
> have simulated. I can make some plots of dR between the lepton and
> photon in W / Z events and maybe we can decide on the best cut to make? 

That would be great. We need some "opposing" distribution, though, like
the dR between the lepton and background photons, which will rise as the
signal falls: the trade-off determines where to place the cut. Otherwise
we'll get a distribution which tells us that the best cone has R = pi ;)
I don't know if you need to simulate pile-up for that at the Tevatron.

> I think that the D0 Run II analyses also corrects for QED radiation,
> in an attempt to present the "true Z pT".
[...]
> For the future we really need to communicate these concerns with the
> electroweak convenors and authors of any current analyses. We should
> define what we want.

We are seeing this from a rather different direction than the experiment
is: from our point of view it's crucial that we have reference data
which can be compared against without having to unfold some model, but
from the point of view of writing a paper that measures the Z boson pT,
the unfolding was necessary to get to the "truth". That's no bad thing,
but I think we need to get the message across that *we* need the
timeless, un-unfolded, model independent measurement at least as much as
we need a best possible inference of what the boson is actually doing.
  In a sense, all we want for MC tuning and validation is enough
different experimental distributions to roughly span the space of
simulated physics. Whether those distributions have a super-clean
physical interpretation is less important than whether they can be
readily constructed from a final state particle record ;)

> For electrons it is probably quite simple:
> "pT of e+e- pair where an electron is defined as a dR = X cone of EM
> objects". 

Sounds fine.

> For muons its a bit more difficult, we don't cluster any photons in the
> muon momenta measurement, but often MIP and/or isolation cuts are made.
> Do we ask that the experiments to correct for this using some QED model
> or do we attempt to model the cuts in our analyses?

I would suggest both: the model-independent part should be preserved as
a matter of principle; the QED correction can be used to make physics
conclusions that we are less interested in for MC tuning purposes. I
would rather model the cuts than attempt to encode some bin-by-bin (or
worse) corrections, but mostly I'd like to have the option!

Andy