[Rivet] Born and dressed level in Rivet

[Andy Buckley]

On 08/10/13 15:52, Ulla Blumenschein wrote:
> Hi David,
> 
> 
> On Tuesday, October 8, 2013, David Grellscheid wrote:
> 
>     Hi Ulla,
> 
>         We didn't identify the FSR photons experimentally but we
>         unfolded to a
>         reference where a  lepton was dressed with the respective technical
>         FSR implementation.
> 
> 
>     I'm sorry, I don't understand fully what you did there. The
>     "reference" you refer to is a certain MC run with a certain setup?
>     In that case isn't your unfolding dependent on that MC author's
>     choice of where to put photon emissions in that specific version of
>     the code?
> 
> 
> we depend on at least three authors of three generators' choices. 
> 
> 
>         We checked that we get a similar correction in the
>         standard generators, (pythia, herwig, sherpa..).
> 
> 
>     That doesn't mean it makes physical sense to split FSR from ISR photons.
> 
> 
> No of course not. We are talking here merely in terms of dominating and
> negligible contributions to the collinear photon radiation.

Recent QED modelling studies by Steve Muanza showed that there is a
substantial "ISR" QED structure in Z->l+l- events only in Pythia8 (with
or without Photos++ to produce the QED FSR). We didn't see the same
structure in invariant masses for Sherpa or Herwig++. Was Py8 included
in the three models used to derive the dressing correction?

In Steve's study there was still relatively little difference between
that and other models once observables were constructed from IR-safe
clustering (and there we/he explicitly excluded photons from hadron
decays as discussed here), so I suspect that as suggested the FSR will
turn out to be dominant.

If there is no significant distinction between the two approaches, then
this sort of separation of photon origins is, I think, preferable to the
dependence so far on particular generators' schemes for representing QED
FSR. It's a lot more portable (unless a generator standard
representation is agreed upon -- good luck!) and I think physically less
contentious.

Cheers,
Andy

-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow / PH Dept, CERN