[Rivet] Born and dressed level in Rivet

[Jon Butterworth]

Dear all,

Happy this seems to be solved (or being solved). Since I've also been 
involved with some discussions with Daniel, Ulla, Tim & others on this, 
I just wanted to make one point here.

It makes some physical sense to separate photons from hadronic decays 
(and e.g. tau decays) from "prompt" ISR/FSR photons. The latter are 
essentially emitted at the matrix element, whereas the timescales for 
hadronisation or tau decay are much longer. Whether it make sense to sum 
of all photons from pi0 or then becomes to some extent a matter of 
choice/taste and may depend on how the calibration etc is really done.

However, try to get "FSR only" photons or "photons from the W or Z decay 
products only" is not physical. Photon emission via ISR or from the 
propagator happen on the same timescale as FSR and are in principle not 
separable (though in many generators they are separated as a decent 
numerical approximation in most of phase space).

So a definition which excludes photons from hadrons & taus, as proposed 
by Andy I think, is good. A definition which only includes FSR photons 
is not. (For our current generators I expect little or no difference, 
but it is important that Rivet and ATLAS do it right and do it the same!)

Cheers,
Jon

On 03/10/2013 17:39, Ulla Blumenschein wrote:
> Hello Oleg,
>
> would be great, if you could test the new proposed dressed-definition
> vs the FSR-only option, just to make sure we don't miss anything
> unexpected. But  I second Tim, currently I can also not think of any
> further substacial contribution. (Have to cross check some slides
> which Evelin had shown a year ago..)
>
> Cheers, Ulla
>

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prof. Jonathan Butterworth,              http://www.hep.ucl.ac.uk/~jmb/
Head, Physics and Astronomy Department            Tel: +44 20 7679 3444
University College London                 Gower St, London WC1E 6BT, UK
ATLAS, CERN                                       Tel: +41 22 76  72340
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