[Rivet] New LHCB analysis --- definition of z_0

[Andy Buckley]

Hi Holger,

Yes, I've thought for a while that we need better tools for particle 
decay analyses. And we did add the safer ideas, like flight length and 
access to direct/all/stable descendants.

We held back from providing "Rivety" access to production and decay 
vertex positions because that gets awkward when there is no decay... but 
I can't see an immediate problem with Vector3 Particle::prodPos(). What 
do you think?

And exactly what quantities are needed to calculate "z0"? I don't like 
the idea of directly encoding special observables on to core objects 
like Particle, but probably we can provide easy access to a few objects 
from which it (or similar quantities) can be easily calculated without 
needing to dip into the HepMC record.

Cheers,
Andy


On 03/11/15 13:21, Holger Schulz wrote:
> Hi all,
>
> I am currently working on the LHCB analysis recently added to contrib
> (LHCB_2015_I1333223).
>
> It's pretty straight forward what they do except for one cut in their
> phase space definition, namely
> a cut on "Distance of Closest Approach in z" i.e. z_0 wrt the primary
> vertex. They use that
> to define prompt particles by requiring this cut to be <0.2 mm. (Which
> is much better I think
> than the whole pythia decay times map that was used in other analyses).
>
> The way it's implemented right now is heavily using genEvent() stuff such as
>
>   * finding primary vertex
>   * finding particle production vertices
>
> and using that information to calculate z_0.
>
> Now I don't think that this horribly wrong at all but it would be nice to
> have a more Rivet like way of doing this, maybe having a Particle.z0()
> function? Not sure though.
>
> What do you think?
>
> Thanks,
> Holger
>
>
>
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-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow