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

[SCHULZ H.]

Hi Andy,

sounds good to me. Right now I use the
end_vertex of one of the particles from the
Beam projection and the production vertex
of each particle to calculate z0.
So I need the vertex positions.
Maybe we also want a Beam::interactionPosition?

Holger


________________________________________
From: Andy Buckley [andy.buckley at cern.ch]
Sent: Tuesday, November 03, 2015 9:12 PM
To: SCHULZ H.; Rivet
Subject: Re: [Rivet] New LHCB analysis --- definition of z_0

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