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

[Holger Schulz]

Hi,

so a FourVector Particle::origin()
and FourVector Beams::IP()

would be sufficient to do the following calculation currently 
implemented in that LHCB analysis.

      const FourVector vppv(vt, vx, vy, vz);
      const FourVector vpivtx(vt, vx, vy, vz);
      const Vector3 diffPV = vpivtx.vector3() - vppv.vector3();
      const Vector3 versor = pp.momentum().p3().unit();
      Vector3 dist(diffPV);
       double vProjMom = dot(diffPV, versor);
       dist -= multiply(versor, vProjMom);
       return dist.mod() * millimeter;

I don't see the need for introducing 4-vectors here in this method.

We probably want  a more generalised method which allows calculation
of the distance of closest approach in z to an arbitrary FourMomentum
or Vector3

so maybe
Particle::z0(FourMomentum);
and/or
Particle::z0(Vector3);

What do you think?

Holger



On 03/11/15 21:56, Andy Buckley wrote:
> On 03/11/15 21:49, David Grellscheid wrote:
>> Hi all,
>>
>>> I can't see an immediate problem with Vector3 Particle::prodPos(). What
>>> do you think?
>>
>> How about Particle::origin(). "production position" is such a clumsy
>> HepMC-ism. Also, it really is a 4-vector. Not all of the hadrons are
>> produced at the same lab time.
>
> I'm down with that :-)
>
> Any gotchas we've not anticipated? I feel like there's a reason that I 
> didn't do this already, but maybe it was just a mix of unease at the 
> broken symmetry with decay, and uncertainty about whether we'd need a 
> more "connected" vertex object than just a 3- or 4-vector (and again 
> re. decays, there's no obvious "null" invalid value of Vector3/4).
>
>> The calculation of z0 should certainly not be a member function, but I
>> don't think that's what Holger suggested.
>
> Holger suggested Particle::z0 ;-)
>
>> If you have the collision
>> point, a particle's origin, and its momentum, you can work out most of
>> these offset calculation tasks (except for the ones that experimentally
>> trace back along curved tracks).
>
> Yep, that should be fine and is what I had in mind. But I don't 
> guarantee that the experimentalists' version is entirely constructed 
> from such sensible things!
>
> Andy
>