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

[Holger Schulz]

Sorry I just realised that this is indeed a calculation
of the actual impact parameter at the point of closest approach,
--- not just in z.

So maybe not put this into Particle but have the
Vector3 Particle::origin()
and
Vector3 Beams::IP()

methods?

Holger



On 04/11/15 08:32, Holger Schulz wrote:
> 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
>>
>