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

[Andy Buckley]

Isn't Beams::IP always going to be (0; 0,0,0)? We used to have a 
PrimaryVertex projection and it was totally pointless.

Andy


On 04/11/15 09:14, Holger Schulz wrote:
> 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
>>>
>>
>


-- 
Dr Andy Buckley, Lecturer / Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow