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[Rivet] New LHCB analysis --- definition of z_0SCHULZ H. holger.schulz at durham.ac.ukWed Nov 4 12:41:52 GMT 2015
Of course. D'oh! ________________________________________ From: Andy Buckley [andy.buckley at cern.ch] Sent: Wednesday, November 04, 2015 12:25 PM To: SCHULZ H. Cc: GRELLSCHEID D.; rivet at projects.hepforge.org Subject: Re: [Rivet] New LHCB analysis --- definition of z_0 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
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