[Rivet] finalstate projection with assymetric beam

[Andy Buckley]

On 14/01/16 15:25, Pierog, Tanguy (IKP) wrote:
> 	Hi,

Hi Tanguy,

> 	In our group we try to develop new analysis for RIVET to test models for
> cosmic ray simulations. For that we use old data with fixed target
> experiments and pion beam for instance. Trying to do some xF (feynman x)
> distributions using pz of course we notice that the pz we get in Rivet are
> all -pz of the hepMC file. We suppose that the "FinalState" projection is the
> only possible reason (or a bug, but this would have been found before I
> guess ...). Is it possible that using "FinaleState" projection the proton
> side (which is the target here) is forced to become the "positive" side
> resulting in a change of the sign of all pz ? If yes, how can it be changed ?
> We tried to find that in the documentation it was not obvious and we didn't
> find any example.

Indeed, there is a rotation built in. This was done extremely early on 
in Rivet's lifetime, in order to enforce the HERA experiments' 
orientation rule... and it's time to reconsider that design, I think!

Are you able to work around the issue for now? Can you let us know what 
are the initial state particles that you're using? It seems like a good 
improvement to aim for in the next non-patch release.

> 	Just in case, we have an other question: is there an easy way to convert from
> lab frame (hepmc) to center-of-mass (rivet) frame (using this projection may
> be ...) ?

There is a LorentzTransform object in Rivet/Math/LorentzTrans.hh. It's 
used by the DIS* projections. Any suggestions for improvements would be 
welcome... we could make it interact more nicely with Particle objects, 
for example, but there haven't yet been any feature requests. Just ask 
if you need assistance with using it.

Andy

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