[Rivet] finalstate projection with assymetric beam

[Andy Buckley]

On 15/01/16 10:40, Pierog, Tanguy (IKP) wrote:
> 	Hi Andy,
>
>>
>> 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.
>>
>
> 	Thanks, we are not completely stupid ;-). So yes we have simple work around
> for the mean time (simply use "-" if pz is involved (xf, rapidity, etc ...)).
> Does this happen only with asymmetric beam because I am not sure that for LHC
> measurements it couldn't be an issue some times (for asymmetric analysis). In
> our case we use a charge pion (211) as projectile and a proton as target.

Yes, it's specific to asymm beams which include one proton.

> 	It would be nice if the possibility to reverse the frame remains but as an
> option and not the default. It could be useful for proton-antiproton for
> instance since not all models can use antiproton as target.

Absolutely, I'll add this to our to-do list.

>> 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.
>>
>
> 	OK. I am not a C++ expert but I thought the "projection" defined at the
> beginning could be used for that. Instead of "FinaleState" some
> like "CenterOfMassFinalState" could be created to get the final particles in
> the center of mass frame even if the hepmc file is generated with asymmetric
> beam (like for p-Pb or lab frame). I don't know if this kind of things are
> developed by you or if we should try and then send it to you to be included
> in the next release.

The only existing projection that does that is DISKinematics, I think. 
We could make a projection like you mention, but everything that 
projections do can also be done inside an analysis code, and since there 
has not previously been a request for that functionality there was no 
incentive for us to make a new feature that no-one wanted! But if you 
would find this useful in *several* analyses -- so there is a chance of 
some benefit from result caching as well as simplifying several analysis 
codes -- then we can take this a bit further.

We will probably need to iterate a bit to understand how the CoM frame 
is to be defined, and whether it also needs to include some rotation 
into a conventional alignment cf. our built-in HERA transformation.

> 	Actually I have the same question about the analysis we are doing. They are
> based on old data (80's and 90's) from collaboration which are not existing
> anymore. What is the policy in that case. Can it be distributed or should it
> get some approval from someone ? Do you have any idea ?

We don't enforce a policy about analyses needing to be approved by 
experiments, particularly not defunct ones. We just much *prefer* that 
analyses being provided during an experiment's lifetime be written, or 
at least be approved by that experiment, but an unofficial analysis is 
far better than nothing. We'll happily take anything you provide!

Andy

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