[Rivet] Access to transverse and longitudinal impact parameters

[Andy Buckley]

On 07/07/14 12:53, Leif Lönnblad wrote:
> Sorry, I thought you were talking about another kind of impact
> parameters (the heavy-ion kind), so forget what I said before.

I had the same reaction!

[By the way, I didn't see Markus in the recipient lists of Leif's
emails, so have added him again. If you didn't get the previous emails,
Markus, the suggestion is in the next quoted block:]

> However, assuming now that you are interested in production/decay
> vertices, the information is still available through the underlying
> GenEvent object. For a given Rivet::Particle, you can access the
> underlying GenParticle and its production_vertex(). If the generator has
> provided vertex positions they are available there.

I would like very much to improve the Rivet Particle class for use in
hadron decay analyses (I guess this is for B physics or similar?), but
have been cautious about adding new facilities to the interface when I
don't know how they would work best.

There are a few features available, listed under "Decay info" on the doc
page: http://rivet.hepforge.org/code/dev/a00222.html
The closest available is flightLength() (which returns -1 as an obvious
invalid value if the particle is stable or a beam particle) but I
avoided adding prodPos() and decayPos() methods because it's less
obvious what an invalid 3- or 4-vector should look like.

If there is enthusiasm for the idea, I could add a prodPos() function,
since all particles other than beams will have production positions and
it's rare for analyses to handle beam particles directly. I'm more
hesitant about adding decayPos() [or any other name for that concept --
not all end vertices are _decays_, but the physical ones are and I think
it's the least confusing name] because the majority of particles handled
by analysis code are stable and hence don't have a decay position.

Suggestions are very welcome, as are code patches!

Andy



> On 2014-07-07 13:28, Markus Radziej wrote:
>> Hi,
>>
>> I was wondering if there is some way to access the impact parameter
>> information via Rivet? I have tried looking into other analyses, but
>> could not find one that works with these variables. I'm not quite sure
>> the information even exists, so I decided to write an email.
>>
>> Thanks for your help,
>> Markus Radziej
>> _______________________________________________
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>> Rivet at projects.hepforge.org
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> 
> 
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-- 
Dr Andy Buckley, Royal Society University Research Fellow
Particle Physics Expt Group, University of Glasgow / PH Dept, CERN