[Rivet] lifetimes in rivet

[Andy Buckley]

On 03/04/10 18:09, Frank Siegert wrote:
> Andy Buckley, Saturday 03 April 2010:
>> On 01/04/10 09:43, Alexander Richards wrote:
>>> Was wondering if there was a nice Rivety was of cutting of particle
>>> lifetimes such as a lifetime projection or something? I think I can
>>> directly get the lifetime of the particles from the GenParticle but
>>> was wondering if there was an easier way?
>>
>> (Copying to Rivet list: please send to the list if possible!)
>>
>> I don't *think* there is a way to get the generator's take on the
>> lifetime from the GenParticle: at least, I don't see anything in the
>> GenParticle interface that does this. HepPDT could probably be used to
>> get this info from the PID, but there's no guarantee that the number
>> matches that used by the generator.
>>
>> If there is such a mechanism in HepMC (and it's used, including in the
>> translation from the Fortran HEPEVT event record), then I'd be very
>> happy: we could move a lot of restrictions on particle lifetime cuts
>> from how the generator has to be configured into the Rivet analysis
>> code, and a lifetime-sensitive projection of some sort would indeed be
>> useful. But I don't know of such a mechanism: maybe someone can correct
>>  me?
> 
> It should be relatively easy (and proper) to do this from the vertex 
> offset in combination with the incoming momentum. At least that must be 
> how the experiments do it when they run a Monte-Carlo, since there is no 
> separate (duplicate) storage of "generated life time".

That's true... but do all generators write in microscopically displaced
vertices as part of their hadron decays? K and B particles yes, but others?

My only experimental interaction with lifetimes has been in placing the
generator level lifetime cut to keep certain G4-handled particles stable
at gen level, but I guess someone will be wanting to know what the true
lifetime of a particular particle was.

Anyway, if this turns out to be reliable enough, then we could provide a
convenience method on Rivet::Particle -- and a projection if there is a
demand for filtering on lifetimes.

Andy

-- 
Dr Andy Buckley
SUPA Advanced Research Fellow
Particle Physics Experiment Group, University of Edinburgh