[Rivet] A comment on InvMassFinalState

[Adam Davison]

Hi Frank,

Thanks, that's interesting. Just thought I'd let you know what my 
experience of using it was.

Cheers,

Adam

Frank Siegert wrote:
> Hi Adam,
>
> Adam Davison, Wednesday 10 June 2009:
>   
>> I've been trying to use the InvMassFinalState class but the interface
>> seems a bit difficult. It returns all particles that could be used to
>> make, for example a Z, excluding duplicates. This means that if there's
>> more than one candidate in an event, it can do some wierd things, like
>> return a list of 3 or 5 particles. This makes it quite hard to apply
>> any further cuts to the candidates, like pt, since in order to identify
>> which particles are actually matching pairs, you have to loop over
>> everything again anyway.
>>     
>
> Yes, I agree that it is awkward, but when I used it (for the ZFinder) I 
> wasn't brave enough to replace it. Unfortunately there are a few places 
> which still seem to use it, and I didn't want to validate them after such 
> a change:
>
> $ grep InvMassFinalState $(svn ls --recursive) | cut -d ":" -f 1 | uniq
> ChangeLog
> include/Rivet/Makefile.am
> include/Rivet/Projections/InvMassFinalState.hh
> src/Analyses/CDF_2006_S6653332.cc
> src/Analyses/CDF_2008_S7541902.cc
> src/Analyses/CDF_2008_S8095620.cc
> src/Analyses/D0_2008_S6879055.cc
> src/Projections/InvMassFinalState.cc
> src/Projections/Makefile.am
> src/Projections/ZFinder.cc
>
>   
>> The ZFinder, which is the only place in Rivet I saw this being used
>> returns if the size of the list isn't 2. Which unless I've
>> misunderstood, potentially throws away events with good Z candidates
>> in.
>>     
>
> Yes, this is a deliberate choice. If we wanted to pick one of the Z 
> candidates for each event, we would introduce a bias (for example towards 
> a known Z mass), so better reject that event. That said, I don't think it 
> will make much of a difference, because the number of events where enough 
> leptons pass cuts and form multiple Z's is quite certainly negligible.
>
> Frank
>
>
>