[Rivet] W finder and PartonJet question

[Andy Buckley]

Hi Kristin,

There is no mechanism in Rivet to find Born-level W/Z or parton jets --
in fact the whole design of Rivet is explicitly to discourage such
approaches because they are not portable between generators, and their
exact physical meaning is also rather vague.

1) If you want to construct these, then turn off QED FSR and any other
EW corrections in your MC: the bare leptons will then be "Born".

3) For parton jets you would need to turn off QCD FSR, hadronization,
and decays in the generator.

At this point you pretty much just have parton level events of the sort
that would appear in an LHE file... and to my mind you might as well
just use an LHE parser and analyse the parton events directly rather
than fight against a system which is explicitly designed to encourage
more "final-state type" analyses!

2) When you say that you reapply the MET cut, do you mean the cut on
nu.pT()? This isn't quite the same as MET: the MET cut that we apply
also includes any other neutrinos in the event (including those from
hadron and tau decays) as well as ET imbalance due to acceptance
limitations. This is something where we might add more options in future
if there is demand, e.g. in response to the ATLAS (& wider) truth
definition workshops. Would this difference explain the change in
numbers that you are seeing? (By the way, it's a good idea to not just
use raw int cut-passed counters but to add the event weights instead --
that way you still get a physical answer if you use weighted generators.)

Andy


On 15/05/14 11:30, kristin Lohwasser wrote:
> Dear all,
> 
> I have a (couple of) question(s) concerning the WFinder implementation
> in Rivet and how to get Parton Jets in Rivet (is this possible at all?).
> 
> Essentially I want to get BORN level electrons, for this I am using the
> WFinder (since as I understand, this is the recommended way to
> reconstruct W's in Rivet -- please correct me, if I am wrong)
> 
> 1) CLUSTERING DEFINITION
> --> For Born level I am not sure, what setting for the Photon clustering
> to use, essentially for the below settings, the WFinder::NOCLUSTER and
> WFinder::CLUSTERNODECAY yield the same results (but these are different
> from my cross check numbers for Born level)
>       WFinder wfinder_born_el(fs, -2.5, 2.5, 20*GeV, PID::ELECTRON,
> 0*GeV, 8000*GeV, 25*GeV, 0.2, WFinder::NOCLUSTER    );
> so I am not sure, whether any of the ENUMS will actually selec W-Born
> level.
> 
> 2) Difference for MET
> I understand, you set the MET cut in the WFinder Declaration (25*GeV in
> the above case).
> However, I apply a MT cut afterwards as well and there I also doublely
> applied the MET cut -- however when I *REMOVED* it, the number of
> selected events *CHANGED*, which should not happen (code follows now):
> 
> 
>       if (!wfinder_born_mu.particles().empty()) {
>         const FourMomentum& nu =
> wfinder_born_el.constituentNeutrinos()[0].momentum();
>         if ( calcMT(nu,
> wfinder_born_el.constituentLeptons()[0].momentum())  > 40*GeV &&
> nu.pT()>25*GeV) {
>           keepevent=true;
> }}
> 
>       if (!keepevent) {
>         MSG_DEBUG("event does not pass mT and MET cuts");
>         vetoEvent;
>       }
> COUNT_PASS_W++;
> 
> 3) How can want one get Parton Jets in Rivet? I tried to find some other
> analysis using this, e.g. ATLAS jets, but it seems, they are not
> implemented. It would be great, if you could suggest me something.
> 
> Thanks & Best
> Kristin
> 
> 
> 
> 
> 


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