[Rivet] WFinder has a problem with purely partonic events

[Holger Schulz]

Hi,

the WFinder or more precisely the MissingMomentum it is based on
has a problem when running on purely partonic events:


EventScale -1 [energy]      alphaQCD=0.120687  alphaQED=0.00776384
PdfInfo: id1=2 id2=-1 x1=0.000319351 x2=0.303109 q=78.7088 xpdf1=2.25358 
xpdf2=0.00657298

                                        GenParticle Legend
        Barcode   PDG ID Name            Stat  ProdVtx DecayVtx   
(       Px,       Py,       Pz,       E ,        m)
      10001        2 u                  3       -2       -1 (       
+0,       +0,    +1.28,    +1.28,-6.12e-07)
      10002       -1 d~                 3       -2       -1 (       
+0,       +0,-1.21e+03,+1.21e+03,        0)
      10003      -11 e+                 3       -1       -2 (    
+19.7,    -19.1,     -171,     +173,-1.91e-06)
      10004       12 nu_e               3       -1       -2 (    
-19.7,    +19.1,-1.04e+03,+1.04e+03,-1.53e-05)
      10005        2 u                 11        0       -2 (       
+0,       +0,    +1.28,    +1.28,-6.12e-07)
      10006       -1 d~                11        0       -2 (       
+0,       +0,-1.21e+03,+1.21e+03,        0)
      10007      -11 e+                 1       -2        0 ( *+19.7,    
-19.1,     -171,     +173,-1.91e-06*)
      10008       12 nu_e               1       -2        0 ( *-19.7,    
+19.1,-1.04e+03,+1.04e+03,-1.53e-05*)

Rivet.Projection.WFinder: TRACE  MET = 27.452 GeV vs. required = 25 GeV
Rivet.Projection.WFinder: DEBUG  Found at least one dressed lepton: 
*(172.942;  19.6765,-19.1428, -170.749) *
Rivet.Projection.WFinder: DEBUG  Found missing 4-momentum: *(172.942; 
-19.6765, 19.1428,  170.749)*

This is the 'culprit' if you want to call it a culprit:

    const FourMomentum missingMomentum(double mass=0*GeV) const { return
    visibleMomentum(mass).reverse(); }

That's probably not the best choice for partonic events.

I would suggest we check in the WFinder is the beams
are defined. If they aren't we can probably safely assume that
this is a partonic event where you would reconstruct the
W from the neutrino anyways and hence use the old method
of reconstructing the W from the neutrino.

Here is a plot (thanks Marek) that illustrates the current situation:

    http://www.physik.uzh.ch/~marek/plots/YFS/Wtest/MC_WINC/W_mass.pdf

red is purely partonic
blue is with parton shower
green is with parton shower and beams

I.e. the WFinder result depends on whether you have particles
travelling down the beam pipe or not.

Not sure whether this is related to the problem Raghav reported.

Cheers,
Holger

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://www.hepforge.org/lists-archive/rivet/attachments/20160520/06277bf7/attachment.html>