Hi Oliver,
the resolution depends a lot on the multiplicity of the recoil (and
the study from Riccardo is clearly an example). Alessio's plots in the
BAD show that bad events are the ones with an odd number of neutrals (you
have an additional background photon there). Does it make sense to have
different tables for the fit as a function of the multiplicity in order to
take care of the good resolution of the events with no neutrals for
instance?
Thanks a lot,
Daniele
On Tue, 14 May 2002, Oliver Buchmueller wrote:
>
>
> Dear Riccardo,
>
> your observation is not a big surprise and actually
> this "smearing effect" is something I have already pointed
> out several times in previous talks
> (e.g see http://www.slac.stanford.edu/~buchmuel/cfit_vub_babarweek.pdf
>  dicussion of fit paramters for X)
> and it is also indicated in the
> kinematic fit part listed in the appendix of the Vub note.
>
> Since we are trying to measure the M_x distribution with an
> INCLUSIVE approach we do NOT know the underlying mass hypothesis
> (D,D*,X_H,...) for the reconstructed Xsystem.
> Therefore, it was decided to describe the Xsystem
> (see paragraph "Energy definition" in the note) only
> with 3 parameters. The energy of the Xsystem is then
> calculated assuming a fixed beta.
>
> Your EXCLUSIVE approach, of course, adds much more information
> because the mass hypothesis for the D is now know. Hence the only
> right parameterization for the Dvector in the fit would
> be a 4(!)vector which includes the reconstructed D mass.
> In fact this is what we already utilize for the reconstructed B
> candidate where, of course, we know the mass hypothesis
> (again see paragraph "Energy definition" in the note)
>
> At the moment you are trying to "fit" the D mass by using
> only a 3Vector and assuming fixed beta.
> Since you have already reconstructed the D meson this
> is obviously the the wrong Ansatz.
> Fitting "3Vector+fixed beta" only makes sense
> for an INCLUSIVE approach where we are dealing with a variety of
> different mass states. Apparently this concept is not so bad for
> INCLUSIVE M_x because we see significant improvements not only
> in the resolution but also in the bias after the cfit.
>
> As far as your EXCLUSIVE study is concerned, there is a option
> in the code to go from a "3Vector+fixed beta" to a full 4Vector
> parameterization (like for the reco B). In fact for the
> moment study we are always running both parameterization
> for the XSystem in parallel .... it works fine.
>
>
> Interesting is your statement that the DATA MC comparison
> gets worse after the fit. This is something which I have
> not seen so far and it is certainly not true for the INCLUSIVE
> Mx distribution. Could you quantify this DATA MC comparison
> or point me to a plot which shows DATA vs MC before and
> after the cfit?
>
> Thanks,
>
> Oliver
>
> On Tue, 14 May 2002, Riccardo Faccini wrote:
>
> > Hi,
> > I have been trying to understand how much can we learn from reconstructing
> > B>Dlnu decays on the recoil of fully reconstructed Bs.
> > To this aim, I have been looking at the distribution of Mx in the D(*)
> > mass range (1.82.1 GeV).
> > In order to clean up the environment, I have requested:
> > 1) either a K+ or a Ks
> > 2) no neutrals
> > 3) I have looked at B0 (Dlnu or D*lnu, D* > D0pi) and Bch
> > (D0lnu) separately.
> >
> > The results are shown in
> > http://babar.roma1.infn.it/~faccini/resoMx/resoVub.html
> >
> > I think we can conclude:
> >
> > a) that without kinematic fitting the resolutions on Mx in data and MC for
> > tracks only are similar (see table at the bottom)
> >
> > b) that the measurement of the D0lnu and D+lnu events in our data show
> > a bit of inefficiency that deserves more attention (although the stat is
> > low...). Within the available statistics, resolutions and biases seem ok
> > (maybe the D* is a bit strange)
> >
> > c) that the kinematic fit has a bad effect on these kinds of events. This
> > is probably due to the fact that the pdf's used in it assume that there is
> > a component with missing particles. In this case some events jump on the
> > wrong part of the pdf and get nasty tails at high MX.
> > This can be seen in
> > babar.roma1.infn.it/~faccini/resoMx/fitNoFitD0lnu.eps
> > where the noFit mass is plotted versus the fitted mass for D0lnu events
> > in cocktail MC.
> >
> > The fact that the fit screws up "good events" is not necessarily a
> > problem, but this means that the fitted mass cannot be used for Dlnu
> > studies.
> >
> > d) after kinematic fitting the agreement between data and MC gets much
> > worse, in particular for the cocktail. Kinematic fitting might be the
> > origin of the fact that we need generic MC in order to get a reasonable
> > agreement with the data.
> >
> > more to come (it looks like a promising sample)
> > ciao
> > ric
> >
> >
> >
> >
> >
> >
> >
> >
> >
>
>
