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Vub measurement using recoil of fully reconstructed Bs

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Subject:
From:
Franco Simonetto <[log in to unmask]>
Date:
15 May 2002 15:38:15 +0200Wed, 15 May 2002 15:38:15 +0200
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Hi,
  the D*ln sample with partially reconstructed D* should fit for this
 kind of study (it has by definition no D mass constraint). The yield
 is approximately  1 evt / 200 B0 with the tight cuts I used for lifetime
analysis,
 leading to a potential sample of several hundred events: however if one B is
already reconstructed, cuts can be relaxed (much less combinatorics), further
increasing the
 overall yield. I presume that these events could be tagged at rootple level
 without need of going back to beta.
 Cheers
        Franco

Riccardo Faccini wrote:

> Hi Oliver,
>  I guess it would be nice if we could discuss today at the meeting. In the
> meanwhile just a couple of comments:
>
> >
> > 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 X-system.
> > Therefore, it was decided to describe the X-system
> > (see paragraph "Energy definition" in the note) only
> > with 3 parameters. The energy of the X-system 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 D-vector 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 3-Vector and assuming fixed beta.
> > Since you have already reconstructed the D meson this
> > is obviously the the wrong Ansatz.
> > Fitting  "3-Vector+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 "3-Vector+fixed beta" to a full 4-Vector
> > parameterization (like for the reco B). In fact for the
> > moment study we are always running both  parameterization
> > for the X-System in parallel .... it works fine.
> >
>
> I agree with you that the fixed beta option is not optimal, but it should
> not create such big biases: if something is correctly reconstructed it
> should not get screwed  up.
>
> I think I found an easier (and more conforting) solution: the events that
> get moved around are actually far from 0 in M_nu^2 (see m_nu^2 vs fitted
> Mx in
> http://babar.roma1.infn.it/~faccini/resoMx/mnuxhadfit.eps
> )
> This means that I was looking at D0lnuX events that were reconstructed as
> D0lnu and the kinematic fit was trying to recover the X on a statistical
> basis.
>
> The only missing point is to understand why (actually, if, the statistics
> might confuse things) the data worsen more than the MC.
> One point I could not get from any of the material you provided is what is
> the impact of the resolution on the Breco and how do you account for it.
> At this point one useful test would be to smear the Breco in the  coctail
> MC and see if we can achieve a resolution similar to the generic one.
>
>  >
> > 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?
>
> You can have a look at the data-cocktail and data-generic comparison of
> the MD masses, all linked from the web page I posted: for istance
> data-cocktail with kfit
> http://babar.roma1.infn.it/~faccini/resoMx/Dcock/Bch1knoneu.eps
> data-generic with kfit:
> http://babar.roma1.infn.it/~faccini/resoMx/Dgen/Bch1knoneu.eps
> data-cocktail without Kfit:
> http://babar.roma1.infn.it/~faccini/resoMx/NoKfitCock/Bch1knoneu.eps
>
> ciao
> ric
> >
> > > 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 (1.8-2.1 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 (D-lnu 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
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >




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