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 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.


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 (1.8-2.1 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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