Exploring the D* way
Ch. de la Vaissiere, 7 octobre 1998
Objectives
Explore channels other than the rare J/psi channel of the book study
Find channels which are also be well signed and pure
Exploit kinematical+geometrical constraints to achieve accuracy on the vertex position of the reconstructed B and remove combinatorial background.
Sample and description of the analysis:
4000 Aslund B0-B0 events with one B0 decaying in J/psi K p
i.
This sample is used to develop prototype kinematical and geometrical fits algorithms.
In this sample, there is only one useful B
0
per event
We attempt to reconstruct decay chains
B
0
--> D*
+/-
+ pions
The D*
+/-
are looked in the
D*
+/-
--&gONT COLOR="#0000FF">D*
+/-
--> D
0
+ pi
+/-
mode.
The D
0
are searched in 4 mode
s
K pi,
K pi pi
0
,
K pi pi
0
pi
0
K pi pi pi
Starting from
D
0
candidates , we construct
D*
+/-
then
B
0
.
Kinematical(mass, Ebeam)+geometrical fits are performed at each step to purify the sample.
Results on
D
0
fits
Many candidates and important combinatorial background except for the K pi mode.
Geometrical and kinematical fits (including pi0s) are performed
Plots of
D
0
fit probabilities fits are given in the following figure
After a probability cut at 1% one finds :
2935 accepted, 414 true D0
0.103
D
0
per
B
0
.B
0
.
The average purity is only 14%
Results on
D*
+/-
--> D
0
+ pi
+/-
selection
After a classical cut on Delta-M between the
D
*
and
D
0
masses, one finds:
499 accepted, 171 true D*
0.042 D* per
B
0
.
The average D* purity is 35%
One applies a fit with 2 kinematical constraints + 6(10) geometrical constraints, and 4 unknowns ; 4(8) n.d.o.f.
Plots of
D
*
fit probabilities fits are given in the following Figure
After a probability cut at 1% one get
214 accepted, 168 true D*
0.042 D* per
B
0
.
The average D* purity is 78%
Results on the reconstruction of
B
0
--> D*
+/-
+ pions.
We add to the
D* ,
1,3 charged pions + 0, 1,2 pi0s
Raw Selection ;
M(D*
+/-
+ pions) ~ M(recoil) ~ M(B)
38 accepted, 27 coil) ~ M(B)
38 accepted, 27 true B
0
We perform a full fit of the decay chain
Highly constrained fit ; <n.d.o.f> = 11
24 candidates accepted, all fully reconstructed B
0
Accuracy on the vertex : < 60 microns> similar to golden Psi channels
Plots of B
0
fit probabilities fits and accuracies are given in the following Figure
Preliminary conclusions
Extrapolation of the 24 reconstructed B
0
to 5M Upsilon(4S) events : 30 K B
0
Kinematical + geometrical fitting are efficient to purify
D*
and
B
0
.
NB: Background may be higher in ordinary
B
0
B
0
events
ASLUND may be optimistic, but it is not the end of the road:
In a near future addition ofIn a near future addition of
channels with K0s
extra
D*
+/-
--> D
+/-
+ pi
0
Reconstruct
B
0
with
D
+/-
+ pions.
Problem
of
B
+/-
lifetimes :
B
+/-
love
D*
0
s but
D*
0
s decay in
D
0
+ pi
0
or
D
0
+ gamma
;
There is no equivalent to the golden chain
D
0
+ pi
+/-
.
Measuring of the
B
+/-
lifetimes with
D*
could be more difficult.
