of 5
V
OLUME
76, N
UMBER
10
PHYSICAL REVIEW LETTERS
4 M
ARCH
1996
Measurement of the
B
Semileptonic Branching Fraction with Lepton Tags
B. Barish,
1
M. Chadha,
1
S. Chan,
1
G. Eigen,
1
J. S. Miller,
1
C. O’Grady,
1
J. Urheim,
1
A. J. Weinstein,
1
F. Würthwein,
1
D. M. Asner,
2
M. Athanas,
2
D. W. Bliss,
2
W. S. Brower,
2
G. Masek,
2
H. P. Paar,
2
J. Gronberg,
3
C. M. Korte,
3
R. Kutschke,
3
S. Menary,
3
R. J. Morrison,
3
S. Nakanishi,
3
H. N. Nelson,
3
T. K. Nelson,
3
C. Qiao,
3
J. D. Richman,
3
D. Roberts,
3
A. Ryd,
3
H. Tajima,
3
M. S. Witherell,
3
R. Balest,
4
K. Cho,
4
W. T. Ford,
4
M. Lohner,
4
H. Park,
4
P. Rankin,
4
J. G. Smith,
4
J. P. Alexander,
5
C. Bebek,
5
B. E. Berger,
5
K. Berkelman,
5
K. Bloom,
5
T. E. Browder,
5,
*
D. G. Cassel,
5
H. A. Cho,
5
D. M. Coffman,
5
D. S. Crowcroft,
5
M. Dickson,
5
P. S. Drell,
5
D. J. Dumas,
5
R. Ehrlich,
5
R. Elia,
5
P. Gaidarev,
5
B. Gittelman,
5
S. W. Gray,
5
D. L. Hartill,
5
B. K. Heltsley,
5
S. Henderson,
5
C. D. Jones,
5
S. L. Jones,
5
J. Kandaswamy,
5
N. Katayama,
5
P. C. Kim,
5
D. L. Kreinick,
5
T. Lee,
5
Y. Liu,
5
G. S. Ludwig,
5
J. Masui,
5
J. Mevissen,
5
N. B. Mistry,
5
C. R. Ng,
5
E. Nordberg,
5
J. R. Patterson,
5
D. Peterson,
5
D. Riley,
5
A. Soffer,
5
P. Avery,
6
A. Freyberger,
6
K. Lingel,
6
C. Prescott,
6
J. Rodriguez,
6
S. Yang,
6
J. Yelton,
6
G. Brandenburg,
7
D. Cinabro,
7
T. Liu,
7
M. Saulnier,
7
R. Wilson,
7
H. Yamamoto,
7
T. Bergfeld,
8
B. I. Eisenstein,
8
J. Ernst,
8
G. E. Gladding,
8
G. D. Gollin,
8
M. Palmer,
8
M. Selen,
8
J. J. Thaler,
8
K. W. Edwards,
9
K. W. McLean,
9
M. Ogg,
9
A. Bellerive,
10
D. I. Britton,
10
E. R. F. Hyatt,
10
R. Janicek,
10
D. B. MacFarlane,
10
P. M. Patel,
10
B. Spaan,
10
A. J. Sadoff,
11
R. Ammar,
12
P. Baringer,
12
A. Bean,
12
D. Besson,
12
D. Coppage,
12
N. Copty,
12
R. Davis,
12
N. Hancock,
12
S. Kotov,
12
I. Kravchenko,
12
N. Kwak,
12
Y. Kubota,
13
M. Lattery,
13
J. K. Nelson,
13
S. Patton,
13
R. Poling,
13
T. Riehle,
13
V. Savinov,
13
R. Wang,
13
M. S. Alam,
14
I. J. Kim,
14
Z. Ling,
14
A. H. Mahmood,
14
J. J. O’Neill,
14
H. Severini,
14
C. R. Sun,
14
S. Timm,
14
F. Wappler,
14
J. E. Duboscq,
15
R. Fulton,
15
D. Fujino,
15
K. K. Gan,
15
K. Honscheid,
15
H. Kagan,
15
R. Kass,
15
J. Lee,
15
M. Sung,
15
C. White,
15
A. Wolf,
15
M. M. Zoeller,
15
X. Fu,
16
B. Nemati,
16
W. R. Ross,
16
P. Skubic,
16
M. Wood,
16
M. Bishai,
17
J. Fast,
17
E. Gerndt,
17
J. W. Hinson,
17
T. Miao,
17
D. H. Miller,
17
M. Modesitt,
17
E. I. Shibata,
17
I. P. J. Shipsey,
17
P. N. Wang,
17
L. Gibbons,
18
S. D. Johnson,
18
Y. Kwon,
18
S. Roberts,
18
E. H. Thorndike,
18
T. E. Coan,
19
J. Dominick,
19
V. Fadeyev,
19
I. Korolkov,
19
M. Lambrecht,
19
S. Sanghera,
19
V. Shelkov,
19
T. Skwarnicki,
19
R. Stroynowski,
19
I. Volobouev,
19
G. Wei,
19
M. Artuso,
20
M. Gao,
20
M. Goldberg,
20
D. He,
20
N. Horwitz,
20
S. Kopp,
20
G. C. Moneti,
20
R. Mountain,
20
F. Muheim,
20
Y. Mukhin,
20
S. Playfer,
20
S. Stone,
20
X. Xing,
20
J. Bartelt,
21
S. E. Csorna,
21
V. Jain,
21
S. Marka,
21
D. Gibaut,
22
K. Kinoshita,
22
P. Pomianowski,
22
and
S. Schrenk
22
(CLEO Collaboration)
1
California Institute of Technology, Pasadena, California 91125
2
University of California, San Diego, La Jolla, California 92093
3
University of California, Santa Barbara, California 93106
4
University of Colorado, Boulder, Colorado 80309-0390
5
Cornell University, Ithaca, New York 14853
6
University of Florida, Gainesville, Florida 32611
7
Harvard University, Cambridge, Massachusetts 02138
8
University of Illinois, Champaign-Urbana, Illinois 61801
9
Carleton University, Ottawa, Ontario, Canada K1S 5B6
and the Institute of Particle Physics, Canada
10
McGill University, Montréal, Québec, Canada H3A 2T8
and the Institute of Particle Physics, Montréal, Québec, Canada
11
Ithaca College, Ithaca, New York 14850
12
University of Kansas, Lawrence, Kansas 66045
13
University of Minnesota, Minneapolis, Minnesota 55455
14
State University of New York at Albany, Albany, New York 12222
15
Ohio State University, Columbus, Ohio 43210
16
University of Oklahoma, Norman, Oklahoma 73019
17
Purdue University, West Lafayette, Indiana 47907
18
University of Rochester, Rochester, New York 14627
19
Southern Methodist University, Dallas, Texas 75275
20
Syracuse University, Syracuse, New York 13244
21
Vanderbilt University, Nashville, Tennessee 37235
22
Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
(
Received 16 October 1995
)
1570
0031-9007
y
96
y
76(10)
y
1570(5)$10.00
© 1996 The American Physical Society
V
OLUME
76, N
UMBER
10
PHYSICAL REVIEW LETTERS
4 M
ARCH
1996
We have used the CLEO II detector and
2.06
fb
2
1
of
Y
s
4
S
d
data to measure the
B
-meson
semileptonic branching fraction. The
B
!
Xe
n
momentum spectrum was obtained over nearly the
full momentum range by using charge and kinematic correlations in events with a high-momentum
lepton tag and an additional electron. We find
B
s
B
!
Xe
n
d
s
10.49
6
0.17
6
0.43
d
%
, with overall
systematic uncertainties less than those of untagged single-lepton measurements. We use this result to
calculate the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element
V
cb
and to set an upper
limit on the fraction of
Y
s
4
S
d
decays to final states other than
B
̄
B
.
PACS numbers: 13.20.He, 13.30.Ce, 14.40.Nd
The semileptonic branching fraction of the
B
meson has
been a persistent puzzle in heavy flavor physics. While
most measurements have been below 11%, theoretical cal-
culations have generally given 12% or higher [1]. This
discrepancy has stimulated much speculation. Recent sug-
gestions of enhancement in the decay channel
b
!
c
̄
cs
can explain the small semileptonic branching fraction [2],
but the accompanying enhancement in
c
-quark production
cannot easily be accommodated by CLEO data [3]. With-
out a satisfactory theoretical explanation it is essential to
continue to reexamine and refine the experimental data on
semileptonic
B
decay.
The most precise measurements of
B
s
B
!
X
,
n
d
are
from studies of the single-lepton momentum spectrum at
the
Y
s
4
S
d
resonance. A recent CLEO II measurement
[4] has a statistical precision of better than 0.5%, but
is limited by systematic effects. Primary decays (
B
!
X
,
n
) must be separated from secondary charm decays
(
b
!
c
!
y
,
n
) by fitting the spectrum with models,
introducing significant theoretical uncertainty. It must
also be assumed that all
Y
s
4
S
d
decays are to
B
̄
B
. While
this is reasonable, the published upper limit on the non-
B
̄
B
fraction is 13% [5].
The ARGUS experiment has reported an analysis that
reduced the model dependence and sensitivity to non-
B
̄
B
decays by using dilepton events [6]. In this Letter we
describe a similar analysis performed with a 10 times larger
data sample. High-momentum lepton tags were used to
separate primary and secondary electrons, giving a nearly
model- and normalization-independent measurement of the
B
semileptonic branching fraction, and an improved limit
on
Y
s
4
S
d
decays to non-
B
̄
B
final states.
Our data sample was collected with the CLEO II de-
tector [7] at the Cornell Electron Storage Ring (CESR).
It consists of an integrated luminosity of
2.06
fb
2
1
at
the
Y
s
4
S
d
, with
2
3
10
6
B
̄
B
events. Continuum back-
ground was studied with
0.96
fb
2
1
at center-of-mass en-
ergies about 60 MeV below the
Y
s
4
S
d
. We identified
electron candidates with momenta greater than
0.6
GeV
y
c
by requiring an energy deposit in the CsI calorimeter
consistent with the measured momentum, and
dE
y
dx
con-
sistent with that expected for an electron. Muon candi-
dates were charged tracks with a minimum momentum
of
1.4
GeV
y
c
that penetrated at least five nuclear interac-
tion lengths of absorber material. Leptons were selected
from the best part of the detector (
j
cos
u
j
,
0.71
for elec-
trons and
j
cos
u
j
,
0.61
for muons), with electron and
muon detection efficiencies above 90%. Hadrons with
momenta greater than
1.0
GeV
y
c
were misidentified as
electrons with a probability of approximately 0.1%, while
the muon misidentification probability for hadrons above
1.4
GeV
y
c
was about 1%.
We selected events with tag leptons of momentum
greater than
1.4
GeV
y
c
. Such leptons are predominantly
from semileptonic decay of one of the two
B
mesons in
an
Y
s
4
S
d
decay. When a tag was found, we searched
for an accompanying electron with minimum momentum
0.6
GeV
y
c
. There are three main sources of these elec-
trons. Semileptonic decay of the other
B
gives an electron
with charge opposite to that of the tag. Semileptonic de-
cay of a
D
meson from the other
B
gives an electron of the
same charge as the tag. Semileptonic decay of a
D
from
the same
B
contributes to the unlike-sign sample, but with
a kinematic signature which makes its contribution easy to
isolate, as is described below. The effect of
B
0
̄
B
0
mixing
is small and can be accounted for explicitly.
At the
Y
s
4
S
d
, the
B
and the
̄
B
are produced nearly at
rest. There is little correlation between the directions of
a tag lepton and an accompanying electron if they are the
daughters of different
B
mesons. If they originate from
the same
B
, however, there is a tendency for the electron
and the tag to be back to back. The strength of the
correlation depends on the electron momentum
p
e
, and
we studied the distribution of
p
e
versus the opening angle
cos
u
,
e
to optimize the separation [8]. For unlike-sign
pairs we applied the diagonal cut
p
e
1
cos
u
,
e
.
1
(
p
e
in GeV
y
c
), which suppresses the background of dileptons
from the same
B
by a factor of 25, while retaining 67% of
the opposite-
B
electron signal.
Systematic effects that may be introduced by this cut
have been studied with a Monte Carlo simulation using
the Isgur-Scora-Grinstein-Wise (ISGW) form-factor model
[9] for the decay modes
B
!
D
,
n
,
B
!
D
p
,
n
, and
B
!
D
pp
,
n
, with an additional component to account
for higher multiplicity nonresonant decays. The signal
efficiency was found to vary insignificantly among these
channels, while the background of lepton-tag pairs from
the same
B
varies somewhat from mode to mode. The
dominant background mode is
D
p
,
n
, while
D
,
n
,
D
pp
,
n
,
and nonresonant decays contribute much less, because
of smaller branching fractions and softer lepton spectra.
Reasonable variations in the fractions of the individual
modes give an uncertainty in this same-side background
of 15%.
1571
V
OLUME
76, N
UMBER
10
PHYSICAL REVIEW LETTERS
4 M
ARCH
1996
FIG. 1. Momentum spectra for electrons with (a) unlike-sign
tags satisfying the diagonal cut, and (b) like-sign tags without
that cut. The points and the histograms represent
Y
s
4
S
d
and
continuum data, respectively.
The raw
Y
s
4
S
d
electron spectra for the unlike-sign
sample with the diagonal cut applied, and for the like-
sign without that cut, are shown in Fig. 1. The raw yield
and background subtractions are summarized in Table I.
The continuum contribution was subtracted by scaling
the lepton yields in the off-resonance data by
2.12
6
0.01
, the ratio of the on- and off-resonance integrated
luminosities, corrected for the energy dependence of
the continuum cross section. Fakes are hadrons that
were misidentified as leptons. Their contributions have
been calculated using misidentification probabilities and
hadron track momentum spectra found in data. The fake
correction is small (
,
3%
for the unlike-sign sample), with
an estimated uncertainty of 50%.
Leptons from
J
y
c
’s produced in
B
decays and elec-
trons from
p
0
and
h
decays to
e
1
e
2
g
were vetoed by
using the invariant mass of oppositely charged lepton pairs.
Electrons from photon conversions in the beam pipe and
drift chamber walls were also suppressed. Residual back-
grounds from these processes were estimated by Monte
Carlo simulation, as was the smaller contribution of lep-
tonic
c
0
decays. Leptons from
B
!
X
t
2
̄
n
t
were also es-
timated by Monte Carlo simulation, assuming a branching
fraction of 2.5%, consistent with the standard model and
recent measurements [10].
The uncertainties in these
backgrounds were taken to be 20%. The background con-
tribution of
D
s
and
L
c
decays was studied with a simula-
tion tuned to match measured rates and momentum spectra
[11,12]. The production of
L
c
included 20%
c
̄
cs
produc-
tion through
B
!
J
c
L
c
s
m
p
d
in addition to the dominant
B
!
L
c
̄
p
s
̄
n
ds
m
p
d
. The uncertainty for both of these con-
tributions was estimated to be 30%. Secondary leptons can
occasionally have momenta above
1.4
GeV
y
c
and con-
tribute false tags. In the CLEO II study of the
B
-meson
single-lepton spectrum [4], we found that approximately
2.8% of the identified leptons above
1.4
GeV
y
c
are sec-
ondary, with an estimated uncertainty of 25%.
After background corrections the electron spectra con-
sist of primary
B
semileptonic decays and secondary
charm semileptonic decays. In both cases the tag and
the electron are from different parent
B
’s. The unlike-
sign and like-sign electron momentum spectra can be ex-
pressed in terms of the primary and secondary branching
fractions as
dN
12
dp
N
,
he
d
B
s
b
d
dp
s
1
2x
d
1
d
B
s
c
d
dp
x
(1)
and
dN
66
dp
N
,
h
d
B
s
b
d
dp
x1
d
B
s
c
d
dp
s
1
2x
d
,
(2)
where
h
is the momentum-dependent efficiency of elec-
tron identification, and
e
is the momentum-dependent
efficiency of the diagonal cut. The number of tags,
N
,
246 465
6
739
, was determined by counting lep-
tons above
1.4
GeV
y
c
, subtracting backgrounds, and cor-
recting for the relative efficiency for selecting dilepton
events compared to single-lepton events [
s
97.9
6
0.5
d
%
].
The correction for
B
̄
B
mixing is given by
x
f
0
x
0
,
where
f
0
is the branching fraction for
Y
s
4
S
d
decay into
B
0
̄
B
0
, and
x
0
is the mixing parameter. We used
x
0.080
6
0.012
, the average of ARGUS [13] and CLEO
[14] measurements made with dileptons at the
Y
s
4
S
d
.
The primary and secondary electron spectra (Fig. 2)
were obtained by solving Eqs. (1) and (2). Integrat-
ing the primary spectrum from 0.6 to
2.6
GeV
y
c
,we
found the partial branching fraction
B
s
B
!
Xe
n
,
p
.
0.6
GeV
y
c
d
s
9.85
6
0.16
6
0.40
d
%
. The systematic
error includes the contributions which have been de-
scribed, a 2% uncertainty in the electron detection effi-
ciency, and a 1% uncertainty in tracking efficiency. This
result is almost completely model independent, with only
slight sensitivity entering through the efficiency and back-
ground estimates. To extract the semileptonic branch-
ing fraction we must correct for the undetected por-
tion of the electron spectrum. We estimated the frac-
tion below
0.6
GeV
y
c
to be
s
6.1
6
0.5
d
%
by using
model predictions [9,15] including corrections for inter-
nal and external bremsstrahlung. This leads to a value for
the
B
-meson semileptonic branching fraction of
B
s
B
!
Xe
n
d
s
10.49
6
0.17
6
0.43
d
%
.
If the semileptonic branching fractions for charged and
neutral
B
mesons differ, the lepton-tagged measurement
of
B
s
B
!
Xe
n
d
could be systematically higher than one
made with generic
Y
s
4
S
d
decays [4].
Nonspectator
effects that could produce such a difference are expected
to be small, and this is supported by measurements of the
B
0
and
B
6
lifetimes [16], and of
B
s
B
2
°!
D
p
0
,
2
̄
n
d
and
B
s
̄
B
0
°!
D
p
1
,
2
̄
n
d
[17]. Utilizing 90% confidence level
limits from these measurements, we find that the
1572
V
OLUME
76, N
UMBER
10
PHYSICAL REVIEW LETTERS
4 M
ARCH
1996
TABLE I. Yield of lepton-tagged electrons and corrections with statistical and systematic
errors, summed over the momentum interval
0.6
2.6
GeV
y
c
.
Unlike sign
Like sign
p
e
1
cos
s
,
,
e
d
.
1
no cut
ON
Y
s
4
S
d
e
yield
13 115
6
115
7699
6
88
Continuum
1365
6
54
6
7
637
6
37
6
3
Fake tag
,
141
6
2
6
71
85
6
1
6
43
Fake
e
214
6
3
6
107
540
6
8
6
270
Leptons from
J
y
c
and
c
0
238
6
6
6
48
154
6
4
6
31
e
from
p
0
or
h
52
6
5
6
10
158
6
9
6
32
e
from
g
conv.
56
6
5
6
11
152
6
8
6
30
B
!
X
t
,
t
!
Y
,
270
6
11
6
54
70
6
5
6
14
Leptons from
L
c
or
D
s
307
6
13
6
87
183
6
8
6
39
Secondary tags
205
6
10
6
51
401
6
13
6
100
e
from same
B
329
6
13
6
49
2
Total background
3177
6
60
6
186
2380
6
43
6
299
Net
e
yield
9938
6
129
6
186
5319
6
98
6
299
systematic upward shift in the lepton-tagged branching
fraction can be no greater than 1.5% of the measured value.
Because of the lepton tag used in this measurement,
we did not need to assume that all
Y
s
4
S
d
decays are to
B
̄
B
. The agreement between the overall rate for lepton
production and the rate in tagged events is evidence that
the fraction
f
of non-
B
̄
B
decays is small. The background-
corrected single-electron spectrum can be expressed as
dN
6
dp
2
N
Y
s
4
S
d
s
1
2
f
d
h
d
B
s
b
d
dp
1
d
B
s
c
d
dp
,
(3)
where
N
Y
s
4
S
d
2 143 400
6
4500
is the number of
Y
s
4
S
d
events. By comparing the integrated single-lepton yield
in the momentum interval
0.6
2.6
GeV
y
c
(
350 460
6
1726
) with that in tagged events, as given by Eqs. (1)
and (2) (
14 384
6
221
for unlike-sign and
5321
6
98
for like-sign), we find
f
s
2
0.11
6
1.43
6
1.07
d
%
,or
f
,
3.4%
at 95% confidence level, assuming no lep-
FIG. 2. Spectra of electrons from
B
!
Xe
n
(filled circles)
and
b
!
c
!
y
,
n
(open circles) obtained by solving Eqs. (1)
and (2). The curves show the best fit to the modified ISGW
model, with 23%
B
!
D
pp
,
n
.
ton production in non-
B
̄
B
decays. Since non-
B
̄
B
decays
have not been observed, their properties are a matter of
speculation. We considered a large variety of possible
mechanisms for lepton production in such decays, primar-
ily involving charmed and charmonium mesons. Event
properties such as multiplicity and charm-quark fragmen-
tation were extensively varied, including two-jet contin-
uum
c
̄
c
and more spherical narrow-resonance topologies.
Based on the worst cases encountered we set the 95% con-
fidence level upper limit on non-
B
̄
B
decays of the
Y
s
4
S
d
to be 4%.
While our determination of the semileptonic branch-
ing fraction did not involve fitting the lepton momentum
spectrum with theoretical models, such fits allow compar-
ison with the predictions of those models, and with the
results of our single-lepton analysis [4]. Good fits and
consistent results were obtained for the Altarelli-Cabibbo-
Corbo-Maiani-Martinelli (ACCMM) spectator model [15]
and for a modified version of the ISGW model [9] in
which the proportion of
B
semileptonic decays to
D
pp
,
n
was allowed to float (Fig. 2).
The branching fraction
B
s
b
!
c
!
ye
n
d
was de-
termined from the secondary electron spectrum. This
spectrum was fitted to model predictions of the charm
semileptonic momentum spectra, boosted according to
measured
D
0
and
D
1
momentum distributions [18]. For
the ACCMM model [15], with charm-decay parameters
tuned to agree with DELCO data [19], the result is
B
s
b
!
c
!
ye
n
d
s
7.8
6
0.2
6
1.2
d
%
, while the re-
sult for the ISGW model [9] is
B
s
b
!
c
!
ye
n
d
s
8.3
6
0.2
6
1.2
d
%
. These branching fractions do not
include the contributions of charmed baryons or
D
s
,
which have been treated as background in our analysis.
Within errors the secondary charm semileptonic branching
fraction is consistent with expectations, and with CLEO II
single-lepton measurements.
1573
V
OLUME
76, N
UMBER
10
PHYSICAL REVIEW LETTERS
4 M
ARCH
1996
The
B
-meson inclusive branching fraction is related
to the Cabibbo-Kobayashi-Maskawa (CKM) matrix ele-
ments
V
ub
and
V
cb
by
B
s
B
!
X
,
n
dy
t
B
g
c
j
V
cb
j
2
1
g
u
j
V
ub
j
2
, where the factors
g
c
and
g
u
must be obtained
from theory. The term involving
V
ub
makes a negligi-
ble contribution [20]. While the computation of
g
c
is
straightforward for any given model, an assumed uncer-
tainty of 20% has been conventional. With the ACCMM
model (
g
c
39
6
8
ps
2
1
d
, our semileptonic branching
fraction measurement and a
B
-meson lifetime of
s
1.61
6
0.04
d
ps [16] lead to
j
V
cb
j
0.041
6
0.001
6
0.004
.
The first error combines all experimental uncertainties,
both statistical and systematic. It is much smaller than
the second error, which is the theoretical uncertainty in
the computation of
g
c
. With the ISGW model (
g
c
41
6
8
ps
2
1
), we find
j
V
cb
j
0.040
6
0.001
6
0.004
.
Shifman, Uraltsev, and Vainshtein [21] have asserted
that the uncertainty in determining
j
V
cb
j
from the in-
clusive rate can be reduced if experimental and theoret-
ical constraints on the quark-mass difference
m
b
2
m
c
are taken into account. Following their prescription, we
find
j
V
cb
j
0.040
6
0.001
6
0.002
. The size of the un-
certainty remains somewhat controversial, however, with
other analyses similar in approach leading to overall un-
certainties in
j
V
cb
j
of 10% or more [22].
In conclusion, we have used events with an electron and
a high-momentum lepton tag to measure
B
s
B
!
Xe
n
d
s
10.49
6
0.17
6
0.43
d
%
, in agreement with other mea-
surements, and below most theoretical expectations. This
measurement is largely model independent, and is insen-
sitive to non-
B
̄
B
decays of the
Y
s
4
S
d
. By comparing
the electron yield in tagged events with the total single-
electron yield, we set a 95% confidence level upper limit
on the fraction of non-
B
̄
B
decays of 4%. We have used
our results to extract the CKM parameter
j
V
cb
j
for differ-
ent theoretical approaches.
We are grateful to the CESR staff for providing ex-
cellent luminosity and running conditions. We thank A.
Vainshtein and M. Shifman for stimulating discussions.
This work was supported by the National Science Fun-
dation, the U.S. Department of Energy, the Heisenberg
Foundation, the Alexander von Humboldt Stiftung, the
Natural Sciences and Engineering Research Council of
Canada, and the A.P. Sloan Foundation.
*Permanent address: University of Hawaii at Manoa,
Honolulu, HI 96802.
[1] I. Bigi, B. Blok, M. A. Shifman, and A. Vainshtein, Phys.
Lett. B
323
, 408 (1994).
[2] A. F. Falk, M. B. Wise, and I. Dunietz, Phys. Rev. D
51
, 1183 (1995); E. Bagan, Patricia Ball, V. M. Braun,
and P. Gosdzinsky, Phys. Lett. B
342
, 362 (1995); M. B.
Voloshin, Phys. Rev. D
51
, 3948 (1995).
[3] T. E. Browder, contribution to the International Euro-
physics Conference on High Energy Physics, Brussels,
Belgium, 1995.
[4] CLEO Collaboration, J. Bartelt
et al.,
Cornell University
Report No. CLEO CONF 93-19.
[5] CLEO Collaboration, S. Henderson
et al.,
Phys. Rev. D
45
, 2212 (1992).
[6] ARGUS Collaboration, H. Albrecht
et al.,
Phys. Lett. B
318
, 397 (1993).
[7] CLEO Collaboration, Y. Kubota
et al.,
Nucl. Instrum.
Methods Phys. Res., Sect. A
320
, 66 (1992).
[8] CLEO Collaboration, J. Gronberg
et al.,
Cornell Univer-
sity Report No. CLEO CONF 94-6.
[9] N. Isgur, D. Scora, B. Grinstein, and M. B. Wise, Phys.
Rev. D
39
, 799 (1989).
[10] L3 Collaboration, M. Acciarri
et al.,
Phys. Lett. B
332
,
201 (1994); ALEPH Collaboration, D. Buskulic
et al.,
Phys. Lett. B
343
, 444 (1995).
[11] CLEO Collaboration, D. Gibaut
et al.,
Phys. Rev. D (to be
published).
[12] CLEO Collaboration, D. Cinabro
et al.,
Cornell University
Report No. CLEO CONF 94-8.
[13] ARGUS Collaboration, H. Albrecht
et al.,
Z. Phys. C
55
,
357 (1992).
[14] CLEO Collaboration, J. Bartelt
et al.,
Phys. Rev. Lett.
71
,
1680 (1993).
[15] G. Altarelli, N. Cabibbo, G. Corbo, L. Maiani, and G.
Martinelli, Nucl. Phys.
B208
, 365 (1982).
[16] S. Komamiya, contribution to the International Euro-
physics Conference on High Energy Physics, Brussels,
Belgium, 1995.
[17] CLEO Collaboration, B. Barish
et al.,
Phys. Rev. D
51
,
1014 (1995).
[18] F. Muheim, contribution to the Eighth Meeting of the
Division of Particles and Fields of the American Physical
Society, Albuquerque, New Mexico, 1994.
[19] DELCO Collaboration, W. Bacino
et al.,
Phys. Rev. Lett.
43
, 1073 (1979).
[20] CLEO Collaboration, J. Bartelt
et al.,
Phys. Rev. Lett.
71
,
4111 (1993).
[21] M. Shifman, N. G. Uraltsev, and A. Vainshtein, Phys. Rev.
D
45
, 2217 (1995).
[22] M. Luke and M. J. Savage, Phys. Lett. B
321
, 88 (1994);
M. Neubert, in Proceedings of the 30th Recontres de
Moriond, Les Arcs, France, 1995 (CERN Report No.
CERN-TH
y
95-107).
1574