of 4
Measurement of the branching fractions of
L
c
1
ò
pK
̄
n
Ñ
p
Ö
M. S. Alam,
1
S. B. Athar,
1
Z. Ling,
1
A. H. Mahmood,
1
H. Severini,
1
S. Timm,
1
F. Wappler,
1
A. Anastassov,
2
J. E. Duboscq,
2
D. Fujino,
2,
*
K. K. Gan,
2
T. Hart,
2
K. Honscheid,
2
H. Kagan,
2
R. Kass,
2
J. Lee,
2
M. B. Spencer,
2
M. Sung,
2
A. Undrus,
2,†
R. Wanke,
2
A. Wolf,
2
M. M. Zoeller,
2
B. Nemati,
3
S. J. Richichi,
3
W. R. Ross,
3
P. Skubic,
3
M. Bishai,
4
J. Fast,
4
J. W. Hinson,
4
N. Menon,
4
D. H. Miller,
4
E. I. Shibata,
4
I. P. J. Shipsey,
4
M. Yurko,
4
L. Gibbons,
5
S. Glenn,
5
S. D. Johnson,
5
Y. Kwon,
5,‡
S. Roberts,
5
E. H. Thorndike,
5
C. P. Jessop,
6
K. Lingel,
6
H. Marsiske,
6
M. L. Perl,
6
D. Ugolini,
6
R. Wang,
6
X. Zhou,
6
T. E. Coan,
7
V. Fadeyev,
7
I. Korolkov,
7
Y. Maravin,
7
I. Narsky,
7
V. Shelkov,
7
J. Staeck,
7
R. Stroynowski,
7
I. Volobouev,
7
J. Ye,
7
M. Artuso,
8
A. Efimov,
8
M. Goldberg,
8
D. He,
8
S. Kopp,
8
G. C. Moneti,
8
R. Mountain,
8
S. Schuh,
8
T. Skwarnicki,
8
S. Stone,
8
G. Viehhauser,
8
X. Xing,
8
J. Bartelt,
9
S. E. Csorna,
9
V. Jain,
9,§
K. W. McLean,
9
S. Marka,
9
R. Godang,
10
K. Kinoshita,
10
I. C. Lai,
10
P. Pomianowski,
10
S. Schrenk,
10
G. Bonvicini,
11
D. Cinabro,
11
R. Greene,
11
L. P. Perera,
11
G. J. Zhou,
11
B. Barish,
12
M. Chadha,
12
S. Chan,
12
G. Eigen,
12
J. S. Miller,
12
C. O’Grady,
12
M. Schmidtler,
12
J. Urheim,
12
A. J. Weinstein,
12
F. Wu
̈
rthwein,
12
D. W. Bliss,
13
G. Masek,
13
H. P. Paar,
13
S. Prell,
13
V. Sharma,
13
D. M. Asner,
14
J. Gronberg,
14
T. S. Hill,
14
D. J. Lange,
14
S. Menary,
14
R. J. Morrison,
14
H. N. Nelson,
14
T. K. Nelson,
14
C. Qiao,
14
J. D. Richman,
14
D. Roberts,
14
A. Ryd,
14
M. S. Witherell,
14
R. Balest,
15
B. H. Behrens,
15
W. T. Ford,
15
H. Park,
15
J. Roy,
15
J. G. Smith,
15
J. P. Alexander,
16
C. Bebek,
16
B. E. Berger,
16
K. Berkelman,
16
K. Bloom,
16
D. G. Cassel,
16
H. A. Cho,
16
D. S. Crowcroft,
16
M. Dickson,
16
P. S. Drell,
16
K. M. Ecklund,
16
R. Ehrlich,
16
A. D. Foland,
16
P. Gaidarev,
16
B. Gittelman,
16
S. W. Gray,
16
D. L. Hartill,
16
B. K. Heltsley,
16
P. I. Hopman,
16
J. Kandaswamy,
16
P. C. Kim,
16
D. L. Kreinick,
16
T. Lee,
16
Y. Liu,
16
G. S. Ludwig,
16
N. B. Mistry,
16
C. R. Ng,
16
E. Nordberg,
16
M. Ogg,
16,
i
J. R. Patterson,
16
D. Peterson,
16
D. Riley,
16
A. Soffer,
16
B. Valant-Spaight,
16
C. Ward,
16
M. Athanas,
17
P. Avery,
17
C. D. Jones,
17
M. Lohner,
17
C. Prescott,
17
J. Yelton,
17
J. Zheng,
17
G. Brandenburg,
18
R. A. Briere,
18
A. Ershov,
18
Y. S. Gao,
18
D. Y.-J. Kim,
18
R. Wilson,
18
H. Yamamoto,
18
T. E. Browder,
19
F. Li,
19
Y. Li,
19
J. L. Rodriguez,
19
T. Bergfeld,
20
B. I. Eisenstein,
20
J. Ernst,
20
G. E. Gladding,
20
G. D. Gollin,
20
R. M. Hans,
20
E. Johnson,
20
I. Karliner,
20
M. A. Marsh,
20
M. Palmer,
20
M. Selen,
20
J. J. Thaler,
20
K. W. Edwards,
21
A. Bellerive,
22
R. Janicek,
22
D. B. MacFarlane,
22
P. M. Patel,
22
A. J. Sadoff,
23
R. Ammar,
24
P. Baringer,
24
A. Bean,
24
D. Besson,
24
D. Coppage,
24
C. Darling,
24
R. Davis,
24
N. Hancock,
24
S. Kotov,
24
I. Kravchenko,
24
N. Kwak,
24
S. Anderson,
25
Y. Kubota,
25
S. J. Lee,
25
J. J. O’Neill,
25
S. Patton,
25
R. Poling,
25
T. Riehle,
25
V. Savinov,
25
and A. Smith
25
~
CLEO Collaboration
!
1
State University of New York at Albany, Albany, New York 12222
2
Ohio State University, Columbus, Ohio 43210
3
University of Oklahoma, Norman, Oklahoma 73019
4
Purdue University, West Lafayette, Indiana 47907
5
University of Rochester, Rochester, New York 14627
6
Stanford Linear Accelerator Center, Stanford University, Stanford, California 94309
7
Southern Methodist University, Dallas, Texas 75275
8
Syracuse University, Syracuse, New York 13244
9
Vanderbilt University, Nashville, Tennessee 37235
10
Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
11
Wayne State University, Detroit, Michigan 48202
12
California Institute of Technology, Pasadena, California 91125
13
University of California, San Diego, La Jolla, California 92093
14
University of California, Santa Barbara, California 93106
15
University of Colorado, Boulder, Colorado 80309-0390
16
Cornell University, Ithaca, New York 14853
17
University of Florida, Gainesville, Florida 32611
18
Harvard University, Cambridge, Massachusetts 02138
19
University of Hawaii at Manoa, Honolulu, Hawaii 96822
*
Permanent address: Lawrence Livermore National Laboratory, Livermore, CA 94551.
Permanent address: BINP, RU-630090 Novosibirsk, Russia.
Permanent address: Yonsei University, Seoul 120-749, Korea.
§
Permanent address: Brookhaven National Laboratory, Upton, NY 11973.
i
Permanent address: University of Texas, Austin TX 78712.
PHYSICAL REVIEW D
1 APRIL 1998
VOLUME 57, NUMBER 7
57
0556-2821/98/57
~
7
!
/4467
~
4
!
/$15.00
4467
© 1998 The American Physical Society
20
University of Illinois, Champaign-Urbana, Illinois 61801
21
Carleton University, Ottawa, Ontario, Canada K1S 5B6
and Institute of Particle Physics, Canada
22
McGill University, Montre
́
al, Que
́
bec, Canada H3A 2T8
and Institute of Particle Physics, Canada
23
Ithaca College, Ithaca, New York 14850
24
University of Kansas, Lawrence, Kansas 66045
25
University of Minnesota, Minneapolis, Minnesota 55455
~
Received 10 September 1997; published 18 February 1998
!
Using data recorded by the CLEO-II detector at CESR, we report new measurements of the branching
fractions for the decays of the charmed baryon
L
c
1
into
pK
2
p
1
p
0
,
pK
̄
0
,
pK
̄
0
p
1
p
2
, and
pK
̄
0
p
0
,
all
measured
relative
to
pK
2
p
1
.
The
relative
branching
fractions
are
0.67
6
0.04
6
0.11,0.46
6
0.02
6
0.04,0.52
6
0.04
6
0.05, and 0.66
6
0.05
6
0.07, respectively.
@
S0556-2821
~
98
!
05705-1
#
PACS number
~
s
!
: 14.20.Lq, 13.30.Eg, 14.65.Dw
Since the first observation of the lowest lying charmed
baryon, the
L
c
1
, there have been many measurements made
of its exclusive decay channels. As it is difficult to measure
the production cross section of the
L
c
1
baryons, decay rates
are typically presented as branching ratios relative to
L
c
1
!
pK
2
p
1
, the most easily observed decay channel. How-
ever, fewer than half of the
L
c
1
hadronic decays are pres-
ently accounted for. Measurement of these modes is of prac-
tical as well as theoretical interest. Here, we present
measurements of the branching fractions of
L
c
1
into
pK
2
p
1
p
0
,
pK
̄
0
,
pK
̄
0
p
1
p
2
, and
pK
̄
0
p
0
, all relative to
pK
2
p
1
. The last of these is the first measurement of this
mode. The other modes have been previously measured but
with considerably larger uncertainties than in the present
study.
The data presented here were taken by the CLEO II de-
tector
@
1
#
operating at the Cornell Electron Storage Ring.
The sample used in this analysis corresponds to an integrated
luminosity of 4.8 fb
2
1
from data taken on the
Y
(4
S
) reso-
nance and in the continuum at energies just above and below
the
Y
(4
S
) . We detected charged tracks with a cylindrical
drift chamber system inside a solenoidal magnet. Photons
were detected using an electromagnetic calorimeter consist-
ing of 7800 cesium iodide crystals.
Particle identification of
p
,
K
2
, and
p
1
candidates was
performed using specific ionization measurements in the drift
chamber and, when present, time-of-flight measurements.
For each mass hypothesis, a combined
x
2
probability
P
i
was
formed (
i
5
p
,
K
,
p
) . Using these probablilities, a normalized
probability
ratio
L
i
was
evaluated,
where
L
i
5
P
i
/(
P
p
1
P
K
1
P
p
) . Well-identified protons peaked near
P
p
5
1.0 while tracks that were identified to not be protons
peak near
P
p
5
0.0. For a track to be used as a proton in this
study, we required it to have
L
p
.
0.8, which eliminated
much of the background, though with considerable diminu-
tion of efficiency. For kaons we applied a looser and more
efficient cut of
L
K
.
0.1. We have chosen these cuts using a
Monte Carlo simulation program to maximize the signifi-
cance of the signals. The proton identification requirement
resulted in an efficiency that is strongly momentum depen-
dent, being over 95% for momenta less than 1 GeV/
c
, fall-
FIG. 1. Invariant mass plots for the 5 different decay modes of
the
L
c
1
.
TABLE I. The number of
L
c
1
’s found with
x
p
(
L
c
)
.
0.5.
Mode
MC width
~
meV
!
Signal
pK
2
p
1
16
10109
6
191
pK
2
p
1
p
0
22
2606
6
165
pK
̄
0
19
1025
6
40
pK
̄
0
p
1
p
2
15
985
6
65
pK
̄
0
p
0
27
774
6
52
4468
57
BRIEF REPORTS
ing to 25% at
p
5
2 GeV/
c
. The kaon identification is rather
efficient, with a mean value of 94% when averaged over all
momenta. In order to reduce the large combinatoric back-
ground, we required
x
p
.
0.5, where
x
p
5
P
L
c
/
A
E
beam
2
2
m
L
c
2
is the scaled momentum of the
L
c
1
candidate. Approximately
60% of
L
c
1
baryons from
cc
̄
continuum events passed this
requirement.
The
K
̄
0
candidates were identified in their decay
K
s
0
!
p
1
p
2
, by reconstructing a secondary vertex from the
intersection of two oppositely charged tracks in the
r
-
f
plane. The secondary vertex was required to be displaced
from the beam spot by at least 1 mm in the direction of flight
of the
K
0
, and each of the daughter particles was required to
be inconsistent with coming from the beam spot. The invari-
ant mass of the
K
0
candidate must lie within 9 MeV/
c
2
@
around 3 standard deviations
~
s
!#
of its nominal value.
The
p
0
candidates were selected through their decay
p
0
!
gg
from pairs of well-defined showers in the CsI calo-
rimeter with a reconstructed invariant mass within 3
s
of the
p
0
mass. In order to reduce the combinatorial background,
each
g
was required to have an energy of at least 50 MeV,
and the
p
0
was required to have a momentum of at least
300 MeV/
c
.
The resulting mass distributions for the 5 modes are
shown in Fig. 1. Each peak was fit to the sum of a Gaussian
signal distribution with width fixed to that obtained from
CLEO’s
GEANT
based Monte Carlo simulation program and
a second order polynomial background distribution. The sig-
nal widths used and the resulting signal yields are tabulated
in Table I.
The efficiency for each
L
c
1
mode was calculated using
the Monte Carlo simulation program
@
2
#
. The reconstruction
efficiency of the
L
c
1
decays has some dependence on the
resonant substructure of these states. In the case of the
pK
2
p
1
mode, the Monte Carlo generator produced a mix-
ture of non-resonant three-body decay together with
D
11
K
2
and
pK
̄
*
0
decays, according to their measured branching
fractions
@
3
#
. These three types of decays had slightly differ-
ent reconstruction efficiencies, so that including the substruc-
ture changes the efficiency by
D
e
/
e
5
0.02 relative to 3-body
phase space. The difference in detection efficiency for
pK
2
p
1
~
non-resonant
!
compared with
pK
2
p
1
resonating
via
pK
̄
*
0
was found to be
'
10% . We have also investigated
the dependence of the reconstruction efficiency of the other
modes on a possible resonant substructure, including all
modes involving an intermediate
K
*
. In no case did the sub-
structure produce as large a change in efficiency as that noted
above. The poor signal to background ratio did not allow a
detailed measurement of the substructure. The efficiency cal-
culation took into account the
K
0
!
K
s
0
and
K
s
0
!
p
1
p
2
branching fractions
~
see Table II
!
.
We have considered many possible sources of systematic
error in the measurement. The main contributors to the sys-
tematic uncertainty came from the following sources: first,
uncertainties in the fitting procedures, which were estimated
by looking at the changes in the yields using different orders
of polynomial background and different signal widths
~
15%
in the case of
pK
2
p
1
p
0
, but much smaller for the other
modes
!
; second, uncertainties due to the unknown mix of
resonant substructure in the multi-body decays
~
up to 3%
depending on the mode
!
; third, uncertainties due to
p
0
find-
ing
~
5%
!
, checked using the partial reconstruction of
D
*
1
!
D
0
p
1
,
D
0
!
K
2
p
1
p
0
decays, the
K
s
0
finding
~
5%
!
,
found by comparing
K
s
0
yields in the data and Monte Carlo
simulation using a large variety of different algorithms, and
track finding
~
1% per track
!
, found by a detailed investiga-
tion of track parameters in
D
*
1
!
D
0
,
D
0
!
K
2
p
1
decays;
and fourth, uncertainties in the reconstruction efficiency due
to the particle identification criteria for protons and kaons
~
4%
!
, found by checking the agreement of Monte Carlo
simulation of the energy-loss and time-of-flight measure-
ments with the data from topologically identified
L
!
p
p
2
and
D
*
1
!
K
2
p
1
p
1
decays.
These uncertainties have been added in quadrature to ob-
tain the total systematic uncertainty for each mode. As the
measurements are of ratios of branching fractions, many of
the systematic uncertainties cancel.
There are three main types of quark decay diagrams that
contribute to
L
c
1
decays. The simplest method is the simple
spectator diagram in which the virtual
W
1
fragments inde-
pendently of the spectator quark. The second method in-
volves the quark daughters of the
W
1
combining with the
remaining quarks. The third method,
W
exchange, involves
the
W
1
combining with the initial
d
quark. Unfortunately all
the decay modes under investigation here can proceed by
more than one of these decay diagrams, and their decay rates
are not amenable to calculation. Furthermore, our results are
expressed as ratios of the branching fraction to that of
L
c
1
!
pK
2
p
1
, and there is no reliable measurement of the
absolute branching fraction of this or any other exclusive
decay mode, further hampering a comparison of experimen-
tal results and theory.
TABLE II. The measured relative branching fractions.
Mode
Relative efficiency
B
/
B
(
pK
2
p
1
)
Previous measurements
pK
2
p
1
1.0
1.0
pK
2
p
1
p
0
0.383
0.67
6
0.04
6
0.11
0.72
2
0.22
1
0.32
@
5
#
pK
̄
0
0.218
0.46
6
0.02
6
0.04
0.44
6
0.07
6
0.05
@
4
#
0.55
6
0.17
6
0.14
@
6
#
0.62
6
0.15
6
0.03
@
7
#
pK
̄
0
p
1
p
2
0.187
0.52
6
0.04
6
0.05
0.43
6
0.12
6
0.04
@
4
#
0.98
6
0.36
6
0.08
@
5
#
pK
̄
0
p
0
0.115
0.66
6
0.05
6
0.07
57
4469
BRIEF REPORTS
In conclusion, we have measured new branching fractions
of the
L
c
1
into 4 decay modes, measured relative to the nor-
malizing mode
L
c
1
!
pK
2
p
1
. The results for three of these
modes are in agreement with, and more accurate than, pre-
vious measurements. We have made the first measurement of
the decay rate of
L
c
1
!
pK
̄
0
p
0
. These measurements help
account for the total width of the
L
c
1
and increase the un-
derstanding of charmed baryon decays.
@
1
#
Y. Kubota
et al.
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~
1992
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1996
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!
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6
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et al.
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!
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et al.
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~
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!
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4470
57
BRIEF REPORTS