We report the observation of the Cabibbo-suppressed decays Λ_c^+→pK^-K^+ and Λ_c^+→pφ using data collected with the CLEO II detector at CESR. The latter mode, observed for the first time with significant statistics, is of interest as a test of color suppression in charm decays. We have determined the branching ratios for these modes relative to Λ_c^+→pK^-π^+ and compared our results with theory

Alexander, J. P.

Barish, B.

Chadha, M.

Chan, S.

Cowen, D. F.

Eigen, G.

Miller, J. S.

O'Grady, C.

Urheim, J.

Weinstein, Alan J.

Würthwein, F.

English

Caltech Authors - Main

PHYSICAL REVIEW D
PARTICLES AND FIELDS
THIRD SERIES, VOLUME 53, NUMBER 3 1 FEBRUARY 1996
priority
d
generallyObservation of the Cabibbo-suppressed charmed baryon decayLc
1
˜pf
J. P. Alexander,1 C. Bebek,1 B. E. Berger,1 K. Berkelman,1 K. Bloom,1 T. E. Browder,1,* D. G. Cassel,1 H. A. Cho,1
D. M. Coffman,1 D. S. Crowcroft,1 M. Dickson,1 P. S. Drell,1 D. J. Dumas,1 R. Ehrlich,1 R. Elia,1 P. Gaidarev,1
M. Garcia-Sciveres,1 B. Gittelman,1 S. W. Gray,1 D. L. Hartill,1 B. K. Heltsley,1 S. Henderson,1 C. D. Jones,1
S. L. Jones,1 J. Kandaswamy,1 N. Katayama,1 P. C. Kim,1 D. L. Kreinick,1 T. Lee,1 Y. Liu,1 G. S. Ludwig,1 J. Masui,1
J. Mevissen,1 N. B. Mistry,1 C. R. Ng,1 E. Nordberg,1 J. R. Patterson,1 D. Peterson,1 D. Riley,1 A. Soffer,1 P. Avery,2
A. Freyberger,2 K. Lingel,2 C. Prescott,2 J. Rodriguez,2 S. Yang,2 J. Yelton,2 G. Brandenburg,3 D. Cinabro,3 T. Liu,3
M. Saulnier,3 R. Wilson,3 H. Yamamoto,3 T. Bergfeld,4 B. I. Eisenstein,4 J. Ernst,4 G. E. Gladding,4 G. D. Gollin,4 M. Palmer,4
M. Selen,4 J. J. Thaler,4 K. W. Edwards,5 K. W. McLean,5 M. Ogg,5 A. Bellerive,6 D. I. Britton,6 E. R. F. Hyatt,6
R. Janicek,6 D. B. MacFarlane,6 P. M. Patel,6 B. Spaan,6 A. J. Sadoff,7 R. Ammar,8 P. Baringer,8 A. Bean,8 D. Besson,8
D. Coppage,8 N. Copty,8 R. Davis,8 N. Hancock,8 S. Kotov,8 I. Kravchenko,8 N. Kwak,8 Y. Kubota,9 M. Lattery,9
M. Momayezi,9 J. K. Nelson,9 S. Patton,9 R. Poling,9 V. Savinov,9 S. Schrenk,9 R. Wang,9 M. S. Alam,10 I. J. Kim,10 Z. Ling,10
A. H. Mahmood,10 J. J. O’Neill,10 H. Severini,10 C. R. Sun,10 F. Wappler,10 G. Crawford,11 R. Fulton,11 D. Fujino,11
K. K. Gan,11 K. Honscheid,11 H. Kagan,11 R. Kass,11 J. Lee,11 M. Sung,11 C. White,11 A. Wolf,11 M. M. Zoeller,11 X. Fu,12
B. Nemati,12 W. R. Ross,12 P. Skubic,12 M. Wood,12 M. Bishai,13 J. Fast,13 E. Gerndt,13 J. W. Hinson,13 T. Miao,13
D. H. Miller,13 M. Modesitt,13 E. I. Shibata,13 I. P. J. Shipsey,13 P. N. Wang,13 L. Gibbons,14 S. D. Johnson,14 Y. Kwon,14
S. Roberts,14 E. H. Thorndike,14 T. E. Coan,15 J. Dominick,15 V. Fadeyev,15 I. Korolkov,15 M. Lambrecht,15
S. Sanghera,15 V. Shelkov,15 T. Skwarnicki,15 R. Stroynowski,15 I. Volobouev,15 G. Wei,15 M. Artuso,16 M. Gao,16
M. Goldberg,16 D. He,16 N. Horwitz,16 S. Kopp,16 G. C. Moneti,16 R. Mountain,16 F. Muheim,16 Y. Mukhin,16 S. Playfer,16
S. Stone,16 X. Xing,16 J. Bartelt,17 S. E. Csorna,17 V. Jain,17 S. Marka,17 D. Gibaut,18 K. Kinoshita,18 P. Pomianowski,18
B. Barish,19 M. Chadha,19 S. Chan,19 D. F. Cowen,19 G. Eigen,19 J. S. Miller,19 C. O’Grady,19 J. Urheim,19 A. J. Weinstein,19
F. Würthwein,19 D. M. Asner,20 M. Athanas,20 D. W. Bliss,20 W. S. Brower,20 G. Masek,20 H. P. Paar,20 J. Gronberg,21
C. M. Korte,21 R. Kutschke,21 S. Menary,21 R. J. Morrison,21 S. Nakanishi,21 H. N. Nelson,21 T. K. Nelson,21 C. Qiao,21
J. D. Richman,21 D. Roberts,21 A. Ryd,21 H. Tajima,21 M. S. Witherell,21 R. Balest,22 K. Cho,22 W. T. Ford,22
M. Lohner,22 H. Park,22 P. Rankin,22 and J. G. Smith22
1Cornell University, Ithaca, New York 14853
2University of Florida, Gainesville, Florida 32611
3Harvard University, Cambridge, Massachusetts 02138
4University of Illinois, Champaign-Urbana, Illinois, 61801
5Carleton University, Ottawa, Ontario, Canada K1S 5B6
and the Institute of Particle Physics, University of Ottawa Campus, Ottawa, Ontario, Canada K1N 9B4
6McGill University, Montréal, Québec, Canada H3A 2T8
and the Institute of Particle Physics, University of Ottawa Campus, Ottawa, Ontario, Canada K1N 9B4
7Ithaca College, Ithaca, New York 14850
8University of Kansas, Lawrence, Kansas 66045
9University of Minnesota, Minneapolis, Minnesota 55455
*Permanent address: University of Hawaii at Manoa.
RAPID COMMUNICATIONS
Rapid Communications are intended for important new results which deserve accelerated publication, and are therefore given
in the editorial office and in production. A Rapid Communication inPhysical Review Dshould be no longer than five printed pages an
must be accompanied by an abstract. Page proofs are sent to authors, but because of the accelerated schedule, publication is
not delayed for receipt of corrections unless requested by the author.530556-2821/96/53~3!/1013~5!/$06.00 R1013 © 1996 The American Physical Society
R1014 53J. P. ALEXANDERet al.10State University of New York at Albany, Albany, New York 12222
11Ohio State University, Columbus, Ohio, 43210
12University of Oklahoma, Norman, Oklahoma 73019
13Purdue University, West Lafayette, Indiana 47907
14University of Rochester, Rochester, New York 14627
15Southern Methodist University, Dallas, Texas 75275
16Syracuse University, Syracuse, New York 13244
17Vanderbilt University, Nashville, Tennessee 37235
18Virginia Polytechnic Institute and State University, Blacksburg, Virginia, 24061
19California Institute of Technology, Pasadena, California 91125
20University of California, San Diego, La Jolla, California 92093
21University of California, Santa Barbara, California 93106
22University of Colorado, Boulder, Colorado 80309-0390
~Received 25 July 1995!
We report the observation of the Cabibbo-suppressed decaysLc
1→pK2K1 andLc1→pf using data col-
lected with the CLEO II detector at CESR. The latter mode, observed for the first time with significant
statistics, is of interest as a test of color suppression in charm decays. We have determined the branching ratios
for these modes relative toLc
1→pK2p1 and compared our results with theory.
PACS number~s!: 13.30.Eg, 12.39.2x, 14.20.Lqr e
th
w
in
s
c
be
in
on
ly
b
he
l-
-
th
ur
he
cto
y
ity
m
ba
-
s
O
he
a
-
ton
ire
we
bi-
d
tted
ck-
5
ays
-
nerThe strength of color suppression in internalW-emission
charmed meson decays has long been in question. Fo
ample, B(Ds
1→K̄* 0K1)/B(Ds1→fp1).1, @1,2# while
the expectation from color-matching requirements is that
ratio should be about 1/9. Reasonable overall agreement
the experimental data in the charm sector has been obta
using factorization and taking the largeNc limit in a 1/Nc
expansion approach, whereNc is the number of quark color
@3,4#. The Cabibbo-suppressed charmed baryon de
Lc
1→pf, shown in Fig. 1, is also naively expected to
color suppressed. However, using factorization and tak
the limit Nc→` leads to a prediction of no color suppressi
@5#. Since theLc
1→pf decay receives contributions on
from factorizable diagrams, a reliable calculation should
obtained using factorization. Observation of t
Lc
1→pf decay was first reported by the ACCMOR Co
laboration with 2.861.9 events@9#. Last year the E687 Col
laboration published results on the first observation of
Cabibbo-suppressed charmed baryon decayLc
1→pK2K1,
along with an upper limit on the resonant substruct
Lc
1→pf @10#. Herein we present new CLEO results on t
observation of Lc
1→pK2K1 and Lc1→pf decays and
discuss the implications of the results.
We use a data sample recorded with the CLEO II dete
operating at the Cornell Electron Storage Ring~CESR!. The
sample consists ofe1e2 annihilations taken at and slightl
below theY(4S) resonance, for a total integrated luminos
of 3.46 fb21. The main detector components which are i
portant for this analysis are the tracking system and the
rel time-of-flight ~TOF! particle identification system. Addi
tional particle identification~ID! is provided by specific
ionization (dE/dx) information from the tracking system’
main drift chamber. A more detailed description of the CLE
II detector has been provided elsewhere@11#.
To search for theLc
1 signals we studypK2K1 track
combinations found by the tracking system. Thep and K6
candidates are identified by combining information from t
TOF anddE/dx systems to form a combinedx2 probabilityx-
is
ith
ed
ay
g
e
e
e
r
-
r-
P i for each mass hypothesisi 5p,K,p. Using these prob-
abilities P i , a normalized probability ratioLi is evaluated
for each track according to the formul
Li[P i /(P p1P K1P p). Well-identified protons form a
sharp peak nearLp51, while tracks identified asnot being
protons form a peak nearLp50. The remainder of the can
didates fall in the region between 0 and 1. For the pro
involved in each decay mode under study we requ
Lp.0.9, which constitutes a strong cut. For the kaons
apply a loose cut ofLK.0.1. In addition, all protons and
kaons must pass a minimum requirement ofP p.0.001 and
P K.0.001, respectively. In order to reduce the large com
natoric background, the candidateLc
1 scaled momentum
xp5PLc /AEbeam2 2mLc2 is limited to xp.0.5.
ThepK2K1 invariant mass is shown in Fig. 2. The broa
enhancement in the mass region above 2.37 GeV/c2 is a
reflection from the decay modeLc
1→pK2p1, where the
pion has been misidentified as a kaon. The spectrum is fi
to a Gaussian for the signal with width fixed tos54.9 MeV/
c2 determined from Monte Carlo simulation@12#, and a
second-order Chebyshev polynomial for the smooth ba
ground. This fit yields 214650 events for the inclusive
Lc
1→pK2K1 signal with a mean mass of 2285.
61.2 MeV/c2 @13#.
The f candidates are reconstructed through their dec
f→K2K1. Because the width of thef is comparable to
the detector mass resolution, thef signal shape is best de
scribed by a convolution of a Gaussian and a Breit-Wig
function of width G54.43 MeV/c2 @1#. The background
is parametrized by a function of the formb(m)
FIG. 1. The decayLc
1→pf.
on
th
on
c
to
d-
nd
he
l
b
es
tral
e
he
-
ed
er
-
7
m
g
o
eV/
-
53 R1015OBSERVATION OF THE CABIBBO-SUPPRESSED CHARMED . . .5N(m2m0)
aeb(m2m0). The measured Gaussian resoluti
from the fit is s51.660.2 MeV/c2. In order to perform
background subtractions, 1.0121,mKK,1.0273 GeV/c
2 is
designated as the f ‘‘signal’’ region, while
0.990,mKK,1.005 GeV/c
2 and 1.035,mKK,1.050 GeV/
c2 are designated as the ‘‘sideband’’ regions. Integrating
background function over the sideband and signal regi
gives a signal-to-sideband scale factorRf50.56060.016,
which is used in thef background subtraction below.
In order to obtain theLc
1→pf signal, thepK2K1 mass
plot is made both formK2K1 in the f signal region and the
f sideband regions. Figure 3 shows the results. The spe
are fitted to a Gaussian for the signal with width fixed
s54.9 MeV/c2 from Monte Carlo simulation, and a secon
order Chebyshev polynomial for the smooth backgrou
The fit to thepK2K1 mass spectrum corresponding to t
f signal region yields 54612 events with a confidence leve
of 97%. The mean mass for the signal is measured to
2288.261.3 MeV/c2. In fitting the pK2K1 mass corre-
sponding to thef sideband region, the meanLc
1 mass is
fixed to that obtained from thef signal region and thes is
FIG. 2. Invariant mass of inclusivepK2K1 combinations pass
ing all requirements. Nof cut is applied. The region above 2.3
GeV/c2, where there is a large enhancement fro
Lc
1→pK2p1 decays, is not included in the fit.
FIG. 3. Invariant mass ofpK2K1 combinations correspondin
to K2K1 mass in thef signal and sideband regions.e
s
tra
.
e
fixed to the Monte Carlo value as before. This giv
216.469.6 events for thef sidebandLc
1 yield. Since the
true contribution must be positive definite we set the cen
value to zero and use 069.6 as the best estimate of th
Lc
1→pK2K1 contribution. After scaling this byRf and
subtracting we find that the netLc
1→pf yield is 54613
events.
As a check of the nonresonant contribution to t
Lc
1→pf signal we fit theK2K1 mass spectra correspond
ing to the Lc
1 signal and sideband regions as determin
from the inclusivepK2K1 mass spectrum. Thef yield ob-
tained from theLc
1 sideband regions, 2.246,mpKK,2.266
and 2.306,mpKK,2.326 GeV/c
2, is subtracted from that for
the Lc
1 signal region, 2.276,mpKK,2.296 GeV/c
2. Figure
4 shows the fits to theK2K1 spectra from theLc
1 signal and
sideband regions, which yieldf signals of 92.2617.0 events
and 36.5613.5 events, respectively. TheLc
1 sideband
K2K1 mass spectrum in Fig. 4 has been scaled by theLc
1
signal-to-sideband scale factor ofRL
c
150.50260.013, ob-
tained by integrating the background function in Fig. 2 ov
FIG. 4. Fit to K2K1 mass from combinations belonging t
the Lc
1 signal and sideband regions. The region above 1.06 G
c2 is not included in the fit because ofK* 0 feed-up when thep is
misidentified as aK.
FIG. 5. Invariant mass ofpK2p1 combinations found in the
same data sample. TheLc
1→pK2p1 signal is used for normaliza
tion of theLc
1→pf branching ratio.
m
is
a
i
o-
au
n
he
x-
-
t i
e
s
o
on
i
o
ID
0
:1
um
el
ea-
te
ng
ID
n
from
0
at
nd
in
s
is
I
es-
de
s
a
ti-
se
sed
nts
R1016 53J. P. ALEXANDERet al.the Lc
1 signal and sideband regions. This gives 56622
events for theLc
1→pf signal, which is in agreement with
the first method.
A check is also made for a possible reflection fro
Ds
1→fp1, where the pion is misidentified as a proton. It
found that the reflection is a broad enhancement in the m
region above the signal. The effect of this background
minimized by the tight particle-ID requirement on the pr
ton. Consequently, the overall fake rate is less than 1%, c
ing negligible reduction of theLc
1→pf signal yield from
the fit.
The decayLc
1→pK2p1 is used as the normalizatio
mode for theLc
1→pf relative branching ratio. In finding
the Lc
1→pK2p1 yield, the same cuts are applied as in t
Lc
1→pK2K1 analysis to minimize systematic errors, e
cept that the particle ID for thep1 is loosened to a consis
tency requirement:P p.0.001. The Lc
1→pK2p1 mass
spectrum is shown in Fig. 5. The parametrization of the fi
the same as theLc
1→pf mass fit in Fig. 3, except that th
width of the Gaussian is allowed to vary. The fit yield
56836138 observed signal events with a mean
2286.860.2 MeV/c2 and a width of 6.460.2 MeV/c2. If the
width of the Gaussian is fixed to the Monte Carlo predicti
of 5.8 MeV/c2, the yield changes by 4%. This dependence
included in the systematic error.
Monte Carlo simulation is used to determine all aspects
the detection efficiency except particle ID. The particle-
efficiency for protons is obtained using a sample of 33 0
L→pp2 decays with a signal-to-background ratio of 50
@14#. For protons thus identified, the momentum spectr
after the particle-ID cuts (Lp.0.9, P p.0.001) is divided by
the momentum spectrum before these cuts, bin by bin, yi
TABLE I. Calculation of the branching ratios forLc
1→pf and
Lc
1→pK2K1 relative toLc1→pK2p1 and Lc1→pK2K1. The
errors are statistical only.
Decay mode: Lc
1→pf Lc1→pK2K1 Lc1→pK2p1
Raw yield 54613 214650 56836138
Efficiency 0.17860.004 0.216 0.005 0.22460.005
B(f→K2K1) 0.49160.005
Corr. yield 6186138 9916233 253716837
B /B(pK2p1) 0.02460.006 0.03960.009 1
B /B(pK2K1) 0.6260.20 1ss
s
s-
s
f
s
f
0
d-
ing the particle-ID efficiencies versus momentum. The m
sured particle-ID efficiency is incorporated into the Mon
Carlo simulation by randomly rejecting the correspondi
fraction of tracks in each momentum bin. The particle-
(LK.0.1, P K.0.001) efficiency for the kaons is derived i
an analogous manner, except that the kaons are taken
D* decays through the cascade processD* 1→D0p1,
D0→K2p1. A sample of 11 000 suchD0→K2p1 decays
is obtained with an 8:1 signal-to-background ratio@14#. The
particle-ID efficiency for protons is near 90% from 30
MeV/c to 1.1 GeV/c falling off to below 10% by 2.5 GeV/c.
For kaons the particle-ID efficiency remains relatively flat
about 95%.
Using a Monte Carlo sample ofLc
1→pf decays, where
the Lc
1 fragmentation takes place according to the Lu
JETSETMonte Carlo simulation@15#, the full detection effi-
ciency is determined, with the particle-ID portion folded
as described above. ForLc
1→pf, the overall efficiency is
0.17860.004 including the particle-ID efficiency which i
0.42560.011. ForLc
1→pK2K1 and Lc1→pK2p1 the
overall efficiencies are 0.21660.005 and 0.22460.005, re-
spectively.
Since for all the decay modes the requirementxp.0.5
is applied, the relative branching ratio for each mode
found simply by dividing the corrected yields. Table
gives the details, listing only the statistical errors. The
timates for the main sources of systematic error inclu
the Lc
1→pf and Lc1→pK2K1 signal shapes~7% and
11%, respectively! and background shapes~2% and
10%, respectively!, particle-ID efficiency ~6%!, and
the Lc
1→pK2p1 fit ~4%!. In addition, for the
Lc
1→pf mode, varying thef signal and sideband region
gives a 5% variation in the yield. Finally, there is
1.8% contribution to theLc
1→pf systematic error from
the f→K2K1 branching ratio uncertainty. Thus we es
mate 12% systematic error inB(pf)/B(pKp), 17%
in B(pKK)/B(pKp), and 18% in B(pf)/B(pKK).
The final results appear in Table II, along with tho
from NA32 @9# and E687@10# and theoretical predictions
from Cheng and Tseng@5#, Körner and Kra¨mer @6#,
Żenczykowski @7#, and Datta@8#. From Table I we also
find B(Lc
1→pK2K1@non-f#!50.02960.01060.005 for
Lc
1→pK2K1 decays not arising fromLc1→pf.
In summary, we have observed the Cabibbo-suppres
decaysLc
1→pf and Lc1→pK2K1. The results appear in
Table II, which show that the phenomenological treatmeTABLE II. Final results onLc
1→pf andLc1→pK2K1.
Ratio of interest B(pf)/B(pK2p1) B(pf)/B(pK2K1) B(pK2K1)/B(pK2p1)
This experiment 0.02460.00660.003 0.6260.2060.12 0.03960.00960.007
NA32 @9# 0.0460.03
E687 @10# ,0.58 at 90% C.L. 0.0960.02960.010
Cheng and Tseng@5# 0.04560.011
Żenczykowski 0.023
Körner and Kra¨mer @6# 0.05
Datta @8# 0.01
aReference@7#, using Ref.@1# for B(Lc
1→pK2p1).
3
tio
t,
of
ff
n-
ce
erg
tu-
ada,
53 R1017OBSERVATION OF THE CABIBBO-SUPPRESSED CHARMED . . .of the Lc
1→pf decay rate agree within a factor of 2 or
with our result. Our measured branching ra
B(pf)/B(pKK) is consistent with the E687 upper limi
while our measurement ofB(pKK)/B(pKp) differs from
the E687 result by 1.7s. Within the factorization approach
using a 1/Nc expansion, our result supports the validity
taking the largeNc limit in charm baryon decays.liWe gratefully acknowledge the effort of the CESR sta
in providing us with excellent luminosity and running co
ditions. This work was supported by the National Scien
Foundation, the U.S. Department of Energy, the Heisenb
Foundation, the Alexander von Humboldt Stiftung, the Na
ral Sciences and Engineering Research Council of Can
and the A. P. Sloan Foundation.A
tatis-
rac-@1# Particle Data Group, L. Montanet al., Phys. Rev. D50, 1173
~1994!.
@2# Unless otherwise specified, reference to a state also imp
reference to the charge conjugate state.
@3# A. J. Buras, J.-M. Ge´rard, and R. Ru¨ckl, Nucl. Phys.B268, 16
~1986!.
@4# M. Bauer, B. Stech, and M. Wirbel, Z. Phys. C34, 103~1987!.
@5# H. Cheng and B. Tseng, Phys. Rev. D46, 1042~1992!.
@6# J. G. Körner and M. Kra¨mer, Z. Phys. C55, 659 ~1992!.
@7# P. Żencykowski, Phys. Rev. D50, 402 ~1994!.
@8# A. Datta, Report No. UH-511-824-95, April 1995~unpub-
lished!.es
@9# S. Barlaget al., Z. Phys. C48, 29 ~1990!.
@10# P. L. Frabettiet al., Phys. Lett. B314, 477 ~1993!.
@11# Y. Kubota et al., Nucl. Instrum. Methods Phys. Res. Sect.
320, 66 ~1992!.
@12# The Monte Carlo simulation employs the CERNGEANT pack-
age: R. Brunet al., GEANT 3.14, CERN DD/EE/84-1.
@13# The quoted uncertainties in mass measurements refer to s
tical error only.
@14# The remaining background is removed by sideband subt
tion.
@15# T. Sjöstrand, Comput. Phys. Commun.43, 367 ~1987!.


Observation of the Cabibbo-suppressed charmed baryon decay Λ_c^+→pφ

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Observation of the Cabibbo-suppressed charmed baryon decay Λ_c^+→pφ

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