V
OLUME
76, N
UMBER
17
PHYSICAL REVIEW LETTERS
22 A
PRIL
1996
Limits on Flavor Changing Neutral Currents in
D
0
Meson Decays
A. Freyberger,
1
D. Gibaut,
1
K. Kinoshita,
1
P. Pomianowski,
1
S. Schrenk,
1
D. Cinabro,
2
B. Barish,
3
M. Chadha,
3
S. Chan,
3
G. Eigen,
3
J. S. Miller,
3
C. O’Grady,
3
M. Schmidtler,
3
J. Urheim,
3
A. J. Weinstein,
3
F. Würthwein,
3
D. M. Asner,
4
M. Athanas,
4
D. W. Bliss,
4
W. S. Brower,
4
G. Masek,
4
H. P. Paar,
4
J. Gronberg,
5
C. M. Korte,
5
R. Kutschke,
5
S. Menary,
5
R. J. Morrison,
5
S. Nakanishi,
5
H. N. Nelson,
5
T. K. Nelson,
5
C. Qiao,
5
J. D. Richman,
5
D. Roberts,
5
A. Ryd,
5
H. Tajima,
5
M. S. Witherell,
5
R. Balest,
6
K. Cho,
6
W. T. Ford,
6
M. Lohner,
6
H. Park,
6
P. Rankin,
6
J. Roy,
6
J. G. Smith,
6
J. P. Alexander,
7
C. Bebek,
7
B. E. Berger,
7
K. Berkelman,
7
K. Bloom,
7
D. G. Cassel,
7
H. A. Cho,
7
D. M. Coffman,
7
D. S. Crowcroft,
7
M. Dickson,
7
P. S. Drell,
7
D. J. Dumas,
7
R. Ehrlich,
7
R. Elia,
7
P. Gaidarev,
7
B. Gittelman,
7
S. W. Gray,
7
D. L. Hartill,
7
B. K. Heltsley,
7
C. D. Jones,
7
S. L. Jones,
7
J. Kandaswamy,
7
N. Katayama,
7
P. C. Kim,
7
D. L. Kreinick,
7
T. Lee,
7
Y. Liu,
7
G. S. Ludwig,
7
J. Masui,
7
J. Mevissen,
7
N. B. Mistry,
7
C. R. Ng,
7
E. Nordberg,
7
J. R. Patterson,
7
D. Peterson,
7
D. Riley,
7
A. Soffer,
7
C. Ward,
7
P. Avery,
8
C. Prescott,
8
S. Yang,
8
J. Yelton,
8
G. Brandenburg,
9
R. A. Briere,
9
T. Liu,
9
M. Saulnier,
9
R. Wilson,
9
H. Yamamoto,
9
T. E. Browder,
10
F. Li,
10
J. L. Rodriguez,
10
T. Bergfeld,
11
B. I. Eisenstein,
11
J. Ernst,
11
G. E. Gladding,
11
G. D. Gollin,
11
M. Palmer,
11
M. Selen,
11
J. J. Thaler,
11
K. W. Edwards,
12
K. W. McLean,
12
M. Ogg,
12
A. Bellerive,
13
D. I. Britton,
13
R. Janicek,
13
D. B. MacFarlane,
13
P. M. Patel,
13
B. Spaan,
13
A. J. Sadoff,
14
R. Ammar,
15
P. Baringer,
15
A. Bean,
15
D. Besson,
15
D. Coppage,
15
N. Copty,
15
R. Davis,
15
N. Hancock,
15
S. Kotov,
15
I. Kravchenko,
15
N. Kwak,
15
S. Anderson,
16
Y. Kubota,
16
M. Lattery,
16
J. K. Nelson,
16
S. Patton,
16
R. Poling,
16
T. Riehle,
16
V. Savinov,
16
M. S. Alam,
17
I. J. Kim,
17
Z. Ling,
17
A. H. Mahmood,
17
J. J. O’Neill,
17
H. Severini,
17
C. R. Sun,
17
S. Timm,
17
F. Wappler,
17
J. E. Duboscq,
18
R. Fulton,
18
D. Fujino,
18
K. K. Gan,
18
K. Honscheid,
18
H. Kagan,
18
R. Kass,
18
J. Lee,
18
M. Sung,
18
A. Undrus,
18,
* C. White,
18
R. Wanke,
18
A. Wolf,
18
M. M. Zoeller,
18
X. Fu,
19
B. Nemati,
19
S. J. Richichi,
19
W. R. Ross,
19
P. Skubic,
19
M. Wood,
19
M. Bishai,
20
J. Fast,
20
E. Gerndt,
20
J. W. Hinson,
20
T. Miao,
20
D. H. Miller,
20
M. Modesitt,
20
E. I. Shibata,
20
I. P. J. Shipsey,
20
P. N. Wang,
20
M. Yurko,
20
L. Gibbons,
21
S. D. Johnson,
21
Y. Kwon,
21
S. Roberts,
21
E. H. Thorndike,
21
C. P. Jessop,
22
K. Lingel,
22
H. Marsiske,
22
M. L. Perl,
22
S. F. Schaffner,
22
R. Wang,
22
T. E. Coan,
23
J. Dominick,
23
V. Fadeyev,
23
I. Korolkov,
23
M. Lambrecht,
23
S. Sanghera,
23
V. Shelkov,
23
R. Stroynowski,
23
I. Volobouev,
23
G. Wei,
23
M. Artuso,
24
A. Efimov,
24
M. Gao,
24
M. Goldberg,
24
R. Greene,
24
D. He,
24
N. Horwitz,
24
S. Kopp,
24
G. C. Moneti,
24
R. Mountain,
24
Y. Mukhin,
24
S. Playfer,
24
T. Skwarnicki,
24
S. Stone,
24
X. Xing,
24
J. Bartelt,
25
S. E. Csorna,
25
V. Jain,
25
S. Marka
25
(CLEO Collaboration)
1
Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
2
Wayne State University, Detroit, Michigan 48202
3
California Institute of Technology, Pasadena, California 91125
4
University of California, San Diego, La Jolla, California 92093
5
University of California, Santa Barbara, California 93106
6
University of Colorado, Boulder, Colorado 80309-0390
7
Cornell University, Ithaca, New York 14853
8
University of Florida, Gainesville, Florida 32611
9
Harvard University, Cambridge, Massachusetts 02138
10
University of Hawaii at Manoa, Honolulu, Hawaii 96822
11
University of Illinois, Champaign-Urbana, Illinois 61801
12
Carleton University, Ottawa, Ontario, Canada K1S 5B6
and the Institute of Particle Physics, Montréal, Québec, Canada
13
McGill University, Montréal, Québec, Canada H3A 2T8
and the Institute of Particle Physics, Montréal, Québec, Canada
14
Ithaca College, Ithaca, New York 14850
15
University of Kansas, Lawrence, Kansas 66045
16
University of Minnesota, Minneapolis, Minnesota 55455
17
State University of New York at Albany, Albany, New York 12222
18
The Ohio State University, Columbus, Ohio 43210
19
University of Oklahoma, Norman, Oklahoma 73019
20
Purdue University, West Lafayette, Indiana 47907
21
University of Rochester, Rochester, New York 14627
22
Stanford Linear Accelerator Center, Stanford University, Stanford, California 94309
23
Southern Methodist University, Dallas, Texas 75275
0031-9007
y
96
y
76(17)
y
3065(5)$10.00
© 1996 The American Physical Society
3065
V
OLUME
76, N
UMBER
17
PHYSICAL REVIEW LETTERS
22 A
PRIL
1996
24
Syracuse University, Syracuse, New York 13244
25
Vanderbilt University, Nashville, Tennessee 37235
(
Received 10 January 1996
)
Using the CLEO II detector at the Cornell Electron Storage Ring, we have searched for flavor
changing neutral currents and lepton family number violations in
D
0
meson decays. The upper limits
on the branching fractions for
D
0
!
,
1
,
2
and
D
0
!
X
0
,
1
,
2
are in the range
10
2
5
to
10
2
4
, where
X
0
can be a
p
0
,
K
0
s
,
h
,
r
0
,
v
,
̄
K
p
0
,or
f
meson, and the
,
1
,
2
pair can be
e
1
e
2
,
m
1
m
2
,or
e
6
m
7
.
Although these limits are above the theoretical predictions, most are new or an order of magnitude
lower than previous limits.
[S0031-9007(96)00011-7]
PACS numbers: 13.20.Fc, 11.30.Hv, 12.60.–i, 14.40.Lb
In the standard model (SM), flavor changing neutral
currents (FCNC) are expected to be very rare in charm
decays, and lepton family number violations (LFNV) are
strictly forbidden. The FCNC decays,
D
0
!
,
1
,
2
and
D
!
X
,
1
,
2
, can occur at the one loop level in the SM
from penguin and box diagrams as shown in Fig. 1, but
are highly suppressed by the Glashow-Iliopoulos-Maiani
mechanism [1] and by the small quark masses in the
loop. The theoretical estimates for the FCNC branching
fractions [2] are of order
10
2
9
for
D
!
X
,
1
,
2
and
10
2
19
for
D
0
!
,
1
,
2
, due to the additional helicity
suppression.
In addition to these short distance loop diagrams there
are contributions from long distance effects that can be
several orders of magnitude larger [2]. There are two
categories: (1) photon pole amplitudes and (2) vector
meson dominance (VMD). Both involve nonperturbative
QCD factors that are difficult to calculate.
The photon pole model [Fig. 2(a)] is essentially a
W
-
exchange decay with a virtual photon radiating from one
of the quark lines. The amplitude behaves differently
depending on whether the final state meson is a vector
(
V
) or pseudoscalar (
P
). The dilepton mass distribution
for
D
!
V
,
1
,
2
modes peaks at zero (small
q
2
) since
the photon prefers to be nearly real. Contrarily, the pole
amplitude for
D
!
P
,
1
,
2
decays vanishes for small
dilepton mass because
D
!
P
g
is forbidden by angular
momentum conservation.
The VMD model [Fig. 2( b)] proceeds through the
decay
D
!
XV
0
!
X
,
1
,
2
, where
V
0
is an intermediate
r
0
,
v
,or
f
vector meson. The
V
0
mixes with a virtual
photon which then couples to
,
1
,
2
. The dilepton mass
spectrum will have poles at the
r
0
,
v
, and
f
masses due
to real
V
0
mesons decaying into
,
1
,
2
. There will also
FIG. 1. Short distance contributions to FCNC decays in
D
mesons due to (a) box and ( b) penguin diagrams.
be another pole at zero dilepton mass from the photon
propagator if
X
is a vector meson.
Observation of FCNC decays at rates that exceed the
long distance contributions opens a window into physics
beyond the standard model; LFNV decays may suggest
leptoquarks or heavy neutral leptons with non-negligible
couplings to
e
and
m
. Measuring the long distance
contributions is also intrinsically important since our
understanding at the charm sector can then be used to
estimate the long distance effects for
b
!
s
g
, which can
be as large as 20% of the total decay rate [3]. Extracting
j
V
td
y
V
ts
j
from the ratio
B
s
B
!
rg
dy
B
s
B
!
K
p
g
d
is
possible only if the short and long distance contributions
can be separated.
The data were collected with the CLEO II detector at
the Cornell
e
1
e
2
Storage Ring (CESR), which operates
on and just below the
Y
s
4
S
d
resonance. The CLEO
II detector [4] is a large solenoidal detector with 67
tracking layers and a CsI electromagnetic calorimeter that
provides efficient
p
0
reconstruction. We have used an
integrated luminosity of
3.85
fb
2
1
, which corresponds to
,
5
3
10
6
e
1
e
2
!
c
̄
c
events.
We have searched for the FCNC and LFNV decays
D
0
!
,
1
,
2
and
D
0
!
X
,
1
,
2
, where
X
can be a
p
0
,
K
0
s
,
h
,
r
0
,
v
,
̄
K
p
0
,or
f
meson [5]. The
,
1
,
2
pair can be
either
e
1
e
2
or
m
1
m
2
for the FCNC decays, and
e
6
m
7
for the LFNV decays.
Charged tracks, except for pions from
K
0
s
decays, are
required to be consistent with coming from the primary
interaction point. Charged pion and kaon candidates are
required to have
dE
y
dx
and, when available, time-of-
flight information consistent with that of true pions and
kaons.
FIG. 2. Long distance contributions to FCNC decays in
D
mesons due to (a) photon pole amplitude and ( b) vector meson
dominance.
3066
V
OLUME
76, N
UMBER
17
PHYSICAL REVIEW LETTERS
22 A
PRIL
1996
Electrons are identified by requiring that the energy
deposition in the CsI calorimeter be consistent with the
track momentum and the specific ionization loss (
dE
y
dx
)
be consistent with that of true electrons. The electron
candidate must have a momentum greater than 0.4 GeV
y
c
and satisfy
j
cos
u
j
,
0.81
, where
u
is the polar angle
with respect to the beam axis. Electrons from photon
conversions and
p
0
Dalitz decays are rejected.
Muon candidates are selected by requiring the charged
track to penetrate at least three nuclear interaction lengths
of steel, which implicitly places a minimum momentum
cut of 0.9 GeV
y
c
. To further reduce the fake rate from
pions we also require that the track lie in the region
j
cos
u
j
,
0.7
and that the CsI shower energy for the
muon be less than 0.5 GeV.
The
K
0
s
candidates are selected through the decay
mode
K
0
s
!
p
1
p
2
by requiring a decay vertex displaced
from the primary interaction point. The invariant mass
of the
K
0
s
candidates must be within
10
MeV
y
c
2
of
its nominal value. The vector meson candidates are
reconstructed through the decays
r
0
!
p
1
p
2
,
v
!
p
1
p
2
p
0
,
̄
K
p
0
!
K
2
p
1
, and
f
!
K
1
K
2
. We require
the candidates to have an invariant mass within 150, 20,
50, and
8
MeV
y
c
2
of their nominal mass, respectively.
We reconstruct the
p
0
!
gg
decay mode from pairs of
well-defined showers in the CsI calorimeter. The showers
must not be matched to charged tracks and must have a
lateral shower shape consistent with that of true photons.
At least one photon must lie in the barrel region defined
by
j
cos
u
j
,
0.7
. The
p
0
from the decay chain
D
0
!
v
,
1
,
2
,
v
!
p
1
p
2
p
0
(
D
0
!
p
0
,
1
,
2
) must have a
momentum greater than 0.1 (0.6) GeV
y
c
, and individual
photon energies must be at least 0.03 (0.10) GeV,
respectively. The
p
0
from the
D
0
!
p
0
,
1
,
2
mode
has more stringent cuts since its momentum spectrum is
harder. We select
p
0
candidates that have an invariant
mass within 2.5 standard deviations (
s
) of the nominal
mass. The photon four-momenta are kinematically fit to
the nominal
p
0
mass to improve the momentum estimate.
The decay
h
!
gg
is reconstructed in a similar
procedure. In addition,
h
candidates are rejected if either
photon is consistent with coming from a
p
0
. The
h
momentum must be greater than 0.5 GeV
y
c
and each
photon must have an energy of at least 0.15 GeV. We
select
h
candidates that have an invariant mass within
30
MeV
y
c
2
of the nominal mass.
In order to reduce the combinatoric background, we
require the
D
0
candidates to come from
D
p
1
!
D
0
p
1
decays. Although
,
75%
of the
D
0
sample is lost by
imposing the
D
p
1
tag, backgrounds are reduced by
a factor of 20 – 40.
We require the mass difference
M
s
D
p
1
d
2
M
s
D
0
d
to be within
2.0
MeV
y
c
2
(
2
s
) of its
expected value. (The
D
p
tag is not required for the
D
0
!
f
,
1
,
2
modes since their backgrounds are negligible.)
Since charmed mesons from
e
1
e
2
!
c
̄
c
events are
produced with a hard momentum spectrum, we further
FIG. 3. Invariant mass distribution for FCNC
D
0
decays. The
signal region for the
D
0
decay modes is shaded.
reduce the combinatoric background by requiring
x
p
.
0.5
, where
x
p
;
P
D
p
1
y
q
E
2
beam
2
M
2
D
p
1
is the scaled
momentum of the
D
p
1
. Finally, the daughter particles
of the
D
0
candidate are required to lie within 90
±
of the
D
0
momentum vector, which further reduces backgrounds
in the
D
0
!
r
0
,
1
,
2
and
v
,
1
,
2
modes.
The invariant mass spectra for the FCNC and LFNV
decays
D
0
!
,
1
,
2
and
D
0
!
X
,
1
,
2
are shown in
Figs. 3 and 4. We do not observe signals in any of
the decay modes. The background levels are consistent
with expectations from Monte Carlo (MC) simulations.
The background combinatorics in the continuum MC are
FIG. 4. Invariant mass distribution for LFNV
D
0
decays. The
signal region for the
D
0
decay modes is shaded.
3067
V
OLUME
76, N
UMBER
17
PHYSICAL REVIEW LETTERS
22 A
PRIL
1996
predominately from lepton fakes, whereas those in the
BB
̄
MC are mainly from real leptons. We set upper limits on
each mode by assuming all the events within
3
s
of the
D
0
mass (
,
30
MeV
y
c
2
) to be signal events. Assuming that
the LFNV decay rates
D
0
!
Xe
1
m
2
and
D
0
!
Xe
2
m
1
are identical, we combine these two mass spectra together
to obtain a more stringent limit on
D
0
!
Xe
6
m
7
. The
number of events in each signal region is shown in
Table I.
The upper limit on branching fractions for the FCNC
and LFNV decay modes is given by
B
≠
l
n
y
e
N
D
0
,
where
l
n
is the Poisson 90% upper limit for
n
observed
events,
e
is the reconstruction efficiency, and
N
D
0
is the
number of
D
0
mesons in the data, which is obtained
from the observed
D
0
!
K
2
p
1
yield.
We observe
70 770
6
470
events in the decay mode
D
0
!
K
2
p
1
TABLE I.
Summary of upper limits on the FCNC and LFNV
decay modes
D
0
!
,
1
,
2
and
D
0
!
X
,
1
,
2
. The efficiencies
(
E
) are for the phase space model and do not include branch-
ing fractions to the observed final states. The 90% C.L. upper
limits are listed separately for the phase space (nonresonant)
and photon pole amplitude decay models, together with previ-
ous limits.
Decay
Signal
B
s
10
2
5
d
Upper limits
mode
events
E
(%)
Nonres.
Pole
Previous
e
1
e
2
0
14
1.3
···
13 [6]
m
1
m
2
1
9
3.4
···
0.3 [7]
e
6
m
7
211 19
···
10 [8]
p
0
e
1
e
2
0
4.2
4.5
···
p
0
m
1
m
2
3
1.0
54
···
18 [9]
p
0
e
6
m
7
2
2.5
8.6
···
K
0
e
1
e
2
0
4.7
11
···
170 [10]
K
0
m
1
m
2
1
1.4
67
···
26 [9]
K
0
e
6
m
7
0
2.7
10
···
h
e
1
e
2
0
4.2
11
···
hm
1
m
2
0
0.9
53
···
h
e
6
m
7
0
2.3
10
···
r
0
e
1
e
2
2
4.2
10
18
45 [11]
r
0
m
1
m
2
1
0.7
49
45
25 [9]
r
0
e
6
m
7
0
1.9
4.9
5.0
v
e
1
e
2
1
1.9
18
27
vm
1
m
2
0
0.3
83
65
v
e
6
m
7
0
0.9
12
12
K
p
0
e
1
e
2
1
3.4
14
20
K
p
0
m
1
m
2
1
0.4
118
100
K
p
0
e
6
m
7
0
1.4
10
10
f
e
1
e
2
2
4.4
5.2
7.6
fm
1
m
2
0
0.2
41
24
f
e
6
m
7
0
1.5
3.4
3.3
for
x
p
.
0.5
, and
17 300
6
150
events in the decay chain
D
p
1
!
D
0
p
1
with
D
0
!
K
2
p
1
. This corresponds to
5.22
3
10
6
D
0
mesons and
1.38
3
10
6
D
p
1
!
D
0
p
1
decays.
For the FCNC and LFNV modes we compute the
D
0
reconstruction efficiency using a phase space decay
of
D
0
!
X
,
1
,
2
. The efficiencies for
D
0
!
Xe
1
e
2
are about 4 – 10 times greater than those for
D
0
!
X
m
1
m
2
, due to the greater momentum acceptance for
electrons. The efficiencies for the
D
0
!
V
,
1
,
2
vector
decay modes are also determined using a photon pole
amplitude decay in which
D
0
!
V
g
p
!
V
,
1
,
2
. This
leads to a dilepton mass distribution of
d
G
y
dm
2
,,
~
1
y
m
2
,,
. The
D
0
!
Ve
1
e
2
efficiency for the pole model
is about 30% less than that of the phase space model,
primarily due to low mass
e
1
e
2
pairs that resemble
photon conversions. We present upper limits in Table I
using both decay model assumptions.
The main sources of systematic error are due to un-
certainties in the efficiencies for charged particle track-
ing (2% per track),
p
0
and
h
reconstruction (5%), lepton
identification (6%),
K
0
s
reconstruction (5%), and Monte
Carlo statistics (4 – 8%). The total systematic errors are in
the range (9 – 12)%, depending on the mode. We incor-
porate these errors into the upper limits by decreasing the
efficiency by
1
s
.
The upper limits on the branching fractions for the
flavor changing neutral current and lepton family number
violating decay modes are summarized in Table I. The
90% confidence level limits range from a few
3
10
2
5
for
D
0
!
,
1
,
2
,
p
0
,
1
,
2
, and
f
,
1
,
2
, to a few
3
10
2
4
for
the other decay modes. Although these limits are well
above the theoretical predictions [2], the limits for
D
0
!
e
1
e
2
,
e
6
m
7
, and
K
0
̄
e
1
e
2
are an order of magnitude
more restrictive than previous limits [6 – 11]. In addition,
the limits for many other decay modes reported here are
the first published constraints.
We gratefully acknowledge the effort of the CESR staff
in providing us with excellent luminosity and running
conditions. This work was supported by the National
Science Foundation, 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. One
of us (D.F.) would like to thank Gustavo Burdman and
Sandip Pakvasa for stimulating discussions.
*Permanent
address:
BINP, RU-630090 Novosibirsk,
Russia.
[1] S. L. Glashow, J. Iliopoulos, and L. Maiani, Phys. Rev. D
2
, 1285 (1970).
[2] J. L. Hewett, Report No. SLAC-PUB-95-6821; S. Pak-
vasa, Report No. UH-511-787-94; A. J. Schwartz, Mod.
Phys. Lett. A
8
, 967 (1993).
3068
V
OLUME
76, N
UMBER
17
PHYSICAL REVIEW LETTERS
22 A
PRIL
1996
[3] E. Golowich and S. Pakvasa, Phys. Rev. D
51
, 1215
(1995); J. Soares, Report No. TRI-PP-95-6; A. Khod-
jamirian
et al.,
Phys. Lett. B
358
, 129 (1995).
[4] CLEO Collaboration, Y. Kubota
et al.,
Nucl. Instrum.
Methods Phys. Res., Sect. A
320
, 66 (1992).
[5] In this Letter the charge conjugate states are implied.
[6] Mark III Collaboration, J. Adler
et al.,
Phys. Rev. D
37
,
2023 (1988).
[7] E771 Collaboration, T. Alexopoulos
et al.,
Report
No. FERMILAB-PUB-95-297-E; WA92 Collaboration,
M. Adamovich
et al.,
Phys. Lett. B
353
, 563 (1995);
E615 Collaboration, W. C. Louis
et al.,
Phys. Rev. Lett.
56
, 1027 (1986).
[8] ARGUS Collaboration, H. Albrecht
et al.,
Phys. Lett. B
209
, 380 (1988).
[9] E653 Collaboration, K. Kodama
et al.,
Phys. Lett. B
345
,
85 (1995).
[10] Mark III Collaboration, J. Adler
et al.,
Phys. Rev. D
40
,
906 (1989).
[11] CLEO Collaboration, P. Haas
et al.,
Phys. Rev. Lett.
60
,
1614 (1988).
3069