of 5
VOLUME
63,
NUMBER
22
PHYSICAL
REVIEW
LETTERS
27
NOVEMBER
1989
Searches
for
New
Quarks
and
Leptons
Produced
in
Z-Boson
Decay
G.
S.
Abrams,
(I
J
C.
E.
Adolphsen,
(
2
)
D.
Averill,
(3)
J.
Ballam,
<
4
>
B.
C.
Barish,
(
5
)
T.
Barklow,
(
4
)
B.
A.
Barnett,
(
6
)
J.
Bartelt,
(
4
)
S.
Bethke,
0
J
D.
Blockus,
(3)
G.
Bonvicini,
<
7
>
A.
Boyarski,
(
4
)
B.
Brabson,
(JJ
A.
Breakstone,
(
8
)
F.
Bulos,
<
4
>
P.
R.
Burchat,
(
2
)
D.
L.
Burke,
(
4
)
R.
J.
Cence,
(
8
)
J.
Chapman,
(
7
)
M.
Chmeissani,
(
7
)
D.
Cords,
<
4
>
D.
P.
Coupal,
<
4
>
P.
Dauncey,
(
6
)
H.
C.
DeStaebler,
(
4
)
D.
E.
Dorfan,
(
2
)
J.
M.
Dorfan,
(
4
)
D.
C.
Drewer,
<
6
>
R.
Elia,
(
4
)
G.
J.
Feldman,
<
4
>
D.
Fernandes,
<
4
>
R.
C.
Field,
(
4
)
W.
T.
Ford,
<
9
>
C.
Fordham,
(
4
)
R.
Frey,
(
7
)
D.
Fujino,
(
4
)
K.
K.
Gan,
(
4
)
E.
Gero,
<
7
>
G.
Gidal,
(I)
T.
Glanzman,
<
4
>
G.
Goldhaber,
(I)
J. J.
Gomez
Cadenas,
<
2
>
G.
Gratta,
<
2
>
G.
Grindhammer,
<
4
>
P.
Grosse-Wiesmann,
(
4
)
G.
Hanson,
<
4
>
R.
Harr,
(I)
B.
Harral,
<
6
>
F.
A.
Harris,
(
8
)
C.
M.
Hawkes,
(
5
)
K.
Hayes,
<
4
>
C.
Hearty,
(I)
C.
A.
Heusch,
(
2
)
M.
D.
Hildreth,
<
4
>
T.
Himel,
<
4
>
D.
A.
Hinshaw,
(
9
)
S.
J.
Hong,
(
7
)
D.
Hutchinson,
<
4
>
J.
Hylen,
<
6
>
W.
R.
Innes,
<
4
>
R.
G.
Jacobsen,
<
4
>
J.
A.
Jaros,
<
4
>
C.
K.
Jung,
(
4
)
J.
A.
Kadyk,
(I)
J.
Kent,
<
2
>
M.
King,
<
2
>
S.
R.
Klein,
(
4
)
D.
S.
Koetke,
<
4
>
S.
Komamiya,
<
4
J
W.
Koska,
(
7
)
L.
A.
Kowalski,
(
4
)
W.
Kozanecki,
(
4
)
J.
F.
Kral,
0
J
M.
Kuhlen,
<
5
J
L.
Labarga,
<
2
>
A.
J.
Lankford,
(
4
)
R. R.
Larsen,
<
4
>
F.
Le
Diberder,
<
4
>
M.
E.
Levi,
0
J
A.
M.
Litke,
(
2
)
X.
C.
Lou,
OJ
V.
Liith,
<
4
J
J.
A.
McKenna,
<
5
>
J.
A.
J.
Matthews,
(
6
)
T.
Mattison,
(
4
)
B.
D.
Milliken,
<
5
>
K.
C.
Moffeit,
(
4
)
C.
T.
Munger,
<
4
>
W.
N.
Murray,
(3)
J.
Nash,
(
4
)
H.
Ogren,
(3)
K.
F.
O'Shaughnessy,
(
4
)
S.
I.
Parker,
(
8
)
C.
Peck,
(
5
)
M.
L.
Perl,
(
4
)
F.
Perrier,
(
4
)
M.
Petradza,
(
7
)
R.
Pitthan,
(
4
)
F.
C.
Porter,
<
5
>
P.
Rankin,
(
9
)
K.
Riles,
(
4
)
F.
R.
Rouse,
(
4
)
D.
R.
Rust,
OJ
H.
F.
W.
Sadrozinski,
(
2
)
M.
W.
Schaad,
(I)
B.
A.
Schumm,
(I)
A.
Seiden,
(
2
)
J.
G.
Smith,
(
9
)
A.
Snyder
,OJ
E.
Soderstrom,
(
5
)
D.P.
Stoker,
(
6
)
R.
Stroynowski,
(
5
)
M.
Swartz,
(
4
)
R.
Thun,
(
7
)
G.
H.
Trilling,
(I
J
R.
Van
Kooten,
<
4
J
P.
Voruganti,
<
4
>
S.
R.
Wagner,
<
9
>
S.
Watson,
<
2
>
P.
Weber,
(
9
)
A.
Weigend,
<
4
>
A.
J.
Weinstein,
<
5
>
A.
J.
Weir,
<
5
J
E.
Wicklund,
<
5
>
M.
Woods,
<
4
>
D.
Y.
Wu,
<
5
J
M.
Yurko,
(3)
C.
Zaccardelli,
<
2
>
and
C.
von
Zanthier<
2
J
0
J
Lawrence
Berkeley
Laboratory
and
Department
of
Physics,
University
of
California,
Berkeley,
California
947
20
(2JUniversity
of
California,
Santa
Cruz,
California
95064
OJ
Indiana
University,
Bloomington,
Indiana
47
405
<
4
>
Stanford
Linear
Accelerator
Center,
Stanford
University,
Stanford,
California
94309
<
5
>California
Institute
of
Technology,
Pasadena,
California
91125
<
6
>Johns
Hopkins
University,
Baltimore,
Maryland
21218
(7)University
of
Michigan,
Ann
Arbor,
Michigan
48109
<sJUniversity
of
Hawaii,
Honolulu,
Hawaii
96822
<
9
JUniversity
of
Colorado,
Boulder,
Colorado
80309
(Received
13
October
1989)
We
have
searched
for
events
with
new-particle
topologies
in
390
hadronic
Z
decays
with
the
Mark
II
detector
at
the
SLAC
Linear
Collider.
We
place
95%-confidence-level
lower
limits
of
40.7
GeV
/c
2
for
the
top-quark
mass,
42.0
GeV/c
2
for
the
mass
of
a
fourth-generation
charge
-
t
quark,
and
41.3
GeV
/c
2
for
the
mass
of
an
unstable
Dirac
neutral
lepton.
PACS
numbers:
13.38.+c,
12.15.Ff,
14.60.Gh,
14.80.Dq
widths
given
by
the
standard
model.
We
have
searched
for
new
quarks
and
leptons
in
Z-
boson
decay
using
data
from
the
Mark
II
detector
at
the
SLAC
e
+
e
-
Linear
Collider
(SLC)
operating
in
the
e
+
e-
center-of-mass
energy
(E
c.m)
range
from
89.2
to
93.0
GeV.
The
standard
model
predicts
the
existence
of
the
top
quark
and
does
not
exclude
the
possibility
of
new
generations
of
fermions.
The
existence
and
masses
of
these
possible
new
particles
may
provide
information
to
help
understand
the
pattern
of
fermion
masses
and
the
presence
of
generations.
We
specifically
search
for
top
(t)
quarks,
fourth-
generation
charge
-
t
(b')
quarks,
and
unstable
neutral
Dirac
leptons
(L
0
);
the
small
production
rate
of
heavy
sequential
charged
leptons
on
the
Z-boson
resonance
prevents
us
from
searching
for
sequential
charged
lep-
tons
at
this
time.
We
assume
pair
production
of
the
new
particles
through
Z
decay
with
couplings
and
decay
One
expects
t
quarks
to
decay
via
the
virtual-
W
(W*)
charged-current
(CC)
process
t-
bW*.
Naively,
one
also
expects
the
b'
quark
to
decay
via
the
CC
process
b'-
cW*
if
the
mass
of
the
b'
quark
is
less
than
the
mass
of
the
t
quark
(Mb'
<
M
1
)
••
Assuming
that
a new
quark
decays
100%
via
the
CC
mode,
the
UAl
Colla-
boration
1
has
excluded
top
quarks
with
masses
less
than
44
GeV
/c
2
and
b'
quarks
with
masses
less
than
32
GeV/c
2
With
the
same
assumption,
the
CDF
Colla-
boration
2
has
excluded
top
quarks
with
masses
between
40
and
77
GeV/c
2
.
A
b'
quark
may
not
decay
100%
of
the
time
via
the
CC
decay
because
of
increased
suppression
of
transitions
which
cross
two
generations.
3
Consequently,
the
flavor-
changing
neutral-current
(FCNC)
loop
decays
4
of
b'-
b+gluon
and
b'-
by
must
also
be
considered.
©
1989
The
American
Physical
Society
2447