1
SEARCH FOR SINGLE LEPTOQUARK PRODUCTION
p
a
AT s = 161 AND 172 GEV
STEFAN SOLDNER-REMBOLD
for the OPAL collab oration
Universitat Freiburg, Hermann-Herder-Str. 3,
D-79104 Freiburg i. Br., Germany
A search for a rst generation scalar lepto quark (LQ) has b een p erformed using
+
the data collected by the OPAL detector in 1996 at e e centre-of-mass energies
p
s of 161 and 172 GeV. It is assumed that a single lepto quark can b e pro duced
in the pro cess eq! LQ, where the initial state quark originates from a hadronic
uctuation of a quasi-real photon which has b een radiated by one of the LEP
b eams. Lower limits at the 95 % con dence level on the mass of a rst generation
scalar lepto quark of 131 GeV for =0:5 and = 1, coupling values larger than
p
4 and lepto quark charges 1=3or 5=3 are obtained.
em
1 Kinematics and Monte Carlo simulations
Lepto quarks are coloured spin 0 or spin 1 particles carrying b oth baryon and
lepton quantum numb ers. Recently it has b een suggested to search for lep-
1
to quarks in electron-photon collisions at LEP . The photon, which has b een
radiated by one of the LEP b eams, serves as a source of quarks through its
uctuations into hadronic states. The electron-quark interaction pro duces a
lepto quark which is assumed to decay subsequently into an electron or a neu-
b
trino and a quark.
In electron-photon scattering rst generation lepto quarks of charge 1=3,
5=3, 2=3 and 4=3 can b e pro duced. The cross-section to pro duce charge
2=3 and 4=3 lepto quarks is suppressed, since there is less d quark content
in the photon than u quark. The limits will therefore b e given for lepto quark
charges 1=3 or 5=3. The cross-sections in e scattering for b oth charge
states are identical, since it is equally probable to nd a u or a u quark in
the photon. In principle this search is also sensitive to electron-charm states,
since the probability for a photon to split into ccoruu is exp ected to b e ab out
equal for lepto quark masses M>>m . Furthermore it has b een assumed that
c
either left or right handed couplings to fermions vanish. The cross-sections
in e scattering for b oth couplings are identical, whereas the branching ratio
a
To b e published in the pro ceedings of PHOTON'97, Egmond aan Zee
b
Charge conjugation is implied throughout this pap er and p ositrons are referred to as electrons
FREIBURG-EHEP-97-04 2
into eq nal states is 1 for right handed couplings and 1/2 for left handed
2
couplings .
The total cross-section for the pro duction of scalar lepto quarks of mass
M is a convolution of the Weizsacker-Williams e ective photon distribution
f (z), with z b eing the momentum fraction carried by the photon, and the
=e
2
parton distribution functions f (x; ) of the photon, evaluated at the scale
q=
1
= M :
Z
1
2
dz
2 2 2 +
f (z; Q )f (M =(zs);M ): (1) (e e ! LQ+X)=
=e q=
max
2s z
2
M =s
3;4
The Monte Carlo simulation of this pro cess is done with PYTHIA 5.722 . In
2
the simulation the maximum photon virtuality Q used in the Equivalent
max
Photon Approximation equals s=4, but the simulated photon is always real
2 5
(Q = 0). The GRV parametrisation of the parton distribution functions
was used. In the kinematic region relevant for lepto quark pro duction the
variations of the cross-section due to the di erent parameterisations are small.
Interference e ects with deep-inelastic e scattering are also neglected. The
p
total cross-section in PYTHIA for s = 172 is ab out 10{20 % lower than the
p
cross-sections given for s = 175 in Ref. 1. Vector lepto quarks can currently
not b e simulated with PYTHIA. The limits are therefore given only for scalar
lepto quarks. The standard PYTHIA Monte Carlo has b een mo di ed to include
LQ ! d decays in addition to the standard LQ ! eu decays.
e
2 Event Analysis
Jets were reconstructed using a cone jet nding algorithm with a cone size
R = 1 and a cut on the minimum transverse jet energy E of 15 GeV. Tracks
T
and calorimeter clusters were used as input for the jet nding algorithm and for
determining the missing transverse energy E= of the event. A matching algo-
T
rithm b etween tracks and clusters is applied. The electron was identi ed using
6
the standard OPAL neural net electron identi cation . All relevant Standard
Mo del background pro cesses were studied using Monte Carlo generators. The
1
total data sample corresp onds to an integrated luminosity of 20.5 pb .
2.1 The electron plus hadronic jet channel
For this channel the identi ed electron with the largest momentum was as-
sumed to b e the electron from the lepto quark decay. The electron is usually
reconstructed as a jet. Candidate events were selected based on the following cuts:
FREIBURG-EHEP-97-04 3
+ +
In order to reduce background from deep-inelastic e and e e !
events, exactly two jets must have b een found in the event(n = 2).
j
+ + + +
A large number of e e ! and e e ! e e events are rejected
by requiring a minimum numb er of 5 reconstructed tracks (n 5). In
ch
p
s has to b e less than 0.9, where E is addition, the ratio E =
ECAL ECAL
the energy in the electromagnetic calorimeter.
The missing transverse energy E= must b e less than 15 GeV in order to
T
+
+
reduce background from and W W pair pro duction.
An isolation cut is applied on the identi ed electron. The jet with the
smallest angular distance to the electron is chosen to b e the electron jet.
j
The di erence between the energy E of this jet and the energy E of
e
e
+
the electron must b e less than 4 GeV. Most multihadronic e e ! qq
events are removed by this cut.
Events where an electron was scattered at a small angle are rejected by
requiring for the angle of the electron j cos j < 0:85.
e
The total multiplicity n of the quark jet must b e n 7, where n is
q q q
the total numb er of tracks and calorimeter clusters asso ciated to this jet.
The cuts on the transverse momenta of the jets and on the angle j cos j of the
e
electron reduce signi cantly the sensitivity to nd a lepto quark which is lighter
than approximately M =2, the region excluded by the LEP1 searches. These
Z
cuts are necessary to reduce the background from deep-inelastic e events
which b ecomes increasingly imp ortant at small masses.
After all cuts we exp ect a background of 5:2 0:4events from all Standard
Mo del pro cesses. In the data four events are observed with jet-jet invariant
masses M of 36, 37, 62 and 98 GeV. In Fig. 1a the M distribution of the four
jj jj
candidate events is shown together with the sum of all Monte Carlo background
p
distributions. Also shown is a p ossible lepto quark signal for = 4 and
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di erent LQ masses. The mass distribution of the candidate events is consistent
with the exp ectation from the background Monte Carlo simulation.
2.2 The neutrino plus hadronic jet channel
This search has to b e optimized for a single hadronic jet in the detector. Its
j
transverse energy E must b e balanced by the neutrino. The cuts are therefore:
T
In order to reject events with large missing transverse energy due to
badly measured tracks, the ratio of the energy E to the total visible
ECAL
energy E has to b e larger than 20 %. vis
FREIBURG-EHEP-97-04 4
j
Exactly one jet has to be found (n = 1) with E > 15 GeV. The jet
j
T
direction in the lab oratory frame is required to lie within a pseudorapidity
j
range j j < 2 to reject events where a single jet, usually due to an
electron, was found in the forward detectors.
n 5 and n 7.
ch q
The missing transverse energy E= must b e greater than 15 GeV and it
T
j
should b e mainly due to the jet. Therefore we require jE E= j < 3
T
T
j
GeV and E =E > 0:5.
vis
Since no additional cuts on electron variables are necessary, the eciency to
0
detect a lepto quark is higher in the q than in the eq channel. For M =
e
0
100 GeV the eciency is ab out 61 % in the q and 55 % in the eq channel. e 1 1 - - (a) OPAL prel. (b) OPAL prel. √s=161 and 172 GeV √s=161 and 172 GeV
10 LQ→eq 10 LQ→νq´ Events in 20.5 pb Events in 20.5 pb data data M=45 GeV M=45 GeV M=80 GeV M=80 GeV M=120 GeV M=120 GeV M=140 GeV M=140 GeV 1 1 background background
-1 -1 10 10
20 40 60 80 100 120 140 160 180 200 220 20 40 60 80 100 120 140 160 180 200 220 M (GeV)
jj MT=2E/ T (GeV)
p
0
Figure 1: Numb er of (a) LQ ! eq and (b) LQ ! q events exp ected with = 4
e em
1
in 20.5 pb of data after all cuts for M =45;80; 120 and 140 GeV (histograms) and the
candidate events (data p oints). The sum of all background contributions exp ected from the
simulation of the Standard Mo del pro cesses is also shown normalized to the data luminosity.
The lepto quark mass was reconstructed by calculating the transverse mass
M =2E= . The transverse mass M of the two candidate events at 38 and 46
T T T
GeV is shown in Fig. 1b together with the background distribution from the
simulation. The exp ected background rate is 1:81 0:05 events. The transverse
p
mass distribution for a lepto quark pro duction cross-section using = 4
em is also indicated.
FREIBURG-EHEP-97-04 5
3 Mass limit for a scalar lepto quark
The systematic error includes
1
(a) the luminosity measurement λ 0.9
0.8
1%, (b) the mo del dep en- with √ 0.7 S=161/172 GeV
dence of the lepto quark fragmen- 0.6 95 % CL excluded
tation with 4 %, (c) the electron 0.5 ti cation eciency with 2 %
iden 0.4
(d) the Monte Carlo statis-
and 0.3
tics with 1 %. The mo del dep en- LEP1 excluded
dence of the lepto quark fragmen- 0.2
as estimated byvarying tation w β=0.5
β=1
the cut on the average charged
ultiplicity by one unit in the m 0.1
0.09
te Carlo while keeping it Mon 0.08
xed in the data. 0.07 OPAL preliminary
0.06
The limit was obtained sepa- 0.05
0 20 40 60 80 100 120 140 160
= 1 and for =0:5.
rately for M (GeV)
The 95 % con dence level (CL)
upp er limit was calculated tak-
Figure 2: Upp er limit at 95 % CL of the coupling
ing into account the candidates,
as a function of the mass M of the scalar LQ.
the background, the exp erimen-
tal resolution and the systematic
errors. The cross-section was determined using PYTHIA. The upp er limit at
95 % CL of the coupling as a function of the lepto quark mass M is given in
Fig. 2. The mass limits are M>131 GeV for b oth =0:5 and = 1 and for
p
= 4 .
em
References
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