Search for Pair Production of Second-Generation Scalar Leptoquarks in pp Collisions at $\sqrt{s}$ = 7 TeV

Abstract

A search for pair production of second-generation scalar leptoquarks in the final state with two muons and two jets is performed using proton-proton collision data at sqrt(s) = 7 TeV collected by the CMS detector at the LHC. The data sample used corresponds to an integrated luminosity of 34 inverse picobarns. The number of observed events is in good agreement with the predictions from the standard model processes. An upper limit is set on the second-generation leptoquark cross section times beta^2 as a function of the leptoquark mass, and leptoquarks with masses below 394 GeV are excluded at a 95% confidence level for beta = 1, where beta is the leptoquark branching fraction into a muon and a quark. These limits are the most stringent to date.

Full text

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP/2012-193
2012/07/24
CMS-EXO-11-028
Search for pair production of first- and second-generation
scalar leptoquarks in pp collisions at√s = 7TeV
The CMS Collaboration∗
Abstract
Results are presented from a search for the pair production of first- and second-
generation scalar leptoquarks in proton-proton collisions at√s = 7TeV. The data
sample corresponds to an integrated luminosity of 5.0fb−1, collected by the CMS de-
tector at the LHC. The search signatures involve either two charged leptons of the
same-flavour (electrons or muons) and at least two jets, or a single charged lepton
(electron or muon), missing transverse energy, and at least two jets. If the branching
fraction of the leptoquark decay into a charged lepton and a quark is assumed to be
β = 1, leptoquark pair production is excluded at the 95% confidence level for masses
below 830GeV and 840GeV for the first and second generations, respectively. For
β = 0.5, masses below 640GeV and 650GeV are excluded. These limits are the most
stringent to date.
Submitted to Physical Review D
∗See Appendix A for the list of collaboration members
arXiv:1207.5406v1 [hep-ex] 23 Jul 2012

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1Introduction
The structure of the standard model (SM) of particle physics suggests a fundamental relation-
ship between quarks and leptons. There are many models beyond the SM that predict the ex-
istence of leptoquarks (LQ), hypothetical particles that carry both baryon number and lepton
number and couple to both quarks and leptons. Among these scenarios are grand unified the-
ories [1, 2], composite models [3], extended technicolor models [4–6], and superstring-inspired
models [7]. LQs are color triplets with fractional electric charge, and can be either scalar or
vector particles. A leptoquark couples to a lepton and a quark with a coupling strength λ,
and it decays to a charged lepton and a quark with an unknown branching fraction β or to
a neutrino and a quark with branching fraction 1 − β. In order to satisfy constraints from
bounds on flavor-changing-neutral-currents and from rare pion and kaon decays [3, 8], it is as-
sumed that LQs couple only to quarks and leptons of a single generation. LQs are classified as
first-, second-, or third-generation, depending on the generation of leptons they couple to. The
dominant mechanisms for the production of LQ pairs at the Large Hadron Collider (LHC) are
gluon-gluon (gg) fusion and quark-antiquark (qq) annihilation, shown in Fig. 1. The dominant
processes only depend on the strong coupling constant and have been calculated at next-to-
leading order (NLO) [9]. The cross section for production via the unknown Yukawa coupling
λ of an LQ to a lepton and a quark is typically smaller.
This paper reports on a search for pair production of scalar leptoquarks. Several experiments
have searched for pair-produced scalar LQs but none has obtained evidence for them [10–15].
This search uses a data sample corresponding to an integrated luminosity of 5.0fb−1recorded
with the Compact Muon Solenoid (CMS) detector during the 2011 proton-proton run of the
LHC at√s = 7TeV. The analysis performed in this paper considers the decay of leptoquark
pairsintotwochargedleptonsofthesameflavor(eitherelectronsormuons)andtwoquarks; or
into a charged lepton, a neutrino, and two quarks. As a result, two distinct classes of events are
selected: one with two high-transverse-momentum (pT) electrons or muons and at least two
high-pTjets (lljj) and the other with one high-pTelectron or muon, large missing transverse
energy (Emiss
T
), and at least two high-pTjets (lνjj).
g
g
LQ
LQ
g
g
g
LQ
LQ
g
g
LQ
LQ
LQ
q
¯ q
LQ
LQ
g
q
¯ q
LQ
LQ
l
Figure 1: Leading order diagrams for the pair production of scalar leptoquarks.
The CMS detector, described in detail elsewhere [16], uses a cylindrical coordinate system with
the z axis along the counterclockwise beam axis. The detector consists of an inner tracking
system and electromagnetic (ECAL) and hadron (HCAL) calorimeters surrounded by a 3.8T
solenoid. The inner tracking system consists of a silicon pixel and strip tracker, providing
the required granularity and precision for the reconstruction of vertices of charged particles in
the range 0 ≤ φ ≤ 2π in azimuth and |η| < 2.5, where the pseudorapidity η is defined as
η = −ln[tan(θ/2)], and θ is the polar angle measured with respect to the z axis. The crystal

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2Dataset and object reconstruction
ECAL and the brass/scintillator sampling HCAL are used to measure with high resolution the
energies of photons, electrons, and hadrons for |η| < 3.0. The three muon systems surrounding
the solenoid cover a region |η| < 2.4 and are composed of drift tubes in the barrel region
(|η| < 1.2), of cathode strip chambers in the endcaps (0.9 < |η| < 2.4), and of resistive plate
chambers in both the barrel region and the endcaps (|η| < 1.6). Events are recorded based
on a trigger decision using information from either the calorimeter or muon systems. The final
triggerdecisionisbasedontheinformationfromallsubsystems, whichispassedontothehigh-
level trigger (HLT), consisting of a farm of computers running a version of the reconstruction
software optimized for fast processing.
The lljj and lνjj analyses are performed separately and the results are combined as a function
of the branching fraction β and the leptoquark mass MLQfor first and second generations in-
dependently. The analysis in all four decay channels searches for leptoquarks in an excess of
events characteristic of the decay of heavy objects. Various triggers are used to collect events
depending on the decay channel and the data taking periods as described in Section 2. An
initial selection isolates events with high-pTfinal-state particles (two or more isolated leptons
and two or more jets; or one isolated lepton, two or more jets, and large Emiss
emission of a neutrino). Kinematic variables are then identified to further separate a possible
leptoquark signal from the expected backgrounds, and optimized thresholds on the values of
these variables are derived in order to maximize the sensitivity to the possible presence of a
signal in each decay mode. The variables used in the optimization are the invariant mass of
jet-lepton pairs (Mlj), the scalar sum (ST) of the pTof each of the final state objects, and either
the invariant mass of the dilepton pair (Mll) in the lljj channels or Emiss
Major sources of SM background are Z/γ∗+jets, W+jets processes, and tt. Smaller contributions
arise from single top production, diboson processes, and QCD multijet processes. The major
backgrounds are determined either from control samples in data or from Monte Carlo (MC)
simulated samples normalized to data in selected control regions.
T
indicative of the
T
in the lνjj channels.
After final selection, the data are well described by the SM background predictions, and upper
limits on the LQ cross section are set using a CLSmodified frequentist approach [17, 18]. Using
Poisson statistics, 95% confidence level (CL) upper limits are obtained on the leptoquark pair-
production cross-section times branching fraction as a function of leptoquark mass (MLQ). This
is compared with the NLO predictions [9] to determine lower limits on MLQfor β = 1 and
β = 0.5. The lljj and lνjj channels are combined to further maximize the exclusion in β and
MLQ, especiallyforthecase β ∼ 0.5, wherecombiningthetwochannelsincreasesthesensitivity
of the search.
The data and the initial event selection are detailed in Section 2 of this paper, followed by
a description of the signal modeling and background estimates in Section 3 and Section 4,
respectively. Section 5 contains the final event selection, and Section 6 describes the systematic
uncertainties. The results of the search are presented in Section 7 and summarized in Section 8.
2Dataset and object reconstruction
For the first-generation eejj analysis, events are required to pass a double-electron trigger or a
double-photon trigger, with an electron or photon pT> 33GeV. For the first-generation eνjj
analysis, events are required to pass either a single-electron trigger or a trigger based on the
requirement of one electron with pTthreshold between 17 and 30GeV, missing transverse en-
ergy threshold between 15 and 20GeV, and two jets with pTthreshold between 25 and 30GeV.
The trigger thresholds vary according to the run period. For the second-generation leptoquark

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analyses, events are required to pass a single-muon trigger without isolation requirements and
with a pTthreshold of 40GeV. For the eejj channel, the trigger efficiency is greater than 99%.
For the eνjj channel, the electron trigger efficiency is measured to be 95%. For the µµjj and µνjj
channels, the single muon trigger efficiency is measured to be 92%.
Electron candidates [19] are required to have an electromagnetic cluster with pT> 40GeV and
pseudorapidity |η| < 2.5 (2.2) for the eejj (eνjj) analysis, excluding the transition region be-
tween the barrel and the endcap detectors, 1.44 < |η| < 1.57. The eνjj analysis requires lower
electron |η| to reduce the QCD multijet background, with negligible reduction of the signal ac-
ceptance. Electron candidates are required to have an electromagnetic cluster in the ECAL that
is spatially matched to a reconstructed track in the central tracking system in both η and the az-
imuthal angle φ, and to have a shower shape consistent with that of an electromagnetic shower.
Electron candidates are further required to be isolated from additional energy deposits in the
calorimeter and from reconstructed tracks beyond the matched track in the central tracking
system. In addition, to reject electrons coming from photon conversion in the tracker material,
the track associated with the reconstructed electron is required to have hits in all inner tracker
layers.
Muons are reconstructed as tracks in the muon system that are matched to the tracks recon-
structed in the inner tracking system [20]. Muons are required to have pT> 40GeV, and to
be reconstructed in the HLT fiducial volume, i.e. with |η| < 2.1. In addition, muons must be
isolated by requiring that the tracker-only relative isolation be less than 0.1. Here, the relative
isolation is defined as a sum of the transverse momenta of all tracks in the tracker in a cone of
∆R =
the muon pT. To have a precise measurement of the transverse impact parameter of the muon
track relative to the beam spot, only muons with tracks containing more than 10 hits in the
silicon tracker and at least one hit in the pixel detector are considered. To reject muons from
cosmic rays, the transverse impact parameter with respect to the primary vertex is required to
be less than 2mm.
?(∆φ)2+ (∆η)2= 0.3 around the muon track (excluding the muon track), divided by
Jets and Emiss
sures stable particles by combining information from all CMS sub-detectors. The Emiss
tion uses calorimeter estimates improved by high precision inner tracking information as well
as corrections based on particle-level information in the event. Jets are reconstructed using the
anti-kT[22] algorithm with a distance parameter of R = 0.5. The jet energy is calibrated using
pTbalance of dijet and γ+jet events [23]. In the eejj and µµjj (eνjj and µνjj) channels, jets are
required to have pT > 30 (40)GeV, and |η| < 2.4. Furthermore, jets are required to have a
spatial separation from electron or muon candidates of ∆R > 0.3.
T
are reconstructed using a particle-flow algorithm [21], which identifies and mea-
T
calcula-
The initial selection of eejj or µµjj events requires two electrons or two muons and at least two
jets satisfying the conditions described above. The two leptons and the two highest-pTjets are
selected as the decay products from a pair of leptoquarks. The invariant mass of the two elec-
trons (muons) is required to be Mll> 60 (50)GeV. To reduce the combinatorial background,
events with a scalar transverse energy Sll
are rejected.
T= pT(l1) + pT(l2) + pT(j1) + pT(j2) below 250GeV
For the eνjj (µνjj) initial selection, events are required to contain one electron (muon) satisfying
the conditions described above and at least two jets with pT> 40GeV and Emiss
jet pTthreshold is higher than that in the dilepton channels to account for jet pTthresholds
in triggers used in the eνjj channel. A veto on the presence of extra muons (electrons) is also
applied. The angle in the transverse plane between the leading pTjet and the Emiss
is required to be ∆φ(Emiss
T
T
> 55GeV. The
T
vector
, j1) > 0.5 to reject events with misreconstructed Emiss
T
. In order to

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4Background estimate
reduce the contribution from QCD multijet events, the lepton and the Emiss
separated by ∆φ(Emiss
T
,l) > 0.8. In addition, events are rejected if the scalar transverse energy
Slν
T
+ pT(j1) + pT(j2) is below 250GeV.
The initial selection criteria are summarized in Table 1.
T
are required to be
T= pT(l) + Emiss
Table 1: Initial selection criteria in the eejj, µµjj, eνjj, and µνjj channels.
Variable
pT(l1) [GeV]
pT(l2)[GeV]
|η(l1)|
|η(l2)|
pT(j1) [GeV]
pT(j2) [GeV]
∆R(l, j)
Emiss
T
[GeV]
|∆φ(Emiss
|∆φ(Emiss
Mll[GeV]
Mlν
Sll
Slν
eejj
> 40
> 40
< 2.5
< 2.5
> 30
> 30
> 0.3
—
—
—
>60
—
> 250
—
µµjj
> 40
> 40
< 2.1
< 2.1
> 30
> 30
> 0.3
—
—
—
> 50
—
> 250
—
eνjj
> 40
—
< 2.2
—
> 40
> 40
> 0.3
> 55
> 0.5
> 0.8
—
> 50
—
> 250
µνjj
> 40
—
< 2.1
—
> 40
> 40
> 0.3
> 55
> 0.5
> 0.8
—
> 50
—
> 250
T
T
, j1)|
,l)|
T[GeV]
T[GeV]
T[GeV]
3Signal and background modeling
The MC samples for the signal processes are generated for a range of leptoquark mass hypothe-
ses between 250 and 900GeV, with a renormalization and factorization scale µ ≈ MLQ. The
MC generation uses the PYTHIA generator [24] (version 6.422) and CTEQ6.6 parton distribu-
tion functions (PDF). The MC samples used to estimate the contribution from SM background
processes are tt+jets events, generated with MADGRAPH [25, 26]; single-top events (s, t, and tW
channels), generated with MADGRAPH; Z/γ∗+jets events and W+jets events, generated with
SHERPA [27]; VV events, where V either represents a W or a Z boson, generated with PYTHIA;
QCD muon-enriched multijet events, generated with PYTHIA in bins of transverse momentum
of the hard-scattering process from 15GeV to the kinematic limit. The simulation of the CMS
detector is based on GEANT4 [28], and includes multiple collisions in a single bunch crossing
corresponding to the luminosity profile of the LHC during the data taking periods of interest.
4Background estimate
The main processes that can mimic the signature of a leptoquark signal in the lljj channels are
Z/γ∗+jets, tt, VV+jets, W+jets, and QCD multijets. The Z/γ∗+jets background is determined
by comparing events from data and MC samples in two different regions: in the region of low
(L) dilepton invariant mass around the Z boson mass (70 < Mll< 100GeV for electrons and
80 < Mll< 100GeV for muons) and in the region of high (H) mass Mll> 100GeV. The low
mass scaling factor RZ= NL/NMC
L
is measured to be 1.27±0.02 for both eejj and µµjj channels,
where NLand NMC
L
are the number of data and MC events, respectively, in the Z mass window.

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The number of Z/γ∗+jets events above 100GeV is then estimated as:
NH= RZNMC
H,(1)
where NMC
Z/γ∗+jets events is obtained with the selection criteria optimized for different LQ mass hy-
potheses and it is used in the limit setting procedure.
H
is the number of MC events with Mll > 100GeV. The estimated number of
The number and kinematic distributions for the tt events with two leptons of the same flavor is
estimated from the number of data events that contain one electron and one muon. This type of
background is expected to produce ee plus µµ and eµ final state events with equal probability.
In the data the number of ee or µµ events is estimated to be:
Nee(µµ)=1
2×?e(µ)
?µ(e)
×
?trig
ee(µµ)
?trig
eµ
× Neµ,(2)
where ?µand ?eare the muon and electron reconstruction and identification efficiencies and
?trigare the HLT efficiencies to select ee, µµ, and eµ events.
No QCD multijet MC events pass the µµjj final selection. A crosscheck made using a data
control sample containing same sign muons confirms that the QCD multijet background is
negligible in this channel.
For the first-generation LQ analyses the multijet background contribution is estimated from
a data control sample as follows. The probability that an electron candidate passing loose
electron requirements additionally passes all electron requirements is measured as a function
of pTand η in a data sample with one and only one electron candidate, two or more jets and
low Emiss
T
. This sample is dominated by QCD multijet events and is similar in terms of jet
activity to the eejj and eνjj analysis samples. A correction for a small contamination of genuine
electrons passing all electron requirements is derived from MC simulations. The QCD multijet
background in the final eejj (eνjj) selection is predicted by applying twice (once) the above
probability to a sample with two electron candidates (one electron candidate and large Emiss
and two or more jets, which satisfy all the requirements of the signal selections. The resulting
estimate is ∼1% (∼8%) of the total background for the selections corresponding to the region
of leptoquark masses where the exclusion limits are placed.
T
),
Contributions to the lljj background from VV+jets processes and single-top production are
small and they are estimated using MC simulation.
In the lνjj channel, the main backgrounds come from three sources: processes that lead to
the production of a genuine W bosons such as W+jets, tt, single-top production, diboson pro-
cesses (WW, WZ); instrumental background, mostly caused by the misidentification of jets as
leptons in multijet processes, thus creating misidentified electrons or muons and misrecon-
structed Emiss
T
in the final state; and Z boson production, such as Z/γ∗+jets and ZZ processes.
The contribution from the principal background, W+jets, is estimated with MC simulation nor-
malized to data at the initial selection level in the region 50 < MT< 110GeV, where MTis the
transverse mass calculated from the lepton pTand the Emiss
T
.
Intheeνjj analysis, theregion50 < MT< 110GeVisusedtodetermineboththeW+jetsandthe
tt normalization factors using two mutually exclusive selections (less than four jets or at least
four jets with pT> 40GeV and |η| < 2.4) that separately enhance the samples with W+jets and
with tt events. The results of these two selections are used to form a system of equations:
N1= RttN1,tt+ RWN1,W+ N1,QCD+ N1,O;
N2= RttN2,tt+ RWN2,W+ N2,QCD+ N2,O.
(3)

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5Event selection optimization
where Ni, Ni,W, Ni,O, Ni,tt, and Ni,QCDare the number of events in data, W+jets, other MC back-
grounds, tt-MC, and QCD multijet events obtained from data, passing selection i. The solution
of the system yields the following normalization factors: Rtt= 0.82 ± 0.04 (stat.)±0.02 (syst.)
and RW= 1.21 ± 0.03(stat.)±0.02(syst.), where the systematic errors reflect the uncertainties
on the QCD multijet background estimate.
In the µνjj analysis the W+jets normalization factor is calculated in the region 50 < MT <
110GeV as:
RW=N − (Ntt+ RZNZ+ NO)
NW
(4)
where N, Ntt, NZ, NO, and NWare the number of events in data, tt, Z/γ∗+jets, other back-
grounds (QCD, diboson, single top production), and W+jets MC samples, normalized to the
integrated luminosity of the data sample. RWis measured to be 1.21 ± 0.01(stat.). To estimate
the total tt background in the µνjj analysis, the MC prediction is compared with data in a con-
trol sample enriched with tt events (50 < MT< 110GeV, Emiss
The normalization factor of the tt background,
T
> 55GeV, and at least four jets).
Rtt=N − (NW+ RZNZ+ NO)
Ntt
(5)
is calculated to be 0.88 ± 0.02(stat.). The calculation of the normalization factors RWand Rttis
repeated using an iterative procedure.
The contribution from QCD multijet processes after all selection criteria are applied to the µνjj
sample is estimated to be negligible. No QCD MC events survive the full selection criteria
optimized for any leptoquark mass hypothesis. However, as multijet processes are difficult to
accurately model by MC simulation, several crosschecks are made with data control samples
to ensure that the QCD multijet background in the µνjj analysis is negligible. The method used
to determine the QCD multijet background in the eνjj analysis is similar to the one used for the
eejj channel.
5 Event selection optimization
After the initial selection, the sensitivity of the search is optimized by maximizing the Gaussian
signal significance S/√S + B in all channels. Optimized thresholds on the following variables
are applied for each leptoquark mass hypothesis in the lljj channels: Mll, Sll
invariant mass of the dilepton pair, Mll, is used to remove the majority of the contribution from
the Z/γ∗+jets background. The variable Mmin
lj
mass for the assignment of jets and leptons to LQs which minimizes the LQ - LQ invariant
mass difference.
T, and Mmin
lj
. The
is defined as the smaller lepton-jet invariant
Thresholdsonthefollowingvariablesareoptimizedforeachleptoquarkmasshypothesisinthe
lνjj channels: Emiss
T
the dominant W+jets background. The variable Mljis defined as the invariant mass of the
lepton-jet combination which minimizes the LQ - LQ transverse mass difference. In addition,
a lower threshold is applied on the transverse mass of the lepton (electron or muon) and Emiss
in the event, MT> 120GeV.
, Slν
T, and Mlj. A minimum threshold on Emiss
T
is used, primarily to reduce
T
The resulting optimized thresholds are summarized in Tables 2 and 3 for the lljj and the lνjj
channels, respectively.

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Table 2: Optimized thresholds for different mass hypothesis of the lljj signal.
MLQ(GeV)
Sll
Mll> (GeV)
Mmin
lj
> (GeV)
250
330
100
60
350
450
110
160
400
530
120
200
450
610
130
250
500
690
130
300
550
720
130
340
600
770
130
370
650
810
130
400
750
880
140
470
850
900
150
500
900
920
150
520
T> (GeV)
Table 3: Optimized thresholds for different mass hypotheses of the lνjj signal.
MLQ(GeV)
Slν
Emiss
T
Mlj> (GeV)
250
450
100
150
350
570
120
300
400
650
120
360
450
700
140
360
500
800
160
360
550
850
160
480
600
890
180
480
650
970
180
540
750
1000
220
540
850
1000
240
540
T> (GeV)
> (GeV)
After the initial selection criteria are applied, the yields in data are found to be consistent with
SM predictions. Distributions of variables used in the final selection for the eejj, eνjj, µµjj, and
µνjj analyses are shown in Figs. 2–5.
(GeV)
ee
T
S
200400600 80010001200 14001600 18002000
Events / 20 GeV
-1
10
1
10
2
10
3
10
4
10
5
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e-t t
Other backgrounds
QCD multijets
= 400 GeV
LQ
M
]
µ
(GeV)
ej
M
020040060080010001200
Entries / 20 GeV
-1
10
1
10
2
10
3
10
4
10
5
10
6
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e- t t
Other backgrounds
QCD multijets
= 400 GeV
LQ
M
]
µ
Figure 2: eejj channel: the distributions of See
pairs (right) for events that pass the initial selection level. The data are indicated by the points,
and the SM backgrounds are given as cumulative histograms. The expected contribution from
a LQ signal with MLQ= 400GeV is also shown.
T(left) and of Mejfor each of the two electron-jet
(GeV)
ν
e
T
S
200400 600800 10001200 1400160018002000
Events / 20 GeV
-2
10
-1
10
1
10
2
10
3
10
4
10
5
10
6
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
W + jets
t t
QCD multijets
Other backgrounds
= 400 GeV
LQ
M
(GeV)
ej
M
0 2004006008001000 1200 14001600
Events / 20 GeV
-2
10
-1
10
1
10
2
10
3
10
4
10
5
10
6
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
W + jets
t t
QCD multijets
Other backgrounds
= 400 GeV
LQ
M
Figure 3: eνjj channel: the distributions of Seν
initial selection level. The data are indicated by the points, and the SM backgrounds are given
as cumulative histograms. The expected contribution from a LQ signal with MLQ= 400GeV is
also shown.
T(left) and of Mej(right) for events that pass the
The number of events selected in data and estimated backgrounds are then compared at differ-
ent stages of selection. This information is shown in Tables 4–7 for the initial selection and for

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5Event selection optimization
(GeV) (GeV)
µ µ
TT
SS
200200 400400600600800800100010001200120014001400 160016001800
µ µ
20002000
Events / 20 GeV
-2-2
1010
-1-1
1010
11
1010
22
1010
33
10 10
44
1010
55
1010
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e- t t
Other backgrounds
= 400 GeV
LQ
M
]
µ
CMS 2011
= 7 TeVs
1800
Events / 20 GeV
(GeV)
j
(GeV)
j
µµ
MM
00200 200400400600 6008008001000 100012001200
Entries / 20 GeV
-2-2
1010
-1-1
1010
11
1010
22
1010
33
1010
44
1010
55
1010
66
1010
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e-t t
Other backgrounds
= 400 GeV
LQ
M
]
µ
CMS 2011
= 7 TeVs
Entries / 20 GeV
Figure 4: µµjj channel: the distributions of Sµµ
pairs (right) for events that pass the initial selection level. The data are indicated by the points,
and the SM backgrounds are given as cumulative histograms. The expected contribution from
a LQ signal with MLQ= 400GeV is also shown.
T(left) and of Mµjfor each of the two muon-jet
(GeV) (GeV)
ν µ
TT
SS
200200400400600600800800100010001200120014001400160016001800
ν µ
20002000
Events / 20 GeV
-2-2
1010
-1-1
1010
11
1010
22
1010
33
1010
44
1010
55
1010
66
1010
-1
Data, 5.0 fb
W + jets
t t
Other backgrounds
= 400 GeV
LQ
M
CMS 2011
= 7 TeVs
1800
Events / 20 GeV
(GeV)
j
(GeV)
j
µµ
MM
0020020040040060060080080010001000120012001400140016001600
Events / 20 GeV
-2-2
1010
-1-1
1010
11
1010
22
1010
33
1010
44
1010
55
1010
66
1010
-1
Data, 5.0 fb
W + jets
t t
Other backgrounds
= 400 GeV
LQ
M
CMS 2011
= 7 TeVs
Events / 20 GeV
Figure 5: µνjj channel: the distributions of Sµν
initial selection level. The data are indicated by the points, and the SM backgrounds are given
as cumulative histograms. The expected contribution from a LQ signal with MLQ= 400GeV is
also shown.
T(left) and of Mµj(right) for events that pass the

Page 11

9
the final selection for each channel separately.
Table 4: Individual background (BG) sources, expected signal, data and total background event
yields after the initial (first row) and final selections for the eejj analysis. Other BG includes
single top,W+jets, γ+jets, and VV+jets. Only statistical uncertainties are reported.
MLQ
–
400
450
500
550
600
650
750
850
Z+jets
6234± 24
35.7± 1.8
15.2± 1.1
6.55± 0.70
4.65± 0.58
3.04± 0.46
2.14± 0.38
1.04± 0.26
0.81± 0.23
ttQCD Other BG
147.6± 2.3
3.12± 0.56
1.92± 0.60
1.03± 0.42
0.84± 0.42
0.72± 0.41
0.48± 0.40
0.41± 0.40
0.40± 0.40
LQ Signal
–
487.4± 2.2
225.6± 1.0
109.30± 0.46
57.35± 0.23
30.95± 0.14
16.998± 0.065
5.526± 0.023
1.9679± 0.0078
Data
7201
55
26
14
11
8
6
0
0
Total BG
7199± 31
58.8± 3.6
25.2± 2.3
10.2± 1.4
6.60± 0.99
4.34± 0.79
3.18± 0.74
1.45+0.73
−0.47
1.21+0.72
−0.46
768± 19
19.1± 3.1
7.8± 2.0
2.45± 1.10
0.98± 0.69
0.49± 0.49
0.49± 0.49
0.000+0.56
0.000+0.56
49.59± 0.43
0.877± 0.022
0.310± 0.013
0.192± 0.012
0.139± 0.012
0.088± 0.011
0.073± 0.011
0.0092± 0.0020
0.00101± 0.00022
−0.00
−0.00
Table5: Individualbackground(BG)sources, expectedsignal, data, andtotalbackgroundevent
yields after the initial (first row) and final selections for the eνjj analysis. Other BG includes
single top, Z+jets, γ+jets, and VV+jets. Only statistical uncertainties are reported.
MLQ
–
400
450
500
550
600
650
750
850
W+jets
20108± 99
28.7± 3.6
19.7± 2.9
13.3± 2.4
2.98± 0.95
2.45± 0.87
2.03± 0.83
1.45± 0.65
1.22± 0.61
ttQCD
3267± 26
6.20± 0.46
3.01± 0.31
1.72± 0.22
0.65± 0.10
0.57± 0.10
0.335± 0.079
0.287± 0.080
0.251± 0.078
Other
1913± 53
6.01± 0.77
4.13± 0.44
2.80± 0.37
1.46± 0.26
1.29± 0.25
0.76± 0.20
0.65± 0.18
0.61± 0.19
LQ Signal
–
126.01± 0.82
68.38± 0.43
34.70± 0.23
16.25± 0.10
9.442± 0.056
5.202± 0.032
1.851± 0.010
0.6973± 0.0037
Data
34135
43
29
18
10
6
4
4
4
Total BG
34590± 120
58.4± 4.1
39.1± 3.3
24.2± 2.6
8.5± 1.3
6.6± 1.1
4.14± 0.95
3.01± 0.75
2.70± 0.71
9301± 42
17.5± 1.8
12.2± 1.5
6.3± 1.1
3.38± 0.82
2.33± 0.67
1.01± 0.41
0.62± 0.31
0.62± 0.31
Data and background predictions after final selection are also shown in Figs. 6–9, which com-
pare STand the best combination for the lepton-jet invariant mass for a signal leptoquark mass
of 600GeV in the four decay channels considered.
6Systematics Uncertainties
The uncertainty on the integrated luminosity is taken as 2.2% [29].
The statistical uncertainties on the values of RZ(RW) after initial selection requirements are
used as an estimate of the uncertainty on the normalization of the Z/γ∗+jets (W+jets) back-
grounds. The uncertainty on the shape of the Z/γ∗+jets and W+jets distributions is calculated
to be 15% (10%) and 20% (11%) for first (second) generation, respectively, by comparing the
predictions of MADGRAPH samples produced with factorization or renormalization scales and
matrix element–parton shower matching threshold varied up and down by a factor of two.
The uncertainty on the estimate of the tt background in the eejj and µµjj channels is derived
from the statistical uncertainty of the eµjj data sample and the ratio of electron and muon

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10
6Systematics Uncertainties
(GeV)
ee
T
S
0500 1000 150020002500
Events / 100 GeV
-2
10
-1
10
1
10
2
10
3
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e-t t
Other backgrounds
QCD multijets
= 600 GeV
LQ
M
]
µ
(GeV)
ej
M
4006008001000120014001600 180020002200
Entries / 100 GeV
-2
10
-1
10
1
10
2
10
3
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e- t t
Other backgrounds
QCD multijets
= 600 GeV
LQ
M
]
µ
Figure 6: eejj channel: the distributions of See
pairs (right) for events that pass the final selection criteria optimized for a signal LQ mass of
600GeV. The data are indicated by the points, and the SM backgrounds are given as cumulative
histograms. The expected contribution from a LQ signal with MLQ= 600GeV is also shown.
T(left) and of Mejfor each of the two electron-jet
(GeV)
ν
e
T
S
5001000150020002500
Events / 100 GeV
-2
10
-1
10
1
10
2
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
W + jets
t t
QCD multijets
Other backgrounds
= 600 GeV
LQ
M
(GeV)
ej
M
4006008001000120014001600180020002200
Events / 100 GeV
-2
10
-1
10
1
10
2
10
CMS 2011
= 7 TeVs
-1
Data, 5.0 fb
W + jets
t t
QCD multijets
Other backgrounds
= 600 GeV
LQ
M
Figure 7: eνjj channel: the distributions of Seν
the final selection criteria optimized for a signal LQ mass of 600GeV. The data are indicated
by the points, and the SM backgrounds are given as cumulative histograms. The expected
contribution from a LQ signal with MLQ= 600GeV is also shown.
T(left) and of Mej(right) for events that pass
(GeV) (GeV)
µ µ
TT
SS
600 600 8008001000100012001200140014001600160018001800200020002200
µ µ
24002400
Events / 100 GeV
-2-2
1010
-1-1
1010
11
1010
22
1010
33
1010
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e-t t
Other backgrounds
= 600 GeV
LQ
M
]
µ
CMS 2011
= 7 TeV s
2200
Events / 100 GeV
(GeV)
j
(GeV)
j
µµ
MM
400400600 60080080010001000120012001400140016001600180018002000200022002200
Entries / 100 GeV
-2-2
1010
-1-1
1010
11
10 10
22
1010
33
1010
-1
Data, 5.0 fb
* + jets
γ
Z/
[Data driven e- t t
Other backgrounds
= 600 GeV
LQ
M
]
µ
CMS 2011
= 7 TeVs
Entries / 100 GeV
Figure 8: µµjj channel: the distributions of Sµµ
pairs (right) for events that pass the final selection criteria optimized for a signal LQ mass of
600GeV. The data are indicated by the points, and the SM backgrounds are given as cumulative
histograms. The expected contribution from a LQ signal with MLQ= 600GeV is also shown.
T(left) and of Mµjfor each of the two muon-jet

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11
Table 6: Individual background (BG) sources, expected signal, data and total background event
yields after the initial (first row) and final selections for the µµjj analysis. Other BG includes
single top, W+jets, and VV+jets. Only statistical uncertainties are reported.
MLQ
–
400
500
550
600
650
750
850
Z+jets
8413± 46
45.9± 2.4
10.3± 1.1
7.18± 0.92
4.95± 0.74
3.76± 0.64
2.00± 0.46
1.53± 0.41
ttOther BG
197.9± 2.8
3.54+0.69
−0.40
0.88+0.60
−0.21
0.53+0.58
−0.16
0.47+0.58
−0.16
0.24+0.57
−0.12
0.09+0.57
−0.08
0.08+0.57
−0.08
LQ Signal
–
629.3± 4.0
136.70± 0.86
70.49± 0.40
37.39± 0.22
20.56± 0.13
5.875± 0.036
2.327± 0.014
Data
9725
68
14
9
6
5
1
0
Total BG
9638± 53
73.8+4.8
15.3+2.1
9.8+1.6
−1.5
6.1+1.2
−1.0
4.7+1.1
−1.0
2.1+1.1
−0.5
1.6+1.1
−0.4
1028± 27
24.3± 4.1
4.2± 1.7
2.1± 1.2
0.69± 0.69
0.69± 0.69
0.00+0.79
−0.00
0.00+0.79
−0.00
−4.8
−2.0
Table7: Individualbackground(BG)sources, expectedsignal, data, andtotalbackgroundevent
yields after the initial (first row) and final selections for the µνjj analysis. Other BG includes
single top, Z+jets, and VV+jets. Only statistical uncertainties are reported.
MLQ
–
400
500
550
600
650
750
850
W+jets
23000± 160
30.2± 4.2
17.0± 3.1
5.6± 1.7
5.4± 1.7
2.8± 1.1
2.8± 1.1
2.7± 1.1
tt Other BG
2106± 17
5.88± 0.64
3.40± 0.51
2.16± 0.42
1.78± 0.38
1.12± 0.30
0.98± 0.28
0.75± 0.23
LQ Signal
–
118.4± 1.2
34.02± 0.28
15.53± 0.14
9.245± 0.077
4.483± 0.040
1.844± 0.015
0.6934± 0.0052
Data
39287
61
26
12
8
7
6
6
Total BG
38170± 170
61.8± 5.1
30.6± 3.6
12.3± 2.1
10.7± 2.0
5.1± 1.3
4.3± 1.2
3.9± 1.2
13066± 61
25.7± 2.8
10.3± 1.8
4.5± 1.2
3.5± 1.1
1.20± 0.62
0.54± 0.38
0.54± 0.38
(GeV) (GeV)
ν µ
TT
SS
60060080080010001000 12001200140014001600160018001800200020002200
ν µ
24002400
Events / 100 GeV
-2-2
1010
-1-1
1010
11
1010
22
1010
-1
Data, 5.0 fb
W + jets
t t
Other backgrounds
= 600 GeV
LQ
M
CMS 2011
= 7 TeVs
2200
Events / 100 GeV
(GeV)
j
(GeV)
j
µµ
MM
400 40060060080080010001000120012001400140016001600 180018002000200022002200
Events / 100 GeV
-2-2
1010
-1-1
1010
11
1010
-1
Data, 5.0 fb
W + jets
t t
Other backgrounds
= 600 GeV
LQ
M
CMS 2011
= 7 TeVs
Events / 100 GeV
Figure 9: µνjj channel: the distributions of Sµν
the final selection criteria optimized for a signal LQ mass of 600GeV. The data are indicated
by the points, and the SM backgrounds are given as cumulative histograms. The expected
contribution from a LQ signal with MLQ= 600GeV is also shown.
T(left) and of Mµj(right) for events that pass

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12
7 Results
reconstruction uncertainties, which is calculated to be 2% and 3%, respectively. A 5.5% (5%)
uncertainty on the normalization of the estimated tt background in the eνjj (µνjj) channel is
given by the statistical uncertainty on the value of Rttafter the initial selection requirements. A
10% (10%) uncertainty on the shape of the tt background distribution in the eνjj (µνjj) channel
is estimated by comparing the predictions of MADGRAPH samples produced with factorization
or renormalization scales and matrix element–parton shower matching thresholds varied up
and down by a factor of two.
A systematic uncertainty of 50% (25%) on the QCD multijet background estimate for the eejj
(eνjj) channel is estimated from the difference between the number of observed data and the
background prediction in a QCD multijet-enriched data sample with lower jet multiplicity.
PDF uncertainties on the theoretical cross section of LQ production and on the final selection
acceptance have been calculated using the PDF4LHC [30] prescriptions. Uncertainties on the
cross section vary from 10 to 30 % for leptoquarks in the mass range of 200–900GeV, while the
effect of the PDF uncertainties on signal acceptance varies from 1 to 3%. The PDF uncertainties
are not considered for background sources with uncertainties determined from data. An uncer-
tainty on the modeling of pileup interactions in the MC simulation is determined by varying
the mean of the distribution of pileup interactions by 8%.
Energy and momentum scale uncertainties are estimated by assigning a 4% uncertainty on
the jet energy scale, a 1% (3%) uncertainty on the electron energy scale for the barrel (endcap)
region of ECAL, and a 1% uncertainty on the muon momentum scale. The effect of electron
energy, muon momentum, and jet energy resolution on expected signal and backgrounds is
assessed by smearing the electron energy by 1% and 3% in the barrel and endcaps, respectively,
the muon momentum by 4%, and by varying the jet energy resolution by an η-dependent value
in the range 5-14%. In the lνjj analyses, the uncertainty on the energy and momentum scales
and resolutions are propagated to the measurement of Emiss
calculated for the (minor) background sources for which no data rescaling is applied. For the
background sources for which data rescaling is applied, residual uncertainties are calculated
(i.e. relative to the initial selection used to derive the rescaling factor).
T
. The effect of these uncertainties is
Recent measurements of the muon reconstruction, identification, trigger, and isolation efficien-
cies using Z → µµ events show very good agreement between data and MC events [31]. A
∼ 1% discrepancy is observed in the data-to-MC comparison of the muon trigger efficiency.
This discrepancy is taken as a systematic uncertainty per muon, assigned to both signal and
estimated background. The electron trigger and reconstruction and identification uncertain-
ties contribute 3% (4%) to the uncertainty in both signal and estimated background for the eejj
(eνjj) channel.
The systematic uncertainties, and their effects on signal and background are summarized in
Table 8 for all channels, corresponding to the final selection optimized for MLQ= 600GeV.
7 Results
The number of observed events in data passing the full selection criteria is consistent with the
SM background prediction in all decay channels. An upper limit on the leptoquark production
cross section is therefore set using the CLSmodified frequentist approach [17, 18]. A log-normal
probability function is used to integrate over the systematic uncertainties. Uncertainties of
statistical nature are described with Γ distributions with widths determined by the number of
events simulated in MC samples or observed in data control regions.

Page 15

13
Table 8: Systematic uncertainties and their effects on signal (S) and background (B) in all chan-
nels for the MLQ= 600GeV final selection. All uncertainties are symmetric.
SystematicMagnitude eejj
Uncertainties(%)
S(%)
B(%)
Jet Energy Scale421
Background Modeling
−
Electron Energy Scale1(3)
16
Muon Momentum Scale1
−
Muon Reco/ID/Iso1
−
Jet Resolution
(5− 14)
Electron Resolution1(3)
0.51
Muon Resolution4
−
Pileup811
Integrated Luminosity 2.22.2
−
Total513
µµjj
eνjj
µνjj
S(%)
1
−
−
0.5
2
< 0.5
−
< 0.5
0.5
2.2
3
B(%)
2
7
−
5
−
0.5
−
5
< 0.5
−
9
S(%)
5
−
1.5
−
−
< 0.5
< 0.5
−
1
2.2
7
B(%)
8.5
11
4.5
−
−
2
1.5
−
1.5
−
15
S(%)
3
−
−
1
1
< 0.5
−
< 0.5
1
2.2
4
B(%)
8.5
10
−
3
−
2.5
−
2
1.5
−
14
−
11
−
−
0.50.5
−
The 95% CL upper limits on σ × β2or σ × 2β(1 − β) as a function of leptoquark mass are
shown together with the NLO predictions for the scalar leptoquark pair production cross sec-
tion in Figs. 10 and 11. The theoretical cross sections are represented for different values of the
renormalization and factorization scale, µ, varied between half and twice the LQ mass (blue
shaded region). The PDF uncertainties are taken into account in the theoretical cross section
values.
By comparing the observed upper limit with the theoretical cross section values, first- and
second-generation scalar leptoquarks with masses less than 830 (640)GeV and 840 (620)GeV,
respectively, are excluded with the assumption that β = 1 (0.5). This is to be compared with
median expected limits of 790 (640)GeV and 780 (610)GeV.
The observed and expected limits on the branching fraction β as a function of leptoquark mass
can be further improved using the combination of the lljj and lνjj channels, as shown in Fig-
ure 12. These combinations lead to the exclusion of first- and second-generation scalar lepto-
quarks with masses less than 640 and 650GeV for β = 0.5, compared with median expected
limits of 680 and 660GeV.
8Summary
In summary, a search for pair production of first- and second-generation scalar leptoquarks has
been performed in decay channels with either two charged leptons of the same-flavour (elec-
trons or muons) and at least two jets, or a single charged lepton (electron or muon), missing
transverse energy, and at least two jets, using 7 TeV proton-proton collisions data correspond-
ing to an integrated luminosity of 5fb−1. The selection criteria have been optimized for each
leptoquark signal mass hypothesis. The number of observed candidates for each hypothesis
agree with the estimated number of background events. The CLSmodified frequentist ap-
proach has been used to set limits on the leptoquark cross section times the branching fraction
for the decay of a leptoquark pair. At 95% confidence level, the pair-production of first- and
second-generation leptoquarks is excluded with masses below 830 (640)GeV and 840 (650)GeV
for β = 1 (0.5). These are the most stringent limits to date.