Physics Letters B 718 (2013) 815–840 Contents lists available at SciVerse ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Search for new physics in events with opposite-sign leptons, jets, and missing transverse energy in pp collisions at √ s = 7 TeV ✩ .CMS Collaboration � CERN, Switzerland a r t i c l e i n f o a b s t r a c t Article history: Received 30 June 2012 Received in revised form 12 November 2012 Accepted 13 November 2012 Available online 21 November 2012 Editor: M. Doser Keywords: CMS Physics Supersymmetry A search is presented for physics beyond the standard model (BSM) in final states with a pair of opposite- sign isolated leptons accompanied by jets and missing transverse energy. The search uses LHC data recorded at a center-of-mass energy √ s = 7 TeV with the CMS detector, corresponding to an integrated luminosity of approximately 5 fb−1. Two complementary search strategies are employed. The first probes models with a specific dilepton production mechanism that leads to a characteristic kinematic edge in the dilepton mass distribution. The second strategy probes models of dilepton production with heavy, colored objects that decay to final states including invisible particles, leading to very large hadronic activity and missing transverse energy. No evidence for an event yield in excess of the standard model expectations is found. Upper limits on the BSM contributions to the signal regions are deduced from the results, which are used to exclude a region of the parameter space of the constrained minimal supersymmetric extension of the standard model. Additional information related to detector efficiencies and response is provided to allow testing specific models of BSM physics not considered in this Letter. © 2012 CERN. Published by Elsevier B.V. All rights reserved. 1. Introduction In this Letter we describe a search for physics beyond the stan- dard model (BSM) in events containing a pair of opposite-sign leptons, jets, and missing transverse energy (Emiss T ), in a sample of proton–proton collisions at a center-of-mass energy of 7 TeV. The data sample was collected with the Compact Muon Solenoid (CMS) detector [1] at the Large Hadron Collider (LHC) in 2011 and corre- sponds to an integrated luminosity of 4.98 fb−1. This is an update and extension of a previous analysis performed with a data sample of 34 pb−1 collected in 2010 [2]. The BSM signature in this search is motivated by three general considerations. First, new particles predicted by BSM physics sce- narios are expected to be heavy in most cases, since they have so far eluded detection. Second, BSM physics signals may be produced with large cross section via the strong interaction, resulting in sig- nificant hadronic activity. Third, astrophysical evidence for dark matter suggests [3–6] that the mass of weakly-interacting mas- sive particles is of the order of the electroweak symmetry break- ing scale. Such particles, if produced in proton–proton collisions, could escape detection and give rise to an apparent imbalance in the event transverse energy. The analysis therefore focuses on the region of high Emiss T . An example of a specific BSM scenario ✩ © CERN for the benefit of the CMS Collaboration. � E-mail address: cms-publication-committee-chair@cern.ch. is provided by R-parity conserving supersymmetric (SUSY) models, in which the colored squarks and gluinos are pair-produced and subsequently undergo cascade decays, producing jets and leptons [7,8]. These cascade decays may terminate in the production of the lightest SUSY particle (LSP), often the lightest neutralino, which es- capes detection and results in large Emiss T . This LSP is a candidate for a dark matter weakly-interacting massive particle. Another BSM scenario which may lead to similar signatures is the model of uni- versal extra dimensions (UED) [9]. The results reported in this Letter are part of a broad program of BSM searches in events with jets and Emiss T , classified by the number and type of leptons in the final state. Here we describe a search for events containing an opposite-sign isolated lepton pair in addition to jets and Emiss T . We reconstruct electrons and muons, which provide a clean signature with low background. In addition, we reconstruct τ leptons in their hadronic decay modes to improve the sensitivity to models with enhanced coupling to third gener- ation particles. Complementary CMS searches with different final states have already been reported, for example in Refs. [10,11]. Results from the ATLAS Collaboration in this final state using ap- proximately 1–2 fb−1 have been reported in Refs. [12,13]. The analysis strategy is as follows. In order to select dilep- ton events, we use a preselection based on that of the CMS top quark pair (tt̄) cross section measurement in the dilepton chan- nel [14]; the details of this preselection are presented in Section 3. Reasonable agreement is found between the observed yields in data and the predictions from standard model (SM) Monte Carlo 0370-2693/ © 2012 CERN. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physletb.2012.11.036 http://dx.doi.org/10.1016/j.physletb.2012.11.036 http://www.ScienceDirect.com/ http://www.elsevier.com/locate/physletb mailto:cms-publication-committee-chair@cern.ch http://dx.doi.org/10.1016/j.physletb.2012.11.036 816 CMS Collaboration / Physics Letters B 718 (2013) 815–840 (MC) simulation. Two complementary search strategies are pur- sued, which are optimized for different experimental signatures. The first strategy is a search for a kinematic edge [15] in the dilepton (ee, μμ) mass distribution. This is a characteristic fea- ture of SUSY models in which the same-flavor opposite-sign lep- tons are produced via the decay χ̃0 2 → ��̃ → χ̃0 1 �+�− , where χ̃0 2 is the next-to-lightest neutralino, χ̃0 1 is the lightest neutralino, and �̃ is a slepton. The second strategy is a search for an excess of events with dileptons accompanied by very large hadronic activ- ity and Emiss T . We perform counting experiments in four signal regions with requirements on these quantities to suppress the tt̄ background, and compare the observed yields with the predictions from a background estimation technique based on data control samples, as well as with SM and BSM MC expectations. These two search approaches are complementary, since the dilepton mass edge search is sensitive to new physics models that have lower Emiss T and hadronic energy, while the counting experiments do not assume a specific dilepton production mechanism and are also sen- sitive to BSM scenarios that produce lepton pairs with uncorrelated flavor. No specific BSM physics scenario, e.g. a particular SUSY model, has been used to optimize the search regions. In order to illus- trate the sensitivity of the search, a simplified and practical model of SUSY breaking, the constrained minimal supersymmetric exten- sion of the standard model (CMSSM) [16,17] is used. The CMSSM is described by five parameters: the universal scalar and gaug- ino mass parameters (m0 and m1/2, respectively), the universal trilinear soft SUSY breaking parameter A0, the ratio of the vac- uum expectation values of the two Higgs doublets (tanβ), and the sign of the Higgs mixing parameter μ. Throughout the Let- ter, four CMSSM parameter sets, referred to as LM1, LM3, LM6, and LM13 [18], are used to illustrate possible CMSSM yields. The parameter values defining LM1 (LM3, LM6, LM13) are m0 = 60 (330,85,270) GeV, m1/2 = 250 (240,400,218) GeV, tanβ = 10 (20,10,40), A0 = 0 (0,0,−553) GeV; all four parameter sets have μ > 0. These four scenarios are beyond the exclusion reach of previous searches performed at the Tevatron and LEP, and are chosen here because they produce events containing opposite-sign leptons and may lead to a kinematic edge in the dilepton mass distribution. These four scenarios serve as common benchmarks to facilitate comparisons of sensitivity among different analyses. 2. The CMS detector The central feature of the CMS detector is a superconducting solenoid, 13 m in length and 6 m in diameter, which provides an axial magnetic field of 3.8 T. Within the field volume are several particle detection systems. Charged particle trajectories are mea- sured by silicon pixel and silicon strip trackers covering |η| < 2.5 in pseudorapidity, where η = − ln[tan θ/2] with θ the polar an- gle of the particle trajectory with respect to the counterclockwise proton beam direction. A crystal electromagnetic calorimeter and a brass/scintillator hadron calorimeter surround the tracking vol- ume, providing energy measurements of electrons, photons and hadronic jets. Muons are identified and measured in gas-ionization detectors embedded in the steel return yoke outside the solenoid. The detector is nearly hermetic, allowing energy balance measure- ments in the plane transverse to the beam direction. The first level of the CMS trigger system, composed of custom hardware processors, uses information from the calorimeters and muon de- tectors to select, in less than 1 μs, the most interesting events. The High Level Trigger processor farm further decreases the event rate from around 100 kHz to around 300 Hz, before data stor- age. Event reconstruction is performed with the particle-flow (PF) algorithm [19], which is used to form a mutually exclusive collec- tion of reconstructed particles (muons, electrons, photons, charged and neutral hadrons) by combining tracks and calorimeter clus- ters. A more detailed description of the CMS detector can be found elsewhere [1]. 3. Event selection The following samples of simulated events are used to guide the design of the analysis. These events are generated with ei- ther pythia 6.4.22 [20], MadGraph 4.4.12 [21], or powheg [22] MC event generators using the CTEQ 6.6 parton density functions [23]. The tt̄, W + jets, and VV (V = W,Z) samples are generated with MadGraph, with parton showering simulated by pythia using the Z2 tune [24]. The single-top samples are generated with powheg. The Drell–Yan (DY) sample is generated using a mixture of Mad- Graph (for events with dilepton invariant mass above 50 GeV) and pythia (for events with dilepton invariant mass in the range 10–50 GeV), and includes decays to the ττ final state. The sig- nal events are simulated using pythia. The detector response in these samples is then simulated with a Geant4 model [25] of the CMS detector. The MC events are reconstructed and analyzed with the same software as is used to process collision data. Due to the varying instantaneous LHC luminosity, the mean number of inter- actions in a single beam crossing increased over the course of the data-taking period to a maximum of about 15. In the MC simula- tion, multiple proton–proton interactions are simulated by pythia and superimposed on the hard collision, and the simulated sam- ples are reweighted to describe the distribution of reconstructed primary vertices in data [26]. The simulated sample yields are normalized to an integrated luminosity of 4.98 fb−1 using next- to-leading order (NLO) cross sections. Events in data are selected with a set of ee, eμ, μμ, eτ , and μτ double-lepton triggers. Since the online reconstruction of hadronic-τ decays (τh) is difficult, τh triggers are intrinsically prone to high rates. Therefore, for the analysis with two τh only, we use specialized triggers that rely on significant hadronic ac- tivity HT, quantified by the scalar sum of online jet transverse energies with pT > 40 GeV, and Emiss T as well as the presence of two τh. The efficiencies for events containing two leptons pass- ing the analysis selection to pass at least one of these triggers are measured to be approximately 1.00+0.00 −0.02, 0.95 ± 0.02, 0.90 ± 0.02, 0.80 ± 0.05, 0.80 ± 0.05 and 0.90 ± 0.05 for ee, eμ, μμ, eτh, μτh and τhτh triggers, respectively. In the following, the simu- lated sample yields for the light lepton channels are weighted by these trigger efficiencies. For the τh channels the trigger simulation is applied to the MC simulation and then a correction is applied based on the measured data and MC efficiencies for these triggers. Because leptons produced in the decays of low-mass particles, such as hadrons containing b and c quarks, are nearly always in- side jets, they can be suppressed by requiring the leptons to be iso- lated in space from other particles that carry a substantial amount of transverse momentum. The details of the lepton isolation mea- surement are given in Ref. [14]. In brief, a cone is constructed of size R ≡ √ ( η)2 + ( φ)2 = 0.3 around the lepton momen- tum direction. The lepton relative isolation is then quantified by summing the transverse energy (as measured in the calorimeters) and the transverse momentum (as measured in the silicon tracker) of all objects within this cone, excluding the lepton, and dividing by the lepton transverse momentum. The resulting quantity is re- quired to be less than 0.15, rejecting the large background arising from QCD production of jets. The τh decays are reconstructed with the PF algorithm and identified with the hadrons-plus-strips (HPS) algorithm, which considers candidates with one or three charged pions and up to CMS Collaboration / Physics Letters B 718 (2013) 815–840 817 Table 1 Summary of event preselection requirements applied in the light lepton channels, hadronic-τ channels, and the dilepton mass edge search of Section 4. The leading (trailing) lepton is the one with highest (second highest) pT. The requirements on jet multiplicity, scalar sum of jet transverse energies (HT), missing transverse energy (Emiss T ), and dilepton mass are also indicated. Requirement light leptons hadronic-τ edge search leading lepton e or μ, pT > 20 GeV e, μ, or τh, pT > 20 GeV e or μ, pT > 20 GeV trailing lepton e or μ, pT > 10 GeV e, μ, or τh, pT > 20 GeV e or μ, pT > 10 GeV jet multiplicity njets � 2 njets � 2 njets � 2 HT HT > 100 GeV HT > 100 GeV HT > 300 GeV Emiss T Emiss T > 50 GeV Emiss T > 100 GeV Emiss T > 150 GeV dilepton mass veto 76 < mee, mμμ < 106 GeV – – Table 2 Data yields and MC predictions in the light lepton channels after preselection, using the quoted NLO production cross sections σ . The tt̄ → �+�− contribution corresponds to dilepton tt̄ with no W → τ decays, tt̄ → �±τ∓/τ+τ− refers to dilepton tt̄ with at least one W → τ decay, and tt̄ → �± + jets/hadrons includes all other tt̄ decay modes. The quoted cross sections for these processes include the relevant branching fractions. The LM points are benchmark SUSY scenarios, which are defined in the text. The MC uncertainties include the statistical component, the uncertainty in the integrated luminosity, and the dominant uncertainty from the tt̄ cross-section determination. The data yield is in good agreement with the MC prediction, but the latter is not used explicitly in the search. The difference between the ee + μμ versus eμ yields is due to the rejection of ee and μμ events with an invariant mass consistent with that of the Z boson. Sample σ [pb] ee μμ eμ total tt̄ → �+�− 7 1466 ± 179 1872 ± 228 4262 ± 520 7600 ± 927 tt̄ → �±τ∓/τ+τ− 9 303 ± 37 398 ± 49 889 ± 108 1589 ± 194 tt̄ → �± + jets/hadrons 141 50 ± 6.2 15 ± 1.9 90 ± 11 155 ± 19 DY → �� 16 677 193 ± 11 237 ± 13 312 ± 15 741 ± 26 WW 43 55 ± 1.7 66 ± 1.9 151 ± 3.8 272 ± 6.5 WZ 18 13 ± 0.4 15 ± 0.4 25 ± 0.6 53 ± 1.3 ZZ 5.9 2.6 ± 0.1 3.3 ± 0.1 3.3 ± 0.1 9.1 ± 0.3 Single top 102 95 ± 3.1 120 ± 3.7 278 ± 7.3 492 ± 12 W + jets 96 648 47 ± 11 9.8 ± 4.6 59 ± 12 117 ± 16 Total MC 2224 ± 224 2735 ± 281 6069 ± 643 11 029 ± 1137 Data 2333 2873 6184 11 390 LM1 6.8 272 ± 8.3 342 ± 9.7 166 ± 5.7 780 ± 20 LM3 4.9 107 ± 3.7 125 ± 4.1 181 ± 5.5 413 ± 11 LM6 0.4 20 ± 0.6 23 ± 0.7 26 ± 0.8 69 ± 1.7 LM13 9.8 138 ± 6.6 157 ± 7.0 334 ± 12 629 ± 19 two neutral pions [27]. As part of the τh identification procedure, loose isolation is applied for the τh final states. Isolated electrons and muons can be misidentified as τh candidates. For this reason τh candidates are required to fail electron selections and not to match a muon signature in the muon system. Events with two opposite-sign isolated leptons are selected. At least one of the leptons must have pT > 20 GeV, both must have pT > 10 GeV, and the electrons (muons) must have |η| < 2.5 (|η| < 2.4). Electrons in the range 1.44 < |η| < 1.57 are excluded. In events containing a τh candidate, both leptons must satisfy pT > 20 GeV and |η| < 2.1, where the acceptance requirement is tightened so that the τh decay products are contained in the track- ing detector in a manner that is consistent with the requirements of the triggers used for these events. In events with more than one opposite-sign pair that satisfy the selection requirements, the two oppositely-signed leptons with highest pT are chosen. Events with an ee or μμ pair with invariant mass of the dilepton sys- tem between 76 GeV and 106 GeV or below 12 GeV are removed, in order to suppress Z/γ ∗ → �� events, as well as low-mass dilep- ton resonances. Events containing two electrons, two muons, or an electron and a muon are referred to as the “light lepton channels”, while events with at least one τh are referred to as “hadronic-τ channels”. The PF objects are clustered to form jets using the anti-kT clus- tering algorithm [28] with the distance parameter of 0.5. We apply pT- and η-dependent corrections to account for residual effects of nonuniform detector response, and impose quality criteria to reject jets that are consistent with anomalous detector noise. We require the presence of at least two jets with transverse momentum of pT > 30 GeV and |η| < 3.0, separated by R > 0.4 from leptons passing the analysis selection. For each event the scalar sum of transverse energies of selected jets HT must exceed 100 GeV. The Emiss T is defined as the magnitude of the vector sum of the trans- verse momenta of all PF objects, and we require Emiss T > 50 GeV (Emiss T > 100 GeV) in the light lepton (hadronic-τ ) channels. The event preselection requirements are summarized in Ta- ble 1. The data yields and corresponding MC predictions after this event preselection are given in Table 2 (light leptons) and Table 3 (hadronic-τ ). For the light lepton channels, the normalization of the simulated yields has been scaled based on studies of Z → �� in data and in MC simulation, to account for effects of lepton selec- tion and trigger efficiency and to match the integrated luminosity. As expected, the MC simulation predicts that the sample passing the preselection is dominated by lepton pair final states from tt̄ decays (dilepton tt̄). The data yield is in good agreement with the prediction, within the systematic uncertainties of the integrated lu- minosity (2.2%) and tt̄ cross section determination (12%) [29–31]. The yields for the LM1, LM3, LM6, and LM13 benchmark scenarios are also quoted. 4. Search for a kinematic edge We search for a kinematic edge (end-point) in the dilepton mass distribution for same-flavor (SF) light-lepton events, i.e., ee or μμ lepton pairs. Such an edge is a characteristic feature of, for example, SUSY scenarios in which the opposite-sign leptons are produced via the decay χ̃0 2 → ��̃ → χ̃0 1 �+�− . The model of UED can lead to a similar signature with different intermediate parti- cles. In case of a discovery such a technique offers one of the best 818 CMS Collaboration / Physics Letters B 718 (2013) 815–840 Table 3 Data yields and MC predictions in hadronic-τ channels after preselection, using the quoted NLO production cross sections σ . Diboson backgrounds comprise WW, WZ and ZZ events. The sum of simulated events is also split into events with a gener- ated τh (MC, genuine τh) and events with a misidentified τh (MC, misidentified τh); the two contributions are equally important. The channel with two τh decays is not presented because the trigger is not efficient in the preselection region, due to the large HT requirement. The uncertainty indicated represents both statistical and sys- tematic components. Sample σ [pb] eτh μτh total DY → �� 16 677 51 ± 12 47 ± 11 98 ± 22 tt̄ 157.5 165 ± 47 205 ± 58 370 ± 105 Diboson 66.9 11 ± 2.0 10.8 ± 1.9 22 ± 3.6 Single top 102 7.2 ± 2.6 8.1 ± 2.7 15 ± 4.8∑ MC, genuine τh 146 ± 39 167 ± 44 313 ± 83∑ MC, misidentified τh 89 ± 24 103 ± 27 191 ± 51 Total MC 235 ± 62 271 ± 72 505 ± 134 Data 215 302 517 LM1 6.8 36 ± 6.7 46 ± 6.8 82 ± 9.8 LM3 4.9 28 ± 6.0 18 ± 4.6 46 ± 7.6 LM6 0.4 2.8 ± 1.1 4.2 ± 1.3 7.0 ± 1.7 LM13 9.8 90 ± 11 118 ± 12 208 ± 16 possibilities for model-independent constraints of the SUSY mass parameters [15]. In contrast, for the dominant background tt̄ as well as other SM processes such as WW and DY → ττ , the two lepton flavors are uncorrelated, and the rates for SF and opposite-flavor (OF) eμ lepton pairs are therefore the same. Hence we can search for new physics in the SF final state and model the backgrounds using events in the OF final state. Thus the tt̄ background shape is ex- tracted from events with OF lepton pairs, and a fit is performed to the dilepton mass distribution in events with SF lepton pairs. In order to be sensitive to BSM physics over the full dilep- ton mass spectrum, events with a dilepton invariant mass m�� consistent with that of the Z boson are not rejected. This in- creases the DY contribution, which is compensated by an increase in the Emiss T > 150 GeV requirement (see Table 1). We then pro- ceed to search for a kinematic edge in the signal region defined as HT > 300 GeV. The invariant mass distributions of SF and OF lep- ton pairs are in good agreement with each other (see Fig. 1). A fit is performed to the dilepton mass distribution with three candi- date signal shapes, over a range of values on the position of the kinematic edge. The flavor-uncorrelated background, as a function of the invari- ant mass m�� , is parameterized as: B(m��) = m�� a e−bm�� , (1) where a ≈ 1.4 describes the rising edge and b ≈ 0.002 dominates the long exponential tail on the right hand side of the background shape; these parameters are extracted from the fit to data. We parametrize the signal shape with an edge model for two subsequent two-body decays, according to: S(m��) = 1√ 2πσll mmax∫ 0 dy yαe − (m��−y)2 2σ2 ll . (2) For α = 1 this function describes a triangle convoluted with a Gaussian, which accounts for detector resolution effects. The resolution parameters for electrons σee and muons σμμ are con- strained with simulation. The DY contribution, found to be neg- ligible as seen in Fig. 1, is modelled by a Breit–Wigner function with the mass and width parameters fixed at the Z boson mass and width, convoluted with a Gaussian function to account for the detector resolution. Fig. 1. Distribution of events (black points) and the results of the maximum like- lihood fit (blue curve) to the dilepton mass distribution for events containing eμ lepton pairs (top), and ee and μμ lepton pairs (bottom) in the signal region HT > 300 GeV and Emiss T > 150 GeV, that suppresses DY contributions almost completely. The signal hypothesis for a value of the kinematic edge position mmax = 280 GeV, corresponding to the largest local excess, is displayed. The shaded band represents the shape uncertainty of the background model. (For interpretation of the refer- ences to color in this figure legend, the reader is referred to the web version of this Letter.) We perform a simultaneous, extended, unbinned maximum likelihood fit to the distribution of dilepton mass for events con- taining ee, μμ (signal, DY and background model), and eμ pairs (background model only). The value of the kinematic edge posi- tion mmax is varied, and the fit is performed for each value of this parameter. The shape parameters of the flavor-uncorrelated back- ground that are free in the fit are assumed to be common in all categories, and the yields of signal (nS), DY (nDY) and background (nB) in these three categories are constrained using the ratio of muon to electron selection efficiencies Rμe = 1.11 ± 0.05. This quantity is evaluated using studies of DY events in data and in MC simulation. The fit is performed in the signal region HT > 300 GeV and Emiss T > 150 GeV. The SF events overlaid with the signal plus back- ground fit, and the flavor-uncorrelated shape overlaid with OF events, are shown in Fig. 1. The results of the fit are displayed for a value of the kinematic edge position mmax = 280 GeV, where the CMS Collaboration / Physics Letters B 718 (2013) 815–840 819 largest excess is observed. The local significance is 2.1σ includ- ing statistical and systematic uncertainties. However, a correction for the look-elsewhere effect [32] reduces the global significance to 0.7σ . The extracted signal yield including statistical uncertainty (nS = 11+6.5 −5.7) at this point is consistent with the background-only hypothesis, and we derive a 95% confidence level upper limit of nS < 23 events for this kinematic edge position. No evidence for a kinematic edge feature is observed in the dilepton mass distribu- tion. 5. Counting experiments We next proceed to search for an excess of events containing lepton pairs accompanied by large Emiss T and HT. To look for pos- sible BSM contributions, we define four signal regions that reject all but ∼0.1% of the dilepton tt̄ events, by adding the following requirements: • high-Emiss T signal region: Emiss T > 275 GeV, HT > 300 GeV, • high-HT signal region: Emiss T > 200 GeV, HT > 600 GeV, • tight signal region: Emiss T > 275 GeV, HT > 600 GeV, • low-HT signal region: Emiss T > 275 GeV, 125 < HT < 300 GeV. The signal regions are indicated in Fig. 2. These signal regions are tighter than the one used in Ref. [2] since with the larger data sample the tighter signal regions allow us to explore phase space farther from the core of the SM distributions. The observed and estimated yields in the high-Emiss T , high-HT, and tight signal re- gions are used in the CMSSM exclusion limit in Section 7. The low-HT region has limited sensitivity to CMSSM models that tend to produce low-pT leptons, since the large Emiss T and low HT re- quirements lead to the requirement of large dilepton pT. However, the results of this region are included to extend the sensitivity to other models that produce high-pT leptons. 5.1. Light lepton channels The dominant background in the signal regions is dilepton tt̄ production. This background is estimated using a technique based on data control samples, henceforth referred to as the dilepton transverse momentum (pT(��)) method. This method is based on the fact [33] that in dilepton tt̄ events the pT distributions of the charged leptons (electrons and muons) and neutrinos are related, since each lepton–neutrino pair is produced in the two-body decay of the W boson. This relation depends on the polarization of the W bosons, which is well understood in top quark decays in the SM [34–36], and can therefore be reliably accounted for. In dilepton tt̄ events, the values of pT(��) and the transverse momentum of the dineutrino system (pT(νν)) are approximately uncorrelated on an event-by-event basis. We thus use the observed pT(��) distri- bution to model the pT(νν) distribution, which is identified with Emiss T . Thus, we predict the background in a signal region S defined by requirements on Emiss T and HT using the yield in a region S ′ de- fined by replacing the Emiss T requirement by the same requirement on pT(��). To suppress the DY contamination to the region S ′ , we in- crease the Emiss T requirement to Emiss T > 75 GeV for SF events and subtract off the small residual DY contribution using the Rout/in technique [14] based on control samples in data. This technique derives, from the observed DY yield in the Z mass region, the ex- pected yield in the complementary region using the ratio Rout/in extracted from MC simulation. Two corrections are applied to the resulting prediction, following the same procedure as in Ref. [2]. The first correction accounts for the fact that we apply minimum Fig. 2. Distributions of Emiss T vs. HT for data in the light lepton channels (top) and hadronic-τ channels (bottom). The signal regions are indicated as hatched regions. The solid grey region is excluded at the preselection level. requirements to Emiss T in the preselection but there is no corre- sponding requirement on pT(��). Since the Emiss T and pT(��) are approximately uncorrelated in individual dilepton tt̄ events, the application of the Emiss T requirement decreases the normalization of the pT(��) spectrum without significantly altering the shape. Hence, we apply correction factors K , which are extracted from data as K = 1.6 ± 0.1, 1.6 ± 0.4, 1.6 ± 0.4, and 1.9 ± 0.1 for the high-Emiss T , high-HT, tight, and low-HT signal regions, respectively. The uncertainty in K is dominated by the statistical component. The second correction factor KC accounts for the W polarization in tt̄ events, as well as detector effects such as hadronic energy scale; this correction is extracted from MC and is KC = 1.6 ± 0.5, 1.4 ± 0.2, 1.7 ± 0.4, and 1.0 ± 0.4 for the four respective regions. The uncertainty in KC is dominated by MC sample statistics and by the 7.5% uncertainty in the hadronic energy scale in this analysis. Backgrounds from DY are estimated from data with the Rout/in technique, which leads to an estimated DY contribution consistent with zero. Backgrounds from processes with two vector bosons as well as electroweak single top quark production are negligible compared with those from dilepton tt̄ decays. Backgrounds in which one or both leptons do not originate from electroweak decays (misidentified leptons) are assessed using 820 CMS Collaboration / Physics Letters B 718 (2013) 815–840 the “tight-to-loose” (TL) ratio (RTL) method of Ref. [14]. A misiden- tified lepton is a lepton candidate originating from within a jet, such as a lepton from semi-leptonic b or c decays, a muon from a pion or kaon decay-in-flight, a pion misidentified as an elec- tron, or an unidentified photon conversion. The results of the tight-to-loose ratio method confirm the MC expectation that the misidentified lepton contribution is small compared to the dom- inant backgrounds. Estimates of the contributions to the signal region from QCD multijet events, with two misidentified leptons, and in W + jets, with one misidentified lepton in addition to the lepton from the decay of the W, are derived separately. The contri- butions are found to be less than ∼10% of the total background in the signal regions, which is comparable to the contribution in the control regions used to estimate the background from the pT(��) method. We therefore assign an additional systematic uncertainty of 10% on the background prediction from the pT(��) method due to misidentified leptons. As a validation of the pT(��) method in a region that is dom- inated by background, the pT(��) method is also applied in a control region by restricting HT to be in the range 125–300 GeV. Here the predicted background yield is 95 ± 16 (stat) ± 40 (syst) events with Emiss T > 200 GeV, including the systematic uncertain- ties in the correction factors K and KC , and the observed yield is 59 events. The data are displayed in the plane of Emiss T vs. HT in Fig. 2. The predicted and observed Emiss T distributions are displayed in Fig. 3. A summary of these results is presented in Table 4. The SF and OF observed yields in the signal regions are quoted separately, since many SUSY models lead to enhanced production of SF lepton pairs. For all signal regions, the observed yield is consistent with the predictions from MC and from the background estimate based on data. No evidence for BSM contributions to the signal regions is observed in the light lepton channels. 5.2. Hadronic-τ channels In the hadronic-τ channels the background has two compo- nents of similar importance, events with a genuine lepton pair from dilepton tt̄ production and events from semi-leptonic tt̄ and W + jets production with a misidentified τh. Backgrounds are esti- mated separately with techniques based on data control samples. Other very small contributions from DY and diboson production with genuine lepton pairs (“MC irreducible”) are estimated from simulation. The background with genuine lepton pairs is predicted by ex- tending the pT(��) method. To translate the background prediction in the ee, eμ, and μμ channels into a prediction for the eτh, μτh, and τhτh channels, a third correction factor is used. This correction, Kτ = 0.10 ± 0.01 for all signal regions, is estimated from simu- lation and accounts for the different lepton acceptances (∼0.75), branching fractions (∼0.56), and efficiencies (∼0.24) in hadronic-τ channels. This procedure predicts the yield of the dilepton tt̄ back- ground with genuine hadronic τ decays. The background with a reconstructed τh originating from a misidentified jet or a secondary decay is determined using a tight- to-loose ratio for τh candidates measured in a dijet dominated data sample, defined as HT > 200 GeV and Emiss T < 20 GeV. Tight can- didates are defined as those that pass the full τh selection criteria. For the definition of loose candidates, the HPS isolation criterion is replaced by a looser requirement. The loose isolation requirement removes any HT dependence of the tight-to-loose ratio; thus the measurement can be extrapolated to the signal regions. To determine the number of expected events including jets misidentified as τh candidates in the signal region, the identifica- tion requirements for one τh are loosened. The obtained yields are Fig. 3. The observed Emiss T distributions (red points) and Emiss T distributions pre- dicted by the pT(��) method (blue points with shaded uncertainty bands) in data for the region 125 < HT < 300 GeV (top), HT > 300 GeV (middle), and HT > 600 GeV (bottom). The uncertainty bands on the predicted Emiss T distribution are statistical, and also include systematic uncertainties for points in the signal regions, to the right of the vertical dashed line. The ratio of data to predicted background is also included. The error bars include the full uncertainties on the data and predicted background. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.) CMS Collaboration / Physics Letters B 718 (2013) 815–840 821 Table 4 Summary of results in the light lepton channels. The total SM MC expected yields (MC prediction), observed same-flavor (SF), opposite-flavor (OF), and total yields in the signal regions are indicated, as well as the predicted yields from the pT(��) estimate. The expected contributions from three benchmark SUSY scenarios are also quoted. The first uncertainty on the pT(��) method prediction is statistical and the second is systematic; the systematic uncertainty is discussed in the text. The non-SM yield upper limit (UL) is a 95% CL upper limit on the signal contribution. high Emiss T high HT tight low HT MC prediction 30 ± 1.2 31 ± 0.9 12 ± 0.6 4.2 ± 0.3 SF yield 15 11 6 3 OF yield 15 18 5 3 Total yield 30 29 11 6 pT(��) prediction 21 ± 8.9 ± 8.0 22 ± 7.5 ± 6.9 11 ± 5.8 ± 3.8 12 ± 4.9 ± 5.7 Observed UL 26 23 11 6.5 Expected UL 21 19 11 8.6 LM1 221 ± 5.1 170 ± 4.5 106 ± 3.5 6.2 ± 0.9 LM3 79 ± 2.4 83 ± 2.5 44 ± 1.8 2.3 ± 0.4 LM6 35 ± 0.6 33 ± 0.5 26 ± 0.5 0.6 ± 0.1 LM13 133 ± 5.5 113 ± 5.2 65 ± 3.9 4.1 ± 0.9 Table 5 Summary of the observed and predicted yields in the four signal regions for hadronic-τ channels. The first indicated error is statistical and the second is systematic; the systematic uncertainties on the RTL ratio and pT(��) method predictions are discussed in the text. The non-SM yield upper limit is a 95% CL upper limit on the signal contribution in each signal region. high Emiss T high HT tight low HT∑ MC, genuine τh 5.8 ± 2.3 3.7 ± 1.6 2.0 ± 1.2 0.4 ± 0.2∑ MC, misidentified τh 1.4 ± 0.5 2.8 ± 1.3 0.2 ± 0.1 0.2 ± 0.1 Total MC 7.1 ± 2.5 6.5 ± 2.3 2.2 ± 1.2 0.7 ± 0.3 pT(��) prediction 2.1 ± 0.9 ± 0.8 2.2 ± 0.8 ± 0.9 1.1 ± 0.6 ± 0.4 1.2 ± 0.5 ± 0.4 RTL prediction 5.1 ± 1.7 ± 0.8 3.6 ± 1.4 ± 0.5 2.7 ± 1.3 ± 0.4 < 0.9@95%CL MC irreducible 1.3 ± 0.5 ± 0.2 0.7 ± 0.3 ± 0.1 0.2 ± 0.1 ± 0.1 0.1 ± 0.1 ± 0.1 ∑ predictions 8.5 ± 2.0 ± 1.1 6.5 ± 1.6 ± 1.0 4.0 ± 1.4 ± 0.6 1.3 ± 0.5 ± 0.5 Total yield 8 5 1 0 Observed UL 7.9 6.2 3.7 3.1 Expected UL 8.1 7.2 5.7 3.9 LM1 32 ± 11 14 ± 6.1 8.1 ± 4.2 – LM3 11 ± 4.2 11 ± 5.1 8.0 ± 4.9 – LM6 4.5 ± 1.5 5.1 ± 1.6 4.2 ± 1.6 0.4 ± 0.4 LM13 69 ± 17 52 ± 8.2 39 ± 9.8 – multiplied by the probability PTL that a misidentified τh candidate passes the tight τh selection: PTL(pT, η) = RTL(pT, η) 1 − RTL(pT, η) . A summation over PTL evaluated for all τh candidates that pass the loose selection but not the tight selection gives the final back- ground prediction in each signal region. The method is validated in tt̄ simulation, where the agree- ment between the predicted and true yields is within 15%. We correct for a 5% bias observed in the simulation, and assign a 15% systematic uncertainty on the background prediction from the tight-to-loose ratio based on the agreement between prediction and observation in simulation and additional control samples in data. The results in the four signal regions are summarized in Ta- ble 5. The low-HT region includes only eτh and μτh channels, because the τhτh trigger is inefficient in this region. In the high- Emiss T region the τhτh trigger is not fully efficient and an efficiency correction of 3% is applied to MC simulation. Good agreement be- tween predicted and observed yields is observed. No evidence for BSM physics is observed in the hadronic-τ channels. The results of observed yields and predicted backgrounds in all signal regions for different lepton categories are summarized in Fig. 4. Fig. 4. Summary of the background predictions from tight-to-loose ratio, pT(��)- method and MC, and observed yields in the signal regions. 6. Acceptance and efficiency systematic uncertainties The acceptance and efficiency, as well as the systematic uncer- tainties in these quantities, depend on the process. For some of the 822 CMS Collaboration / Physics Letters B 718 (2013) 815–840 Table 6 Summary of the relative uncertainties in the signal efficiency due to the jet and Emiss T scale, for the four benchmark SUSY scenarios in the signal regions used for the counting experiments of Section 5. Signal model high Emiss T high HT tight low HT LM1 22% 33% 40% 19% LM3 26% 34% 42% 18% LM6 11% 15% 19% 10% LM13 26% 31% 40% 14% individual uncertainties, it is reasonable to quote values based on SM control samples with kinematic properties similar to the SUSY benchmark models. For others that depend strongly on the kine- matic properties of the event, the systematic uncertainties must be quoted model-by-model. The systematic uncertainty in the lepton acceptance consists of two parts: the trigger efficiency uncertainty, and the identi- fication and isolation uncertainty. The trigger efficiency for two leptons of pT > 10 GeV, with one lepton of pT > 20 GeV is mea- sured using samples of Z → ��, with an uncertainty of 2%. The simulated events reproduce the lepton identification and isolation efficiencies measured in data using samples of Z → �� within 2% for lepton pT > 15 GeV and within 7% (5%) for electrons (muons) in the range pT = 10–15 GeV. The uncertainty of the trigger effi- ciency (5%) of the τh triggers is estimated with the tag-and-probe method [37]. The τh identification efficiency uncertainty is esti- mated to be 6% from an independent study using a tag-and-probe technique on Z → ττ events. This is further validated by obtaining a Z → ττ enhanced region showing consistency between simula- tion and data. Another significant source of systematic uncertainty is associated with the jet and Emiss T energy scale. The impact of this uncertainty is final-state dependent. Final states characterized by very large hadronic activity and Emiss T are less sensitive than final states where the Emiss T and HT are typically close to the minimum requirements applied to these quantities. To be more quantitative, we have used the method of Ref. [14] to evaluate the systematic uncertainties in the acceptance for three bench- mark SUSY points. The energies of jets in this analysis are known to within 7.5%; the correction accounting for the small difference between the hadronic energy scales in data and MC is not ap- plied [38]. The uncertainty on the LM1 signal efficiency in the region HT > 300 GeV, Emiss T > 150 GeV used to search for the kinematic edge is 6%. The uncertainties for the four benchmark SUSY sce- narios in the signal regions used for the counting experiments of Section 5 are displayed in Table 6. The uncertainty in the inte- grated luminosity is 2.2%. 7. Limits on new physics 7.1. Search for a kinematic edge An upper limit on the signal yield is extracted from the fit to the dilepton mass distribution, assuming the triangular shape (α = 1) of Eq. (2). The 95% CL upper limit is extracted using a hy- brid frequentist–bayesian CLS method [39], including uncertainties in the background model, resolution model and Z -boson yield. We scan the position of the kinematic edge mmax and extract a signal yield upper limit for each value, as shown in Fig. 5. The extracted upper limits on nS vary in the range 5–30 events; these upper lim- its do not depend strongly on the choice of signal shape parameter when using two different shapes specified by a concave (α = 4) and convex curvature (hatched band). Fig. 5. A CLS 95% CL upper limit on the signal yield nS as a function of the end- point in the invariant mass spectrum mmax assuming a triangular shaped signal (black dots and thick line). The hatched band shows the variation of the expected limit (thin line) assuming two alternate signal shapes, with the alternative expected limits corresponding to the boundary of the hatched band. The SUSY benchmark scenarios LM1, LM3 and LM6 are shown with their expected yields and theoretical positions of the corresponding kinematic dilepton mass edges. The LM1 (LM3) yield is scaled to 20% (40%) of its nominal yield. At LM3 and LM6 a three-body decay is present; thus the shape of the kinematic edge is only approximately triangular. Table 7 Summary of results in the light lepton channels used for the CMSSM exclusion of Section 7. Details are the same as in Table 4 except that these results are divided into three non-overlapping regions defined by Emiss T > 275 GeV, HT 300–600 GeV (SR1), Emiss T > 275 GeV, HT > 600 GeV (SR2, same as the “tight” signal region), and Emiss T 200–275 GeV, HT > 600 GeV (SR3). The regions are further divided between same-flavor (SF) and opposite-flavor (OF) lepton pairs. SR1 SR2 SR3 SF yield 9 6 5 OF yield 10 5 13 pT(��) prediction 5.7 ± 5.1 ± 2.8 5.3 ± 4.1 ± 1.9 5.6 ± 3.4 ± 2.1 7.2. Search for an excess of events with large Emiss T and HT In this section we use the results of the search for events with light leptons accompanied by large Emiss T and HT reported in Sec- tion 5 to exclude a region of the CMSSM parameter space. The ex- clusion is performed using multiple, exclusive signal regions based on the high-Emiss T , high-HT, and tight signal regions, divided into three non-overlapping regions in the Emiss T vs. HT plane. The re- sults are further divided between the SF and OF final states in order to improve the sensitivity to models with correlated dilep- ton production leading to an excess of SF events, yielding a total of six signal bins, as summarized in Table 7. The use of multiple, disjoint signal regions improves the sensitivity of this analysis to a specific BSM scenario. The predicted backgrounds in the SF and OF final states are both equal to half of the total predicted back- ground, because the tt̄ events produce equal SF and OF yields. The inputs to the upper limit calculation are the expected background yields and uncertainties from the pT(��) method, the expected sig- nal yields and uncertainties from MC simulation, and the observed data yields in these six regions. The exclusion is performed with the CLS method. In the presence of a signal, the pT(��) background estimate increases due to signal events populating the control re- gions. To correct for this effect, for each point in the CMSSM pa- rameter space this expected increase is subtracted from the signal yields in our search regions. CMS Collaboration / Physics Letters B 718 (2013) 815–840 823 Fig. 6. The observed 95% CL exclusion contour (solid thick red line), the expected exclusion contour (solid thin blue line), the variation in the observed exclusion from the variation of PDF, renormalization and factorization scales, and αS theoretical uncertainties (dashed red lines), the ±1σ uncertainty in the median expected exclusion (dotted blue lines), and the observed exclusion contour based on 34 pb−1 2010 data in the opposite-sign dilepton channel (dark blue shaded region), in the CMSSM (m0,m1/2) plane for tanβ = 10, A0 = 0 GeV and μ > 0. The area below the red curve is excluded by this search. Exclusion limits obtained from the LEP experiments are presented as shaded areas in the plot. The thin grey lines correspond to constant squark and gluino masses. The LM benchmark SUSY scenarios are also indicated. The LM3 and LM13 benchmark scenarios have values of tan β and/or A0 that differ from 10 and 0 GeV, respectively, but both are also excluded by the results of this search; see the text of Section 1 for the full definitions of these scenarios. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this Letter.) The SUSY particle spectrum is calculated using SoftSUSY [40], and the signal events are generated at leading order (LO) with pythia 6.4.22. We use NLO cross sections, obtained with the pro- gram prospino [41]. Experimental uncertainties from luminos- ity, trigger efficiency, and lepton selection efficiency are constant across the CMSSM plane, while the uncertainty from the hadronic energy scale is assessed separately at each CMSSM point taking into account the bin-to-bin migration of signal events. The vari- ation in the observed and expected limits due to the theoreti- cal uncertainties, including renormalization and factorization scale, parton density functions (PDFs), and the strong coupling strength αS [42], are indicated in Fig. 6 as separate exclusion contours. These results significantly extend the sensitivity of our previous results [2]. The LEP-excluded regions are also indicated; these are based on searches for sleptons and charginos [43]. 8. Additional information for model testing Other models of new physics in the dilepton final state can be constrained in an approximate way by simple generator-level stud- ies that compare the expected number of events in the data sam- ple corresponding to an integrated luminosity of 4.98 fb−1 with the upper limits from Section 7. The key ingredients of such stud- ies are the kinematic requirements described in this Letter, the lepton efficiencies, and the detector responses for HT and Emiss T . The trigger efficiencies for events containing ee, eμ or μμ lepton pairs are 100%, 95%, and 90%, respectively. For eτh, and μτh the efficiency is ∼80% [37]. The trigger used for τhτh final states has an efficiency of 90%. We evaluate the light lepton, hadronic-τ , Emiss T , and HT selec- tion efficiencies using the LM6 benchmark model, but these effi- ciencies do not depend strongly on the choice of model. Jets at the generator-level are approximated as quarks or gluons produced prior to the parton showering step satisfying pT > 30 GeV and |η| < 3. Generator-level leptons are required to satisfy pT > 10 GeV and |η| < 2.5 and not to overlap with a generator-level jet within R < 0.4. For generator level τh the visible decay products are re- quired to satisfy the tighter pT > 20 GeV and |η| < 2.1 selection. The generator-level Emiss T is the absolute value of the vector sum of the transverse momenta of invisible particles, e.g., neutrinos and lightest supersymmetric particles. The lepton selection efficiencies as a function of generator-level pT are displayed in Fig. 7. The effi- ciency dependence can be parameterized as a function of pT as f (pT) = ε∞ { erf [ (pT − C)/σ ]} + εC { 1 − erf [ (pT − C)/σ ]} , (3) where erf indicates the error function, ε∞ gives the value of the efficiency plateau at high momenta, C is equal to 10 GeV, εC gives the value of the efficiency at pT = C , and σ describes how fast the transition is. The parameterization is summarized in Table 8 for electrons, muons, and taus. The Emiss T and HT selection efficiencies are displayed in Fig. 8 as a function of the generator-level quantities. These efficiencies are parameterized using the function: f (x) = ε∞ 2 ( erf ( (x − C)/σ ) + 1 ) , (4) where ε∞ gives the value of the efficiency plateau at high x, C is the value of x at which the efficiency is equal to 50%, and σ describes how fast the transition is. The values of the fitted pa- rameters are quoted in Table 9. This efficiency model has been validated by comparing the yields from the full reconstruction with the expected yields using generator-level information only and the efficiencies quoted above. In addition to the LM1, LM3, LM6 and LM13 benchmarks con- sidered throughout this Letter, we have tested several additional benchmarks (LM2, LM4, LM5, LM7, and LM8) [18]. In general we observe agreement between full reconstruction and the efficiency model within approximately 15%. 824 CMS Collaboration / Physics Letters B 718 (2013) 815–840 Fig. 7. The efficiency to pass the light lepton (top), and hadronic-τ (bottom) selec- tion as a function of the generator-level pT (visible τh pT). These efficiencies are calculated using the LM6 MC benchmark. Table 8 Values of the fitted parameters in Eq. (3) for the lepton selection efficiencies of Fig. 7. Parameter e μ τh C 10 GeV 10 GeV 10 GeV ε∞ 0.78 0.89 0.44 εC 0.34 0.62 0.31 σ 18 GeV 30 GeV 13 GeV 9. Summary We have presented a search for physics beyond the stan- dard model in the opposite-sign dilepton final state using a data sample of proton–proton collisions at a center-of-mass energy of 7 TeV. The data sample corresponds to an integrated luminosity of 4.98 fb−1, and was collected with the CMS detector in 2011. Two complementary search strategies have been performed. The first focuses on models with a specific dilepton production mechanism leading to a characteristic kinematic edge in the dilepton mass distribution, and the second focuses on dilepton events accompa- nied by large missing transverse energy and significant hadronic activity. This work is motivated by many models of BSM physics, Fig. 8. The efficiency to pass the signal region Emiss T (top), and HT (bottom) require- ments as a function of the generator-level quantities. The vertical lines represent the requirements applied to the reconstruction-level quantities. These efficiencies are calculated using the LM6 MC benchmark, but they do not depend strongly on the underlying physics. Table 9 Values of the fitted parameters in Eq. (4) for the Emiss T and HT selection efficiencies of Fig. 8. Parameter Emiss T > 150 GeV Emiss T > 200 GeV Emiss T > 275 GeV ε∞ 1.00 1.00 1.00 C 157 GeV 211 GeV 291 GeV σ 33 GeV 37 GeV 39 GeV Parameter HT > 125 GeV HT > 300 GeV HT > 600 GeV ε∞ 1.00 1.00 0.99 C 124 GeV 283 GeV 582 GeV σ 56 GeV 75 GeV 93 GeV such as supersymmetric models or models with universal extra dimensions. In the absence of evidence for BSM physics, we set upper limits on the BSM contributions to yields in the signal re- gions. Additional information has been provided to allow testing whether specific models of new physics are excluded by these re- sults. The presented result is the most stringent limit to date from the opposite-sign dilepton final state accompanied by large miss- ing transverse energy and hadronic activity. CMS Collaboration / Physics Letters B 718 (2013) 815–840 825 Acknowledgements We congratulate our colleagues in the CERN accelerator depart- ments for the excellent performance of the LHC machine. We thank the technical and administrative staff at CERN and other CMS institutes, and acknowledge support from: FMSR (Austria); FNRS and FWO (Belgium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MoST, and NSFC (China); COLCIENCIAS (Colombia); MSES (Croatia); RPF (Cyprus); MoER, SF0690030s09 and ERDF (Estonia); Academy of Finland, MEC, and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NKTH (Hungary); DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Korea); LAS (Lithuania); CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mexico); MSI (New Zealand); PAEC (Pakistan); MSHE and NSC (Poland); FCT (Portugal); JINR (Armenia, Belarus, Georgia, Ukraine, Uzbekistan); MON, RosAtom, RAS and RFBR (Russia); MSTD (Serbia); MICINN and CPAN (Spain); Swiss Funding Agencies (Switzerland); NSC (Taipei); TUBITAK and TAEK (Turkey); STFC (United Kingdom); DOE and NSF (USA). Individuals have received support from the Marie- Curie programme and the European Research Council (European Union); the Leventis Foundation; the A.P. Sloan Foundation; the Alexander von Humboldt Foundation; the Belgian Federal Science Policy Office; the Fonds pour la Formation à la Recherche dans l’In- dustrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technologie (IWT-Belgium); the Council of Science and Industrial Research, India; and the HOM- ING PLUS programme of Foundation for Polish Science, cofinanced from European Union, Regional Development Fund. Open access This article is published Open Access at sciencedirect.com. It is distributed under the terms of the Creative Commons Attribu- tion License 3.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are credited. References [1] S. 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Pimiä, D. Piparo, G. Polese, L. Quertenmont, A. Racz, W. Reece, J. Rodrigues Antunes, G. Rolandi 33, T. Rommerskirchen, C. Rovelli 34, M. Rovere, H. Sakulin, F. Santanastasio, C. Schäfer, C. Schwick, I. Segoni, S. Sekmen, A. Sharma, P. Siegrist, P. Silva, M. Simon, P. Sphicas ∗,35, D. Spiga, M. Spiropulu 3, M. Stoye, A. Tsirou, G.I. Veres 18, J.R. Vlimant, H.K. Wöhri, S.D. Worm 36, W.D. Zeuner CERN, European Organization for Nuclear Research, Geneva, Switzerland 834 CMS Collaboration / Physics Letters B 718 (2013) 815–840 W. Bertl, K. Deiters, W. Erdmann, K. Gabathuler, R. Horisberger, Q. Ingram, H.C. Kaestli, S. König, D. Kotlinski, U. Langenegger, F. Meier, D. Renker, T. Rohe, J. Sibille 37 Paul Scherrer Institut, Villigen, Switzerland L. Bäni, P. Bortignon, M.A. Buchmann, B. Casal, N. Chanon, Z. Chen, A. Deisher, G. Dissertori, M. Dittmar, M. Dünser, J. Eugster, K. Freudenreich, C. Grab, D. Hits, P. Lecomte, W. Lustermann, A.C. Marini, P. Martinez Ruiz del Arbol, N. Mohr, F. 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