CMS PAS EXO-10-018

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2011/04/13Head Id: 35888Archive Id: 23779:23784MArchive Date: 2011/02/01Archive Tag: trunk

Search for a Heavy Bottom-like Quark in pp Collisions at√s = 7 TeV

The CMS Collaboration

Abstract

A search for pair-produced bottom-like quarks in pp collisions at√

s = 7 TeV is con-ducted with the CMS experiment at the LHC. The decay b′ → tW is considered inthis search. The b′b′ → tW−tW+ process can be identified by the distinctive signa-ture of trileptons and same-sign dileptons. With a data sample corresponding to anintegrated luminosity of 34 pb−1, no excess above the standard model backgroundpredictions is observed and a b′ quark with a mass between 255 and 361 GeV/c2 isexcluded at the 95% confidence level.

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PDFAuthor: Yeong-Jyi Lei, Kai-Feng ChenPDFTitle: Search for b’ to tW in pp collisions at sqrt“(s“) = 7 TeVPDFSubject: CMSPDFKeywords: CMS, physics, exotica

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The standard model with three generations of quarks describes remarkably well almost all par-1ticle physics phenomena observed to date. Although adding a fourth generation of massive2fermions is an obvious extension of the model, it became less popular when limits were ob-3tained on the number of light neutrino flavours [1–5]. In addition, precise measurements of4the electroweak parameters disfavour such a possibility [6, 7]. Recently, however, there has5been renewed interest in the fourth generation [8–12], since the electroweak parameters only6exclude a degenerate fourth generation. Indirect bounds on the Higgs boson mass can be re-7laxed [13, 14], and an additional generation of quarks may possess enough intrinsic matter and8anti-matter asymmetry to be relevant for the baryon asymmetry of the Universe [15].9

A search for a heavy bottom-like quark (b′) is presented in pp collisions at a centre-of-mass10energy of 7 TeV with the Compact Muon Solenoid (CMS) detector at the large Hadron Collider11(LHC). The decay chain b′b′ → tW−tW+ → bW+W−bW−W+ is expected to be dominant12if the mass of the b′ quark (Mb′) is larger than the sum of top-quark and W-boson masses13and the coupling between b′ and top preponderates over other quarks [16]. A b′ mass below14this threshold is disfavoured by results from several previous experiments [17]. As each W15boson can decay leptonically into eν or µν in 22% of the cases, the full decay chain may lead16to distinctive signatures with two isolated leptons with the same electric charge (same-sign17dileptons) or three isolated leptons in the final state, which covers 7.3% of the total decays18and is expected to happen very rarely in the standard model. A similar search in these decay19channels has been carried out by the CDF experiment [17], setting a lower limit of 338 GeV/c220at the 95% confidence level (CL) on the mass of b′ quark.121

The central feature of the CMS detector is a large-solid-angle magnetic spectrometer, with an22axial magnetic field of 3.8 T provided by a superconducting solenoid. Charged particle tra-23jectories are measured by a silicon pixel detector and strip tracker. A lead tungstate crystal24electromagnetic calorimeter (ECAL), with a lead-silicon preshower detector in the end-caps,25and a brass/scintillator hadron calorimeter (HCAL) are placed outside of the tracker, which al-26together provide high resolution measurements for electrons/photons and hadronic jets. The27hermetic design of the detector allows good measurement of missing transverse energy. Muons28are detected by the tracker and a gas-ionization detector embedded in the steel magnetic field29return yoke. A detailed description of the CMS detector can be found in Ref. [19].30

This analysis is based on a data sample corresponding to an integrated luminosity of 34 pb−131recorded during 2010 run. A two-level trigger system [20] selects events for further analysis.32The events in this search are selected by requiring the presence of at least two electrons or at33least one muon in the trigger. Given the trigger efficiencies measured in data for single-trigger34objects, the trigger efficiency for the final state in the kinematic region of the off-line analysis35has been determined to be more than 99% from simulation studies.36

Candidate muons are reconstructed with a global fit of trajectories using hits in the tracker37and the muon system. Muons are required to have transverse momenta pT > 20 GeV/c and38|η| < 2.4, where η is the pseudorapidity, defined as η = − ln[tan θ/2] and θ is the polar angle39relative to the counterclockwise proton beam direction as measured from the nominal interac-40tion vertex. As discussed in Ref. [21], the muon candidate must be associated with hits in the41silicon strip and the pixel detector, the segments in the muon chamber, and have a high-quality42global fit to the track trajectory. The efficiency for these muon selection criteria is 99% or higher.43In addition, the track is required to be consistent with originating from the primary interaction44vertex.45

1A new analysis from CDF, in the lepton plus multijet channel, sets the b′ quark mass limit at 372 GeV/c2 [18].

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Reconstruction of electron candidates starts from clusters of energy deposits in the ECAL,46which are then matched to hits in the silicon tracker. Electron candidates are required to have47pT > 20 GeV/c. Candidates are required to be reconstructed in the fiducial volume of the bar-48rel (|η| < 1.478) or in the end-caps (1.55 < |η| < 2.4). The electron candidate track is required49to be consistent with originating from the interaction vertex. Electrons are identified using50variables which include the ratio between the energy deposited in the HCAL and the ECAL,51the shower width in η, and the distance between the calorimeter shower and the particle tra-52jectory in the tracker, measured in both η and azimuthal angle (φ). The selection criteria are53optimized [22] to reject the background from hadronic jets while maintaining an efficiency of5485% for the electrons from W or Z decays.55

Electrons and muons from W → `ν (` = e, µ) decays are expected to be isolated from other56particles in the detector. A cone of ∆R < 0.3, where ∆R =

√(∆η)2 + (∆φ)2, is constructed57

around the lepton candidate direction. The scalar sum of the track transverse momenta and58calorimeter energy deposits inside the cone projected onto the transverse plane is calculated,59excluding contributions from the lepton candidate. A barrel (end-cap) electron candidate is60rejected if this scalar sum exceeds 9% (6%) of the candidate pT, while the scalar sum for a61muon candidate is not allowed to exceed 20%. Electron candidates are further required to be62separated from the selected muon candidates; any electron candidate within a ∆R < 0.1 cone63of a muon candidate is rejected to remove misidentified electrons due to muon bremsstrahlung.64Electron candidates which are identified as coming from photon conversions are also rejected.65

Hadronic jets are clustered from the particles reconstructed with a use of all CMS sub-detectors66by the particle-flow global event reconstruction described in Ref. [23–26], with the anti-kT al-67gorithm [27]. The energy calibration [28] is performed separately for each particle type, the68resulting jet energies require only a small correction accounting for thresholds and residual69inefficiencies. Jet candidates are required to have a minimum pT of 25 GeV/c and |η| < 2.4.70Neutrinos from W boson decays escape the detector and thus produce a significant energy im-71balance in the detector. An important quantity is the missing transverse energy, E/T, which72describes the imbalance of detected energy perpendicular to the beam direction. It is deter-73mined as the vectorial sum of the transverse momenta of all particles reconstructed by the74particle-flow algorithm [25, 29].75

Events are required to have at least one well reconstructed interaction vertex [30]. Events76with two same-sign leptons or with three leptons (with two of them are oppositely charged)77are selected. Events with fewer than four (two) jets are rejected for the same-sign dilepton78(trilepton) channel. In addition, events with an oppositely-charged muon or electron pair with79|M`+`− − MZ| < 10 GeV/c2 are rejected in order to suppress the background from Z decays.80The backgrounds due to charge misidentification are substantially larger for electrons than81muons, thus events with same-sign electron pair with |Me±e± −MZ| < 10 GeV/c2 are also dis-82carded. For each event, the scalar quantity ST = ∑ pT(jets) +∑ pT(leptons) + E/T is determined83and a minimum ST of 350 GeV is required.84

Selection efficiencies for signal events are estimated using samples simulated with the MAD-85GRAPH/MADEVENT generator (v4.4.26) [31] with up to two additional partons in the hard86interactions. Two additional quarks are implemented as a straightforward extension to the87standard model configuration of the generator. The events are subsequently processed with88PYTHIA (v6.420) [32] to provide parton showering and hadronization of the particles, and then89passed through a simulation of the CMS detector based on GEANT4 [33]. The signal efficiency90varies from 3.1 to 4.6% for b′ masses between 300 and 500 GeV/c2. These efficiencies include91the W decay branching fractions. The jet multiplicities for the trilepton and same-sign dilepton92

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Figure 1: Jet multiplicity distributions for the same-sign dilepton channel (left), and the trilep-ton channel (right). The star in the right plot represents the single measured event, which failsto satisfy the requirement on jet multiplicity. The open histogram is the signal contribution ex-pected from a b′ with Mb′ = 400 GeV/c2. The light blue and dark green filled histograms showthe contributions from tt + jets and tt + W(Z) + jets respectively. The shaded histogram repre-sents electroweak processes (W(Z) + jets, dibosons). All selections are applied except the onecorresponding to the plotted variable. The vertical dotted lines indicate the minimum numbersof jets required in events selected for each of the channels.

channels are shown in Fig. 1. The distributions of dilepton invariant mass M`` and ST are pre-93sented in Fig. 2. The expected distributions of the b′ signal are normalized with the production94cross section calculated at the next-to-leading order (NLO) in αs [34], for a b′ with 400 GeV/c295mass.96

The search is performed in the following sequence. Background sources are examined with97simulated samples. Data-driven methods are used to estimate the contributions of the relevant98sources from data. The background estimation methods are validated on simulated samples.99Finally, the observed yield in data, the background derived from data, signal efficiencies, and100the integrated luminosity are converted to a limit on the pp→ b′b′ production cross section.101

The expected yields and efficiencies for signal and background from simulations are summa-102rized in Table 1. The background contributions from pp→ tt+ jets and W/Z+ jets are normal-103ized to the CMS measured inclusive pp → tt, W, and Z cross sections [35, 36]. The simulated104samples for pp → tt + jets and W/Z + jets processes include initial state b and c quarks in105the hard interactions. Production of dibosons is estimated with NLO cross sections given by106MCFM [37]. The tt + W/Z and same-sign WW + jj processes are calculated using the MAD-107GRAPH generator at leading order (LO) in αs. The total background yield is estimated to be1080.33. The dominant background contribution comes from pp → tt + jets events; contributions109from other processes are very small.110

For the same-sign dilepton channel, there are two types of tt background: single-lepton tt111events with an extra misidentified or non-isolated lepton, or dilepton tt events with a charge-112misidentified electron. Background yields are re-estimated from data as follows.113

Leptons chosen with relaxed selection criteria are denoted as “loose” muon or “loose” electron.114Leptons chosen with the full selection criteria defined above are denoted as “tight” muons115and “tight” electrons. The background events with a misidentified or non-isolated lepton are116

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Figure 2: The invariant mass distribution (left) of two muons with opposite charges or electronsof any charge, M(``), and the ST distribution (right) including same-sign dilepton and trileptonchannels. The star in the right plot represents the measured event, which fails to satisfy therequirement on ST. The open histogram is the signal contribution expected from a b′ withMb′ = 400 GeV/c2. The light blue and dark green filled histograms show the contributionsfrom tt + jets and tt + W(Z) + jets respectively. The shaded histogram represents electroweakprocesses (W(Z) + jets, dibosons). All selections are applied except the one corresponding tothe plotted variable. Events with an electron pair or an opposite sign muon pair, with M(``)falling in the region defined by the vertical dotted lines on the left plot, are rejected in order tosuppress the background from Z events. The vertical dotted line in the right plot indicates thelower ST threshold used in the analysis.

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Table 1: Summary of expected signal and background production cross sections, selection effi-ciencies e, expected yields in simulations, and the observed event yield in data. The cross sec-tions are obtained from leading order predictions, next-to-leading order predictions, or CMSmeasurements.

Process Cross section e [%] Yieldb′b′, Mb′ = 300 GeV/c2 7.29 pb (NLO) 3.08 7.7b′b′, Mb′ = 350 GeV/c2 2.94 pb (NLO) 3.75 3.8b′b′, Mb′ = 400 GeV/c2 1.30 pb (NLO) 3.99 1.8b′b′, Mb′ = 450 GeV/c2 0.617 pb (NLO) 4.34 0.91b′b′, Mb′ = 500 GeV/c2 0.310 pb (NLO) 4.58 0.49tt + jets 1.9× 102 pb (CMS) 4.1× 10−3 0.27tt + W + jets 0.144 pb (LO) 0.67 0.033tt + Z + jets 0.094 pb (LO) 0.50 0.016W + jets 3.0× 104 pb (CMS) < 1.0× 10−5 < 0.11Z + jets 2.9× 103 pb (CMS) < 9.2× 10−5 < 0.09WW 43 pb (NLO) < 8.2× 10−4 < 0.012WZ 18 pb (NLO) < 8.1× 10−4 < 0.005ZZ 5.9 pb (NLO) 3.0× 10−3 0.006Same-sign WW + jj 0.15 pb (LO) 3.9× 10−2 0.002Background sum - - 0.33Data-driven background yield - - 0.32Observed yield in data - - 0

estimated using a control region with one tight lepton and one loose lepton, with the rest of the117selection criteria exactly the same as for signal. The background contribution is calculated from118the yields observed in the control regions multiplied by the ratios of the number of electrons or119muons passing tight and loose cuts. These ratios are determined from data by taking the ratios120between the number of events in the control region with two loose leptons, and the control121region with one loose plus one tight lepton. The background contribution from electron charge122misidentification is determined from control regions with oppositely-charged electron pairs or123from e-µ events. The charge misidentification rate (0.6± 0.1%) is determined by measuring the124Z boson events reconstructed using two electron candidates with the same electric charge, and125is normalized to the yield of Z→ e+e− events.126

For the trilepton channel, the background yield in the signal region is estimated using a control127region with the same criteria as for the signal, but requiring only two leptons with opposite128charges. The normalization between the background in the signal region and the background129in the control region is determined from simulations.130

The background yield in the signal region, including both trilepton and same-sign dilepton131channels, is estimated from the data-based technique to be 0.32. In order to validate the proce-132dure, and to assign a systematic uncertainty, the same study is repeated on a weighted mixture133of fully simulated samples representing the potential background. The weight for each physics134process in the simulated events is derived from the cross sections, as listed in Table 1. Applying135this estimation procedure to the samples of simulated events gives an estimated background136of 0.21 events. This has to be compared to the expectation of 0.33 events obtained by counting137directly the total number of simulated background events satisfying the signal selection. The138difference between these two yields is conservatively included in the systematic uncertainties.139

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The background estimation procedure for trilepton events is assumed to have a systematic un-140certainty of 100% on the normalization ratio, which was taken in that case from the simulation.141The sum of these two uncertainties, which arise from the potential bias of the control-region142methods, provide the dominant uncertainty of 56% on the background yield.143

The relative uncertainty on the integrated luminosity measurement is estimated to be 11% [38]144and is included in the limit calculations. The effect of this uncertainty in the background esti-145mation cancels when the absolute normalization of backgrounds are taken from the measured146yields in the control regions. The statistical uncertainties on the yields in the control regions147are included in the uncertainty on the backgrounds. The QCD multijet contribution is esti-148mated using a control region with two (three) loose leptons for same-sign dilepton (trilepton)149channel, while the rest of the criteria are the same as for signal. The QCD yield in the sig-150nal region is obtained from the yields observed in the control region, multiplied by the ratios151of the number of leptons passing tight and loose cuts. The contribution is calculated to be152smaller than 0.09 events and considered as a systematic uncertainty of 29% on background153estimation. The uncertainties on the background cross sections are included by varying the154normalization on the relevant processes as follows: ±39% for tt + jets [35], ±3% (±4%) for W155(Z) [36], ±(27 to 42)% for dibosons, and ±50% for other processes. Lepton selection efficiencies156are measured using inclusive Z samples; the resulting differences between data and simulated157samples are smaller than 2%. An additional systematic uncertainty was assigned with a magni-158tude of 50% on the efficiency difference between simulated Z and b′ samples due to the effects159of different event topologies. This results in 5.8% and 5.4% uncertainties for the electrons and160muons, respectively. Weighted averages including trilepton and same-sign dilepton final states161in the appropriate proportions of selected muons and electrons result uncertainties of 13% and1621.5% in signal efficiency and background estimation, respectively. Uncertainty sets given by163CTEQ6 [39] are used to determine the uncertainties from parton distribution functions (PDFs).164Weights for each simulated event are recalculated, and the variations are summed in quadra-165ture. The systematic effects of the jet energy scale uncertainty, jet resolution, E/T resolution, and166jets from pile-up are found to be small [28, 29]. The total uncertainties on the signal selection167efficiency and on the background estimation are evaluated to be 13% and 65%, respectively,168and are summarized in Table 2.169

The background yield in the signal region is 0.32 with a total relative uncertainty of 65%. No170events are observed in the data, which is consistent with the background expectation. An event171is found below the ST threshold in the same-sign dilepton channel (Fig. 2), and another event172is rejected by the jet multiplicity requirement in the trilepton channel (Fig. 1). These two events173are consistent with the expected total background yield of 0.69, if the requirements on ST and174jet multiplicity in trilepton channel are relaxed to 200 GeV/c2 and 1, respectively.175

For each b′ mass hypothesis, cross sections, selection efficiencies and associated uncertainties176are estimated (table 1). From these, and from the estimated background yield and zero selected177events, upper limits on b′b′ cross sections at the 95% CL are derived using a Bayesian method178with a log-normal prior for integration over the nuisance parameters [40]. The resulting upper179limits obtained on the b′ cross section are 3.00, 2.46, 2.31, 2.13, and 2.01 pb for mass hypotheses180of 300, 350, 400, 450, and 500 GeV/c2, respectively. These limits are plotted as the solid line181in Fig. 3, while the dotted line represents the limit expected with the available sample size, as-182suming the presence of standard model processes alone. By comparing to the NLO production183cross section for pp→ b′b′, a lower limit of 361 GeV/c2 is extracted for the mass of the b′ quark184at the 95% CL.185

In summary, a search for a heavy bottom-like quark produced in proton-proton collisions at186

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Table 2: Summary of relative systematic uncertainties for signal selection efficiencies (∆e/e)and for background estimations (∆B/B). The ranges represent the dependence on the input b′

mass.

∆e/e [%] ∆B/B [%]Accuracy of control-region method - 56Norm: QCD multijet - 29Norm: tt + jets - 0.5Norm: W(Z) + jets - 1.0Norm: dibosons - 0.9Norm: other processes - 5.5Jet energy scale 1.1 – 2.1 1.0Jet energy resolution 0.1 – 0.6 1.5Missing energy resolution 0.1 – 1.2 5.6Lepton selection 13 1.5Pile-up 1.0 – 1.2 < 0.1PDF 0.5 – 1.0 1.0Control region statistics - 13Simulated sample statistics 2.4 – 3.0 -Total 13 65

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Figure 3: The exclusion limits at the 95% CL on the pp → b′b′ production cross section. Thesolid line represents the observed limits, while the dotted line represents the limit expectedwith the available sample size, assuming the presence of standard model processes alone. Com-paring with NLO production cross sections, b′ mass less than 361 GeV/c2 is excluded with anassumption of 100% b′ → tW decay branching fraction.

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√s = 7 TeV has been presented. The production of pp→ b′b′ → ttW+W− has been studied in187

a data set corresponding to an integrated luminosity of 34 pb−1 collected by the CMS detector188during 2010. Final states with the signatures of trileptons or same-sign dileptons are very rare189in standard model processes, and background contributions have been estimated to be very190small. No events are found in the signal region defined in the analysis. With an assumption of191100% b′ → tW decay branching fraction, the b′ mass range from 255 to 361 GeV/c2 has been192excluded at the 95% CL.193

We wish to congratulate our colleagues in the CERN accelerator departments for the excellent194performance of the LHC machine. We thank the technical and administrative staff at CERN and195other CMS institutes, and acknowledge support from: FMSR (Austria); FNRS and FWO (Bel-196gium); CNPq, CAPES, FAPERJ, and FAPESP (Brazil); MES (Bulgaria); CERN; CAS, MoST, and197NSFC (China); COLCIENCIAS (Colombia); MSES (Croatia); RPF (Cyprus); Academy of Sci-198ences and NICPB (Estonia); Academy of Finland, ME, and HIP (Finland); CEA and CNRS/IN2P3199(France); BMBF, DFG, and HGF (Germany); GSRT (Greece); OTKA and NKTH (Hungary);200DAE and DST (India); IPM (Iran); SFI (Ireland); INFN (Italy); NRF and WCU (Korea); LAS201(Lithuania); CINVESTAV, CONACYT, SEP, and UASLP-FAI (Mexico); PAEC (Pakistan); SCSR202(Poland); FCT (Portugal); JINR (Armenia, Belarus, Georgia, Ukraine, Uzbekistan); MST and203MAE (Russia); MSTD (Serbia); MICINN and CPAN (Spain); Swiss Funding Agencies (Switzer-204land); NSC (Taipei); TUBITAK and TAEK (Turkey); STFC (United Kingdom); DOE and NSF205(USA).206

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