Available on the CERN CDS information server CMS PAS B2G-12-019

CMS Physics Analysis Summary

Contact: cms-pag-conveners-b2g@cern.ch 2013/09/10

Search for pair-produced vector-like quarks of charge −1/3in lepton+jets final state in pp collisions at

√s = 8 TeV

The CMS Collaboration

Abstract

Results are presented from an inclusive search for the pair production of vector-likebottom quark partners b′ that decay into tW, bZ or bH final states. The search is per-formed using a sample of proton-proton collisions at

√s = 8 TeV collected with the

CMS detector and corresponding to an integrated luminosity up to 19.8 fb−1 . Thesignal region is defined by events containing one electron or muon, missing trans-verse momentum, and at least four jets with large transverse momenta, where onejet is likely to originate from the decay of a bottom quark. Events are further catego-rized based on the number of jets that are consistent with the decay of boosted W, Zor H bosons. No significant excess of events is observed with respect to the standardmodel expectations. Assuming a strong pair-production mechanism, 95 percent con-fidence level limits between 582 and 732 GeV are set on the b′ quark mass for variousdecay branching ratios. If the b′ decays exclusively into a top quark and a W boson,b′ quark masses below 732 GeV are excluded at the 95 percent confidence level.

1

1 IntroductionVector-like quarks, for which both the left- and right-handed chiralities have the same repre-sentation under the SU(2) symmetry group, easily evade the electroweak precision measure-ments constraints and can cancel the top quark radiative corrections to the Higgs mass, therebysolving the mass hierarchy problem in the particle physics. Vector-like quarks appear in littleHiggs [1, 2] and composite Higgs [3, 4] models, as well as in models with extra dimensions [5]and in non-minimal supersymmetric extensions [6, 7] of the standard model (SM) of particlephysics. These quarks can naturally have masses of the order of 1 TeV.

In many extensions beyond the SM (BSM) vector-like quarks mix with the ordinary quarksof the third generation and decay into top or bottom quarks, accompanied by W, Z or Higgsbosons. Unlike for chiral quarks, flavor changing neutral currents (FCNC) are not suppressed,and vector-like quarks can decay into different final states more democratically.

So far, most of the searches for vector-like quarks performed at the Tevatron and LHC ex-periments considered scenarios where they decay exclusively into one final state. This notepresents an inclusive search for the pair production of vector-like bottom quarks b′ that decayinto a top quark and a W boson (tW), a bottom quark and a Z boson (bZ), or a bottom quarkand a Higgs boson (bH). An analogous search for vector-like top quark partners t′ is performedby the CMS experiment for the t′ channel in Ref. [8]. The current experimental constraints onthe mass of the vector-like b′ quark are set by previous LHC searches. Assuming an exclu-sive decay into tW, the b′ quark mass must be greater than 675 GeV [9], and for an exclusivedecay into bZ, the b′ quark mass must be greater than 700 GeV [10]. The exclusion limits areweakened if the b′ → tW and b′ → bZ branching ratios are less than one.

The search presented in this note is performed using events with exactly one charged lepton(electron or muon) with high transverse momentum (pT), four or more energetic jets at least oneof which is consistent with the decay of a bottom quark, and missing transverse momentum( /ET). Precisely one leptonic W boson decay is selected in this final state. The W boson may beproduced either directly through the decay of the b′ quark or subsequently through the decayof b′ quark products, such as top quark into Wb and/or Higgs boson into WW.

The dominant SM process that results in the same signature is production of tt with associatedjets. Nonetheless, tt events are characterized by smaller lepton and jet transverse momenta. Inaddition, the decay products of b′ quarks often result in highly boosted W, Z or Higgs bosonsthat are merged into a single jet. To identify these boosted objects, a tagging algorithm is em-ployed that identifies substructure within jets consistent with the hadronic decay of W, Z orHiggs boson, and is described in Ref [11]. We refer to this procedure as “V-tagging”.

The search for heavy quarks is performed by classifying events based on the number of V-tagged jets. For each V-tag multiplicity, the scalar sum (ST) of the transverse momenta of thelepton, the jets, and /ET, is used to test for the presence of a new physics signal in the data.

2 The CMS Detector and Data SamplesThe data were recorded during 2012 by the Compact Muon Solenoid (CMS) experiment atthe LHC, which delivered proton-proton collisions at

√s = 8 TeV. The data correspond to an

integrated luminosity of 19.8 fb−1. The CMS detector uses a polar coordinate system with thez axis pointing along the counterclockwise circulating beam. The x axis points towards thecenter of the Large Hadron Collider (LHC) ring, and the y axis points up. Angular coordinatesare specified by the pseudorapidity η = − ln[tan(θ/2)], where θ is the polar angle measured

2 3 Event Reconstruction

with respect to the positive z axis, and by φ, the azimuthal angle about this axis.

The e+jets events were collected with triggers requiring at least one electron candidate with apT greater than 27 GeV. The µ+jets events were collected with triggers that required at leastone muon candidate with a pT greater than 40 GeV. The integrated luminosity of the electron(muon) dataset corresponds to 19.2 (19.8) fb−1.

The signal process pp→ b′b′ is simulated with up to two additional hard partrons using theMADGRAPH 5.1.1 [12] event generator with CTEQ6L1 [13] parton distribution functions (PDF),and using PYTHIA 6.424 [14] for hadronization. The signal samples are generated for differentb′ quark masses, and for each mass point six distinct samples are created corresponding todifferent decays of b′ quarks: tWtW, tWbZ, tWbH, bZbZ, bHbH, bZbH. The entire phase spacecorresponding to different branching ratios of the b′ quarks is scanned using a mixture of thesesix samples.

The background processes tt+jets, W+jets, and Z+jets, are simulated using the MADGRAPH 5.1.1event generator with CTEQ6L1 parton distribution functions (PDF). The diboson processes(WW, WZ, and ZZ) are generated using the PYTHIA 6.424 event generator with CTEQ6M PDF.The single top quark production via tW, s and t channels is simulated using the POWHEG 1.0 [15–17] event generator with CTEQ6M PDF [13], with PYTHIA again used for the parton showerand hadronization. The multijet background is determined using data and will be explainedin section 3. The generated events are processed through a CMS detector simulation based onGEANT4 [18]. Additional minimum-bias events (pileup) are generated with PYTHIA and super-imposed on the hard-scattering events to simulate multiple collisions within the same bunchcrossing. All the simulated events are weighted to reproduce the distribution of the number ofinteraction vertices observed in data.

3 Event ReconstructionEvents are reconstructed using the CMS particle-flow (PF) algorithm [19–21], which identifiesall observable particles in an event by combining the information from tracks, energy depositedin the ECAL and HCAL, and signals in the preshower detector and the muon systems. Thisprocedure separates all particles into five categories: muons, electrons, photons, and chargedand neutral hadrons. Energies are separately calibrated for each particle type. The missingtransverse momentum in an event is defined as the negative vector sum of the transverse mo-menta of all objects from the PF algorithm. Events must also have a well-reconstructed primaryvertex, which is chosen as the one with the largest value for the scalar sum of the p2

T of the as-sociated tracks. Charged hadrons not associated with primary vertex are also removed.

Electron candidates are reconstructed from clusters of energy deposited in the ECAL. The clus-ters are first matched to track seeds in the pixel detector. The track trajectories of electroncandidates are reconstructed using a dedicated modeling of the electron energy loss and fittedwith a Gaussian-sum filter [22].

Muon candidates are identified through a combination of reconstruction algorithms using hitsin the central silicon tracker and signals in the outer muon system [23]. A standalone muonalgorithm uses only information from the muon chambers. The tracker muon algorithm beginswith tracks found in the inner tracker, and associates these with matching segments in themuon chambers. In this analysis, all muons have to pass the global muon algorithm, whichstarts off with the standalone muons and then performs a global fit to the hits in the trackerand the muon system for each muon candidate.

3

Events are required to have four or more jets, with at least one b-tag jet and are classifiedinto boson multiplicity. Therefore, jets are reconstructed using two different jet clustering al-gorithms implemented in FASTJET version 3.0 [24]. The first is the anti-kT jet clustering algo-rithm [25], with a distance parameter R = 0.5 and is used to identify jets, with at least one b-tagjet in the event. The second is Cambridge-Aachen algorithm [26] with jet pruning [27], in whichsoft and wide angle clusters are removed from the jet constituents within a distance parameter,R = 0.8. This algorithm attempts to identify the hadronic decay products of boosted W, Z andH bosons, and hence in V-tagging.

Jets from the first algorithm are referred to as AK5 jets. The combined secondary vertex (CSV)algorithm identifies jets from the decay of bottom quarks [28]. The CSV algorithm provides op-timal b-tagging performance by combining information on the impact parameter significanceand the properties of the secondary vertex. The variables are combined using a likelihood-ratiotechnique to compute a b-tagging discriminant. The small differences in the performance of theb-tagging algorithm between data and simulation are accounted for by pT- and η-dependentdata-to-simulation scale factors [28]. The selection on the CSV discriminant is chosen such thatlight quarks are tagged at a mis-tag rate of about 1% and b-quark jets are identified with anefficiency of about 70%.

Jets from the second algorithm are referred to as CA8 jets. The wide cone size of R = 0.8is chosen to ensure that all decay products are captured in the same jet. This requirement issatisfied for jets with pT greater than 200 GeV. The two sub-jets identified with the pruningalgorithm [27] are required to have an invariant mass between 50 and 150 GeV to be consistentwith W, Z or Higgs boson. The ratio of the most massive sub-jet to the mass of the pruned jet isrequired to be below 0.4. A CA8 jet is considered V-tagged if it satisfies these requirements. TheCA8 jets are required to match at least one of the AK5 jets within a ∆R < 0.5. The V-taggingefficiency is measured in data and simulation, and is approximately 50%, as determined inRef. [29]. The measurement in data is performed using jets from hadronic W-boson decays ina sample of semileptonic tt events. The correction scale factor between data and simulation ismeasured to be 93.3%.

4 Event SelectionCharged leptons from the decay of W and Z bosons are required to be isolated from jets. Thelepton isolation can be expressed in terms of the quantity I`r , defined as the scalar sum of thepT of charged hadrons, neutral hadrons, and photons in a cone of ∆R =

√(∆φ)2 + (∆η)2 < 0.3

around the lepton momentum vector, divided by the lepton pT. The isolation requirements areoptimized to be I`r < 0.1 and I`r < 0.12 for electrons [21] and muons [23], respectively. Theelectrons (muons) also must have pT > 30 GeV (pT > 45 GeV), and |η| < 2.5 (|η| < 2.1). Thelepton trajectories are required to have a transverse impact parameter of less than 0.02 cm inmagnitude, and a longitudinal impact parameter along the beam direction of less than 1 cm inmagnitude, relative to the primary vertex.

The final selection requires events to have exactly one isolated lepton and at least four AK5jets with |η| < 2.4 and pT > 200, 60, 40, 30 GeV and at least one b-tagged jet. The minimumnumber of jets, and the jet pT requirements are optimized to enhance sensitivity to the b′b′signal. The thresholds for lepton pT are driven by the trigger conditions. Jets that are within acone of ∆R < 0.3 of the lepton direction are not considered. At least one AK5 jet, not matchedto a V-tagged CA8 jet, must be b-tagged. Jets from b′ decays are produced more centrally ascompared to those from SM processes. Therefore, centrality C, defined as the scalar sum of thejet transverse momenta divided by the scalar sum of the jet energies, is required to be greater

4 5 Systematic Uncertainties

than 0.4. The event is also required to have /ET > 20 GeV to suppress the SM backgrounds.

Table 1 lists the number of events observed and the number expected for all backgroundsources, based on the corresponding cross sections, efficiencies and acceptances for each back-ground after all selection criteria have been applied, and the total integrated luminosity. Thecross sections for tt+jets production and Z+jets processes are taken from CMS measurements [30,31]. The single top quark cross sections are approximate NNLO calculations obtained fromRef. [32–34]. The cross section for W+jets is computed to NNLO using FEWZ [35]. The crosssections for the diboson processes WW, WZ, and ZZ are calculated to NLO using MCFM [36].The multijet events are obtained from a background enriched data sample defined as I`r > 0.25in both channels. In electron channel, the multijet events can be additionaly selected if electronidentification criteria fails. The contribution for multijet processes is estimated directly froma fit to the /ET distribution in data. In the fit, we constrain the tt+jets contribution within itsmeasured uncertainty, while W+jets, Z+jets, single top and diboson production processes areconstrained within their theoretical uncertainties. The normalization of multijet contributionis allowed to float freely. This fit is performed independently for each of the V-tag categoriesused in the analysis, and separately for muon and electron channels in each V-tag category.

Table 1: Number of data and expected events with statistical uncertainties in the electron andmuon channels after the full event selection.

Background process e+jets events µ+jets eventstt +jets 11397± 85 9550± 79W+jets 1247± 37 1137± 37Multijet 1072± 19 505± 4Single top 775± 17 683± 17Z+jets 222± 22 238± 23tt V+jets 92± 1 82± 1Diboson (WW, WZ, ZZ) 43± 2 34± 2Total background 14846± 99 12229± 91Data 14640 11695

5 Systematic UncertaintiesThe systematic uncertainties affect both the number of selected events and the shapes of thekinematic distributions. The uncertainty on the normalization is applied using a lognormalconstraint on the parameter in question in the fit to the data. In order to evaluate system-atic uncertainties that affect both the shape and normalization an alternative set of templateshapes is constructed that incorporates the systematic uncertainties in question. These alterna-tive shapes are included in the statistical treatment using template morphing.

The tt +jets cross section uncertainty is taken to be 10%, based on measurements of the dif-ferential tt cross section in the high-pT regime explored with this analysis selection. A 50%uncertainty is assigned on the non-tt +jets electroweak background contribution, while a 100%uncertainty is applied to the QCD multijet contribution. Trigger and lepton identification cor-rection scale factors to account for differences between data and simulation are obtained fromdata-driven techniques using Z boson decays. The respective uncertainties are 2% in bothchannels.

The shape uncertainty due to the jet-energy scale is treated based on shifting the jet energy

5

Table 2: List of systematic uncertainties included in the likelihood fit. Parameters labeled “Dis-tribution” affect both shape and normalization of the ST distributions. The quoted uncertaintiescorrespond to their effect on normalization only.

Parameter type Source Uncertainty (%)

Distribution

Q2 scales for tt+jet 9.1Matching partons 6.5

Jet energy scale 5.5b-tagging Scale Factor 2.1V-tagging Scale Factor 1.0Jet energy resolution < 1

Pile-up < 1

Normalization

Lepton ID/reco/trigger 2.0Luminosity 4.4

tt cross section 10Other Electroweak Backgrounds 50

QCD Multijet Background 100

scales up and down by one standard deviation [37]. This effect is integrated into the limit-setting calculation by constructing alternative shapes, used in template morphing. These vari-ations in shapes changes the acceptance in the total background prediction by about 5.5%. Dif-ferences in the jet-energy resolutions between data and simulation are accounted for by usinga similar procedure. The jet-energy resolution has less than a 1% effect on the acceptance. Thesystematic uncertainties related to the b-tagging scale factors are estimated by varying the pT-and η-dependent values of the b-tagging scale factors by one standard deviation. This accountsfor a 2.1% change in the acceptance. The uncertainty on the V-tagging scale factor is evaluatedsimilarly, resulting in a 1% effect.

The factorization and renormalization scales (Q2) are assumed to be same and the uncertaintydue to them is estimated by simulating tt events in which Q scales are increased or decreasedsimultaneously by a factor of two relative to their nominal value. This systematic uncertaintyaccounts for the largest deviation in the tt +jets ST spectrum with a 9.1% effect on the normal-ization. The uncertainty arising from matching matrix-element partons with parton showers isestimated using two tt simulated samples, with matching threshold shifted up or down by afactor of two relative to its default value (40 GeV).

The list of systematic uncertainties is summarized in Table 2.

6 Statistical TreatmentStudies of various kinematic variables have shown that ST, defined as a scalar sum of AK5jets transverse energies, lepton pT and /ET, is the most powerful discriminator to separate b′b′signal from the SM background.

At high b′ masses, we expect a b′ quark to decay into a boosted W boson along with a top quarkor a boosted Z or H boson along with a bottom quark. We categorize events into 0-, 1- and ≥ 2V-tag categories, and perform a one-dimensional fit to data using the ST distributions, simul-taneously across each V-tag category in both the electron and muon channels to test for thepresence of a b′b′ signal in the data. The search is performed by fitting the observed distribu-

6 8 Summary

tions of ST for each V-tag multiplicity to signal and background templates. The fit is performedfor the combination of e+jets and µ+jets channels and for 0, 1, and ≥ 2 V-tags. Fig 1 show theST distributon for data and background in 0-, 1- and ≥ 2 V-tag categories in electron and muonchannels prior to the fit. The uncertainty bands shown in Fig 1 are taken by adding all the sys-tematic uncertainties in quadrature on a bin-by-bin basis. The data to background ratio plotsshows a data deficit in ST distributions between 1200 GeV and 2000 GeV, and it was checkedthat the mismodeling is resolved by replacing the tt+jets templates with 1σ Q2 and matchingpartons scale up templates. Therefore, the deviations are fully covered by these largest uncer-tainties as shown in Table 2. The tails of the ST distributions are rebinned to ensure that thestatistical uncertainty in each bin is below 25%.

The dominant SM background is from tt production. All the non-tt+jets backgrounds are com-bined into a single template, and varied together in the fit. The upper limits on the b′b′-quarkpair production are derived using the Bayesian approach, using the THETA framework [38].The expected rate for the signal as a function of b′-quark mass is determined by the approxi-mate NNLO cross section [39], the signal efficiencies and the measured luminosity. The uncer-tainties described in Section 5 are taken into account and integrated into the likelihood fit, asdescribed above.

7 ResultsThe data fit to the 2-dimensional distribution of ST vs V-tag multiplicity shows no excess overstandard model predictions. Fig 2 shows the limits for the benchmark scenario, where the b′

decays to tW, bH, and bZ with branching fractions as BR(tW) = 0.50, BR(bH) = 0.25, and BR(bZ)= 0.25. In this case we set the observed mass limit at 700 GeV, while the expected limit is 689GeV, at the 95% CL.

The 95% confidence level (CL) limits corresponding to BR(tW) = 1, BR(bH) = 1, and BR(bZ) = 1are shown in Fig 3 for combination of both electron and muon channels. Assuming b′ decaysexclusively into tW, b′ quark masses below 732 GeV are excluded at 95% CL. It can be seenthat this analysis is most sensitive to the tW decay mode. Independent analyses are optimizedfor the bH and bZ decay modes by selecting additional lepton candidates. However, we retainsensitivity to the bH and bZ decay modes when those leptons are not reconstructed. When theb′ quark decays to tW, a larger number of boosted W bosons can be reconstructed due to thesubsequent top quark decays, compared to the bH or bZ decay mode. As a result, the b′ signalin the tW decay mode will populate more strongly the 1 and ≥2 V-tag categories, leading toan improved signal-to-background ratio in these channels. The b′ signal in the bH or bZ finalstates tends to populate the 0 V-tag category containing a much larger background. Therefore,with the specific analysis selections applied here, we expect the sensitivity to the tW decaymode to be much higher than for the bH or bZ decay modes.

To explore the effect of different branching ratios a scan of the entire phase space is performed.The branching ratios are varied in steps of 0.1. The result of the scan is presented in Fig 4.

8 SummaryAn inclusive search for pair production of vector-like bottom quarks is performed in lep-ton+jets events. The analysis is based on a data sample of proton-proton collisions at

√s = 8

TeV corresponding to an integrated luminosity of 19.2 fb−1 (19.8 fb−1) in the electron (muon)channel. The search is performed under the assumption that b′ quarks decay into tW, bZ or

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8 References

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bH) = 0.25→BR(b' bZ) = 0.25→BR(b'

µCombined e+

Figure 2: The expected and observed 95% C.L. upper limits on the signal strength parameterµ = σ/σTheory assuming BR(tW) = 0.50, BR(bH) = 0.25, and BR(bZ) = 0.25 for the combinedelectron and muon channels.

bH. Events are selected requiring an electron or a muon, missing transverse momentum, andat least four jets, one of which is identified as a bottom jet. Events are categorized based on thenumber of boosted W, Z, or Higgs bosons reconstructed as a single jet in the detector with asubstructure consistent with the hadronic decay of the boson. A combined fit is performed tothe scalar sum of the transverse momenta of all final reconstructed objects as a function of thenumber of identified bosons.

No significant deviations from SM expectation are found. The limits on the mass of a heavyquark for various decay branching ratios are computed, assuming a strong pair-productionmechanism. For all the combinations, with varying branching fractions for b′ → tW, b′ → bZ,and b′ → bH, we exclude b′ masses below 582 GeV at 95% confidence level. For the scenariowhen b′ decays exclusively into tW, the b′ quark masses below 732 GeV are excluded at 95%CL. The exclusion limit for the exclusive tW decay mode increases with this analysis relativeto previous results [9], from 675 GeV to 732 GeV. The new analysis techniques implemented,including boosted object reconstruction, significantly improve the sensitivity of this analysiscompared to prior methods, increasing the expected limit from 625 GeV to 797 GeV for theexclusive tW decay mode.

References[1] N. Arkani-Hamed et al., “The Minimal Moose for a Little Higgs”, JHEP 0208 (2002) 021,

doi:10.1088/1126-6708/2002/08/021.

[2] M. Perelstein, M. E. Peskin, and A. Pierce, “Top Quarks and Electroweak SymmetryBreaking in Little Higgs Models”, Phys. Rev. D 69 (2004) 075002,doi:10.1103/PhysRevD.69.075002.

References 9

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ObservedExpected

Exp.σ1 ± Exp.σ2 ±

-1CMS Preliminary, 19.8 fb = 8 TeVsat bZ) = 1→BR(b'

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Figure 3: The expected and observed 95% C.L. upper limits on the signal strength parameterµ = σ/σTheory assuming BR(tW) = 1 (top left), BR(bH) = 1 (top right), and BR(bZ) = 1 (bottom)for the combined electron and muon channels.

10 References

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Figure 4: Expected and observed limit results with varied branching fraction of tW, bH and bZin steps of 0.1. The shaded regions represent regions with small expected sensitivity, preciselythat the expected limit is less than 500 GeV.

[3] P. Lodone, “Vector-like quarks in a composite Higgs model”, JHEP 12 (2008) 029,doi:10.1088/1126-6708/2008/12/029, arXiv:0806.1472.

[4] B. Dobrescu and C. T. Hill, “Electroweak Symmetry Breaking via Top CondensationSeesaw”, Phys. Rev. Lett. 81 (1998) 2634, doi:10.1103/PhysRevLett.81.2634.

[5] R. Contino et al., “Warped/Composite Phenomenology Simplified”, JHEP 0705 (2007)074, doi:10.1088/1126-6708/2007/05/074.

[6] S. P. Martin, “Extra Vector-Like Matter and the Lightest Higgs Scalar Boson Mass inLow-Energy Supersymmetry”, Phys. Rev. D 81 (2010) 035004,doi:10.1103/PhysRevD.81.035004.

[7] S. P. Martin, “Raising the Higgs Mass with Yukawa Couplings for Isotriplets inVector-like Extensions of Minimal Supersymmetry”, Phys. Rev. D 82 (2010) 055019,doi:10.1103/PhysRevD.82.055019.

[8] CMS Collaboration, “Inclusive search for top partners in single- and multiple-leptonfinal states at sqrt(s)=8 TeV”, CMS Physics Analysis Summary CMS-PAS-B2G-12-015(2013).

[9] CMS Collaboration, “Search for Heavy Quarks Decaying into a Top Quark and a W or ZBoson Using Lepton + Jets Events in pp Collisions at

√s = 7 TeV”, JHEP 01 (2013) 154,

doi:10.1007/JHEP01(2013)154.

[10] CMS Collaboration, “Search for pair-produced vector-like quarks of charge -1/3 indilepton+jets final state in pp collisions at sqrt(s) = 8 TeV”,.

[11] CMS Collaboration, “Search for heavy resonances in the W/Z-tagged dijet massspectrum in pp collisions at sqrt(s) = 7 TeV”, Submitted to PLB (2012)arXiv:1212.1910.

[12] J. Alwall et al., “MadGraph v5: going beyond”, JHEP 06 (2011) 128,doi:10.1007/JHEP06(2011)128, arXiv:1106.0522.

References 11

[13] J. Pumplin et al., “New generation of parton distributions with uncertainties from globalQCD analysis”, JHEP 07 (2002) 012, doi:10.1088/1126-6708/2002/07/012,arXiv:hep-ph/0201195.

[14] T. Sj..ostrand, S. Mrenna, and P. Z. Skands, “PYTHIA 6.4 Physics and Manual”, JHEP 05

(2006) 026, doi:10.1088/1126-6708/2006/05/026, arXiv:hep-ph/0603175.

[15] P. Nason, “A New method for combining NLO QCD with shower Monte Carloalgorithms”, JHEP 11 (2004) 040, doi:10.1088/1126-6708/2004/11/040,arXiv:hep-ph/0409146v1.

[16] S. Frixione, P. Nason, and C. Oleari, “Matching NLO QCD computations with PartonShower simulations: the POWHEG method”, JHEP 11 (2007) 070,doi:10.1088/1126-6708/2007/11/070, arXiv:0709.2092.

[17] S. Alioli, P. Nason, C. Oleari, and E. Re, “A general framework for implementing NLOcalculations in shower Monte Carlo programs: the POWHEG BOX”, JHEP 06 (2010) 043,doi:10.1007/JHEP06(2010)043, arXiv:1002.2581v1.

[18] GEANT4 Collaboration, “GEANT4—a simulation toolkit”, Nucl. Instrum. Meth. A 506(2003) 250, doi:10.1016/S0168-9002(03)01368-8.

[19] CMS Collaboration, “Particle–Flow Event Reconstruction in CMS and Performance forJets, Taus, and Emiss

T ”, CMS Physics Analysis Summary CMS-PAS-PFT-09-001, (2009).

[20] CMS Collaboration, “Commissioning of the Particle-Flow Reconstruction inMinimum-Bias and Jet Events from pp Collisions at 7 TeV”, CMS Physics AnalysisSummary CMS-PAS-PFT-10-002, (2010).

[21] CMS Collaboration, “Commissioning of the particle-flow event reconstruction withleptons from J/Ψ and W decays at 7 TeV”, CMS Physics Analysis SummaryCMS-PAS-PFT-10-003, (2010).

[22] W. Adam et al., “Reconstructions of electrons with the Gaussian-sum filter in the CMStracker at the LHC”, J. Phys. G 31 (2005) N9, doi:10.1088/0954-3899/31/9/N01,arXiv:physics/0306087.

[23] CMS Collaboration, “Performance of CMS muon reconstruction in pp collision events at√(s) = 7 TeV”, JINST 7 (2012) P10002, doi:10.1088/1748-0221/7/10/P10002,

arXiv:1206.4071.

[24] M. Cacciari, G. P. Salam, and G. Soyez, “Fastjet User Manual”, Eur. Phys. J. C 72 (2012)1896, doi:10.1140/epjc/s10052-012-1896-2, arXiv:1111.6097.

[25] M. Cacciari, G. P. Salam, and G. Soyez, “The Anti-k(t) jet clustering algorithm”, JHEP 04(2008) 063, doi:10.1088/1126-6708/2008/04/063, arXiv:0802.1189.

[26] J. M. B. et al., “Jet substructure as a new Higgs search channel at the LHC”, Phys. Rev.Lett. 100 (2008) 242001, doi:10.1103/PhysRevLett.100.242001,arXiv:0802.2470.

[27] S. D. Ellis, C. K. Vermilion, and J. r. Walsh, “Techniques for Improved Heavy ParticleSearches with Jet Substructure”, Phys. Rev. D 80 (2009) 051501,doi:10.1103/PhysRevD.80.051501, arXiv:0903.5081.

12 References

[28] CMS Collaboration, “ Identification of b-quark jets with the CMS experiment”, JINST 8(2013) P04013, doi:doi:10.1088/1748-0221/8/04/P04013,arXiv:arXiv:1211.4462.

[29] CMS Collaboration, “Search for ttbar resonances in boosted all-hadronic final state”,CMS Physics Analysis Summary CMS-PAS-B2G-12-005 (2013).

[30] CMS Collaboration, “Top pair cross section in e/mu+jets at 8 TeV”, CMS PhysicsAnalysis Summary CMS-PAS-TOP-12-006 (2012).

[31] CMS Collaboration, “Inclusive W/Z cross section at 8 TeV”, CMS Physics AnalysisSummary CMS-PAS-SMP-12-01 (2012).

[32] N. Kidonakis, “Next-to-next-to-leading-order collinear and soft gluon corrections fort-channel single top quark production”, Phys. Rev. D 83 (2011) 091503,doi:10.1103/PhysRevD.83.091503, arXiv:1103.2792.

[33] N. Kidonakis, “Next-to-next-to-leading logarithm resummation for s-channel single topquark production”, Phys. Rev. D 81 (2010) 054028,doi:10.1103/PhysRevD.81.054028, arXiv:1001.5034.

[34] N. Kidonakis, “Two-loop soft anomalous dimensions for single top quark associatedproduction with a W− or H−”, Phys. Rev. D 82 (2010) 054018,doi:10.1103/PhysRevD.82.054018, arXiv:1005.4451.

[35] R. Gavin, Y. Li, F. Petriello, and S. Quackenbush, “FEWZ 2.0: A code for hadronic Zproduction at next-to-next-to-leading order”, Comput. Phys. Commun. 182 (2011) 2388,doi:10.1016/j.cpc.2011.06.008, arXiv:1011.3540.

[36] J. Campbell and R. K. Ellis, “Next-to-leading order corrections to W+ 2 jet and Z+ 2 jetproduction at hadron colliders”, Phys. Rev. D 65 (2002) 113007,doi:10.1103/PhysRevD.65.113007, arXiv:hep-ph/0202176.

[37] “CMS Internal TWiki: Jet Energy Corrections”,.

[38] J. Ott, “Search for Resonant Top Quark Pair Production in the Muon+Jets Channel withthe CMS Detector”. PhD thesis, Karlsruhe U., Dec, 2012. Presented 14 Dec 2012.

[39] M. Aliev et al., “HATHOR – HAdronic Top and Heavy quarks crOss section calculatoR”,Comput. Phys. Commun. 182 (2011) 1034, doi:10.1016/j.cpc.2010.12.040,arXiv:1007.1327.