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Cms Pas Exo-11-055 Available on the CERN CDS information server CMS PAS EXO-11-055 CMS Physics Analysis Summary Contact: [email protected] 2011/08/23 Search for heavy narrow resonances decaying to t¯t in the muon+jets channel The CMS Collaboration Abstract A search for narrow heavy resonances decaying to top quark pairs is performed us- ingp a data sample of pp collisions collected by the CMS experiment at the LHC at s = 7 TeV corresponding to an integrated luminosity of 1.14 ± 0.05 fb−1. This search considers a sample of t¯t candidates in the muon+jets topology, t¯t ! (W+b)(W−b¯) ! (qq¯ 0b)(m−n¯b¯) (or charge conjugate), and focuses on heavy resonances resulting in en- ergetic top quarks whose decay products are narrowly collimated. To cope with this topology, a dedicated event selection and reconstruction of the invariant t¯t mass is deployed. Sub-picobarn limits (at 95% confidence level) are set on s(pp ! Z0 ! t¯t) for invariant Z0 masses above 1.35 TeV/c2. 1 1 Introduction The top quark, discovered in 1995 by both Tevatron experiments [1, 2], is the only known fermion with a mass of the order of the electroweak symmetry breaking (EWSB) scale. It plays a special role in many beyond the Standard Model (BSM) theories of EWSB. In models with top condensation, such as technicolor and topcolor, the role of the Standard Model (SM) Higgs boson is taken by a composite particle that is a tt bound state [3]. These models predict ad- ditional heavy gauge bosons, e.g., color-singlet Z0 [4–6], color octets, such as colorons [7] or axigluons [8, 9], which couple strongly to top quarks. Models with two Higgs doublets such as minimal supersymmetric models predict that the pseudoscalar Higgs may couple strongly to top quarks [10]. The relative weakness of gravity compared to other forces has been addressed in the context of extra dimensions, e.g., in Randall-Sundrum [11] and ADD models [12]. Here, TeV-scale gravitons can decay, in some cases preferentially, to top quark pairs [13]. In each of these BSM mechanisms, the production of top quark pairs at hadron colliders distorts the tt invariant mass spectrum relative to the SM expectation, as described in [13]. Previous searches for physics beyond the SM in tt production have been performed by the Tevatron experiments [14–17]. The Tevatron experiments provide limits on the production cross sections of narrow resonances decaying to tt as a function of the invariant mass of the resonance. These limits exclude a Topcolor Z0 [18] for masses below 900 GeV/c2 at 95% C.L. First searches for Z0 ! tt at the LHC have been performed by CMS in the semileptonic channel for masses close to the tt production threshold [19], in the all-hadronic decay mode focusing on masses above 1 TeV/c2 [20], and by ATLAS in the semileptonic channel [21]. Using a model-independent approach, we place upper limits on the cross sections of narrow resonances decaying to top quark pairs. This study focuses on a search for new heavy particles above a mass of 1 TeV/c2, and considers a sample of tt candidate events in the muon+jets topol- ogy tt ! (W+b)(W−b¯) ! (qq¯ 0b)(m−n¯b¯) (or charge conjugate) where one W-boson decays to a muon and a neutrino, and the other W-boson decays hadronically. The main aspects of the analysis presented in this paper are the following. We use a dedicated event selection appropriate for high momentum top quarks yielding narrowly collimated de- cay products. In particular jets from top quark decays are allowed to overlap and to be recon- structed as a single jet (Section 3). In the reconstruction of the invariant tt mass, the assignment of jets to top quarks is based on topological criteria which favor back-to-back highly boosted top quark pairs (Section 4). The Z0 signal and the SM tt, single top, W+jets and Z+jets back- grounds are obtained from simulation. These backgrounds are further checked with the data. The QCD background is estimated from data using a sideband technique (Section 5). The sta- miss tistical analysis makes use of templates of the HT,lep (scalar sum of muon pT and ET ) and Mtt distributions for the signal and the various backgrounds. We consider the effect of various sources of systematic uncertainty on the shape of the templates (Section 6). To extract upper limits on the cross section Z0 ! tt, we integrate the Bayesian posterior probability distribu- tion over the model parameters, namely the templates’ normalization factors and the nuisance parameters (Section 7). 2 Data and Simulation p In this article, we analyzed proton-proton collisions at a center-of-mass energy of s = 7 TeV recorded by the CMS detector. The central feature of the CMS detector is a superconducting solenoid providing a field of 3.8 T. Located within the solenoid are the silicon pixel and strip tracker, the crystal electromagnetic calorimeter and the brass/scintillator hadron calorimeter. 2 3 Event Selection Table 1: Samples of simulated events used in this analysis. Cross section values are evaluated NLO or beyond NLO for all processes. The single top samples only include leptonic decays of the W boson, including decays to t. The uncertainties include pdf and Q2 scale uncertainties. Process s × BR [pb] QCD tt+jets 157.5 ± 24 W+jets 31314 ± 1558 Z+jets 3048 ± 132 Single-Top, t channel, lepton decay 21.5 ± 0.9 Single-Top, tW production, lepton decay 5.2 ± 0.4 Muons are measured combining the information from the silicon-based inner detector with gas-ionisation detectors embedded in the steel return yoke. A detailed description of the CMS detector can be found in [22]. The data sample corresponds to an integrated luminosity of 1.14 ± 0.05 fb−1. We compare the measured distributions in the data sample with predictions using simulated events. Standard Model top quark production is simulated with the tree-level matrix element generator MADGRAPH [23] interfaced to PYTHIA [24] for the parton showering using the MLM [25] matching algorithm. The same combination of MADGRAPH and PYTHIA is used for the description of the main SM backgrounds to QCD top quark pair production. W and Z boson production is simulated in association with jets (abbreviated as W+jets and Z+jets). Also, the electroweak production of single top quarks is simulated using the MAD- GRAPH generator. The combination of MADGRAPH and PYTHIA was also used to produce the model of a generic high-mass resonance decaying to top quark pairs. In particular, a model with a Z0 that has the same fermion couplings as the Standard Model Z boson, with masses of 0.75, 1, 1.25, 1.5, 2, and 3 TeV/c2, was used. In the simulation, the branching ratio Z0 ! tt is set to 100%. The width of the resonance was set to 1% of mZ0 which is well below the experimental resolution and can thus serve as a universal model for narrow resonances with natural widths much smaller than the experimental resolution of 6–13% (cf. Section 4). All generated events were processed with a GEANT4-based [26] detector simulation and reconstructed using the CMS simulation and reconstruction software [27]. Table 1 summarizes the samples of simulated events used for signal and background processes, showing both the number of simulated events as well as the cross section used for each sample. The cross section for QCD tt is based on a NLO calculation with MCFM [28]. The production cross sections for W and Z are NNLO values calculated with FEWZ [29]. The cross section for single top tW production is from [30], whereas single top in the t channel is from [31]. For QCD multijet background, we make use of a data-driven method as described in Section 5. 3 Event Selection We select candidate high-mass Z0 ! tt events from the data sample where one W boson from a top quark decays leptonically into a muon and a neutrino, and the other W boson decays hadronically. We use a particle-flow [32] based reconstruction. Jets are reconstructed from particle-flow can- didates with the anti-kT jet clustering algorithm [33, 34] with a size parameter R = 0.5. We select events passing a trigger requiring at least one muon with pT above a certain threshold. 3 CMS generator study QCD t t CMS generator study QCD t t 0.18 s = 7TeV 2 s = 7TeV 2 Z’, M=1TeV/c 0.1 Z’, M=1TeV/c Z’, M=3TeV/c2 Z’, M=3TeV/c2 0.16 0.14 event fraction event fraction 0.08 0.12 0.1 0.06 0.08 0.04 0.06 0.04 0.02 0.02 0 0 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 min ∆ R(q1, q2, bhad) ∆ R(blep,lep) (a) (b) Figure 1: Kinematic properties of Standard Model tt and Z0 signal on generator level: (a) mini- mum DR between any two of the three quarks of the hadronically decaying top quark, (b) DR between the muon and the b quark from the leptonically decaying top quark. The largest fraction of the data was taken with a trigger threshold of 30 GeV/c. Lower thresh- olds were used in the initial periods with a lower instantaneous luminosity. The trigger does not impose an isolation requirement on the muon candidate. The trigger efficiency is 92 ± 1% for events with one reconstructed muon with pT > 35 GeV/c and jhj < 2.1.
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