Abstract
A search for new particles that decay into top quark pairs (tt¯) is performed with the ATLAS experiment at the LHC using an integrated luminosity of 4.7 fb−1 of proton–proton (pp) collision data collected at a center-of-mass energy s=7 TeV. In the tt¯→WbWb decay, the lepton plus jets final state is used, where one W boson decays leptonically and the other hadronically. The tt¯ system is reconstructed using both small-radius and large-radius jets, the latter being supplemented by a jet substructure analysis. A search for local excesses in the number of data events compared to the Standard Model expectation in the tt¯ invariant mass spectrum is performed. No evidence for a tt¯ resonance is found and 95% credibility-level limits on the production rate are determined for massive states predicted in two benchmark models. The upper limits on the cross section times branching ratio of a narrow Z′ resonance range from 5.1 pb for a boson mass of 0.5 TeV to 0.03 pb for a mass of 3 TeV. A narrow leptophobic topcolor Z′ resonance with a mass below 1.74 TeV is excluded. Limits are also derived for a broad color-octet resonance with Γ/m=15.3%. A Kaluza–Klein excitation of the gluon in a Randall–Sundrum model is excluded for masses below 2.07 TeV.4 MoreReceived 13 May 2013DOI:https://doi.org/10.1103/PhysRevD.88.012004This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2013 CERN, for the ATLAS Collaboration
Highlights
Despite its many successes, the Standard Model (SM) of particle physics is believed to be an effective field theory valid only for energies up to the TeV scale
This paper presents the results of a search for production of new particles decaying to top quark pairs using a data set corresponding to an integrated luminosity of 4:7 fbÀ1 of 7 TeV center-of-mass energy proton–proton collisions collected by the ATLAS experiment in 2011
In the boosted analysis this requirement is independent of any large-radius jet in the event, i.e. the b-tagged jet may originate from the hadronic top quark decay and overlap with the large-radius jet or it may originate from the leptonic top quark decay and be near the lepton
Summary
The Standard Model (SM) of particle physics is believed to be an effective field theory valid only for energies up to the TeV scale. This paper presents the results of a search for production of new particles decaying to top quark pairs using a data set corresponding to an integrated luminosity of 4:7 fbÀ1 of 7 TeV center-of-mass energy proton–proton (pp) collisions collected by the ATLAS experiment in 2011. The search is carried out using the lepton plus jets decay channel where the W boson from one top quark decays leptonically (to an electron or a muon, and a neutrino) and the other top quark decays hadronically This search uses a combination of resolved and boosted reconstruction schemes, defined by the cases when the top quark decay products are well separated or merged in the detector, respectively. Using 4:7 fbÀ1 of integrated luminosity, this analysis improves the previous ATLAS lepton plus jets analyses in that it uses a large-radius R 1⁄4 1:0 jet trigger, applies b-tagging in the boosted selection, utilizes an optimized method for charged lepton isolation, and combines the resolved and boosted reconstruction analyses
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