A search for ttbar resonances in the lepton plus jets final state with ATLAS using 4.7 fb^-1 of pp collisions at sqrt(s) = 7 TeV

TL;DR

This paper reports a search for new particles decaying into top quark pairs in proton-proton collisions at 7 TeV, using ATLAS data, finding no evidence for such resonances and setting limits on various theoretical models.

Contribution

First search for ttbar resonances in lepton plus jets final state at 7 TeV with ATLAS, using jet substructure techniques and setting limits on multiple models.

Findings

No evidence for ttbar resonances was observed.

Excluded a leptophobic Z' resonance below 1.74 TeV.

Excluded a Kaluza-Klein gluon below 2.07 TeV.

Abstract

A search for new particles that decay into top quark pairs (ttbar) 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 sqrt(s)=7 TeV. In the ttbar --> WbWb decay, the lepton plus jets final state is used, where one W boson decays leptonically and the other hadronically. The ttbar 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 ttbar invariant mass spectrum is performed. No evidence for a ttbar 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 Gamma/m = 15.3%. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.07 TeV.