Abstract
A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb$^{-1}$ of proton-proton collision data collected at a centre-of-mass energy of $\sqrt{s}=8$ TeV. The lepton-plus-jets final state is used, where the top pair decays to $W^+bW^-\bar{b}$, with one $W$ boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour $Z'$ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with $\Gamma/m =$ 15% decaying to $t\bar{t}$. These range from 2.5 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.
Highlights
Background contributions estimated from dataData are used to estimate the magnitudes and uncertainties of two important background contributions: with jets (W +jets) and multi-jet production.7.1 W +jets scale factorsFor the W +jets background, data are used to derive scale factors that are applied to correct the normalisation and flavour fractions given by Alpgen Monte Carlo (MC) simulations of this background.For both the resolved- and boosted-topology event selection criteria, the normalisation scale factors are determined by comparing the measured W boson charge asymmetry in data [101, 102] with that predicted by Alpgen MC simulation
Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with Γ/m = 15% decaying to tt
In order to decrease the statistical uncertainty on the scale factors, a relaxed set√of selection criteria that does not include the b-tagging, ∆φ(jet, ) > 2.3, jet mass and d12 requirements is used
Summary
The first class of models explored produces spin-1 colour-singlet vector bosons, Z. This search uses topcolour-assisted technicolour ZTC2 [3, 19, 20] as a benchmark. This is a leptophobic boson, with couplings only to first- and third-generation quarks, referred to as Model IV in ref. Constraints on ZTC2 have been set by the CDF [23, 24] and D0 [25] collaborations using data from proton-antiproton collisions at the Tevatron.
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