Abstract
A search is performed for electroweak production of a vector-like top quark partner T of charge 2/3 in association with a standard model top or bottom quark, using 2.3 fb−1 of proton-proton collision data at sqrt{s}=13 TeV collected by the CMS experiment at the CERN LHC. The search targets T quarks decaying to a top quark and a Higgs boson in fully hadronic final states. For a T quark with mass above 1 TeV the daughter top quark and Higgs boson are highly Lorentz-boosted and can each appear as a single hadronic jet. Jet substructure and b tagging techniques are used to identify the top quark and Higgs boson jets, and to suppress the standard model backgrounds. An excess of events is searched for in the T quark candidate mass distribution in the data, which is found to be consistent with the expected backgrounds. Upper limits at 95% confidence level are set on the product of the single T quark production cross sections and the branching fraction mathrm{mathcal{B}}left(mathrm{T}to mathrm{t}mathrm{H}right) , and these vary between 0.31 and 0.93 pb for T quark masses in the range 1000-1800 GeV. This is the first search for single electroweak production of a vector-like T quark in fully hadronic final states.
Highlights
Signal and background modelingThe single T quark production cross section and the branching fraction B(T → tH) are highly model dependent
Background estimationThe main backgrounds in the signal region are tt+jets, multijets, and, to a lesser extent, W+jets
Studies on samples generated using larger widths have established that the reconstructed M (T) distributions do not change significantly compared to the narrow width approximation for T quarks having a width of up to 10% of their masses
Summary
The single T quark production cross section and the branching fraction B(T → tH) are highly model dependent. A next-to-next-to-leading order (NNLO) tt+jets production cross section of 832+−4561 pb [21], corresponding to a top quark mass of 172.5 GeV, is used to estimate the tt+jets rate. The W+jets events cross section is estimated to be 95.1 ± 3.6 pb, calculated at NNLO with Fewz 3.1 [28]. This cross section is calculated in the region of phase space where the sum of the parton transverse momenta pT is greater than 600 GeV, in keeping with the signal event topology requiring highly boosted jets. The W+jets production sample, simulated with Madgraph 5.1.3.30, is restricted to this phase space and to events with hadronic W boson decays. The generated signal events are processed using a Geant4-based [33, 34] simulation of the CMS detector
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