Abstract
A search is performed for electroweak production of a vector-like top quark partner T of charge 2/3 in association with a top or bottom quark, using proton-proton collision data at sqrt{s} = 13 TeV collected by the CMS experiment at the LHC in 2016. The data sample corresponds to an integrated luminosity of 35.9 fb−1. The search targets T quarks over a wide range of masses and fractional widths, decaying to a top quark and either a Higgs boson or a Z boson in fully hadronic final states. The search is performed using two experimentally distinct signatures that depend on whether or not each quark from the decays of the top quark, Higgs boson, or Z boson produces an individual resolved jet. Jet substructure, b tagging, and kinematic variables are used to identify the top quark and boson jets, and also to suppress the standard model backgrounds. The data are found to be consistent with the expected backgrounds. Upper limits at 95% confidence level are set on the cross sections for T quark-mediated production of tHQq, tZQq, and their sum, where Q is the associated top or bottom heavy quark and q is another associated quark. The limits are given for each search signature for various T quark widths up to 30% of the T quark mass, and are between 2 pb and 20 fb for T quark masses in the range 0.6–2.6 TeV. These results are significantly more sensitive than prior searches for electroweak single production of T → tH and represent the first constraints on T → tZ using hadronic decays of the Z boson with this production mode.
Highlights
Background estimation and validationNone of the SM backgrounds are expected to result in a resonance in the five-jet invariant mass, the spectrum of the background five-jet invariant mass distribution should have a monotonically decreasing shape
The second new region is denoted as the 2T 1L (2M 1L) signal region; in order to have significant numbers of events in this background-dominated region and to keep events with similar kinematics to the 3T signal region, the b tagging criteria are relaxed to two medium and one loose b-tagged jets but excluding three medium b-tagged jets
The five-jet invariant mass distributions for the quantum chromodynamics (QCD) multijet 3T region, the tt 2T 1L region, and the 3M signal region are presented in figure 2 together with a potential signal for mT = 0.7 TeV corresponding to a product of cross section times branching fraction of 600 fb
Summary
The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. The energy of electrons is determined from a combination of the electron momentum at the primary interaction vertex as determined by the tracker, the energy of the corresponding ECAL cluster, and the energy sum of all bremsstrahlung photons spatially compatible with originating from the electron track. The energy of charged hadrons is determined from a combination of their momentum measured in the tracker and the matching ECAL and HCAL energy deposits, corrected for zero-suppression effects and for the response function of the calorimeters to hadronic showers. Additional pp interactions within the same or nearby bunch crossings (pileup) can contribute additional tracks and calorimetric energy depositions to the jet momentum To mitigate this effect, tracks identified to be originating from pileup vertices are discarded, and an offset correction is applied to correct for remaining contributions. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in ref. [33]
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