Abstract
A search for vectorlike quarks is presented, which targets their decay into a Z boson and a third-generation Standard Model quark. In the case of a vectorlike quark T (B) with charge +2/3e (-1/3e), the decay searched for is T→Zt (B→Zb). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb-1 of pp collisions at s=13 TeV. The final state used is characterized by the presence of b-tagged jets, as well as a Z boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair and single production of vectorlike quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transverse-momentum large-radius jets, as well as - in the case of the single-production selections - the presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vectorlike quarks of mT>1030 GeV (mT>1210 GeV) and mB>1010 GeV (mB>1140 GeV) in the singlet (doublet) model. In the case of 100% branching ratio for T→Zt (B→Zb), the limits are mT>1340 GeV (mB>1220 GeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses. © 2018 CERN.
Highlights
In the Standard Model (SM), the electromagnetic and weak interactions arise from a SUð2ÞL × Uð1ÞY gauge symmetry that is spontaneously broken by the EnglertBrout-Higgs mechanism
This paper focuses solely on the search for T and B quarks, which could couple to SM quarks by
The NNPDF3.0 NNLO parton distribution functions (PDFs) set was used and the samples were normalized to the next-toleading order (NLO) cross sections calculated with SHERPA
Summary
In the Standard Model (SM), the electromagnetic and weak interactions arise from a SUð2ÞL × Uð1ÞY gauge symmetry that is spontaneously broken by the EnglertBrout-Higgs mechanism. In the case of single production, the T quark is expected to decay into Zt. In the case of pair production, the search targets only one VLQ decay into a Z boson and a thirdgeneration quark explicitly, so that it is sensitive to all decays that include at least one Z boson in the final state, i.e., TT → ZtZt and BB → ZbZb, and TT → ZtWb, TT → ZtHt, BB → ZbWt, and BB → ZbHb. thaTt hwe asovpeerarflol rmaneadlywsiisthstpraffistffie1⁄4gy is based TeV data on a [23], search which exploited the leptonic Z boson decays Z → eþe− and. The two single-production channels are referred to as the dilepton channel (SP 2l), and the trilepton channel (SP ≥ 3l)
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