Search for pair and single production of vectorlike quarks in final states with at least one Z boson decaying into a pair of electrons or muons in pp collision data collected with the ATLAS detector at

Abstract

The ATLAS CollaborationA 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 → Z b). 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-transversemomentum 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 m T > 1030 GeV (m T > 1210 GeV) and m B > 1010 GeV (m B > 1140 GeV) in the singlet (doublet) model. In the case of 100% branching ratio for T → Zt (B → Z b), the limits are m T > 1340 GeV (m B > 1220 GeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses.

Searching for dark photons with maverick top partners

“…Never-the-less, for completeness we summarize their results here. In these traditional modes, the LHC excludes VLQ masses M T 1.1 − 1.4 TeV in pair production searches [55][56][57] and M T 1 − 1.2 TeV in single production searches [58][59][60]. Single production of an SU (2) L singlet T depends on the mixing angle θ t L and decouples as θ t L → 0 [16] weakening the above limit.…”

“…Single production of an SU (2) L singlet T depends on the mixing angle θ t L and decouples as θ t L → 0 [16] weakening the above limit. Taking this into account, LHC searches for single T production have been cast into constraints on θ t L which are comparable to EW precision constraints for M T 1 TeV [58,61].…”

Exotic decays of top partners: Mind the search gap

“…ATLAS and CMS pursue an active search program for vector-like quarks, with several searches for pairproduced T targeting decays into T → t h, t Z, b W extended to the √ s = 13 TeV data [6][7][8][9][10][11][13][14][15]. Both collaborations provide bounds in benchmarks as well as in "branching ratio triangle" summary plots, assuming BR(T → t h) + BR(T → t Z) + BR(T → b W ) = 1.…”

“…Following the minimal coset [4], they are expected to decay into a top or bottom, plus a Standard Model (SM) massive boson: for a top partner of charge 2/3, T , the standard decay channels are thus T → b W + , t Z and t h [5]. Both ATLAS and CMS at the Large Hadron Collider (LHC) have extensive search programmes tailored to search for these states, leading to current bounds in the range 1.30 ÷ 1.42 TeV depending on their branching ratios (BRs) [6][7][8][9][10][11][12][13][14][15].…”

Viable low-scale model with universal and inverse seesaw mechanisms

“…All these ratios can be rewritten in terms of the heaviest mass, Models with vectorlike fermions are being tested at the LHC. The ATLAS collaboration has reported several analyses, in particular [49][50][51]. At the moment, mass exclusion limits for exotic isosinglet quarks give M B > 1.…”