Pair Production Of Vector-like Quarks Research Articles

Search for pair production of vector-like quarks in leptonic final states in proton-proton collisions at sqrt{s} = 13 TeV

A search is presented for vector-like T and B quark-antiquark pairs produced in proton-proton collisions at a center-of-mass energy of 13 TeV. Data were collected by the CMS experiment at the CERN LHC in 2016–2018, with an integrated luminosity of 138 fb−1. Events are separated into single-lepton, same-sign charge dilepton, and multi-lepton channels. In the analysis of the single-lepton channel a multilayer neural network and jet identification techniques are employed to select signal events, while the same-sign dilepton and multilepton channels rely on the high-energy signature of the signal to distinguish it from standard model backgrounds. The data are consistent with standard model background predictions, and the production of vector-like quark pairs is excluded at 95% confidence level for T quark masses up to 1.54 TeV and B quark masses up to 1.56 TeV, depending on the branching fractions assumed, with maximal sensitivity to decay modes that include multiple top quarks. The limits obtained in this search are the strongest limits to date for textrm{T}overline{textrm{T}} production, excluding masses below 1.48 TeV for all decays to third generation quarks, and are the strongest limits to date for textrm{B}overline{textrm{B}} production with B quark decays to tW.

Search for pair production of vectorlike quarks in the fully hadronic final state

The results of two searches for pair production of vector-like T or B quarks in fully hadronic final states are presented, using data from the CMS experiment at a center-of-mass energy of 13 TeV. The data were collected at the LHC during 2016 and correspond to an integrated luminosity of 35.9 fb$^{-1}$. A cut-based analysis specifically targets the bW decay mode of the T quark and allows for the reconstruction of the T quark candidates. In a second analysis, a multiclassification algorithm, the "boosted event shape tagger," is deployed to label candidate jets as originating from top quarks, and W, Z, and H. Candidate events are categorized according to the multiplicities of identified jets, and the scalar sum of all observed jet momenta is used to discriminate signal events from the quantum chromodynamics multijet background. Both analyses probe all possible branching fraction combinations of the T and B quarks and set limits at 95% confidence level on their masses, ranging from 740 to 1370 GeV. These results represent a significant improvement relative to existing searches in the fully hadronic final state.

Hunting for vectorlike quarks

We analyze decays of vectorlike quarks in extensions of the standard model and a two Higgs doublet model. We identify several typical patterns of branching ratios of the lightest new up-type quark, t4, and down-type quark, b4, depending on the structure of Yukawa couplings that mix the vectorlike and standard model quarks (we assume only mixing with the third generation) and also on their doublet or singlet nature. We find that decays into heavy neutral or charged Higgs bosons, when kinematically open, can easily dominate and even be close to 100%: b4 → Hb at medium to large tan β, t4 → Ht at small tan β and b4 → H±t, t4 → H±b at both large and small tan β. The pair production of vectorlike quarks leads to 6t, 4t2b, 2t4b and 6b final states. The decay modes into W, Z and h follow the pattern expected from the Goldstone boson equivalence limit that we generalize to scenarios with all possible couplings. We also discuss in detail the structure of Yukawa couplings required to significantly deviate from the pattern characteristic of the Goldstone boson equivalence limit that can result in essentially arbitrary branching ratios.

Exotic signals of vectorlike quarks

Vectorlike fermions are an important target for hadron collider searches. We show that the vectorlike quarks may predominantly decay via higher-dimensional operators into a quark plus a couple of other standard model fermions. Pair production of vectorlike quarks of charge 2/3 at the LHC would then lead to a variety of possible final states, including , or . Additional channels (, , , etc) arise in the case of a vectorlike quark of charge −1/3. If the vectorlike quark decays into three light quarks, then the signal is more difficult to observe, and the vectorlike quark mass is almost unconstrained by current searches.

Search for pair production of vector-like quarks in the [formula omitted] channel from proton–proton collisions at [formula omitted

A search is presented for the production of vector-like quark pairs, TT‾ or YY‾, with electric charge of 2/3 (T) or −4/3 (Y), in proton–proton collisions at s=13TeV. The data were collected by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.8fb−1. The T and Y quarks are assumed to decay exclusively to a W boson and a b quark. The search is based on events with a single isolated electron or muon, large missing transverse momentum, and at least four jets with large transverse momenta. In the search, a kinematic reconstruction of the final state observables is performed, which would permit a signal to be detected as a narrow mass peak (≈7% resolution). The observed number of events is consistent with the standard model prediction. Assuming strong pair production of the vector-like quarks and a 100% branching fraction to bW, a lower limit of 1295 GeV at 95% confidence level is set on the T and Y quark masses.

Search for production of vector-like quark pairs and of four top quarks in the lepton-plus-jets final state in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV with the ATLAS detector

A search for pair production of vector-like quarks, both up-type ($T$) and down-type ($B$), as well as for four-top-quark production, is presented. The search is based on $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 20.3 fb$^{-1}$. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets. Dedicated analyses are performed targeting three cases: a $T$ quark with significant branching ratio to a $W$ boson and a $b$-quark ($T\bar{T} \to Wb$+X), and both a $T$ quark and a $B$ quark with significant branching ratio to a Higgs boson and a third-generation quark ($T\bar{T} \to Ht$+X and $B\bar{B} \to Hb$+X respectively). No significant excess of events above the Standard Model expectation is observed, and 95% CL lower limits are derived on the masses of the vector-like $T$ and $B$ quarks under several branching ratio hypotheses assuming contributions from $T \to Wb$, $Zt$, $Ht$ and $B \to Wt$, $Zb$, $Hb$ decays. The 95% CL observed lower limits on the $T$ quark mass range between 715 GeV and 950 GeV for all possible values of the branching ratios into the three decay modes, and are the most stringent constraints to date. Additionally, the most restrictive upper bounds on four-top-quark production are set in a number of new physics scenarios.

Search for pair-produced vector-like quarks with the ATLAS detector

The high energy frontier opened by the LHC is allowing us to explore physics scenarios where new physics might lay. The need to go beyond the Standard Model (SM) comes from various unanswered questions such as where does the matter-antimatter asymmetry comes from? What is the nature of Dark Matter? How can the hierarchy problem be solved? The recent discovery of an Higgs-like boson tends to disfavour the existence of a heavy 4th generation of quarks which would change the Higgs SM cross section and branching ratio in a way that is not experimentally observed. At the same time, vector-like quarks become a more compelling possibility due to their important role stabilizing the Higgs boson mass against r adiative corrections. The purpose of this poster is to review the latest results in the searches for pair production of vector-like quarks at the ATLAS experiment.