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

Abstract: 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.

Overview of the Paper on Vector-like Quark Pairs and Four Top Quarks Production in ATLAS Detector

The paper titled "Search for production of vector-like quark pairs and of four top quarks in the lepton-plus-jets final state in $pp$ collisions at $\sqrt{s}=8\tev$ with the ATLAS detector" presents a comprehensive analysis from the ATLAS Collaboration at CERN. This study investigates the possibility of producing vector-like quark pairs and four top quarks in proton-proton collisions, specifically targeting the lepton-plus-jets final state.

Research Background and Purpose

Vector-like quarks are theoretical particles predicted in models that extend the Standard Model (SM) of particle physics, including Little Higgs and Composite Higgs models. These quarks are expected to couple preferentially to third-generation quarks and could provide insight into the mechanism behind electroweak symmetry breaking. The paper also explores scenarios predicting enhanced rates of four top-quark events, a rare occurrence in the SM, but potentially more common in specific beyond the Standard Model (BSM) theories.

Experimental Setup and Methodology

The research utilized data from $pp$ collisions at $\sqrt{s}=8\tev$, recorded by the ATLAS detector in 2012, with an integrated luminosity of 20.3 fb$^{-1}$. The ATLAS detector's capability to identify final states with isolated high-energy leptons and jets was pivotal in these investigations.

Three key analyses were conducted:
- $T \to Wb$ Analysis: Targeting vector-like $T$ quarks decaying into a $W$ boson and a $b$ quark.
- $T \to Ht$ Analysis: Focused on decays where $T$ quarks produce Higgs ($H$) bosons and top quarks ($t$).
- $B \to Hb$ Analysis: Investigating vector-like $B$ quarks decaying into a Higgs boson and a $b$ quark.

Dedicated techniques identifying jets and determining lepton isolation were employed to suppress SM background, primarily from $\ttbar$ production and $W/Z$+jets processes.

Results and Interpretations

No significant excess events were observed above the SM predictions. Consequently, stringent upper limits were established on the masses of vector-like $T$ and $B$ quarks under multiple branching ratio hypotheses:
- Vector-like $T$ Quarks: The paper sets 95% confidence level (CL) lower limits on $T$ quark mass, ranging between 715 GeV and 950 GeV.
- Vector-like $B$ Quarks: Similar analyses on $B$ quarks produced constraints, concluding that masses below certain thresholds depending on decay branching ratios are highly improbable.

These limits on vector-like quarks are the most stringent results obtained to date, significantly advancing the exclusion zones for these particles.

Implications and Future Work

The stringent mass lower bounds for vector-like $T$ and $B$ quarks suggest limited feasibility for their existence at mass scales below the derived limits, under the given branching ratios. The work significantly contributes to the ongoing exploration of BSM physics, guiding future searches to higher energy and luminosity regimes, possibly at $\sqrt{s}=13\tev$.

Additionally, the results contribute to refining theoretical models. The non-observation of significant deviations reinforces constraints on BSM theories predicting vector-like quarks and processes involving four top quark production.

Concluding Remarks

This paper exemplifies the vital role of high-energy collider experiments and sophisticated detector technology in probing new physics. The methodologies highlighted will continue to play a crucial role in future experimental physics endeavors, particularly as models evolve and new theoretical predictions arise.