1 Introduction
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ATLAS.bib \addbibresourcepaper_bib.bib \AtlasTitleSearch for pair production of a new heavy quark that decays into a boson and a light quark in collisions at  TeV with the ATLAS detector \AtlasRefCodeEXOT-2014-10 \PreprintIdNumberCERN-PH-EP-2015-212 \AtlasDateAugust 4, 2019 \AtlasJournalPRD \AtlasAbstractA search is presented for pair production of a new heavy quark () that decays into a boson and a light quark () in the final state where one boson decays leptonically (to an electron or muon plus a neutrino) and the other boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb of collisions at TeV collected by the ATLAS detector at the LHC. No evidence of production is observed. New chiral quarks with masses below 690 GeV are excluded at 95% confidence level, assuming BR. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of BR versus BR. \pdfstringdefDisableCommands\pdfstringdefDisableCommands

1 Introduction

The recent observation of the long-predicted Higgs boson by the ATLAS [Aad:2012tfa] and CMS [Chatrchyan:2012ufa] collaborations now completes the Standard Model (SM). In spite of its success, the SM cannot account for dark matter and the matter/antimatter asymmetry in the Universe and also fails to provide insight into the fermion mass spectrum, nonzero neutrino masses, why there are three generations of fermions, or why parity is not violated in the strong interaction. Furthermore, the observed Higgs boson is unnaturally light, requiring fine tuning to cancel radiative corrections that would naturally result in a mass many orders of magnitude larger, a discrepancy known as the hierarchy problem [Susskind:1978ms].

A variety of models have been proposed to address the various shortcomings of the SM. For example, a primary motivation for supersymmetry (SUSY) is to solve the hierarchy problem [Dimopoulos:1981zb]. In SUSY models, the quadratically divergent radiative corrections to the Higgs-boson mass due to SM particles are automatically canceled by the corrections from the supersymmetric partners. Models such as Little Higgs, Composite Higgs, and Topcolor take a different approach, proposing that electroweak symmetry breaking happens dynamically as the result of a new strong interaction [ArkaniHamed:2002qy, Contino, Dobrescu:1997nm, Hill:1994hp, Hill2003235]. Grand unified theories (GUTs) go further, unifying the electroweak and strong forces by proposing that the SM gauge symmetry is the low-energy limit of a single fundamental symmetry group such as or  [Kang:2007ib, Sultansoy], which could potentially explain the observed spectrum of fermions and even provide insight into the unification of the electroweak and strong forces with gravity. A feature in many of these, and other models [PhysRevD.41.1286, other1, other2], is the prediction of vectorlike quarks (VLQs), hypothetical spin- particles that are triplets under the color gauge group and have identical transformation properties for both chiralities under the electroweak symmetry group. Furthermore, massive VLQs would respect gauge invariance without coupling to the Higgs field. This allows VLQs to avoid constraints from Higgs-boson production [nochiral]; if the Higgs sector is minimal, these constraints rule out additional chiral quarks. However, some two-Higgs-doublet models are able to avoid those constraints and accommodate a fourth generation of quarks [BarShalom:2012ms].

In the models of interest, the VLQs have some mixing with the SM quarks, allowing them to decay to SM quarks and either a , , or Higgs boson; however, the exact nature of the coupling depends on the model. For example, in composite Higgs models, the VLQs are involved in a seesaw mechanism with the SM quarks, so the lightest VLQ couples almost exclusively to the heaviest SM quarks (- and -quarks) [Contino]. However, there are also models that predict \TeV-scale VLQs that could preferentially decay to light SM quarks ( or [Kang:2007ib, Sultansoy, Chakdar:2013tca]. For example, the left-right mirror model (LRMM) [Chakdar:2013tca] predicts three generations of heavy “mirror” quarks, with the lightest mirror generation coupling to the lightest SM generation. The two lightest mirror quarks could be pair-produced at the LHC via the strong interaction and would then decay to , , or ( or ). The LRMM would provide an explanation for tiny neutrino masses, parity violation in weak interactions, parity conservation in strong interactions, and could be the first step toward uncovering the symmetry structure of a GUT. Another model predicting VLQs that decay to light quarks is the GUT with isosinglet quarks [Kang:2007ib, Sultansoy]. In this model, after the symmetry is broken down to the SM group structure, VLQ partners to the -, -, and -quarks are predicted. If the VLQs have the same mass ordering as their SM partners, the lightest VLQ would couple predominately to first-generation SM quarks ( or ). The values for the branching ratios to the three decay modes (, , ) depend on parameters in the model. The values in the isosinglet model range from approximately (0.6, 0.3, 0.1) to (0.5, 0.25, 0.25), while the LRMM allows branching ratios from approximately (0.6, 0.4, 0) to (0, 0, 1), depending on the VLQ mass and mixing angles.

If such new quarks exist, they are expected to be produced predominantly in pairs via the strong interaction for masses up to ) in LHC collisions with a center-of-mass energy of 8 \TeV. Single production of a new heavy quark, , would be dominant for very high quark masses, but the production rate is model-dependent and could be suppressed if the coupling to SM quarks is small. To date, there have been two analyses of LHC data sensitive to VLQs that decay to light quarks, both using 1.04 

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