Review of Particle Physics: Top Quark review

1 61. Top Quark 61. Top Quark Revised September 2023 by T.M. Liss (City Coll. of New York), F. Maltoni (CP3 U. catholique de Louvain; Bologna U.) and A. Quadt (Göttingen U.). 61.1 Introduction and SM theory overview In the Standard Model (SM), the left-handed top quark is the Q = 2/3, T3 = +1/2 member of the weak-isospin doublet containing the bottom quark, while the right-handed top is an SU (2)L singlet (see, e.g., the review “Electroweak Model and Constraints on New Physics” here). Its phenomenology is driven by its large mass, mt ' v√2 , where v = 246 GeV is the vacuum expectation value of the Higgs field, i.e., it is the only quark whose Yukawa coupling to the Higgs boson is of order unity. As it is heavier than the W boson, it is the only quark that decays semi-weakly into a real W boson and a b quark. This results in a lifetime that is shorter than the time that is needed for strong interactions to modify its properties or to bind it into a hadron. In this respect, the top is the only quark that, in its brief life, behaves as a free one. Because of its large mass and Yukawa coupling to the Higgs boson, it also gives important contributions (via loops) to Standard Model precision observables measured at lower scales. For these reasons, the top quark provides a unique laboratory to test our understanding of mat- ter and fundamental interactions at the ElectroWeak (EW) symmetry-breaking scale and beyond. In addition, since it is abundantly produced at the Large Hadron Collider (LHC), a precise knowl- edge of its properties (mass, couplings, production cross sections, decay branching ratios, etc.) can be achieved. This review provides a concise discussion of the experimental and theoretical issues involved in the determination of the top-quark properties. 61.1.1 Properties 61.1.1.1 Mass The top quark mass determines to a large extent the unique phenomenology of this quark. In the SM quark masses are derived parameters from the Yukawa coupling and the vacuum expectation value. At the loop level, with the Higgs boson discovery and per mil measurement of its mass, the values of the W -boson and top quark mass are correlated, so that their precise determinations provides a strong test of the SM (see Section 10 "Electroweak Model and constraints on new physics" of this Review). At present there is some tension at the 1.7 σ level, between the indirect top quark (pole) mass determination from electroweak precision data (176.12 ± 1.9 GeV) and the combination of direct measurements at the LHC that yields 172.52 ± 0.14(stat.) ± 0.30(syst.) GeV [1]. The top mass value is also critical in the issue of vacuum stability in the SM [2–4]. At high scales, the Higgs quartic coupling λ evolves to increasingly small values as mt grows. Above mt = 171 GeV, i.e., very close to the most precise measurements, λ becomes negative at the Planck scale leading to a meta-stable electroweak vacuum, while for slightly larger values, mt > 176 GeV the electroweak vacuum would become unstable. Current top-quark measurements therefore point to a Higgs quartic coupling which is nearly vanishing at the Planck scale. While being quite suggestive, in absence of a clear UV picture this argument allows only to conclude that comparison between our best SM predictions and the data does not imply new physics below the Planck scale (see Section 11.2.3 of "Status of Higgs Boson Physics" in this Review). Given its importance and the large samples of top quarks produced at the LHC, the top mass determination has been one of the most important goals in the precision measurement campaign of the ATLAS and CMS collaborations. The basic methodology rests on the idea of fitting mt- dependent kinematic distributions to fully-exclusive (Monte Carlo) predictions, via the full or partial reconstruction of the system of the t and ¯t decay products. These are called “direct measurements” R.L. Workman et al. (Particle Data Group), Prog. Theor. Exp. Phys. 2022, 083C01 (2022) and 2023 update 1st December, 2023 11:10am