Thirty years after the discovery of the top quark: the field enters an age of refinement and subtlety

TL;DR

Thirty years after the top quark's on-shell discovery, the field has entered an era of refinement and subtlety. The paper surveys new observations of top-quark processes such as $t\bar{t}\gamma\gamma$ and $tWZ$, high-precision determinations of $m_t$ and differential $t\bar{t}$ cross-sections, and extensive exploration in extreme phase spaces. It also covers direct searches for vector-like quarks and indirect probes through EFT, and reports on quantum effects like entanglement and quasi-bound-state phenomena near the $t\bar{t}$ threshold. Together these results illustrate the role of the top quark as both a precision SM probe and a sensitive portal to beyond-SM physics, enabled by improved object reconstruction, calibration, and statistical inference.

Abstract

Thirty years after the first observation of on-shell top quarks the investigation of the heaviest elementary particle remains a thriving field of basic research, as was illustrated by the 18th edition of the annual Workshop on Top-Quark Physics hosted by Hanyang University in Seoul, Korea. Observing new scattering processses involving top quarks, precision measurements of top-quark properties, and the usage of top quarks as a means of exploration remain key elements of research, but are most recently complemented by the observation of even more subtle effects based on the application of refined experimental techniques. This article summarises the most striking experimental results presented at the conference.

Figures (2)

• Figure 1: Ratio of the unfolded differential cross-sections as a function of $p_\mathrm{T}(t\;\mathrm{or}\; \bar{t})$. The shaded band indicates the total measurement uncertainty. The cross-sections are compared with theoretical predictions obtained from POWHEG+PYTHIA$\,$8 with different PDF sets. The uncertainty on the theoretical predictions are the MC statistical uncertainties only.
• Figure 2: Observed (points with statistical error bars) and predicted (stacked colored histograms) $m(t\bar{t})$ distribution in three out of nine bins of the two angular observables $c_\mathrm{hel}$ and $c_\mathrm{han}$ quantifying the degree of spin correlation between the top quark and the top antiquark. In the upper panels, the $t\bar{t}$ histogram shows the fixed-order perturbative prediction in quantum chromodynamics after the fit to the data that includes the pseudoscalar $\eta_t$ signal model (whose contribution is not drawn). The shown event rates are divided by the bin width. The lower panels display the ratio of the data to the prediction (without the pseudoscalar signal), with the $\eta_t$ signal overlaid at its best fit cross-section (red line). The gray band indicates the postfit uncertainty.