ATLAS Top-Quark Physics
The top quark is the heaviest known elementary particle, with a mass of approximately 173 GeV, comparable to that of a gold atom. Its exceptionally large mass makes it particularly interesting for understanding the mechanism of electroweak symmetry breaking and searching for physics beyond the Standard Model (SM). The top quark has a Yukawa coupling to the Higgs boson close to unity, much larger than that of any other known fermion, suggesting that it may play a special role in the electroweak sector.
Discovered in 1995 by the CDF and DØ experiments at Fermilab, the top quark differs from other quarks in another remarkable way: its lifetime of approximately 5 × 10−25 seconds is shorter than the typical timescale of hadronization. It therefore decays before forming bound hadronic states, preserving information about its spin and other properties in the angular distributions of its decay products. This makes the top quark a unique laboratory for precision tests of the Standard Model and searches for new interactions.
The Large Hadron Collider (LHC) produces top quarks abundantly, predominantly in top-quark–antiquark pairs (tt̄) through the strong interaction, but also individually through electroweak processes. The large datasets collected by the ATLAS experiment at centre-of-mass energies of 13 and 13.6 TeV enable increasingly precise measurements of top-quark properties, studies of rare production processes, and searches for phenomena beyond the Standard Model.
At the University of Siegen, our research spans precision measurements, top-quark interactions with electroweak and Higgs bosons, rare top-quark production and decays, and searches for new physics. Advanced machine-learning techniques and the identification of jets originating from bottom and charm quarks are central ingredients of many of these analyses.
Precision measurements of top-quark properties
Precise measurements of top-quark production and decay provide stringent tests of quantum chromodynamics (QCD) and electroweak interactions. Our group studies top-quark spin correlations using dileptonic tt̄ events, exploiting the fact that the spin information of the top quarks is transferred to their decay products. These measurements probe the quantum properties of top-quark pairs and can reveal deviations from the Standard Model.
Another important research direction is the determination of the top-quark mass and decay width. We investigate improved theoretical descriptions of the W+W−bb̄ final state, including interference between singly and doubly resonant top-quark production, to reduce modelling uncertainties and improve the extraction of these fundamental parameters.
Recent ATLAS measurements have revealed a significant enhancement of the tt̄ production cross section close to the production threshold. This observation is consistent with the formation of short-lived, QCD-induced quasi-bound top-quark pairs, often referred to as toponium. Understanding these effects provides a new opportunity to study strong interactions and top-quark dynamics near threshold.
Top quarks in association with electroweak bosons
The production of top-quark pairs together with photons or electroweak bosons provides direct access to the interactions of the top quark with the gauge sector. These processes are particularly sensitive to possible deviations from the Standard Model and can be interpreted within the framework of Standard Model Effective Field Theory (SMEFT).
Our group has contributed to measurements of tt̄Z and tt̄γ production, including differential cross sections and studies of the top-quark charge asymmetry. More recently, ATLAS achieved the first observation of tt̄γγ production, a particularly rare process involving two top quarks and two photons.
We also investigate rare processes involving single top quarks produced in association with additional electroweak bosons, including tWZ production. With the increasing LHC datasets, such measurements provide complementary probes of the electroweak structure of the Standard Model and possible new interactions.
Top quarks and the Higgs boson
The large top-quark Yukawa coupling makes processes involving both top quarks and Higgs bosons especially important. The associated production of a Higgs boson with a top-quark pair (tt̄H) provides direct access to this coupling, while precision measurements of tt̄ production can probe effects from virtual Higgs-boson exchange.
Our group contributes to measurements of top-associated Higgs production, with particular emphasis on Higgs-boson decays into charm-quark pairs, H → cc̄, in tt̄H events. These analyses rely on advanced heavy-flavour identification and connect top-quark physics with the broader Higgs-physics programme and the Cluster of Excellence Color meets Flavor. Further details can be found on our ATLAS Higgs-Physics page.
Rare top-quark production and searches for new physics
Processes involving several top quarks are particularly sensitive to new physics because of the top quark's large mass and its potentially enhanced interactions with new heavy particles. A prominent example is the simultaneous production of four top quarks (tt̄tt̄), one of the rarest Standard Model processes observed at the LHC.
Following the first evidence reported by ATLAS in 2020, four-top-quark production was observed in 2023 with a significance of 6.1 standard deviations. The measured cross section, 22.5+6.6−5.5 fb, is consistent with the Standard Model prediction within uncertainties. Four-top production provides an important probe of the top-quark Yukawa coupling, possible new particles, and effective four-fermion interactions.
Our research also explores rare top-quark interactions that could provide direct evidence of physics beyond the Standard Model. These include flavour-changing neutral-current (FCNC) interactions, in particular decays of the type t → Hq, where q is an up or charm quark. Such decays are extremely suppressed in the Standard Model, so their observation at experimentally accessible rates would constitute a clear sign of new physics.
We have also contributed to searches for charged-lepton-flavour violation involving top quarks, including interactions that produce a muon, a tau lepton and an up-type quark. Another research direction investigates heavy right-handed Majorana neutrinos produced in top-quark events. These searches probe possible extensions of the Standard Model associated with lepton-flavour violation and the origin of neutrino masses.
Quark flavour and precision measurements of the CKM matrix
Top-quark events also offer new opportunities to study quark flavour mixing. In 2026, ATLAS reported the first measurement of the Cabibbo–Kobayashi–Maskawa (CKM) matrix element |Vcb| using on-shell W-boson decays in tt̄ events, with an important contribution from our group.
The analysis exploits the rare decay W → cb̄, corresponding to the decay chain t → bcb̄, and relies on the identification of both bottom- and charm-quark jets. The result, |Vcb| = (50+11−14) × 10−3, is compatible with measurements from B-meson decays. Although currently less precise, this method provides a complementary determination using on-shell W-boson decays rather than hadronic B-meson decays and demonstrates the potential of top-quark events for precision flavour physics.
Advanced analysis methods and outlook
Many of our top-quark analyses rely on advanced machine-learning methods for event classification, reconstruction and the identification of heavy-flavour jets. In particular, transformer-based flavour-tagging algorithms and improved secondary-vertex reconstruction are essential for analyses with multiple bottom and charm quarks, such as tt̄H (H → cc̄), rare top-quark decays and the measurement of |Vcb|. Further details of our work on these methods can be found on the ATLAS Flavour-Tagging page.
With the increasing Run-3 datasets and the forthcoming High-Luminosity LHC, these techniques will allow us to investigate progressively rarer processes and improve the precision of top-quark measurements. Our goal is to exploit the unique properties of the top quark to deepen our understanding of the Standard Model and explore possible new physics at the highest accessible energies.