Transforming sensitivity to new physics with single-top-quark events
4 December 2025 | By
The top quark, the heaviest known elementary particle, has captivated physicists since its discovery in the 1990s and holds a privileged place within the Standard Model of particle physics. It has a very short lifetime and is thus the only quark that decays before forming bound states, allowing physicists to study its properties directly through its decay products. The top quark’s strong interaction to the Higgs boson also makes it a powerful probe of the Higgs mechanism, which gives particles their mass. Because of its large mass, it could also be especially sensitive to interactions with as-yet-unknown particles or forces at higher energy scales. If such effects exist, the Large Hadron Collider (LHC), often called a “top-quark factory”, is the ideal place to uncover them. Searching for subtle deviations in how top quarks are produced and decay therefore offers unique clues about physics beyond the Standard Model.
While most top quarks at the LHC are produced in pairs, proton–proton collisions can also yield single top quarks through electroweak interactions. During LHC Run 2 (2015–2018), the ATLAS experiment recorded more than 42 million single-top-quark events at a centre-of-mass energy of 13 TeV. In a new analysis by the ATLAS Collaboration, researchers used this vast dataset to test whether the top quark is hiding signs of new physics. Their findings set comprehensive limits on a broad class of new effects in top-quark physics, encompassing potential new forces of nature and the particles that transmit them. Those effects are described by a theory known as Effective Field Theory (EFT).
A window into new physics
EFTs provide a model-independent framework to describe the possible influence of new particles that are too heavy to be produced directly by an accelerator. New particles and interactions would modify the way elementary particles interact with each other. Studies of single-top-quark production in the t-channel are especially powerful for investigating these effects. It is the only process that produces a strongly polarised top quark – that is, where the top quark’s spin is aligned in a given direction – which allows researchers to precisely study the angular structure of its decay. Any deviation from Standard-Model expectations in these angular correlations could signal new interactions, some of which might even violate the combined symmetry of charge and parity (CP violation).
Mapping angular information with Fourier precision
To fully exploit this sensitivity, ATLAS physicists developed a novel technique to analyse the pattern of decay products radiated during the decay of top quarks. The underlying formalism, published in 2017, provides a complete mathematical description of the most common decay of a single top quark: where the top quark decays into a W boson and a bottom quark, with the W boson subsequently decaying into a lepton and its associated neutrino (t → Wb → ℓνb, see Figure 1).
Each single-top-quark event is characterised by four angles that describe the decay geometry and capture the spin correlations and possible CP-violating effects. The novel technique developed by ATLAS analyses the four-dimensional data in the domain of frequencies, a method made famous by the French mathematician Joseph Fourier. Some examples of possible radiation patterns in top-quark decay are shown in Figure 2. Fourier techniques are widely used in fields such as cosmology, medicine, image processing, and music, helping identify patterns hidden in complex data by, for example, separating signals from noise, analysing brain scans, efficiently encoding visual and audio information, or breaking down musical sounds into their fundamental tones. Fourier techniques also play a key role in detector signal processing, helping to clean, reconstruct and interpret the fast electronic pulses produced in the ATLAS subdetectors. Similarly, at the accelerator level, Fourier analysis is essential for monitoring beam stability, identifying oscillations in the proton bunches, and optimising LHC performance.
Beyond its elegance and efficiency, ATLAS’ Fourier-based method provides a complete description of the decay kinematics, capturing both the total production rate and subtle angular distributions.
The new result represents the most complete study of single-top-quark production and decay carried out by the ATLAS Collaboration. Physicists achieved groundbreaking levels of sensitivity to potential new interactions in the top-quark sector, establishing a new benchmark. Through comparisons with Standard-Model predictions, researchers also placed stringent constraints on multiple parameters governing top-quark interactions.
The new result represents the most complete study of single-top-quark production and decay carried out by the ATLAS Collaboration.
Tightening the bounds on new interactions
ATLAS researchers studied seven EFT parameters that describe the fundamental interaction of top quarks, which can be observed in their production and decay. Those interactions appear as dark circles in Figure 1. Some of these parameters can take both a magnitude and a phase. If the phase were non-zero, it would point to a difference between matter and antimatter known as CP violation.
The results agree with the Standard Model predictions (see Figure 3), and they set the most stringent constraints so far on several of these parameters. The analysis improves upon previous ATLAS and CMS results by combining information from both angular distributions and total production rates. It is also the first study to simultaneously extract several of these parameters sensitive to CP violation using top-quark events.
A framework for the future
This work demonstrates a powerful new way to explore the top quark’s interactions and to test the Standard Model with high precision. The Fourier-based framework can be readily extended to other processes where angular information carries key insights, including top-quark-pair production, top-quark production associated with a W boson, or even Higgs-boson production.
As the LHC enters its high-luminosity phase, the much larger datasets expected from Run 3 and beyond will allow ATLAS to refine this method further, perhaps revealing the first hints of new physics through the delicate angular fingerprints left by nature’s heaviest known particle.
