The Particle Physics Group of the Institute for Corpuscular Physics (IFIC), a joint center of the Spanish National Research Council (CSIC) and the University of Valencia (UV), has been contributing to the ATLAS experiment at the Large Hadron Collider (LHC, for its initials in English) for more than three decades. Their work covers both data analysis and the construction, operation, and upgrade of the detector itself. All its researchers actively participate in studies related to the top quark and the Higgs boson, as well as in the search for dark matter or signs of supersymmetry, among other potential indications of new physics beyond the Standard Model.

A major new step toward understanding the interaction between the Higgs boson and the top quark — and how it may reveal new physics

The ATLAS experiment, one of the major detectors at the LHC, has taken an important step forward in understanding how the Higgs boson, the particle that gives mass to the others, interacts with the top quark, the heaviest known elementary particle. For the first time, scientists from the ATLAS Collaboration have carried out a search specifically optimized to study the joint production of a single top quark and a Higgs boson (known as tH), an extremely rare process within the Standard Model and one that has not yet been observed experimentally. This result brings the scientific community closer to more precisely measuring the coupling between these two particles — a key interaction in the Standard Model of particle physics.

The associated production of the Higgs boson with a pair consisting of a top quark and a top antiquark has already been observed at ATLAS and CMS, but producing a Higgs boson alongside a single top quark remains one of the least explored processes. This mechanism is especially interesting because, due to the way the Higgs–top coupling enters the process, comparing its production rate with the Standard Model prediction could reveal hints of so-called CP violation, a phenomenon related to the asymmetry between matter and antimatter. Since the Standard Model does not fully explain why the Universe is composed almost exclusively of matter, any clue in this direction is of particular relevance.

IFIC’s role in the analysis

PhD researchers trained at IFIC, Pablo Martínez Agulló and Jesús Guerrero Rojas, under the supervision of researchers Susana Cabrera Urbán and Carlos Escobar Ibáñez (who also coordinates the ATLAS working group responsible for these analyses and others that study different Higgs boson decays, as well as the combination of all experimental channels to increase sensitivity), played an essential role in this work, which forms part of their doctoral theses. Their analysis focused on Higgs boson decays into different types of particles, allowing them to explore several experimental channels sensitive to tH production. These collisions can produce several leptons, a type of light particle: the most common are electrons, muons (heavier electrons), and tau leptons (heavier and shorter-lived).

The most sensitive analysis to date — and what comes next

Although the tH process is extraordinarily rare (with an inclusive cross section of 74.3 fb) and has not yet been experimentally observed, the new ATLAS analysis has set the most stringent limits obtained so far for this process. Specifically, ATLAS determines that the production rate of the tH process in LHC collisions does not exceed about 14 times the Standard Model prediction with 95% statistical confidence. This advance not only highlights the enormous experimental challenge of studying such a faint signal among millions of collisions, but also confirms that the techniques developed — from combining multiple channels to advanced artificial intelligence methods for lepton identification, Higgs decay reconstruction, and signal–background discrimination — are performing at the required level for future searches.

In alternative scenarios where the interaction between the Higgs boson and the top quark behaves differently from Standard Model expectations, such as those where the interaction changes under the exchange of matter and antimatter (known as a CP-odd component), the experiment obtains an even more restrictive upper limit compared to the Standard Model scenario, of 2.4. Although these figures indicate that the process cannot yet be claimed as observed, they represent an essential step forward, showing that the experiment’s sensitivity is improving rapidly and that the methods developed already meet the requirements for future data-taking campaigns — particularly at the High-Luminosity LHC (HL-LHC). These future datasets will allow much more sensitive studies of the Higgs–top coupling, one of the most important interactions for understanding why particles have mass and what role the Higgs boson plays in the evolution of the Universe.

This study has been published in the prestigious journal Journal of High Energy Physics (JHEP): https://doi.org/10.1007/JHEP10(2025)093.