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University of Liverpool physicists are celebrating a major step towards one of the most sought-after discoveries in particle physics, as a new result from the ATLAS experiment at CERN provides tantalising hints of Higgs boson pair production.
More than a decade after the discovery of the Higgs boson, one of the biggest unanswered questions in particle physics is how the Higgs boson interacts with itself. The Standard Model of particle physics predicts that Higgs bosons should interact through a process known as the Higgs self-coupling. Measuring this interaction is crucial to understanding the shape of the Higgs potential, which underlies the mechanism through which elementary particles acquire mass.
The Higgs potential also has profound implications for our understanding of the Universe. It played a central role in the evolution of the early Universe, and its shape may have influenced the transition that occurred as the Universe cooled after the Big Bang. Understanding the Higgs self-coupling therefore provides an important window into the conditions of the early Universe and may help explain why matter came to dominate over antimatter. The stability of the Higgs potential is also connected to the ultimate fate of our Universe.
University of Liverpool researchers connected to the Particle Physics frontier, led by Professor Carl Gwilliam, are working to shed light on this fundamental interaction by searching for the simultaneous production of two Higgs bosons – known as di-Higgs or HH production – at the Large Hadron Collider (LHC).
This process is extraordinarily rare and can only be observed in the high-energy proton-proton collisions at the LHC. Even there, it is predicted to occur only once in every trillion collisions, making it around a thousand times less likely than the production of a single Higgs boson. Despite extensive searches, Higgs boson pair production has yet to be definitively observed.
The search for this rare process is one of the major goals of the ATLAS experiment at the LHC. ATLAS is a global collaboration involving researchers from universities and laboratories around the world, with the UK being one of the largest contributors. The University of Liverpool is one of the biggest UK groups involved in ATLAS, with Liverpool researchers contributing to the development and operation of the experiment as well as to searches for new physics and precision measurements of the Higgs boson and other Standard Model (SM) particles.
In its search for the first evidence of this process, the ATLAS Collaboration has now released a new result using the channel in which one Higgs boson decays into two bottom quarks and the other into two tau leptons (HH → bbττ). This is one of the most sensitive channels for searching for Higgs boson pair production. The analysis uses the complete dataset collected by the LHC during Run 2 between 2015 and 2018, together with the first two years of Run 3 data collected in 2022 and 2023
The new result has been made possible in part by substantial improvements in particle reconstruction and identification algorithms. Liverpool Postdoctoral Researcher Dr Jordy Degens leads the ATLAS tau reconstruction and identification group, while the wider Liverpool ATLAS group played a key role in calibrating the algorithms used to identify bottom quarks.
To further separate the potential HH signal from SM background processes that can mimic it, the analysis uses a state-of-the-art transformer-based multivariate discriminant. Liverpool researchers also played a leading role in estimating the remaining backgrounds, with doctoral students Mr Bhuepsh Dixit and Mr Lennox Wood studying backgrounds arising from top-quark pair production and Z-boson production, respectively.
The analysis finds a moderate excess of events in the data compared with the background-only expectation (Figure 1). This corresponds to a significance of 2.6 standard deviations (σ), while a significance of 1.2σ would be expected from Standard Model HH production (Figure 2). The result represents the most sensitive probe of Higgs boson pair production to date.
Professor Carl Gwilliam said: “This result shows intriguing hints of Higgs pair production. While it falls short of the 3σ needed to claim first evidence of the di-Higgs process, it shows that this will be possible once the full Run 3 LHC dataset already recorded is analysed, which is extremely exciting.”
The paper has been submitted to European Physical Journal C, and the results are among a wide range of new ATLAS results being presented at the 43rd International Conference on High Energy Physics (ICHEP) in Brazil.
The LHC is currently shut down for a major upgrade that will significantly increase its data collection rate. The definitive observation of Higgs boson pair production is one of the flagship goals of the resulting High-Luminosity LHC (HL-LHC) programme.
This result marks a key milestone for particle collider physics and the worldwide effort to understand the Higgs boson. For Liverpool researchers, and the ATLAS-UK collaboration in general, it represents another significant step in the quest to measure the Higgs boson's self-interaction. As the precision of these measurements continues to improve, they will provide a unique window into the Higgs field itself, helping to reveal how our Universe acquired its present form and whether new physics shaped its earliest moments.
Fig 1: The combined run-2 and -3 event yields in data (black points) compared to that expected from the signal (red histogram) and backgrounds (other histograms) as a function of the log of the signal to background ratio, S/B. The lower panel shows the statistical significance (“pull”).of the data relative to the background, compared to the best fit signal contribution (solid line) and the SM prediction (dashed line)
Fig 2: Observed and expected HH significances obtained from the run-2 and run-3 datasets, along with their combination.