Unveiling the Higgs boson’s Secrets: New ATLAS Results Illuminate Rare Decays and Probe Beyond the Standard Model
The 2012 discovery of the Higgs boson at the Large Hadron Collider (LHC) marked a monumental achievement in particle physics. Though, the story didn’t end there. Understanding how the Higgs boson interacts with other particles, and whether its behaviour perfectly aligns with the predictions of the Standard Model, remains a central quest for physicists. Recent results presented by the ATLAS Collaboration at the 2025 European Physical Society Conference on High Energy Physics (EPS-HEP) represent significant progress on this front, focusing on two exceptionally rare Higgs boson decay pathways and pushing the boundaries of our knowledge. These findings, built upon years of meticulous data collection and analysis, offer tantalizing glimpses into potential new physics beyond our current understanding.
Why Rare Decays Matter: Probing the Foundations of reality
The Higgs boson, responsible for giving mass to fundamental particles, doesn’t decay in a single, predictable way.It can transform into various combinations of other particles, each with a specific probability known as a branching fraction. While some decays are relatively common, others are incredibly rare. It’s precisely these rare decays that hold the key to unlocking deeper truths about the universe.
The ATLAS Collaboration focused on two particularly elusive processes: the decay of the Higgs boson into a pair of muons (H→μμ) and its decay into a Z boson and a photon (H→Zγ).
H→μμ: A Window into Fermion Mass Generation: The decay into two muons is exceptionally rare, occurring in only about 1 in 5,000 Higgs boson decays. However, it’s crucial because it provides the most sensitive probe of the Higgs boson’s interaction with second-generation fermions - the muons themselves. Understanding this interaction is vital to unraveling the mystery of how different generations of particles acquire their mass. Discrepancies from Standard Model predictions could point to new particles or forces influencing this process.
H→Zγ: Searching for New Physics in a Quantum loop: The decay into a Z boson and a photon is even more intriguing. This decay doesn’t happen directly; it proceeds through a quantum “loop” involving virtual particles. This loop is a theoretical construct, and the particles contributing to it are not necessarily limited to those already known to us. The presence of new, undiscovered particles within this loop would subtly alter the decay rate, offering a potential signature of physics beyond the Standard Model.The Challenge of Finding Needles in a Haystack
Identifying these rare decays is a formidable task. The signal – the evidence of the Higgs boson decaying in these specific ways – is incredibly faint, buried within a vast background of events produced by other, more common processes. Imagine searching for a few specific grains of sand on a massive beach.
For H→μμ, researchers meticulously searched for a slight excess of muon pairs with a combined mass of 125 GeV – the known mass of the Higgs boson. This signal is easily obscured by the sheer number of muon pairs created through other interactions.The H→Zγ decay presents an even greater challenge. The Z boson itself decays only about 6% of the time into detectable leptons (electrons or muons), further reducing the signal strength.Moreover, the increased collision rate in LHC Run 3, while providing more data, also leads to more overlapping events and “jets” of particles that can mimic real photons, complicating the analysis.
Advanced Techniques for Unprecedented Sensitivity
To overcome these hurdles, the ATLAS physicists employed a suite of sophisticated techniques. They combined data from the first three years of LHC Run 3 (165 fb-1,collected between 2022-2024) with the complete Run 2 dataset (140 fb-1,from 2015-2018),maximizing the available statistics. Crucially, they also:
Refined Background Modeling: Developed advanced algorithms to accurately model and subtract the background noise, isolating the potential signal.
Categorized production Modes: Analyzed events based on how the Higgs boson was produced, allowing for more targeted searches.
* Optimized Event Selection: Implemented improved criteria for selecting events likely to contain the desired decay signatures.Evidence for H→μμ and Enhanced Sensitivity for H→Zγ
The results are compelling. Previous searches using the full Run 2 dataset hinted at the H→μμ decay, reaching a significance of 2 standard deviations. Now, with the combined Run
