Physicists from the ATLAS Collaboration at CERN’s Large Hadron Collider (LHC) have found evidence of the Higgs boson decaying into a ‘virtual’ photon and a real photon, which in turn produces a pair of leptons. This pathway, known as H→γ*γ→llγ, occurs in just one in about 10,000 Higgs decays, making it one of the particle’s most elusive behaviors.
Event display of a candidate Higgs boson decay to a photon and a lepton pair, from proton-proton collisions recorded with the ATLAS detector in 2022. Image credit: ATLAS Collaboration / CERN.
“Every decay mode of the Higgs boson offers a different window into the fundamental laws of nature,” the ATLAS physicists said in a statement.
“Rare decays are particularly interesting as they provide unique opportunities to test the Standard Model in regimes where subtle effects of new physics could become visible.”
“Recent studies of such rare Higgs-boson decays — including decays to a photon and a Z boson and to a pair of muons — have begun to unlock this potential, with more still to be explored.”
“One particularly elusive decay is the Higgs-boson decay to a virtual photon and a photon.”
Unlike an ordinary photon, which is stable and massless, the virtual photon involved in this process exists only fleetingly, has a non-zero mass and decays instantly.
Because the decay proceeds through a lepton pair rather than a second photon, it offers a lens for examining subtle properties of the Higgs boson, including tests of so-called CP symmetry.
To gather enough data on the rare event, the ATLAS researchers pooled results from two separate runs of the Large Hadron Collider: three years of Run-3 collisions from 2022 to 2024 and the full Run-2 dataset gathered between 2015 and 2018.
They searched for collision events containing a photon paired with two low-mass leptons emitted so close together that they nearly overlap, particularly challenging when the leptons are electrons rather than muons.
While the ATLAS experiment can readily identify nearby muons, identifying two close-by electrons is extremely difficult.
To solve the problem, the scientists built a machine-learning tool based on a boosted decision tree to spot these so-called merged electrons, along with specialized calibration and detection systems.
Combining the two datasets roughly doubled the amount of information available for analysis, and the resulting signal — a small bump in the data near the Higgs boson’s known mass of 125 GeV — closely matched theoretical predictions.
The statistical significance of the finding reached 3.4 standard deviations, enough to qualify as evidence, though short of the traditional five-sigma threshold physicists require to claim a discovery.
“This result demonstrates the remarkable capabilities of the ATLAS experiment and the power of innovative analysis techniques, which allow physicists to explore some of the Higgs boson’s rarest decays,” the physicists said.
“As the High-Luminosity LHC era approaches, researchers will be able to explore these elusive processes with unprecedented precision, providing new opportunities to test the Standard Model and search for signs of new physics.”
The results will be published in the Journal of High Energy Physics.
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ATLAS Collaboration. 2026. Measurement of the Higgs boson decay to a low-mass dilepton system and a photon in pp collisions at s√= 13 and 13.6 TeV with the ATLAS detector. Journal of High Energy Physics, in press; arXiv: 2608.03369
