The Higgs Left a Three-Particle Whisper
ATLAS has found evidence for a Higgs decay expected only about once in every 10,000 cases. One photon escapes intact while another exists briefly enough to split into charged matter.
The One-in-Ten-Thousand Decay
The Higgs boson has a rare route out. Instead of producing two ordinary photons, it can produce one real photon and one virtual photon — a fleeting quantum intermediary with non-zero mass that immediately becomes an electron-positron or muon-antimuon pair.
ATLAS physicists report evidence for that process, written H→γ*γ→ℓℓγ, after combining proton-collision data from two runs of the Large Hadron Collider. The decay is expected in only about one in 10,000 Higgs decays.
A particle that vanishes almost instantly opened a new window into the Higgs.
Finding a Bump in the Noise
The ATLAS analysis used 139 inverse femtobarns of Run 2 data collected from 2015 to 2018 and 164 inverse femtobarns from the first three years of Run 3. Researchers selected events containing a photon plus a low-mass pair of electrons or muons, then searched their combined mass for a small excess near the Higgs mass of 125 gigaelectronvolts.
The two leptons can emerge so close together that the detector sees overlapping electron showers. ATLAS built specialised triggers, calibrations and a machine-learning classifier to recover those merged signatures.
- The combined signal strength was 1.03 times the Standard Model prediction.
- Its uncertainty was approximately plus 0.35 and minus 0.32.
- The observed significance reached 3.4 standard deviations.
That is strong evidence, not the five-sigma threshold particle physicists conventionally call a discovery. The result also resolves an earlier Run 2 measurement that sat above the Standard Model but carried much larger uncertainty.
Why a Virtual Photon Matters
This decay happens through quantum loops involving other virtual particles. Because one photon carries mass before splitting into charged leptons, its geometry exposes information unavailable when the Higgs produces two real photons. Larger datasets could use that geometry to probe charge-parity properties and search for subtle departures from the Standard Model.
No departure appears here. The measured rate agrees closely with the current theory. That agreement is still valuable: new physics often has fewer places to hide after a rare prediction survives contact with data.
The Higgs did not break the rules. It whispered through one of their narrowest passages.