First evidence for rare Higgs-boson decay

2 February 2021

A report from the ATLAS experiment

Evidence for the decay of the Higgs boson to a photon and a low-mass electron or muon pair, propagated predominantly by a virtual photon (γ*), H → γ*γ → ℓℓγ (where ℓ = e or μ), has been obtained at the LHC. At an LHC seminar today, the ATLAS collaboration reported a 3.2σ excess over background of H → ℓℓγ decay candidates with dilepton mass mℓℓ < 30 GeV.

The H → ℓℓγ decay is particularly interesting as it is a loop process

The measurement of rare decays of the Higgs boson is a crucial component of the Higgs-boson physics programme at the LHC, since they probe potential new interactions with the Higgs boson introduced by possible extensions of the Standard Model. The H → ℓℓγ decay is particularly interesting in this respect as it is a loop process and the three-body final state allows the CP structure of the Higgs boson to be probed. However, the small expected signal-to-background ratio and the typically low dilepton invariant mass make the search for H → ℓℓγ highly challenging.

ATLAS H to 2 leptons gamma plot

Bump hunt

The analysis performed by ATLAS searched for H → e+eγ and H → μ+μγ decays. Special treatment was needed in particular for the electron channel: a dedicated electron trigger was developed as well as a specific identification algorithm. The predicted mℓℓ spectrum rises steeply towards lower values, with a kinematic cutoff at twice the final-state lepton mass. At such low electron–positron invariant masses, and given the large transverse momentum of their system, the electromagnetic showers induced by the electron and the positron in the ATLAS calorimeter can merge, requiring a specially developed reconstruction. Furthermore, a dedicated identification algorithm was developed for these topologies, and its efficiency was measured in data using photon detector-material conversions at low radius into an electron–positron pair from Z → ℓℓγ events.

Rare decay candidate in ATLAS

The signal extraction is performed by searching in the ℓℓγ invariant mass (mℓℓγ) range between 110 and 160 GeV for a narrow signal peak over smooth background at the mass of the Higgs boson. The sensitivity to the H → ℓℓγ signal was increased by separating events in mutually exclusive categories based on lepton types and event topologies. ATLAS reports evidence in data for a H → ℓℓγ signal emerging over the background with a significance of 3.2σ (see figure). The Higgs boson production cross section times H → ℓℓγ branching fraction, measured for mℓℓ < 30 GeV, amounts to 8.7+2.8–2.7 fb. It corresponds to a signal strength – the ratio of the measured cross section times branching fraction to the Standard Model prediction – of 1.5 ± 0.5. With this, ATLAS has also extended the invariant-mass range of the lepton pair for the related Higgs-boson decay into a photon and a Z boson to lower masses, opening the door to future studies of three-body Higgs-boson decays and investigations of its underlying CP structure.

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