Astroparticle Physics News

Rare Higgs boson decay lighting the way

The Higgs boson was first observed at the LHC  by ATLAS and CMS in 2012, and confirmed in 2013. Many of the predicted properties of the Higgs boson have since been observed at the LHC. New evidence of a rare Higgs boson decay indicates such detailed studies may be the new norm, and by studying the decay angles may shed light on CP symmetry violation.

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It was announced on February 2, 2021, that ATLAS has found evidence for a rare Higgs boson decay – the Higgs boson decaying into a photon (\(\gamma\)) and a lepton pair (\(\ell \ell \)). Confirming predicted decay branches for the Higgs boson enables further investigation of its properties and testing of the Standard Model.


Higgs Boson decay branches to two leptons and a photon:




The Higgs boson can decay into a photon and lepton pair, namely electron-positron or muon-antimuon pairs, in three main unique ways. ATLAS observed the branch produced by an intermediate virtual photon, emphasized in blue on the left. The ATLAS team chose to investigate events where the dilepton mass was less than 30 GeV, so that this decay branch would dominate over the other two. The likelihood of the resulting Higgs boson signal being a false positive is 3.2𝜎.

Evidence of rare Higgs boson decay branches is expected to become more frequent with studies of the vast amounts of data from the High Luminosity LHC programme, of which TRIUMF is a collaboration institute.

Studying this decay branch specifically lights the way to studying charge conjugation and parity (CP) symmetry in Higgs boson reactions and if these decays have a preferred direction. Investigating the origins of CP symmetry violation is part of the search for physics beyond the Standard Model.