The Higgs boson has a decay that happens once in 10,000 times, and ATLAS found it

The ATLAS collaboration at CERN published a result this week that I thought was worth reading about. They have evidence for one of the rarest ways a Higgs boson can fall apart.

When the Large Hadron Collider makes a Higgs boson, it lasts for a tiny fraction of a second and then decays into other particles. Most of the routes it can take are fairly common. This one isn't. The Higgs goes to a real photon plus a "virtual" photon, and the virtual photon turns straight into two leptons, usually two electrons or two muons. Written out, that's H to gamma-star gamma to two leptons plus a photon. It happens in about one out of every 10,000 Higgs decays.

A virtual photon isn't a normal particle of light. It's a short-lived in-between state with a small mass that isn't quite zero, and it can't travel anywhere on its own. So what the detector actually sees is a photon and two leptons that all came from almost the same point.

ATLAS measured a signal about 1.5 times what the Standard Model predicts, with an uncertainty of 0.5, at 3.2 sigma. That counts as evidence, not a discovery. The measured value also sits well inside the error bars of the prediction.

The part I like is what this opens up. This decay gives physicists a way to get at CP symmetry, which is the question of whether particle interactions look the same when you swap particles for antiparticles and flip everything as if in a mirror. Collecting more data on this decay is how they plan to test it.

Source: https://www.sci.news/physics/higgs-boson-decay-virtual-photon-dilepton-15070.html