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The tau lepton is a heavy cousin of the electron, living less than a trillionth of a second. It usually decays into lighter particles, but never directly into three muons: such a transition is forbidden by known laws of particle physics. If it did happen, it would mean unknown forces are at play.

The Standard Model, the best map of the microworld, predicts that the tau decay into three muons is practically impossible. But many theories that try to go beyond it allow this process with a probability accessible to observation. So searching for this decay is like checking whether there's a door in the wall where the map doesn't show one.

ATLAS physicists analyzed proton collision data at 13 TeV collected over three years. They looked for events where a W boson is produced, decaying into a tau and a neutrino, and the tau then turns into three muons. Special algorithms picked such events out of billions. In the end, no signal was found: the data are consistent with background.

The absence of the decay allowed setting a new limit: the probability of tau decay into three muons is less than 8.7×10⁻⁸: that is, less than once in 11 million tau decays. This is 20% stricter than the previous limit and provides a benchmark for future theories and experiments.

Original abstract

as written by the authors arXiv:2603.18099

A search for charged lepton flavour violation in $$\tau \to 3\mu$$ decays is performed in $$pp$$ collisions at a centre-of-mass energy of 13 TeV using ATLAS data collected between 2016 and 2018, corresponding to an integrated luminosity of 137 $$\text{fb}^{-1}$$. The search is optimised for the electroweak $$W \to \tau u$$ production channel, due to its higher trigger efficiency, but also considers potential signal events from heavy-flavour production. Data are collected using two-muon and three-muon triggers and a multivariate analysis is used to separate the signal from the background. An unbinned likelihood fit is then performed to the resulting three-muon invariant mass spectrum and the data are found to be compatible with the background-only hypothesis. The observed (expected) limit on the branching ratio $$B(\tau\to3\mu)$$ is found to be $$8.7\times 10^{-8}$$ ($$7.5\times 10^{-8}$$) at 90% CL.