In a new result presented at Moriond EW, physicists at the ATLAS Collaboration tested lepton flavour universality between muons and electrons. The precision of the result stands as the best yet-achieved in W-boson decays by a single experiment and surpasses the world average.

The ATLAS Collaboration has released a new study into a key building block of matter: leptons. This type of particle comes in three different families (flavours) and, according to the Standard Model, should follow strict rules. For instance, except for their mass, leptons of different flavours have identical properties – a feature known as lepton flavour universality. This was recently corroborated by a key measurement of the W-boson decay rates into leptons by the ATLAS Collaboration.

This week, at the LHCP 2020 conference, the ATLAS Collaboration presented a precise measurement of lepton flavour universality using a brand-new technique. Physicists examined collision events where pairs of top quarks decay to pairs of W bosons, and subsequently into leptons. They then measured the relative probability that this lepton is a muon or a tau-lepton – a ratio known as R(τ/μ). According to the Standard Model, R(τ/μ) should be unity – but there has been long-standing tension with this prediction, ever since it was measured at the Large Electron-Positron (LEP) collider in the 1990s.