Lepton Flavour Violation at the LHC : a search for $Z\to e\tau$ and $Z\to\mu\tau$ decays with the ATLAS detector

In the Standard Model of particle physics, three lepton families (flavours) are part of the fundamental blocks of matter. All families have the same properties, except their mass. In addition, each family acts independently of the others, which is known as Lepton Flavour Conservation. Such conservation is assumed in the equations of the Standard Model, without a fundamental theoretical motivation. Since the formulation of the Standard Model, neutrino oscillation experiments have demonstrated that Lepton Flavour Conservation is violated in Nature. Yet, there is no experimental evidence that such violation occurs in processes involving only charged leptons. An observation of Lepton Flavour Violation among charged leptons would be an exciting sign of new particles or new interactions beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider at CERN sets a new strong constraint on Lepton Flavour Violation effects in weak interactions, searching for $Z$ boson decays into a $\tau$-lepton and another lepton of different flavour ($e$ or $\mu$) with opposite electric charge. Using a combination of LHC Run 1 and Run 2 proton-proton collision data, the branching fractions for these decays are now measured by the ATLAS experiment to be less than $9.5\times10^{-6}$ ($\mu\tau$) and $8.1\times10^{-6}$ ($e\tau$) at 95% confidence level, superseding the otherwise best limits set by the LEP experiments more than two decades ago.