Searches for lepton flavour violation in the tau -> 3mu decay with the ATLAS Experiment

Flavour is not a fundamental symmetry and we know that it is violated in the
Standard Model (SM), both in the neutrino- and quark-sectors, via mixing. It
is thus natural to expect charged lepton flavour violation (cLFV) but, despite
hints of flavour anomalies at g-2 experiment and B factories, it has never
been seen in nature and its observation would be unambiguous evidence of
beyond the SM physics. While stringent limits have been set in the electron
and muon channels, the constraints in the tau case are much weaker. Since
new physics is likely to couple more strongly to the heavier tau, this makes it
an ideal place to search. The project will search for LFV tau decays using the
full run-2 and partial run-3 ATLAS data using the golden tau -> 3 muon
channel, with its clean final state. The student will first analyse tau produced
in heavy flavour decays, developing methods to estimate the backgrounds
and multi-variate techniques to extract the signal. Following this, the work
will be extended to taus produced in W boson decays and the results of the
two searches combined. Together, these have the potential to provide
sensitivity beyond the current limit and, if new physics is not observed, set
the most stringent limit on LFV tau decays. The group has previous wide
experience in both W boson measurements and searches in tau final states.