A search for charged lepton-flavour violating tau decays to 3 muons with the ATLAS Experiment

Student: Conor McPartland
Supervisor: Carl Gwilliam

Particle physicists describe nature by a standard model that organises the fundamental building blocks of matter into 3 families known as flavours, one of these families is the charged lepton family made up of the electron and its heavier siblings the muon and tau. The standard model does not necessarily conserve flavour as has been seen with quarks and neutrinos, for example neutrinos have been shown to oscillate between flavours. Given these observations, it is natural to question if charged leptons violate flavour. If this is found to be the case it would be unambiguous evidence of physics beyond the standard model. Searches for lepton-flavour violating decays of electrons and muons have led to stringent limits. On the other hand, the constraints on tau decays are much less stringent. Given that new physics is more likely to be strongly coupled to the heavier tau, it makes a good starting point to search for lepton flavour violations.

In order to search for the tau -> 3 muon decay, this project will make use of the large data set of high-energy proton--proton collisions provided by the Large Hadron Collider to the ATLAS experiment from 2015 to 2018. Large samples of tau leptons are produced in the decay of heavy flavour hadrons and W bosons and both channels will be investigated. Using state-of-the-art analysis methods including machine learning methods, these channels together will provide a world-leading sensitivity far exceeding the current limits. If no such decay is discovered new, more stringent limits, should be able to be set.