Lepton Flavour Universality – measuring BR(W->τν)/BR(W->μν) at the LHC – Christopher Young (CERN)

In the Standard Model on of the fundamental principles is that the couplings of the W-boson to the different lepton flavours (e, mu, tau) should be identical. Therefore, outside of small phase space effects, the branching ratio of W->lν should be identical for all flavours of lepton. The most precise previous measurements of the branching ratios of the W boson to the different lepton flavours were from the Large Electron-Positron Collider (LEP) at CERN and showed some tension with the Standard Model. This seminar will describe an ATLAS analysis at the LHC measuring the ratio BR(W->τν)/BR(W->μν) with higher precision than the previous LEP measurements to test this tension with the Standard Model expectation. Top–anti-Top events are used to provide a large sample of W-bosons with which to perform the measurement. One top is then used to trigger and tag the events and the other is used to test the ratio in question.