Test of lepton universality with W boson decays

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This project aims to exploit the large LHCb Run-II dataset to make a precision test of lepton universality with W boson decays.In the standard model the W boson decays with equal branching fraction, up to tiny corrections, to the three generations of leptons.For more than a decade there has been an anomalous measurement, by experiments at the LEP collider, of the W branching ratio to third generation leptons.This is yet to be tested by any other experiment. Furthermore there have been several recent measurements by three different experiments, including LHCb, which hint at a violation of lepton universality in beauty hadron decays.LHCb is well suited to make the first test of the LEP anomaly, using muonic decays of the tau lepton.This is thanks to its ability to trigger on soft muons, and to resolve the subtle impact parameter signature of the tau decay.The idea will be to measure the ratio of the decay rate of W->mu nu compared to that of W -> tau nu, with the single muon final state.Developing a strategy to subtract the background from high transverse momentum charm hadron decays will be one of the main challenges.Novel statistical methods will be required in order to exploit different control samples and observables to constrain the effect of this background.This promises to be a very excited research project, which addresses a timely and important physics question, and has the potential for a high impact result.