Time will tell for rare beauty decays

The standard model of particle physics is arguably the most successful theoretical paradigm ever developed, accounting for a huge range of fundamental phenomenon with stunning (though at times somewhat frustrating) accuracy. That said, there are several well-rehearsed reasons for believing that it is incomplete: its inability to explain the nature of dark matter, the observed cosmological matter-antimatter asymmetry, or the hierarchy problem between the electroweak and Planck scales, to name a few.

Theories that address these problems often predict the existence of new particles at or just above the electroweak scale, which is currently being forensically probed by experiments at the Large Hadron Collider. However, despite an extraordinary range of direct and indirect searches, firm experimental evidence of physics beyond the standard model has proved elusive.

In recent years, several measurements performed by the LHCb experiment of rare beauty decays have revealed significant tensions with the predictions of the standard model, which could indicate the presence of new fundamental particles. Observed in transitions of beauty quarks into a strange quark and two charged leptons (b?sl+l-), the 'B anomalies' have generated substantial interest in the theoretical community and motivated a range of theoretical proposals, which generally posit leptoquarks or Z prime bosons with masses in the TeV range.

Though apparently compelling evidence for the violation of lepton flavour universality was recently overturned, a number of significant tensions persist in the branching ratios and angular observables of b?sµ+µ- decays. However, since these observables are affected by substantial theoretical uncertainties from hadronic physics, the overall picture is rather unclear at present.

I will conduct a new programme of time-dependent measurements of rare beauty decays in order to test the standard model, search for signs of new physics and clarify the B anomalies. These measurements will provide complementary information to time-integrated observables and have not yet been explored at any experiment due to limited data samples. However, in the next few years the upgraded LHCb experiment will collect around five times more rare beauty decays than recorded so far.

My track record in rare decays and time-dependent analyses, and my leading role at the LHCb experiment makes me perfectly placed to explore this exciting new frontier.

The proposed programme includes time dependent studies of Bs0?µ+µ-, Bs0?fµ+µ-, B0?KSµ+µ- and B0?KSe+e- decays, giving access a range of new observables. I will perform the world's most precise measurement of the Bs0?µ+µ- effective lifetime, which can reveal new physics contributions not apparent in the branching fraction.

A similar measurement of the Bs0?fµ+µ- effective lifetime will be used to probe new physics that could be inducing the observed 3.6s tension in the decay's branching ratio as well as a milder tension in the longitudinal polarisation.

Core to this project will be achieving a substantive leap in the ability to 'tag' initial flavour of neutral B mesons. Using state-of-the-art recurrent neural networks, I aim to more than double the current tagging power for rare decays, making a range of tagged time-dependent CP-asymmetry measurements possible for the first time. These measurements give access to entirely new observables, many of which are essentially free from hadronic uncertainties, offering clean null tests of the standard model.

In addition, I will ensure the research achieves wider impact through a dedicated public engagement programme of talks, articles and podcasts.