Isospin breaking effects in hadronic precision observables

ended

Lattice quantum chromodynamics (QCD) allows for first-principle theory predictions of hadronic observables in the non-perturbative regime of the strong interaction. In this approach QCD is solved by means of Monte Carlo simulations in finite volumes on a Euclidean space-time lattice. For high precision observables such as the anomalous magnetic moment of the muon the prediction of hadronic contributions computed by Lattice QCD has reached a level of precision at which isospin breaking effects (IBE), i.e. effects caused by the electric charge of quarks due to quantum electrodynamics (QED) and the mass non-degeneracy of up and down quarks, cannot be neglected and therefore have to be included into the theory computation. Due to an existing tension between the experimental determination and the theory prediction the anomalous magnetic moment of the muon is a candidate for BSM physics. The hadronic vacuum polarisation contribution to this observable dominates the overall uncertainty of the theory prediction. As the uncertainties of the experimental results will significantly decrease in the upcoming years a precise prediction is of high importance. With a few exceptions IBE are commonly only included at the level of valence quark effects in quark-connected contributions. This project aims to compute IBE in the valence quark sector in the hadronic vacuum polarisation contribution and in related observables by means of Lattice QCD+QED, also focussing on operator renormalisation and quark-disconnected contributions, where noise reduction techniques have to be applied. In particular, the renormalisation constant of the local vector current is determined based on the vector Ward identity and valence-quark IBE in quark-connected and quark-disconnected contributions are computed. The fellowship allows the fellow to pursue research goals in this field and to further sharpen his profile. It marks the next crucial step in his academic career aiming for a long-term position.