Hadronic Contributions to Precision Observables and New Physics Searches

• Applicant: Dr. Franziska Hagelstein
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449369623

The Standard Model (SM) of particle physics is remarkably successful. Yet, there are clear indications for physics beyond the SM (BSM). The search for BSM in lab-based experiments proceeds at three different frontiers: energy, precision, and intensity. These experimental efforts must be accompanied by theoretical studies of the SM predictions. The accuracy of the SM predictions is often limited by hadronic contributions, which are hard to calculate due to the complicated non-perturbative nature of quantum chromodynamics (QCD). The main aim of this project is to systematically improve the evaluation of hadronic contributions to precision observables, such as the muon anomalous magnetic moment (g-2) and the hyperfine structure of muonic hydrogen. To achieve this, we employ two approaches: i) the low-energy effective-field theory (EFT) of QCD, called Chiral Perturbation Theory (ChPT), and ii) dispersion relations (DRs) which allow for data-driven evaluations of hadronic contributions. In addition, we will study BSM candidates, like axion-like particles, called to explain deviations between experiment and SM.This project is divided into five topical areas:1) g-2. The accuracy of the SM prediction for the muon g-2 must be improved to complement the anticipated precision of the new Fermilab experiment. We will employ a new DR approach, based on the Schwinger sum rule, for the evaluation of hadronic contributions to g-2. We will also perform a feasibility study for the measurement of muon structure functions, aiming for a data-driven evaluation of hadronic contributions.2) µH. The forthcoming measurements of the hyperfine splitting in muonic hydrogen rely on theoretical inputs. We will revise the proton-structure corrections with a dispersive analysis based on a new global fit of proton structure function, implementing rigorous theory constraints. We will also extend the ChPT calculation to next-to-leading order.3) LFV. We will derive bounds on BSM interactions from new experimental limits for charged lepton flavour violating (LFV) processes, e.g. from Mu3e (PSI), using an EFT framework.4) ALPs. Axion-like particles (ALPs) will be studied to establish exclusion limits for their mass and couplings from precision quantities, such as g-2 or atomic spectroscopy. We will also perform feasibility studies for the confirmation of the neutral 17 MeV boson (X17) at low-energy electron- or positron-beam facilities, such as ELSA (Bonn), MAMI / MESA (Mainz), JLab and Frascati.5) DRs. To pursue a formal development of the dispersive framework, we will extend the well-known sum rules for electromagnetic moments and polarizabilities of spin-1/2 particles to spin-1, spin-3/2 and spin-2 particles. They will be used in the contexts of atomic spectroscopy, scattering experiments and gravitational-wave detection.

• DFG Programme: Independent Junior Research Groups