Aims and scope

The CRC 1044 studies the role of hadron­s, which are subatomic particles built up from quarks and gluons, within the broader context of particle physics, atomic phy­sics, and nuclear astrophysics. To answer physics questions at both the highest and low­est energy scales, hadron physics plays a central and connecting role. In nearly all questions at the forefront of the mentioned research fields, the progress is limited by a missing quantitative knowledge of the strong interaction in the non-perturbative do­main of Quantum Chromo Dynamics (QCD). On the one hand, advancing this low-en­ergy frontier of the Standard Model has a direct impact on central questions in physics. The CRC aims firstly to make advances in the low-energy frontier of the Standard Model as it will have a direct impact on central quesitons in physics. Secondly, the CRC aims to gain new insights on the structure of hadrons, as well as on the question of how hadrons emerge out of ther constituent quarks and gluons.

In CRC 1044, we are in the position to fulfill these research goals by a strategic cooperation between the Mainz Microtron (MAMI), the Beijing Spectrometer BES-III, and the new Mainz Energy Recovering Superconducting Accelerator MESA, which is presently being constructed as a main initiative within the excellence cluster PRISMA. With its beam with a hitherto unrivaled intensity, it will allow for new measurements reaching from a search for light hypothetical particles, beyond the Standard Model of particle physics, till ultra-high precision measurements of the proton charge radius.   Through a unique combination of measurements in electron scattering (MAMI and MESA), in electron-positron physics (BESIII), with state-of-the-art theoretical calculations, CRC 1044 will allow to decisively advance the low-energy frontier.

The highlights of CRC 1044 are the most precise measurement worldwide of the weak mix­ing angle in electron-proton scattering, as well as measurements and theoretical cal­culations which will lead to an improved knowledge of the anomalous magnetic mo­ment of the muon. A new measure­ment campaign of nucleon form factors and polarizabilities, combined with more refined theoretical analyses, allows to importantly improve on the limiting fac­tors in the interpretation of high precision tests of the Lamb shift in muonic atoms and to shed light on the proton radius puzzle. A photon-photon physics program, aimed at extracting me­son transition form factors, allows to derive more precise constraints on the light-by-light hadronic contribution to the anomalous magnetic moment of the muon. Further­more, studies of spectroscopy will allow for an interpretation of recently discovered new exotic mesons, composed of four quarks. Finally, measurements in nuclear systems allow to study the nuclear equation of state, addressing important questions in astrophysics, such as the detailed structure of neutron stars.