FOR 5327:  Photon-photon interactions in the Standard Model and beyond - exploiting the discovery potential from MESA to the LHC

• Subject Area: Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 458854507

Photon-photon interactions play a central role in the Standard Model (SM) of particle physics. It is fascinating to realize that these gamma-gamma interactions, and, in particular, the light-by-light (LbL) scattering, are effects of purely quantum origin. A precise calculation of these effects in the SM is, however, not always straightforward - major obstacles are posed by the non-perturbative nature of the strong interaction, described by quantum chromodynamics (QCD). The Research Unit (RU) intends to overcome these obstacles with important consequences for various aspects of SM physics. The team of researchers of the RU has broad and complementary expertise in various aspects of photon-photon physics, both in theory and experiment. By including international experts, this RU will expand its expertise and advance the understanding of photon-photon interactions as a precision and discovery tool, and thereby have a decisive impact on hadron and particle physics. More specifically, the RU will perform: - the world’s best determination of meson transition form factors (Project TFF). Measurements at the BESIII experiment in Beijing will be complemented by determinations from Dalitz decays at A2-MAMI in Mainz as well as dispersive studies in phenomenology and lattice QCD calculations.- partial wave analyses of exotic mesons in the charmonium sector (Project XYZ). This will be achieved through a collaborative effort between the BESIII collaboration and theorists within the RU and will allow to determine the nature of these exotic states of hadronic matter.- measurements at the ATLAS experiment at the LHC and theoretical interpretations of the direct light-by-light reaction gamma-gamma into gamma-gamma (Project LBL). - world-class searches for axion-like particles in the MeV to TeV mass range (Project ALP). These searches will be carried out in very different physics environments at BESIII, ATLAS, A2-MAMI, and - at a later stage - at the new MESA facility in Mainz.As can be seen from these issues, photon-photon physics may hold the key to new discoveries, whether these are new exotic mesons, ALPs, or precision effects beyond the SM. The most important and pressing of these SM tests is the relation of photon-photon physics to the anomalous magnetic moment of the muon, for which very recently a deviation on the level of 4.2 standard deviations between the SM prediction and its direct measurement has been observed. By a joint effort of experiment, dispersive analyses and lattice QCD calculations (Joint Research Project, JRP), we will constrain the HLbL contribution to the anomalous light-by-light contribution with the precision level targeted by the future precision of the Fermilab (US) and JPARC (Japan) experiments.

• DFG Programme: Research Units
• International Connection: China, Poland, Switzerland

Projects

• Axion-like Particle Searches (Project ALP) (Applicants Denig, Achim ;  Vanderhaeghen, Marc )
• Coordination Funds (Applicant Denig, Achim )
• Exotic Meson Spectroscopy (Project XYZ) (Applicants Danilkin, Igor ;  Denig, Achim ;  Redmer, Christoph Florian ;  Vanderhaeghen, Marc )
• Hadronic Light-by-Light Contribution to the Muon g-2 (Applicants Danilkin, Igor ;  Hagelstein, Franziska ;  Meyer, Ph.D., Harvey Byron ;  Wittig, Hartmut )
• Light-by-Light Scattering (Project LBL) (Applicants Meyer, Ph.D., Harvey Byron ;  Pascalutsa, Ph.D., Vladimir )
• Meson Transition Form Factors (Project TFF) (Applicants Danilkin, Igor ;  Denig, Achim ;  Redmer, Christoph Florian ;  Vanderhaeghen, Marc )

• Spokesperson: Professor Dr. Achim Denig