Theory for the FCC-ee : Report on the 11th FCC-ee Workshop Theory and Experiments: Geneva, Switzerland 08 - 11 Jan 2019

The Future Circular Collider (FCC) at CERN, a proposed100km circular facility with several collidersin succession, culminates in a100TeV proton–proton collider. It offers a vast new domain of explorationin particle physics, with orders-of-magnitude advances in terms of precision, sensitivity, and energy.The implementation plan published in 2018 foresees, as a first step, an electroweak factory electron–positron collider. This high-luminosity facility, operating at centre-of-mass energies between 90 and365GeV, will study the heavy particles of the Standard Model (SM), Z, W, and Higgs bosons, andtop quarks with unprecedented accuracy. The physics programme offers great discovery potential:(i) through precision measurements, (ii) through sensitive searches for symmetry violations, forbidden,or extremely rare decays, and (iii) through the search for direct observation of new particles withextremely small couplings. The electroweak factorye+e−collider constitutes a real challenge to thetheory and to precision calculations, triggering the need for the development of new mathematicalmethods and software tools. A first workshop in 2018 focused on the first FCC-ee stage, the Tera-Z, andconfronted the theoretical status of precision Standard Model calculations on the Z boson resonanceto the experimental demands.The second workshop, in January 2019, extended the scope to the next stages, with the pro-duction of W bosons (FCC-ee-W), the Higgs boson (FCC-ee-H), and top quarks (FCC-ee-tt). In par-ticular, the theoretical precision in the determination of the crucial input parameters,αQED,αQCD,MW, andmt, at the level of FCC-ee requirements was thoroughly discussed. The requirements onStandard Model theory calculations were spelt out, so as to meet the demanding accuracy of theFCC-ee experimental potential. The discussion of innovative methods and tools for multiloop calcu-lations was deepened. Furthermore, phenomenological analyses beyond the Standard Model were dis-cussed, including effective theory approaches. The reports of 2018 and 2019 serve as white papers ofthe workshop results and subsequent developments.