Precise Standard Model corrections to New Physics searches with unitarity tests of the Cabibbo-Kobayashi-Maskawa quark mixing matrix

• Applicant: Dr. Mikhail Gorshteyn
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 495329596

The discovery of the Higgs boson has completed the quest for the particles required by the symmetries of the Standard Model (SM) and breaking thereof. Extremely successful in particle physics, SM fails to describe a body of cosmological observations, which motivates searches for hidden particle and interaction sectors beyond SM (BSM). If new heavy particles are present in nature, they should become visible at the energy frontier reachable with colliders. No signs of BSM have been seen at the LHC so far, suggesting that BSM particles may be heavier than originally expected, a hypothesis that can be tested with the future colliders. Surprisingly, several hints to possible BSM signals are observed at lower energies with the elements of the Cabibbo-Kobayashi- Maskawa (CKM) quark mixing matrix. Statistically significant tensions are observed in b-quark decays. The unitarity constraint in the top row shows a 2.3σ deficit and may point to the existence of non-SM right-handed currents. For a BSM signal to be unambiguously identified, precise theoretical calculations of SM corrections are vital. Due to confinement, weak processes involving quarks are only accessible with their bound states, hadrons. Quantum Chromodynamics (QCD) that describes this binding and the properties of the compound is very complex, and a perturbation expansion at low energies has only limited applicability. These difficulties can be overcome by supplementing theoretical calculations with inputs from data. Quantities extracted from other high-precision experiments and used as input for CKM unitarity tests, in their turn, rely on theory assumptions that must be controlled. This calls for approaches that are data-driven but reexamine all theory ingredients entering the quantities deduced from data. In the first funding period I developed such a data-driven method for extracting Vud from superallowed nuclear beta decays and applied it to the pertinent corrections. The method combines ab initio nuclear theory calculations with the nuclear charge radii deduced from the spectra of muonic atoms and parity-violating electron scattering. Application of this strategy to the Al-26m --> Mg-26 superallowed beta decay led to a reduction of the unitarity deficit by half. In this extension project, I will pursue the innovative data-driven approach that builds upon my expertise in radiative corrections to electroweak processes and is enriched by the cooperation with experts on nuclear theory, QED calculations and phenomenology. It will deliver crucial theoretical ingredients for constraining BSM with CKM unitarity, and will address (i) nuclear structure effects in β decay, muonic atoms and electron scattering, resulting in (ii) Vud from superallowed decays with reduced nuclear uncertainties, and (iii) direct bounds on scalar BSM with superallowed beta decays.

• DFG Programme: Research Grants
• International Connection: India, Israel, USA
• Co-Investigators: Professorin Dr. Sonia Bacca; Professorin Dr. Concettina Sfienti
• Cooperation Partners: Professor Dr. Ben Ohayon; Professor Dr. Bijaya Kumar Sahoo; Dr. Chien-Yeah Seng