Precise predictions for beautiful observables Using b-hadron decays to probe the Standard Model and seek for New Physics

• Applicant: Dr. Nico Gubernari
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558599025

The Standard Model of Particle Physics (SM), despite its impressive experimental success, cannot be the ultimate theory of Nature. In fact, it is unable to explain phenomena such as gravity, dark matter, and the matter-antimatter asymmetry. Direct searches for new particles in high-energy collider experiments have so far found no evidence of New Physics (NP). Flavour physics provides complementary means to test the SM up to very high energy scales using indirect searches. These searches look for deviations in the experimental measurements from the calculated SM predictions. In this way, potential effects of NP can be probed at energy scales beyond the reach of terrestrial collider experiments. The thorough study of b-hadron (e.g. B-meson and Λ_b-baryon) decays over the last two decades has led to strong constraints on NP, which is one of the major achievements of flavour physics to date. Nevertheless, more accurate and precise theoretical predictions are needed to further constrain NP and potentially claim a new discovery. This task is particularly challenging for these decays, since the accuracy of most current predictions is spoiled by the hadronic matrix elements (MEs), which are extremely difficult to calculate. The overarching objective of this Emmy Noether group is to make a breakthrough in the theoretical precision of b-hadron decays. This objective will be achieved by developing new tools for calculations and analyses (such as new B and Λ_b distribution amplitudes and novel parametrizations for the MEs) and by calculating the unknown power corrections to the MEs in b-hadron decays (such as B→Kμμ, B→πμμ, Λ_b→Λμμ, and Λ_b→Λ_cμν). My proposal is timely, as the experimental precision of several key observables has surpassed the precision of the SM predictions. In addition, the LHC Run 3 and the Belle II programme will collect an enormous amount of data in the coming years, which will lead to a further reduction of the experimental uncertainties. Therefore, a massive improvement on the theoretical side is urgently needed to fully exploit the experimental data. This improvement will have an impact on the field in three main directions. First, more precise predictions will allow us to further constrain NP. Second, they are decisive for understanding whether the tensions between theory and experiment observed in b-hadron decays are due to an underestimation of theoretical uncertainties or are real signs of NP. Third, a higher precision of the theoretical predictions will result in a better determination of the fundamental parameters of the SM (e.g. |V_ub| and |V_cb|).

• DFG Programme: Emmy Noether Independent Junior Research Groups
• International Connection: France
• Cooperation Partner: Dr. Méril Reboud