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High precision flavour physics using lattice QCD

Final Report Summary - HPFPLATTICEQCD (High precision flavour physics using lattice QCD)

The flavour physics program plays a dominant role in testing the Standard Model (SM) of particle physics and searching for New Physics (NP), providing information which is complementary to direct searches in colliders, such as the LHC at CERN. NP effects could be unveiled through the observation of deviations from the SM predictions via high-precision measurements of low-energy observables in high-luminosity experiments.

The tests that flavour physics provide are limited not by the available energy but by the available precision in both experiment and theory. The main problem in the comparison of experiment and theory is that experimental measurements are of hadrons, not quarks, as the theory is. So, in order to connect the underlying quark level parameters to experimental measurements, a theoretical description of how quarks are confined into hadrons is needed with high precision. The confinement is produced by the strong interactions (Quantum Chromodynamics (QCD)), whose dynamics can be parametrised in terms of so-called hadronic matrix elements. The only way to do this ab initio and with errors at small as required by experiment is using numerical simulations of the theory, lattice QCD.

The main scientific objective of the research in this grant was the high precision calculation of the hadronic matrix elements needed for the analysis of the current and the forthcoming experimental flavour data using lattice QCD. The final goals are to perform the most stringent tests of the SM, try to unveil NP, and put constraints on the possible new theories.

We have carried out a number of projects studying different flavour observables. We have compared our theoretical predictions with experiment and checked for consistency of the SM description. Our results are all state-of-the-art calculations providing in almost all cases the most precise results for the quantities studied and significantly reducing uncertainties in previous studies (if any). These results are being used (and will be used) by other groups in the search for NP hints and to establish constraints on NP theories.

Furthermore, we have found several tensions at the 2-3 standard deviations level between our SM theoretical predictions and experimental measurements, as well as internal inconsistencies in the SM description. Several of those discrepancies point towards an emerging global tension, which could be an indication of NP. Additional work is still needed to confirm or disregard those tensions. Nevertheless, during these four years we have made very important progress towards that direction, identify several observables that are showing up hints of tensions and are sensitive to the discovery of NP, as well as developing new methodologies that will allow to increase the precision for those and other observables.

The PI of the project has now a permanent position at the host institution and is working on establishing her own group. The CIG has been very useful, among other things, in promoting her within the host institution. She has been selected to be the PI of one of the national projects (the main source of local funding) that the host group is applying for.