Periodic Reporting for period 1 - SNPF (Searching New Physics using Flavour)
Berichtszeitraum: 2023-10-01 bis 2025-09-30
The Standard Model of particle physics provides an exceptionally precise description of known elementary particles and their interactions, yet it cannot explain several fundamental phenomena such as the nature of dark matter or the origin of matter-antimatter asymmetry in the Universe. One of the most promising ways to reveal signs of new physics is through high-precision studies of flavour physics, where quarks change type (“flavour”). Experiments in Europe and worldwide are now reaching unprecedented accuracy, and fully exploiting their results requires theoretical predictions of comparable precision.
Lattice QCD is the only first-principles method capable of computing the effects of the strong interaction with systematically improvable uncertainties. As lattice calculations have become increasingly precise, it has become essential to also include contributions that were previously neglected, such as the effects of electromagnetic interactions and strong isospin breaking. Incorporating these corrections consistently is crucial for producing reliable Standard-Model predictions for precision flavour observables.
The SNPF project addressed two central challenges in this area, focusing on objectives that strengthen the theoretical foundations needed to test the Standard Model and to search for signs of new physics:
- Objective 1: Precision determination of |Vus| through lattice QCD+QED calculations.
The aim is to improve the theoretical description of hadronic processes that determine the CKM matrix element |Vus|, a key parameter governing weak interactions and an essential ingredient in tests of CKM unitarity.
- Objective 2: First-principles determination of long-distance contributions to neutral D-meson mixing.
These long-distance effects dominate the theoretical uncertainty in a process highly sensitive to potential contributions from new physics.
Together, these objectives provide more robust theoretical input for interpreting precision flavour experiments and for exploring possible deviations from the Standard Model.
Lattice QCD is the only first-principles method capable of computing the effects of the strong interaction with systematically improvable uncertainties. As lattice calculations have become increasingly precise, it has become essential to also include contributions that were previously neglected, such as the effects of electromagnetic interactions and strong isospin breaking. Incorporating these corrections consistently is crucial for producing reliable Standard-Model predictions for precision flavour observables.
The SNPF project addressed two central challenges in this area, focusing on objectives that strengthen the theoretical foundations needed to test the Standard Model and to search for signs of new physics:
- Objective 1: Precision determination of |Vus| through lattice QCD+QED calculations.
The aim is to improve the theoretical description of hadronic processes that determine the CKM matrix element |Vus|, a key parameter governing weak interactions and an essential ingredient in tests of CKM unitarity.
- Objective 2: First-principles determination of long-distance contributions to neutral D-meson mixing.
These long-distance effects dominate the theoretical uncertainty in a process highly sensitive to potential contributions from new physics.
Together, these objectives provide more robust theoretical input for interpreting precision flavour experiments and for exploring possible deviations from the Standard Model.
The project made significant progress in improving lattice predictions for key flavour-physics observables. A major achievement was the development of a new finite-volume formulation of QED, called QEDr, which removes important systematic effects present in traditional formulations. This method potentially enhances the precision of lattice calculations that include electromagnetic corrections, which are now essential as simulations reach sub-percent accuracy.
In parallel, the project carried out a study of hadronic tau decays in the us channel in full QCD+QED. This process provides an independent way to determine the CKM matrix element Vus and offers a technically clean environment to test and validate methods required for future calculations of radiative corrections in semileptonic decays. To address the challenges posed by photon-induced effects in Euclidean correlation functions, the project applied advanced spectral-reconstruction techniques. This represents the first successful use of such methods in a full QCD+QED calculation. Preliminary numerical results were obtained and presented at several international conferences.
The project also developed a comprehensive theoretical framework for computing long-distance contributions to neutral D-meson mixing from first principles. This work extends spectral-reconstruction techniques to the more complex structure of charm mixing and provides a thorough renormalisation strategy. In addition, high-performance computing resources were secured to launch the first numerical study of long-distance charm mixing. Preliminary correlation functions have been generated, and variance-reduction methods were implemented to control statistical noise in diagrams involving quark loops.
Overall, the project delivered essential theoretical developments and initiated the numerical work needed to obtain more reliable Standard-Model predictions in key flavour-physics processes.
In parallel, the project carried out a study of hadronic tau decays in the us channel in full QCD+QED. This process provides an independent way to determine the CKM matrix element Vus and offers a technically clean environment to test and validate methods required for future calculations of radiative corrections in semileptonic decays. To address the challenges posed by photon-induced effects in Euclidean correlation functions, the project applied advanced spectral-reconstruction techniques. This represents the first successful use of such methods in a full QCD+QED calculation. Preliminary numerical results were obtained and presented at several international conferences.
The project also developed a comprehensive theoretical framework for computing long-distance contributions to neutral D-meson mixing from first principles. This work extends spectral-reconstruction techniques to the more complex structure of charm mixing and provides a thorough renormalisation strategy. In addition, high-performance computing resources were secured to launch the first numerical study of long-distance charm mixing. Preliminary correlation functions have been generated, and variance-reduction methods were implemented to control statistical noise in diagrams involving quark loops.
Overall, the project delivered essential theoretical developments and initiated the numerical work needed to obtain more reliable Standard-Model predictions in key flavour-physics processes.
The project delivered several advances that push the methodological and scientific boundaries of lattice-QCD research.
The new QED-r formulation offers an improved way to treat electromagnetic effects in finite-volume lattice simulations, addressing typical issues affecting traditionally used formalisms such as QEDL. This development potentially improves the reliability of lattice calculations that require electromagnetic corrections.
The successful use of spectral-reconstruction techniques in a full QCD+QED calculation represents another important step forward. It enables the study of processes that were previously inaccessible, particularly those involving multiple final-state particles with photon interactions.
In the charm sector, the project introduced the first practical framework for computing long-distance contributions to neutral D-meson mixing from first principles. This offers a long-sought pathway to reducing one of the dominant theoretical uncertainties in a key flavour-physics observable.
Together, these results provide a solid foundation for future high-precision studies. Completing the full numerical analyses will require additional large-scale simulations and coordinated international efforts, but all major conceptual and methodological challenges identified at the start of the project have now been overcome.
The new QED-r formulation offers an improved way to treat electromagnetic effects in finite-volume lattice simulations, addressing typical issues affecting traditionally used formalisms such as QEDL. This development potentially improves the reliability of lattice calculations that require electromagnetic corrections.
The successful use of spectral-reconstruction techniques in a full QCD+QED calculation represents another important step forward. It enables the study of processes that were previously inaccessible, particularly those involving multiple final-state particles with photon interactions.
In the charm sector, the project introduced the first practical framework for computing long-distance contributions to neutral D-meson mixing from first principles. This offers a long-sought pathway to reducing one of the dominant theoretical uncertainties in a key flavour-physics observable.
Together, these results provide a solid foundation for future high-precision studies. Completing the full numerical analyses will require additional large-scale simulations and coordinated international efforts, but all major conceptual and methodological challenges identified at the start of the project have now been overcome.