BCFS · Beyond Colours and Flavours on Supercomputers

Beyond Colours and Flavours on Supercomputers

The overarching goal of particle physics is the discovery of new fundamental particles in order to advance our understanding of the universe. Our current best knowledge of the interactions of fundamental particles is summarised in the Standard Model of Particle Physics (SM). Searches for "New Physics" (NP) beyond the Standard Model (BSM) are often categorised as direct and indirect searches where the former aim to directly observe a new particle and the latter aim to infer the existence of some unknown particle as an explanation of observational inconsistencies. NP searches can only be successful, through the interplay between high-precision experimental measurements (for example performed by the experiments at the Large Hadron Collider at CERN) and high-precision theoretical predictions. This project contributes to direct and indirect NP searches by providing first-principle theory predictions which can be compared to experimental measurements. The strong force (Quantum Chromodynamics or QCD) governs the interactions of quarks and gluons (the fundamental constituents of protons and neutrons) into bound states, called hadrons. The project "Beyond Colours and Flavours on Supercomputers" aims to predict hadronic observables with control over all sources of uncertainty. We use the well established framework of Lattice Quantum Chromodynamics (LQCD) to provide such first-principle predictions, which cannot be computed with analytical methods. LQCD numerically simulates a finite space-time grid on which a representative sample of field configurations is generated via Markov Chain Monte Carlo methods. This is achieved via the use of large scale numerical simulations on supercomputers. On these configurations we can compute certain objects from which we can extract hadron masses and other relevant observables such as matrix elements. Our work focuses on the computation of observables which are currently displaying some degree of disagreement ("tension") between theory and experiment, with the goal to substantiate whether this originates from statistical fluctuations or is indeed a sign of NP. To achieve this, it is paramount to make pure theory predictions from first principles, to avoid the introduction of any un-quantifiable uncertainties. We aim to compute "semi-leptonic form factors" which parameterise decays of a heavy quark (such as a "charm" or "bottom" quark) to a lighter quark (for example "up", "down" or "strange"). Since the target precision for many state-of-the-art observables is below the percent-level accuracy, it is important to consider and quantify all effects that contribute at this level of precision. We therefore aim to predict isospin breaking effects arising from the mass difference of the up and the down quark (strong isospin breaking) and from the electric charge of quarks (weak isospin breaking). This is achieved in the theoretically sound framework of massive QED.

Data: CORDIS, © European Union