Subventions et des contributions :

Subvention ou bourse octroyée s'appliquant à plus d'un exercice financier. (2017-2018 à 2020-2021)

The Standard Model (SM) of particle physics has successfully withstood decades of intensive testing culminating in the long-awaited discovery of the Higgs boson in 2012 at the LHC. Yet, we know that the SM is incomplete since it does not accommodate gravity and neither does it account for neutrino oscillations, dark matter and for the matter/antimatter asymmetry. Despite intensive efforts, to this date no New Physics (NP) discoveries have been made at the LHC. However, an intriguing number of anomalies, i.e. discrepancies between the data and SM predictions, have been reported in rare exclusive B decays. These are decays of the B meson (containing a heavy b quark) to a lighter meson (no b quark) accompanied by a photon or a pair of leptons. These exclusive decays are relatively clean to measure experimentally but the theory behind them is challenging. There are two reasons for this: first, although we are able to compute (using perturbation theory) the underlying b-quark decay in the SM, our final predictions for observables have a residual dependence on an energy scale called the renormalization scale thus introducing a systematic uncertainty. Secondly, quarks and gluons are permanently confined into mesons and this has to be carefully accounted for using non-perturbative methods.

The LHC Run II has started in 2015 and in the next five years, the precision of the data is expected to at least double. These more precise LHC data will be complemented by independent measurements from the Belle II detector at the SuperKEKB collider in Japan scheduled to start taking data in 2017. Clearly theory input will be essential to interpret the forthcoming data as well as to guide future experiments. In this research, we aim to increase the reliability of our theoretical predictions for the rare exclusive B decays in two ways. First, we are applying a new theoretical procedure, the so-called Principle of Maximum Conformality, to reduce the renormalization scale uncertainty. Thus, our research can help to identify unambiguously NP signals in these rare decays. Secondly, we are using an alternative (and complementary) non-perturbative method, known as light-front holography, to account for the confinement of quarks and gluons in mesons.

Besides its phenomenological applications, light-front holography in itself is worth investigating. It is an example of what are known as holographic dualities, i.e. mathematical equivalences between strongly-coupled quantum theories in physical spacetime and weakly-coupled gravitational theories in higher dimensional spaces. It may well be that these dualities have a deeper physical meaning aside from being mere mathematical tools. In this research, we also aim to explore the underlying links between light-front holography and another standard non-perturbative method. This will help to understand better the unsolved problem of quark confinement in hadrons.