Light-front holographic QCD and exclusive B decays

粒子物理学的标准模型（SM）成功地经受住了数十年的密集测试，最终在2012年LHC发现了期待已久的希格斯玻色子。然而，我们知道SM是不完整的，因为它不包含引力，也不解释中微子振荡，暗物质和物质/反物质不对称性。尽管付出了巨大的努力，但到目前为止，LHC还没有新物理学（NP）的发现。然而，在罕见的排他性B衰变中报告了一些有趣的异常现象，即数据与SM预测之间的差异。这些是B介子（含有重B夸克）衰变为轻介子（没有B夸克），伴随着一个光子或一对轻子。这些独特的衰变在实验上相对干净，但它们背后的理论具有挑战性。这有两个原因：第一，虽然我们能够计算（使用微扰理论）在SM的基本b夸克衰变，我们的最终预测的观测有一个剩余的依赖于能量尺度称为重整化尺度，从而引入了系统的不确定性。其次，夸克和胶子被永久地限制在介子中，这必须用非微扰方法仔细解释。

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.