Light-front holographic QCD and exclusive B decays

粒子物理的标准模型(SM)已经成功地经受住了数十年的密集测试，最终于2012年在大型强子对撞机上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，因为它没有考虑重力，也没有考虑中微子振荡、暗物质和物质/反物质的不对称性。尽管进行了密集的努力，但到目前为止，大型强子对撞机还没有发现新的物理(NP)。然而，在罕见的独占B衰变中，报告了一些有趣的异常情况，即数据与SM预测之间的差异。这些是伴随着一个光子或一对轻子的B介子(包含一个重的b夸克)到一个较轻的介子(没有b夸克)的衰变。这些独占的衰变在实验上是相对干净的，但背后的理论是具有挑战性的。这有两个原因：首先，虽然我们能够计算(使用微扰理论)SM中潜在的b-夸克衰变，但我们对可观测到的最终预测对称为重整化标度的能量标度有剩余依赖，从而引入了系统不确定性。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地解释。

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.