Light-front holographic QCD and exclusive B decays

粒子物理学的标准模型（SM）成功地经受住了几十年的密集测试，最终在2012年在大型强子对撞机上发现了人们期待已久的希格斯玻色子。然而，我们知道SM是不完整的，因为它不考虑重力，也不考虑中微子振荡、暗物质和物质/反物质的不对称性。尽管付出了巨大的努力，但到目前为止，大型强子对撞机还没有发现新的物理学（NP）。然而，在罕见的B衰变中出现了一些有趣的异常现象，即数据与SM预测之间的差异。这是B介子（含重B夸克）到较轻介子（不含B夸克）的衰变，伴随着一个光子或一对轻子。这些排他的衰变在实验上是相对干净的，但其背后的理论是具有挑战性的。这有两个原因：首先，尽管我们能够计算（使用摄动理论）SM中潜在的b-夸克衰变，但我们对可观测值的最终预测对称为重整化尺度的能量尺度有残余依赖，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加以解释。******大型强子对撞机第二期运行于2015年开始，在未来五年内，数据的精度预计将至少提高一倍。这些更精确的大型强子对撞机数据将由日本SuperKEKB对撞机Belle II探测器的独立测量结果补充，该探测器计划于2017年开始采集数据。显然，理论输入对于解释即将到来的数据以及指导未来的实验至关重要。在这项研究中，我们的目标是通过两种方式提高我们对罕见的B衰变的理论预测的可靠性。首先，我们正在应用一个新的理论程序，即所谓的最大一致性原则，以减少重整化尺度的不确定性。因此，我们的研究可以帮助在这些罕见的衰减中明确地识别NP信号。其次，我们正在使用一种替代（和补充）的非微扰方法，称为光前全息，来解释介子中夸克和胶子的限制。******除了现象学上的应用，光前全息术本身也值得研究。它是所谓全息对偶性的一个例子，即物理时空中的强耦合量子理论和高维空间中的弱耦合引力理论之间的数学等价。很可能这些二元性除了仅仅是数学工具之外，还有更深层次的物理意义。在这项研究中，我们还旨在探索光前全息术与另一种标准的非摄动方法之间的潜在联系。这将有助于更好地理解尚未解决的强子中的夸克约束问题。

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.