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。大型强子对撞机第二次运行已于2015年启动，预计在未来五年内，数据的精度至少会翻一番。这些更精确的大型强子对撞机数据将得到来自日本SuperKEKB对撞机Belle II探测器的独立测量的补充，该探测器计划于2017年开始采集数据。显然，理论输入将对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独占B衰变的理论预测的可靠性。首先，我们正在应用一种新的理论程序，即所谓的最大共形原理，来减少重整化标度的不确定性。因此，我们的研究可以帮助明确地识别这些罕见衰变中的NP信号。其次，我们正在使用另一种(和互补的)非微扰方法，称为光前全息术，来解释夸克和胶子在介子中的限制。除了它的现象学应用，光前全息术本身也值得研究。这是一个被称为全息对偶的例子，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和另一种标准的非微扰方法之间的潜在联系。这将有助于更好地理解强子中夸克禁闭这个悬而未决的问题。

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.