Hadron phenomenology using holographic light-front Quantum Chromodynamics

自然界中有四种已知的基本相互作用，本论文研究了其中的一种：强相互作用，它使夸克在强子中结合在一起，使核子在原子核中结合在一起。因此，它是强子物理和核物理的基本相互作用。我们确实有一个强相互作用的基本理论：量子色动力学（QCD），它告诉我们夸克如何通过交换胶子相互作用，以及胶子如何通过交换其他胶子相互作用。然而，当相互作用的粒子之间的耦合很强时，QCD方程很难精确求解。实验上，孤立的夸克（或胶子）从未被观察到，也就是说，我们的探测器只能看到强子。从QCD的第一性原理理解强子内部夸克和胶子的永久约束至今仍是一个悬而未决的问题。因此，强子物理学在很大程度上仍然是现象学。但现象学是理论与实验相互指导、相互加强的东西，是科学进步的关键因素。对强耦合QCD的新见解来自于Juan Maldacena（普林斯顿大学）在90年代末提出的猜想，该猜想是关于四维时空中特殊类型的量子理论（没有引力）与高维时空中的经典（非量子）引力理论之间的数学等价。这个等价的有用性是由于这样一个事实：如果量子理论是强耦合的，它的引力对偶就是弱耦合的。因此，人们可以通过求解容易求解的引力对偶来获得关于难以求解的量子理论的信息。不幸的是，QCD的引力对偶是未知的。但引力对偶的近似版本的QCD是已知的，其中一个被称为光前全息QCD。本研究聚焦于光前全息量子光盘的现象学，以了解强子结构以及后者如何影响B介子稀有衰变的物理特性。这种衰变对于标准模型（SM）之外的新物理学来说是非常敏感的探测。近年来，在欧洲的大型强子对撞机（LHC）上观察到许多SM预测与实验数据之间的差异。对强子结构的更好理解将有助于解释这些差异。同时，本研究将有助于解释和指导美国杰弗逊实验室等其他机构未来对强子结构的实验。这项研究通过向加拿大下一代研究人员介绍高度复杂的数学和计算工具，并使他们有机会使用来自世界各地粒子对撞机的真实世界数据，从而有助于培训他们。它还维持了加拿大在解决当代粒子物理学中最基本问题之一的世界级研究前沿的突出作用。

There are four known fundamental interactions in Nature and this proposal investigates one of the them: the strong interaction which is responsible for binding quarks together in hadrons and nucleons together in atomic nuclei. It is thus the underlying interaction for both hadronic and nuclear physics. We do have a fundamental theory for the strong interaction: Quantum Chromodynamics (QCD) and it tells us how quarks interact by exchanging gluons and also how gluons interact between themselves by exchanging other gluons. However, the equations of QCD are extremely difficult to solve exactly when the coupling between the interacting particles is strong. Experimentally, isolated quarks (or gluons) are never observed, i.e. our detectors only see hadrons. Understanding the permanent confinement of quarks and gluons inside hadrons from first principles in QCD remains to this day an open problem. Consequently, hadronic physics remain largely phenomenological. But phenomenology is what allows theory and experiment to mutually guide and reinforce each other, it is a key element to progress in science.  New insights into strongly-coupled QCD comes from conjecture, proposed by Juan Maldacena (Princeton) in the late nineties, of a mathematical equivalence between special classes of quantum theory (without gravity) in 4-dimensional spacetime and a classical (not quantum) gravity theory in a higher dimensional spacetime. The usefulness of this equivalence is due to the fact that if the quantum theory is strongly-coupled, its gravity dual is weakly-coupled. Hence, one can obtain information on the hard-to-solve quantum theory by solving instead its easy-to-solve gravity dual.  Unfortunately, the gravity dual to QCD is not known. But gravity duals to approximate versions of QCD are known and one of them is referred to as light-front holographic QCD. This research focuses on the phenomenology of light-front holographic QCD to understand hadronic structure and how the latter affects the physics of rare decays of the B meson. Such decays are extraordinarily sensitive probes to new physics beyond the Standard Model (SM).  In the recent years, a number of discrepancies between SM predictions and experimental data have been observed at the Large Hadron Collider (LHC) in Europe. A better understanding of hadronic structure will shed light on these discrepancies. At the same time, this research will serve to interpret and guide future experiments on hadronic structure from other facilities like the Jefferson Lab in the US.  This research contributes to the training of the next generation of Canadian researchers by introducing them to highly sophisticated mathematics and computational tools and giving them the opportunity to use real world data coming from particle colliders around the world. It also sustains the prominent Canadian role at the forefront of world-class research in addressing one of the most fundamental questions in contemporary particle physics.