Hadron phenomenology using holographic light-front Quantum Chromodynamics

自然界中有四种已知的基本相互作用，这个方案研究了其中之一：负责将强子中的夸克和原子核中的核子结合在一起的强相互作用。因此，它是强子物理和核物理的基本相互作用。我们确实有一个关于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，以及胶子之间如何通过交换其他胶子相互作用。然而，当相互作用粒子之间的耦合很强时，QCD方程很难精确求解。在实验上，孤立的夸克(或胶子)从未被观测到，即我们的探测器只看到强子。从QCD中的第一原理理解强子中夸克和胶子的永久限制直到今天仍然是一个悬而未决的问题。因此，强子物理在很大程度上仍然是唯象的。但现象学是让理论和实验相互指导和相互促进的，它是科学进步的关键因素。 对强耦合QCD的新见解来自20世纪90年代末由Juan Maldacena(普林斯顿大学)提出的猜想，即在4维时空中的特殊类型的量子理论(无引力)和在更高维时空中的经典(非量子)引力理论之间的数学等价。这种等价性之所以有用，是因为如果量子理论是强耦合的，那么它的引力对偶就是弱耦合的。因此，人们可以通过求解其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的引力对偶尚不清楚。但与QCD近似版本的引力对偶是已知的，其中一种被称为光前全息QCD。本研究主要从光波前全息QCD的唯象来理解强子结构，以及后者如何影响B介子稀有衰变的物理过程。这种衰变是对超越标准模型(SM)的新物理的异常灵敏的探测。近年来，在欧洲的大型强子对撞机(LHC)上观察到了SM预测与实验数据之间的一些差异。对强子结构的更好理解将有助于阐明这些差异。同时，这项研究将有助于解释和指导美国杰斐逊实验室等其他设施未来对强子结构的实验。 这项研究有助于培训下一代加拿大研究人员，向他们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。它还保持了加拿大在解决当代粒子物理学中最基本的问题之一方面处于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.