Hadron phenomenology using holographic light-front Quantum Chromodynamics

自然界中有四种已知的基本相互作用，本提案研究了其中之一：强相互作用，它负责将强子中的夸克结合在一起，并将原子核中的核子结合在一起。因此，它是强子物理和核物理的基础相互作用。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中夸克和胶子的永久禁闭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唯象的。而现象学是理论与实验相互指导、相互促进的学科，是科学进步的关键要素。 对强耦合QCD的新认识来自于90年代末普林斯顿大学的胡安·马尔达塞纳（Juan Maldacena）提出的猜想，即四维时空中的特殊量子理论（不含引力）与高维时空中的经典（非量子）引力理论之间的数学等价性。这种等价的有用性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的引力对偶还不为人所知。但是，引力波近似的QCD版本是已知的，其中之一被称为光阵面全息QCD。本研究着重于光前全像QCD的唯象，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。这种衰变是对标准模型（SM）之外的新物理学的非常敏感的探针。 近年来，在欧洲大型强子对撞机（LHC）上观察到了一些SM预测与实验数据之间的差异。更好地理解强子结构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未来的强子结构实验，如美国杰斐逊实验室。 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的世界数据。它还保持了加拿大在解决当代粒子物理学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.