A Charcuterie Platter of QCD Matter

Abstract

One of the greatest current challenges in theoretical high energy physics is to understand the dynamics of Quantum Chromodynamics (QCD). In this thesis, I address a variety of questions in QCD using Effective Field Theory (EFT). The first question deals directly with the observed phenomenology of QCD: How can we use EFT to disentangle the complicated three-dimensional dynamics of how quarks and gluons, the fundamental degrees of freedom of QCD, combine to form the observed bound states in nature called hadrons? I initiate a new formalism using Heavy Quark Effective Theory to study this dynamical process known as hadronization. I shed new light on the transverse momentum-dependent fragmentation process of heavy (charm and bottom) quarks by making use of the fact that heavy quarks with masses much larger than the strong interaction scale decouple from the rest of the hadronization cascade. I also present exciting heavy quark phenomenology at existing colliders and the upcoming Electron-Ion Collider. The second question investigates the field theory structure of QCD: What can we learn about the nonperturbative structure of the quantum field theory through the abstruse emergent phenomenon in QCD called “confinement”, which traps quarks and gluons inside hadrons? I study a class of cleverly constructed observables known as energy correlators by using fieldtheory based methods to determine the leading nonperturbative contribution, and examine the universality of the nonperturbative matrix element that gives rise to this contribution. I also show that including the nonperturbative contribution has a significant impact on the extraction of the strong coupling constant, a fundamental parameter of the Standard Model, using tools such as factorization and resummation from EFT. Last but not least, the final question explores the underlying symmetry properties of QCD and its potential completions: How robust is the axion solution to the strong CP (ChargeParity) problem, and what are some of its implications beyond the realm of QCD? I examine the axion quality problem in post-inflationary QCD axion models with different symmetry properties and identify a new tension with standard cosmology. I further show that the axion string-domain wall dynamics is more complicated than commonly expected, undermining the reliability of a unique mass prediction for axion dark matter in post-inflationary models. I showcase the importance of considering both high-energy extensions and the EFT at low energy, and uncover new complexity of the axion solution to the strong CP problem.