Quantum Chromodynamics (QCD) is the theory that describes the strong interaction, one of the fundamental forces in nature. While QCD has been remarkably successful in describing the behavior of quarks and gluons, there are several unresolved issues that researchers are actively investigating. Some of these issues include:
Confinement: One of the outstanding problems in QCD is understanding confinement. QCD predicts that quarks and gluons are confined within composite particles such as protons and neutrons, preventing the observation of free quarks or gluons in isolation. The mechanism behind this confinement is not yet fully understood.
Mass Generation: QCD predicts that the fundamental quarks have zero mass, but in reality, quarks have observed masses. The mechanism by which quarks acquire their masses, known as mass generation or chiral symmetry breaking, is still not completely understood.
Strong CP Problem: The strong CP problem is related to the conservation of the combined symmetry of charge conjugation (C) and parity (P) in QCD. QCD should allow for a violation of this symmetry, known as CP violation, but experimental observations indicate that CP violation in the strong interaction is extremely small. Understanding the origin of this small CP violation and why it is suppressed is an active area of research.
Lattice QCD: Solving QCD directly from its equations is extremely challenging due to its non-perturbative nature. Lattice QCD is a numerical approach that discretizes space and time to perform calculations on a lattice. While lattice QCD has been successful in studying certain aspects of QCD, there are still computational challenges in simulating systems with realistic parameters, such as including dynamical quarks and dealing with finite volume effects.
Quark-Gluon Plasma: At extreme temperatures and densities, it is predicted that the behavior of quarks and gluons changes, leading to the formation of a new state of matter called quark-gluon plasma (QGP). Experimental studies of QGP are being carried out in heavy-ion collision experiments, such as those conducted at the Large Hadron Collider (LHC). Understanding the properties of QGP and its phase transition is an ongoing research topic.
These are just a few examples of the unresolved issues in QCD. Researchers are actively working on these and other questions to deepen our understanding of the strong interaction and advance our knowledge of the fundamental forces and particles in the universe.