QCDalpha

QCDalpha refers to the strong coupling constant in quantum chromodynamics (QCD), often denoted by the Greek letter alpha_s. It is a fundamental parameter that characterizes the strength of the strong nuclear force, which binds quarks together to form protons and neutrons, and holds atomic nuclei together. Unlike the electromagnetic coupling constant in quantum electrodynamics, alpha_s is not constant but varies with the energy scale at which the interaction is probed. This phenomenon is known as asymptotic freedom.

At high energy scales (short distances), alpha_s is small, meaning the strong force is weak. This allows quarks and gluons, the force carriers of QCD, to behave almost as free particles. Conversely, at low energy scales (long distances), alpha_s becomes large, making the strong force very strong. This leads to color confinement, where quarks and gluons are never observed in isolation, but are instead bound within composite particles called hadrons.

The precise value of alpha_s at a reference energy scale is determined experimentally through various measurements, such as deep inelastic scattering experiments or the decay rates of heavy mesons. The running of alpha_s with energy is a crucial prediction of QCD and has been extensively verified. Understanding the behavior of alpha_s is essential for making accurate predictions in high-energy physics and for comprehending the structure of matter.