Reggeons

Reggeons are theoretical particles or more precisely, trajectories of particles, that arise in the context of Regge theory. This theory was developed in the early 1960s by Tullio Regge to describe the scattering of hadrons, which are composite particles made of quarks, such as protons and neutrons. Instead of treating individual hadrons as fundamental entities, Regge theory views them as excitations on these Reggeon trajectories.

Reggeon trajectories are characterized by their spin and their mass. In Regge theory, the scattering amplitude of two hadrons can be described as a sum over contributions from these Reggeon trajectories. At high energies, the dominant contribution comes from the exchange of the trajectory with the lowest intercept, which is known as the Pomeron. The Pomeron is thought to be responsible for the roughly constant total cross-section observed in high-energy hadron-hadron scattering.

The concept of Reggeons has been crucial in understanding the behavior of strong interactions at high energies. While initially developed in a phenomenological context, Reggeon calculus has also been used in quantum chromodynamics (QCD), the fundamental theory of strong interactions, to describe phenomena like gluon radiation at high energies. Calculations involving Reggeons in QCD have led to important insights into the evolution of parton distribution functions and the behavior of the strong force in different kinematic regimes. The study of Reggeons continues to be an active area of research in theoretical particle physics.