Explain what a TMD is in physics and what its purpose is.
In physics, TMD stands for Transverse Momentum Dependent. TMDs are a class of non-perturbative functions that describe the distribution of partons (quarks and gluons) inside a proton with respect to their transverse momentum. These distributions are crucial for understanding the internal structure of protons and for making predictions for a wide variety of high-energy scattering processes, such as deep inelastic scattering and hadron-hadron collisions.
The purpose of TMDs is to provide a way to account for the non-perturbative effects that arise in high-energy scattering processes, where the interactions between partons cannot be treated using perturbation theory. By incorporating TMDs into theoretical calculations, physicists can make more accurate predictions for the outcomes of these scattering processes and better understand the underlying dynamics of particle interactions.