Experimental Studies of Transverse Momentum Dependent Parton Distributions

The use of "hard" scattering processes to reveal the microscopic structure of matter has a rich history dating back more than 100 years to the experiments of Rutherford, Geiger, and Marsden, who discovered the atomic nucleus and placed upper limits on its size by observing large-angle deflections of α particles passing through thin gold foils. More recently, the methods of lepton scattering, pioneered by Hofstadter and others at Stanford in the 1950s, have been used for decades to precisely map the "one-dimensional" structure of nucleons and nuclei via measurements including elastic form factors and inelastic structure functions. In the deep inelastic (DIS) or "scaling" regime, nucleon structure functions have been successfully interpreted in terms of universal longitudinal momentum distributions of the nucleon's point-like quark constituents (or partons) within the framework of perturbative quantum chromodynamics (pQCD), over many orders of magnitude in energy and momentum transfer. The parton distribution functions (PDFs) measured in DIS processes are universal, and can be used, in principle, to predict the results of all high-energy collisions involving hadrons within the Standard Model. Over the last three decades, advances in theoretical understanding have established the complementary frameworks of Generalized Parton Distributions (GPDs) and Transverse Momentum Dependent parton distributions (TMDs) for the interpretation of hard exclusive and semi-inclusive scattering processes, respectively, in terms of the three-dimensional, spin-dependent structure of hadrons in coordinate and momentum space. In this talk, I will survey the global experimental status of TMDs and the outlook for near-future precision studies of TMDs at existing and planned experimental facilities.