Experimental results on TMDs

QCD factorisation for semi-inclusive deep inelastic scattering (DIS) at low transverse momentum in the current-fragmentation region has been established recently, providing a rigorous basis to study the transverse-momentum–dependent (TMD) distribution and fragmentation functions of partons from semi-inclusive DIS data using different spin-dependent and spin-independent observables. The main focus of the experiments were the measurements of various single- and double-spin asymmetries in hadron electro-production (ep↑ → ehX) with unpolarised, longitudinally and transversely polarised targets. The joint use of a longitudinally polarised beam and longitudinally and transversely polarised targets allowed to measure double-spin asymmetries related to leading-twist distribution functions describing the transverse momentum distribution of longitudinally and transversely polarised quarks in a longitudinally and transversely polarised nucleons (helicity and worm-gear TMDs). The single-spin asymmetries measured with transversely polarised targets, provided access to specific leading-twist parton distribution functions: the transversity, the Sivers function and the so-called “pretzelosity” function. In this review we present the current status and some future measurements of TMDs worldwide.