Review

Experimental results on TMDs

¹ Jefferson Lab, New Port News, VA, USA
² University of Trieste and INFN, Trieste, Italy
³ University of Ferrara and INFN, Trieste, Italy

^* e-mail: Andrea.Bressan@cern.ch

Received: 14 October 2015
Accepted: 22 October 2015
Published online: 13 June 2016

Abstract

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 ( 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.