Transverse polarisation of quarks in hadrons

Abstract

We review the present state of knowledge regarding the transverse polarisation (or transversity) distributions of quarks. After some generalities on transverse polarisation, we formally define the transversity distributions within the framework of a classification of all leading-twist distribution functions. We describe the QCD evolution of transversity at leading and next-to-leading order. A comprehensive treatment of non-perturbative calculations of transversity distributions (within the framework of quark models, lattice QCD and QCD sum rules) is presented. The phenomenology of transversity (in particular, in Drell-Yan processes and semi-inclusive leptoproduction) is discussed in some detail. Finally, the prospects for future measurements are outlined.

Evolution of the helicity and transversity. Transverse-momentum-dependent parton distributions

“…Compare our results with results of collinear evolution of g 1 and h 1 presented, for example in Ref. [52]. As in the case of collinear evolution, also in TMD evolution h 1 becomes smaller than g 1 under evolution and the difference grows with Q.…”

“…(50,51,52). This means also that the gluon TMD cannot be studied via scaling violations at least at this order.…”

Leading-twist single-transverse-spin asymmetries: Drell–Yan and deep-inelastic scattering

“…There is also the possibility of a nonzero transversity of a quark in an unpolarized hadron, with the transversity being correlated with the transverse momentum of the quark. This is defined by [2,7,8] …”

Azimuthal and single spin asymmetries in hard scattering processes

“…The rich, low-energy aspects of topics iii) and iv), which may be confronted only with nonperturbative approaches, will also remain uncovered. A comprehensive overview on these subjects and on related topics can be found in several review papers, addressing both their theoretical and experimental aspects [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]. Here we will focus on the high-energy regime and the treatment in the context of pQCD approaches of transverse SSA's and, marginally, of the transverse polarization of spin 1/2 particles in unpolarized processes.…”