We study the Sivers effect in the transverse single spin asymmetries (SSA) for pion and kaon production in semi-inclusive deep inelastic scattering (SIDIS) processes. We perform a fit of Awhich, by including recent high statistics experimental data for pion and kaon production from HER-MES and COMPASS Collaborations, allows a new determination of the Sivers distribution functions for quarks and antiquarks with u, d and s flavours. Estimates for forthcoming SIDIS experiments at COMPASS and JLab are given.

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The Sivers distributions recently extracted from semi-inclusive deep inelastic scattering data [1] are used to compute estimates for Sivers asymmetries in Drell-Yan processes which are being planned at several facilities (RHIC, COMPASS, J-PARC, PAX, PANDA, NICA (JINR) and SPASCHARM (IHEP)). Most of these asymmetries turn out to be large and could allow a clear test of the predicted sign change of the Sivers distributions when active in SIDIS and Drell-Yan processes. This is regarded as a fundamental test of our understanding, within QCD and the factorization scheme, of single spin asymmetries.

The high energy and large p(T) inclusive polarized process, (A,SA) + (B,SB) -> C + X, is considered under the assumption of a generalized QCD factorization scheme. For the first time all transverse motions, of partons in hadrons and of hadrons in fragmenting partons, are explicitly taken into account; the elementary interactions are computed at leading order with noncollinear exact kinematics, which introduces many phases in the expressions of their helicity amplitudes. Several new spin and k(perp)-dependent soft functions appear and contribute to the cross sections and to spin asymmetries; we put emphasis on their partonic interpretation, in terms of quark and gluon polarizations inside polarized hadrons. Connections with other notations and further information are given in some Appendixes. The formal expressions for single and double spin asymmetries are derived. The transverse single spin asymmetry A(N), for p(uparrow) p -> pi X processes is considered in more detail, and all contributions are evaluated numerically by saturating unknown functions with their upper positivity bounds. It is shown that the integration of the phases arising from the noncollinear kinematics strongly suppresses most contributions to the single spin asymmetry, leaving at work predominantly the Sivers effect and, to a lesser extent, the Collins mechanism

We present a phenomenological analysis of the cos 2φ asymmetry recently measured by the COM-PASS and HERMES collaborations in unpolarized semi-inclusive deep inelastic scattering. In the kinematical regimes explored by these experiments the asymmetry arises from transverse-spin and intrinsic transverse-momentum effects. We consider the leading-twist contribution, related to the socalled Boer-Mulders transverse-polarization distribution h ⊥ 1 (x, k 2 T ), and the twist-4 Cahn contribution, involving unpolarized transverse-momentum distribution functions. We show that a reasonably good fit of the preliminary data sets from COMPASS and HERMES is achieved with a Boer-Mulders function consistent with the main theoretical expectations. Our conclusion is that the COMPASS and HERMES measurements represent the first experimental evidence of the Boer-Mulders effect in SIDIS.

We present a global re-analysis of the most recent experimental data on azimuthal asymmetries in semi-inclusive deep inelastic scattering, from the HERMES and COMPASS Collaborations, and in e + e − → h1 h2 X processes, from the Belle Collaboration. The transversity and the Collins functions are extracted simultaneously, in the framework of a revised analysis in which a new parameterisation of the Collins functions is also tested.

The unpolarised transverse momentum dependent distribution and fragmentation functions are extracted from HERMES and COMPASS experimental measurements of SIDIS multiplicities for charged hadron production. The data are grouped into independent bins of the kinematical variables, in which the TMD factorisation is expected to hold. A simple factorised functional form of the TMDs is adopted, with a Gaussian dependence on the intrinsic transverse momentum, which turns out to be quite adequate in shape. HERMES data do not need any normalisation correction, while fits of the COMPASS data much improve with a y-dependent overall normalisation factor. A comparison of the extracted TMDs with previous EMC and JLab data confirms the adequacy of the simple gaussian distributions. The possible role of the TMD evolution is briefly considered.PACS numbers: 13.88.+e, 13.60.-r, 13.85.Ni I.Although the HERMES and COMPASS data cover similar Q 2 regions (1 ≤ Q 2 ≤ 10 GeV 2 ), they differ in the experimental set-up, in the statistics, in the binning choices and in the explored x B range; in addition, there seems to be some discrepancy between the two measurements. We then fit the HERMES and the COMPASS multiplicities separately. A simultaneous fit of both sets of data would lead to poor results and is not presented here.Recently, another study of the unpolarised TMDs has appeared [28], which follows a procedure somehow similar to that of this work, but which considers only the HERMES set of experimental data and does not include any attempt to check for signs of scale evolution.After a short Section II devoted to the formalism, we present our main results in Section III. In Section IV we briefly discuss the possible role, and look for possible signs, of TMD evolution. In Section V we compare our present results with those of previous analyses [9,11] and check their consistency with other measurements of SIDIS cross sections and P T -distributions [10,12,13,29] which were not included in our fits. Further comments and concluding discussions are presented in Section VI.

We present an update of a previous global analysis of the experimental data on azimuthal asymmetries in semiinclusive deep inelastic scattering (SIDIS), from the HERMES and COMPASS Collaborations, and in e + e − → h1h2X processes, from the Belle Collaboration. Compared to the first extraction, a more precise determination of the Collins fragmentation function and the transversity distribution function for u and d quarks is obtained.