Stefano Melis

Sivers effect for pion and kaon production in semi-inclusive deep inelastic scattering

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

Update on transversity and Collins functions from SIDIS and e+e− data

\ensuremath A^{\sin(\phi_h-\phi_S)}_{UT}

Boer-Mulders effect in unpolarized SIDIS: An analysis of the COMPASS and HERMES data on the

which, by including recent high-statistics experimental data for pion and kaon production from HERMES 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.

\cos 2 \phi

We present an update of a previous global analysis of the experimental data on azimuthal asymmetries in semi-inclusive deep inelastic scattering (SIDIS), from the HERMES and COMPASS Collaborations, and in e + e − → h 1 h 2 X 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.

asymmetry

We present a phenomenological analysis of the cos2{phi} asymmetry recently measured by the COMPASS 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 so-called Boer-Mulders transverse-polarization distribution h{sub 1}{sup perpendicular}(x,k{sub T}{sup 2}), 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.

Unpolarised transverse momentum dependent distribution and fragmentation functions from SIDIS multiplicities

A bstractThe 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.

Non-perturbative QCD effects in qT spectra of Drell-Yan and Z-boson production

A bstractThe factorization theorems for transverse momentum distributions of dilepton/boson production, recently formulated by Collins and Echevarria-Idilbi-Scimemi in terms of well-defined transverse momentum dependent distributions (TMDs), allows for a systematic and quantitative analysis of non-perturbative QCD effects of the cross sections involving these quantities. In this paper we perform a global fit using all current available data for Drell-Yan and Z-boson production at hadron colliders within this framework. The perturbative calculable pieces of our estimates are included using a complete resummation at next-to-next-to-leading-logarithmic accuracy. Performing the matching of transverse momentum distributions onto the standard collinear parton distribution functions and recalling that the corresponding matching coefficient can be partially exponentiated, we find that this exponentiated part is spin-independent and resummable. We argue that the inclusion of higher order perturbative pieces is necessary when data from lower energy scales are analyzed. We consider non-perturbative corrections both to the intrinsic nucleon structure and to the evolution kernel and find that the non-perturbative part of the TMDs could be parametrized in terms of a minimal set of parameters (namely 2-3). When all corrections are included the global fit so performed gives a χ2/d.o.f. ≲ 1 and a very precise prediction for vector boson production at the Large Hadron Collider (LHC).

Collins functions for pions from SIDIS and new e+e- data: A first glance at their transverse momentum dependence

New data from the BABAR Collaboration, on azimuthal asymmetries measured in e+e- annihilations into pion pairs at Q2=112  GeV2, allow us to take the first, direct glance at the p⊥ dependence of the Collins functions, in addition to their z dependence. These data, together with previous e+e-→h1h2 Belle measurements and the available semi-inclusive deep inelastic scattering (SIDIS) data on the Collins asymmetry, are simultaneously analyzed in the framework of the generalized parton model, assuming two alternative Q2 evolution schemes and exploiting two different parametrizations for the Collins functions. The corresponding results for the transversity distributions are presented. Analogous data, newly released by the BESIII Collaboration, on e+e- annihilations into pion pairs at the lower Q2 of 13  GeV2, offer the possibility to explore the sensitivity of these azimuthal correlations on transverse-momentum-dependent evolution effects.

Transversity and Collins Fragmentation Functions: Towards a New Global Analysis

We present an update of a previous global analysis of the experimental data on azimuthal asymmetries in semi‐inclusive 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.

Azimuthal asymmetries in unpolarized Drell-Yan processes and the Boer-Mulders distributions of antiquarks

Using a previous extraction of the quark Boer-Mulders distributions from semiinclusive deep inelastic scattering data, we fit the unpolarized Drell-Yan data on the