General helicity formalism for two-hadron production in e+e− annihilation within a TMD approach

We present the complete leading-order results for the azimuthal dependences and polarization observables in e+ e− - > h1 h2 + X processes, where the two hadrons are produced almost back-to-back, within a transverse momentum dependent (TMD) factorization scheme. We consider spinless (or unpolarized) and spin-1/2 hadron production and give the full set of the corresponding quark and gluon TMD fragmentation functions (TMD-FFs). By adopting the helicity formalism, which allows for a more direct probabilistic interpretation, single- and double-polarization cases are discussed in detail. Simplified expressions, useful for phenomenological analyses, are obtained by assuming a factorized Gaussian-like dependence on intrinsic transverse momenta for the TMD-FFs

Transverse Λ polarization in e+e− annihilations and in SIDIS processes at the EIC within TMD factorization

We present a phenomenological study on the role of charm contribution and SU(2) isospin symmetry in the extraction of the Λ polarizing fragmentation functions from e+e−→Λ↑(Λbar↑)h+X annihilation processes. We adopt the well-established transverse-momentum-dependent factorization formalism, within the Collins-Soper-Sterman evolution scheme at next-to-leading logarithm accuracy, carefully exploiting the role of the nonperturbative component of the polarizing fragmentation function. We then discuss the impact of these results on the predictions for transverse Λ, Λbar polarization in semi-inclusive deep inelastic scattering processes at typical energies of the future Electron-Ion Collider

Transverse Λ\Lambda polarization in e+e−e^+e^- processes within a TMD factorization approach and the polarizing fragmentation function

We perform a re-analysis of Belle data for the transverse $\Lambda$ and $\bar\Lambda$ polarization in $e^+ e^-$ annihilation processes within a transverse momentum dependent (TMD) factorization approach. We consider two data sets, one referring to the associated production of $\Lambda$'s with a light unpolarized hadron in an almost back-to-back configuration, and one for the inclusive $\Lambda$ production, with the reconstruction of the thrust axis. We adopt the Collins-Soper-Sterman framework and employ the recent formulation on the factorization of single-inclusive hadron production. This extends a previous phenomenological study carried out in a more simplified TMD approach, leading to a new extraction of the polarizing fragmentation function (FF). While confirming several features of the previous analysis, here we include the proper QCD scale dependence of this TMD FF and carefully exploit the role of its nonperturbative component. The compatibility, within a unique TMD factorization scheme, of double and single-inclusive hadron production is discussed in detail. Moreover, we consider another data set for inclusive $\Lambda$ production, at much larger energies, from the OPAL Collaboration, in order to test the consistency of the entire approach. Finally, we elaborate on some fundamental issues like the role of $SU(2)$ isospin symmetry and the heavy quark contributions.Comment: 45 pages, 14 figure

Transverse Λ polarization in unpolarized pp → jet Λ↑ X

In this Letter, we investigate the spontaneous transverse polarization of Λ hyperons produced in unpolarized pp collisions inside a jet, by adopting a TMD approach where transverse momentum effects are included only in the fragmentation process. We will present predictions based on the parametrizations of the Λ polarizing fragmentation function as extracted from fits to Belle e+e− data. These estimates will be compared against preliminary STAR data. We will then be able to explore the universality properties of the quark polarizing fragmentation function and, for the first time, the role of its gluon counterpart

Transverse momentum dependent fragmentation functions and their role in spin-1/2 polarized hadron production

The study of transverse single-spin asymmetries (SSAs) is closely related to our understanding of strong interactions and to the exploration of the three-dimensional structure of hadrons in terms of quarks and gluons. The aim of this thesis is to study two processes that play a fundamental role in this context: the double and the single-inclusive hadron production in e+e- collisions. They allow for a clear and deep comprehension of the hadronization mechanisms and polarization effects, in terms of spin and transverse momentum dependent fragmentation functions (TMD-FFs). Within the helicity formalism it is possible to define, for a spin-1/2 hadron, eight leading-twist TMD-FFs with a straightforward partonic interpretation. These allow one to derive, and to present, the complete expressions for all azimuthal dependences and polarization observables in double-hadron production in unpolarized e+e- annihilation processes. These have been used, within a parton model picture and employing a Gaussian model for the transverse momentum dependence of TMD-FFs, to analyze the Belle data for the Λ transverse polarization, produced in e+e- → h1 h2 + X and e+e- → h1 + X. This allowed to obtain the first ever extraction of the Λ polarizing FF, giving the distribution of transversely polarized spin-1/2 hadrons in the fragmentation of unpolarized quarks. Due to the universality of the TMD-FFs, we used the polarizing FF to give estimates for the Λ transverse polarization in Semi-inclusive Deep Inelastic Scattering (SIDIS) processes, and for typical energies reachable at the future Electron-Ion Collider (EIC). Finally, we present a renewed analysis of Belle data by employing proper TMD factorization theorems and exploiting the Collins-Soper-Sterman (CSS) evolution equations both for double and single-inclusive hadron production case. A comparison between the old and new extraction is presented, along with the main differences between the functions extracted in the two frameworks

First extraction of the Λ polarizing fragmentation function from Belle e+e− data

We present a thorough phenomenological analysis of the experimental data from Belle Collaboration for the transverse $\Lambda$ and $\bar\Lambda$ polarisation, measured in $e^+e^-$ annihilation processes, for the case of inclusive (plus a jet) and associated production with a light charged hadron. This allows for the first ever extraction of the quark polarising fragmentation function for a $\Lambda$ hyperon, a transverse momentum dependent distribution giving the probability that an unpolarised quark fragments into a transversely polarised spin-1/2 hadron.Comment: 6 pages, 4 figures (10 plots

Transverse Λ polarization in e + e − processes within a TMD factorization approach and the polarizing fragmentation function

Abstract We perform a re-analysis of Belle data for the transverse Λ and Λ ¯ $$\overline{\Lambda}$$ polarization in e + e − annihilation processes within a transverse momentum dependent (TMD) factorization approach. We consider two data sets, one referring to the associated production of Λ’s with a light unpolarized hadron in an almost back-to-back configuration, and one for the inclusive Λ production, with the reconstruction of the thrust axis. We adopt the Collins-Soper-Sterman framework and employ the recent formulation on the factorization of single-inclusive hadron production. This extends a previous phenomenological study carried out in a more simplified TMD approach, leading to a new extraction of the polarizing fragmentation function (FF). While confirming several features of the previous analysis, here we include the proper QCD scale dependence of this TMD FF and carefully exploit the role of its nonperturbative component. The compatibility, within a unique TMD factorization scheme, of double and single-inclusive hadron production is discussed in detail. Moreover, we consider another data set for inclusive Λ production, at much larger energies, from the OPAL Collaboration, in order to test the consistency of the entire approach. Finally, we elaborate on some fundamental issues like the role of SU(2) isospin symmetry and the heavy quark contributions