Testing the universality of the Collins function in pion-jet production at RHIC

Umberto D'Alesio,Francesco Murgia,Cristian Pisano

doi:10.1016/j.physletb.2017.08.023

Umberto D'Alesio, Francesco Murgia + Show 1 more

Open Access

https://doi.org/10.1016/j.physletb.2017.08.023

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Abstract

By adopting a generalised parton model approach at leading order in QCD, including spin and intrinsic parton motion effects, we study the Collins azimuthal asymmetries for pions within a large-pT jet produced at mid-rapidity in polarised hadronic collisions. Using available information on the quark transversity distributions and the pion Collins functions, as extracted from semi-inclusive deeply inelastic scattering and e+e−→h1h2X processes, we compute estimates for the Collins asymmetries in kinematical configurations presently investigated at RHIC by the STAR Collaboration. Collins-like asymmetries, involving linearly polarised gluons, are also considered. Our predictions, compared against available preliminary data, show a very good agreement, even if some discrepancies, to be further scrutinized both theoretically and experimentally, appear in the transverse momentum dependence of the Collins asymmetry. These results are in favour of the predicted universality of the Collins function and of a mild, if any, evolution with the hard scale of the asymmetries.

Highlights

Transverse single-spin asymmetries (SSAs) are a long-standing challenge and a striking issue for collinear leadingtwist perturbative QCD
All free parameters entering the parametrisation of the transversity distribution and of the Collins functions are fixed by fitting experimental data for the Collins asymmetry in semi-inclusive deeply inelastic scattering (SIDIS) pion production by the HERMES and Compass Collaborations, and for two-hadron azimuthal correlations in e+e− → ππ X processes by the Belle and BaBar
In this letter we have studied the Collins and Collins-like azimuthal asymmetries in the distribution of hadrons within jets produced in polarised proton-proton collisions

Summary

Introduction

Transverse single-spin asymmetries (SSAs) are a long-standing challenge and a striking issue for collinear leadingtwist perturbative QCD. All free parameters entering the parametrisation of the transversity distribution and of the Collins functions are fixed by fitting experimental data for the Collins asymmetry in SIDIS pion production by the HERMES and Compass Collaborations, and for two-hadron azimuthal correlations in e+e− → ππ X processes by the Belle and BaBar. No additional free parameter is introduced in our analysis, and the theoretical curves, compared with the preliminary STAR experimental data on the Collins asymmetry in p↑ p → jet π X, are to be considered as direct predictions based on a TMD factorisation scheme. Notice that in all cases the extraction of the quark transversity distribution is constrained only up to x 0.3

Results the

5: Collins-like asymmetry

Conclusions