Probing the Collins effect at photon-photon colliders

We propose to study the Collins effect in the production of almost back-to-back unpolarized hadrons (mainly charged pions) by looking at their azimuthal distributions around the jet thrust-axis in the process $γγ\to h_1h_2X$. The quasi-real photons in the initial state can be emitted either by two scattering leptons or two charged heavy nuclei in ultra-peripheral collisions. We provide numerical estimates for kinematic configurations in principle accessible at the Future Circular Collider and at the planned experiments on ultra-peripheral collisions at the LHC. We find that the corresponding asymmetries can be sizable, even up to around 10-15%. The proposed measurements would thus shed light on the fragmentation mechanism of quarks into hadrons and, in principle, would allow one to distinguish among different existing models for the Collins functions.