First insight into transverse-momentum-dependent fragmentation physics at photon-photon colliders

Future planned lepton colliders, both in the circular and linear configurations, can effectively work as virtual and quasireal photon-photon colliders and are expected to stimulate an intense physics program in the next few years. In this paper, we suggest to consider photon-photon scattering as a useful source of information on transverse-momentum-dependent fragmentation functions (TMD FFs), complementing semi-inclusive deeply inelastic scattering and e±e- annihilation processes, which provide most of the present phenomenological information on TMD FFs. As a first illustrative example, we study two-hadron azimuthal asymmetries around the jet thrust axis in the process ℓ±ℓ-− → γ*γ → q¯q → h1h2 X, in which in a circular lepton collider one tagged, deeply virtual photon scatters off an untagged quasireal photon, both originating from the initial lepton beams, producing inclusively an almost back-to-back light-hadron pair with large transverse momentum, in the γ*γ center-of-mass frame. Similar processes, in a more complicated environment due to the presence of initial hadronic states, can also be studied in ultraperipheral collisions at the LHC and the planned future hadron colliders.