Particle-level parton densities and uncertainties for the resolved photon at NLO
We present a new set of parton distribution functions (PDFs), PL$γ$26, for the resolved photon at next-to-leading order (NLO), the first to be fitted to a wide range of jet measurements from several experiments. Jet production in photoproduction at HERA and in $γγ$ collisions at LEP is sensitive to both the quark and the gluon content of the photon, but the measurements lie at low scales, where non-perturbative corrections are large. We therefore choose a novel approach and fit the PDFs to these measurements at particle level, using NLO predictions from the general-purpose event generator SHERPA, matched to the parton shower and including hadronisation and multi-parton interactions (MPI). As part of the procedure, we tune the MPI model separately for photon--proton and photon--photon collisions. Modelling these non-factorisable effects in the fit, rather than through external correction factors, lets us include many more measurements and data points. We estimate the experimental uncertainties with replicas and compare the fitted PDFs with previous sets.