Hard underlying event correction to inclusive jet cross sections

Jets observed in hadron-hadron scattering contain a contribution from the underlying event that is produced by spectator interactions taking place incoherently with the major parton-parton collision, due to the extended composite structure of the colliding hadrons. Using a recent measurement of the double parton interaction rate, we calculate that the underlying event may be 2 - 3 times stronger than generally assumed, as a result of semi-hard perturbative multiple-parton interactions. This can have an important influence on the inclusive jet cross section at moderate values of E_T, persisting at the 5 - 10% level to the largest observable E_T. We show how the underlying event can be measured accurately using a generalization of the method first proposed by Marchesini and Webber.