Abstract
An essential part of high-energy hadronic collisions is the soft hadronic activity that underlies the primary hard interaction. It includes soft radiation from the primary hard partons, secondary multiple parton interactions (MPI), and factorization-violating effects. The invariant mass spectrum of the leading jet in and events is directly sensitive to these effects, and we use a QCD factorization theorem to predict its dependence on the jet radius , jet , jet rapidity, and partonic process for both the perturbative and nonperturbative components of primary soft radiation. We prove that the nonperturbative contributions involve only odd powers of , and the linear term is universal for quark and gluon jets. The hadronization model in Pythia8 agrees well with these properties. The perturbative soft initial state radiation (ISR) has a contribution that depends on the jet area in the same way as the underlying event, but this degeneracy is broken by dependence on the jet . The size of this soft ISR contribution is proportional to the color state of the initial partons, yielding the same positive contribution for and , but a negative interference contribution for . Hence, measuring these dependencies allows one to separate hadronization, soft ISR, and MPI contributions in the data.
- Received 11 June 2014
DOI:https://doi.org/10.1103/PhysRevLett.114.092001
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