TNT Colloquium: The Entropy of QCD scattering processes [Duke]
Tuesday, October 22, 2019 - 3:30pm
Scattering in QCD presents a conundrum: at the hard scattering region,we can have unitary quantum mechanical evolution of a pure state.However, the distribution of radiation observed in the final stateoften obeys a semi-classical probability distribution. Typically, suchcollapse of unitary evolution is explained in terms of decoherence byan environment coupled to the observed system. However, in a systemlike e^+e^- --> hadrons, there is no clear candidate for an externalbath. I will explain how soft radiation induced in the scatteringcreates the bath that decoheres the hard scattering degrees offreedom, and how to calculate the (entanglement)-entropy between thehard and soft degrees of freedom as a function of the boundary inhilbert space between the two regions. We will see that the KLNtheorem is flipped on its head, instead of being a requirement for awell-defined measurement, the KLN theorem should be thought of as adynamical process which precisely projects out the final states thatcan be observed after decoherence. Time permitting, I will comment onimplications for jet substructure observables and machine learning, aswell as applications to the structure of the initial state in DIS.