TNT Colloquium: The Entropy of QCD scattering processes [Duke]

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 state
often obeys a semi-classical probability distribution. Typically, such
collapse of unitary evolution is explained in terms of decoherence by
an environment coupled to the observed system. However, in a system
like e^+e^- --> hadrons, there is no clear candidate for an external
bath. I will explain how soft radiation induced in the scattering
creates the bath that decoheres the hard scattering degrees of
freedom, and how to calculate the (entanglement)-entropy between the
hard and soft degrees of freedom as a function of the boundary in
hilbert space between the two regions. We will see that the KLN
theorem is flipped on its head, instead of being a requirement for a
well-defined measurement, the KLN theorem should be thought of as a
dynamical process which precisely projects out the final states that
can be observed after decoherence. Time permitting, I will comment on
implications for jet substructure observables and machine learning, as
well as applications to the structure of the initial state in DIS.