# 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.