The problem of evaluating the parton distribution function is formulated in terms of the entanglement entropy. The entanglement between the part of the hadron probed in a hard scattering and the rest of the hadron is found to be related to the conventional parton distribution.
Using nonlinear evolution equations of QCD, we compute the entanglement entropy resolved by hard scattering at a given Bjorken x and momentum transfer q2=‐Q2. At small x the relation between the entanglement entropy S(x) and the parton distribution xG(x) becomes very simple: S(x)=ln[xG(x)]. In this small x, large rapidity Y regime, all partonic microstates have equal probabilities, and the entanglement entropy is maximal—so at small x, hard scattering probes a maximally entangled state. We propose the entanglement entropy as an observable that can be studied in hard scattering. This will require event‐by-event measurements of hadronic final states, and would allow to study the transformation of entanglement entropy into the Boltzmann one. We compare our predictions to the available experimental data from the LHC.