Quark fragmentation in the θ vacuum

QCD vacuum is a superposition of degenerate states with different topological numbers that are connected by tunneling (the θ vacuum). The tunneling events are due to configurations of gauge fields (e.g., the instantons) that induce local P-odd domains in Minkowski space-time. We study the quark fragmentation in this topologically nontrivial QCD background. We find that even though QCD globally conserves P and CP symmetries, two new kinds of P-odd fragmentation functions emerge. We study their experimental manifestations in dihadron production in e+e- collisions, and find two interesting dihadron correlations: the cos(φ₁+φ₂) correlation usually referred to as the Collins effect, and a P-odd ∼sin(φ₁+φ ₂) correlation that vanishes in the cross section summed over many events, but survives on the event-by-event basis.