# Quark fragmentation in the $θ$-vacuum - High Energy Physics - Phenomenology

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Abstract: The vacuum of Quantum Chromodynamics is a superposition of degenerate stateswith different topological numbers that are connected by tunneling the$\theta$-vacuum. The tunneling events are due to topologically non-trivialconfigurations of gauge fields e.g. the instantons that induce local $\p$-odddomains in Minkowski space-time. We study the quark fragmentation in thistopologically non-trivial QCD background. We find that even though QCD globallyconserves $\p$ and $\cp$ symmetries, two new kinds of $\p$-odd fragmentationfunctions emerge. They generate interesting dihadron correlations: one is theazimuthal angle correlation $\sim \cos\phi 1 + \phi 2$ usually referred to asthe Collins effect, and the other is the $\p$-odd correlation $\sim \sin\phi 1+ \phi 2$ that vanishes in the cross section summed over many events, butsurvives on the event-by-event basis. Using the chiral quark model we estimatethe magnitude of these new fragmentation functions. We study their experimentalmanifestations in dihadron production in $e^+e^-$ collisions, and comment onthe applicability of our approach in deep-inelastic scattering, proton-protonand heavy ion collisions.

Author: ** Zhong-Bo Kang RIKEN BNL, Dmitri E. Kharzeev BNL**

Source: https://arxiv.org/