Imaginary Rotation Breaks Charge Conjugation in Hot QCD

Abstract: Starting from the identity $$\mathrm{e}<sup>{2π\mathrm{i}J_z}=(-1)<sup>{F}=\mathrm{e}<sup>{\mathrm{i}πQ}$$ for the angular momentum, fermion parity, and quark number operators, valid on the color-singlet physical state space, we derive a density-operator identity that ties imaginary rotation to an imaginary quark chemical potential, $ρ(Ω<em>I/T+2π,θ_q)=ρ(Ω_I/T,θ_q+π)$. It follows that the imaginarily rotating system of $SU(3)$ QCD at $(Ω_I/T,θ_q)=(2π,0)$ is mapped exactly onto the Roberge-Weiss point $(0,π)$, where charge conjugation is spontaneously broken above the Roberge-Weiss endpoint. This conclusion depends on no model and no approximation. Minimizing the one-loop effective potential on the rotation axis over the entire $SU(3)$ Weyl alcove, we further obtain analytically, for three massless quark flavors, the second-order transition point $Ω</em>{I,C}/(πT)=(22-2\sqrt{22})/9$ at which $\mathrm{Im}\,L$ rises continuously, and the first-order transition point $Ω_{I,\mathrm{lock}}/(πT)=2/3+2\sqrt{111}/27$ at which the minimum locks into the nontrivial center elements. In the massless one-loop approximation a continuous degeneracy appears in full QCD and in pure Yang-Mills theory alike, but on the full-QCD side it is merely accidental, lifted by a finite strange-quark mass. Finally, we state the conditions under which the exact relations and the on-axis predictions can be tested in lattice QCD.