Fractional Superfluid Vortices and Worldsheet Wess--Zumino--Witten Models in Two-Color QCD and $Sp(2N)$ Gauge Theories

We study superfluid vortices in two-color QCD and more general pseudoreal gauge theories with even flavor number $N_f=2m$ at finite baryon density. The antisymmetric diquark condensate admits fractional minimal vortices with baryon-phase winding $\nu =1\mathrm{/}m$, corresponding to $1\mathrm{/}m$ of the conventional circulation quantum, each localizing an $Sp(2)\simeq SU(2)$ core sector. For vortices embedded in mutually orthogonal flavor planes, the leading-order chiral action factorizes exactly. Off-diagonal flavor zero modes are non-normalizable at finite separation but become localized at coincidence, enhancing the normalizable sector from $[Sp(2){]}^m$ to $Sp(2m)$. For fundamental fermions of an $Sp(2N)$ gauge theory, the bulk Wess–Zumino–Witten (WZW) term descends compatibly: a cluster of $n$ coincident minimal vortices carries an $Sp(2n{)}_N$ WZW theory, while a flavor-symmetric vortex with positive integer baryon-phase winding $\nu$ carries $Sp(2m{)}_{N\nu }$. At unit winding this yields the vortex map to $Sp(2m{)}_N$, with central charge $c=Nm(2m+1)\mathrm{/}(N+m+1)$ at the WZW fixed point, and suggests a broader nonsupersymmetric four-dimensional/two-dimensional correspondence.