出版社：SISSA, Scuola Internazionale Superiore di Studi Avanzati
摘要：We revisit two-color two-flavor Quantum Chromodynamics (QCD) which suffers from the sign
problem of the Dirac determinant at finite baryon or quark density if a finite isospin chemical
potential breaks two-flavor degeneracy. We discuss the eigenvalue distribution of the single-flavor
Dirac operator to find the quartet structure, i.e. if l +m is an eigenvalue where m is the current
quark mass in a continuum theory, l ¤ +m, ¡l +m, and ¡l ¤ +m must be eigenvalues as well.
As a result the product of these four eigenvalues makes a non-negative real number, unless l is
real. The sign problem remains harmful for real l because not a quartet but a pair of l +m and
¡l +m appears then and its product is not necessarily non-negative. In such a case, however, the
sign problem has a different nature from the ordinary one in three-color QCD at finite density.
We can rather identify it as the sign problem in the parity broken (or Aoki) phase inherent in the
Wilson fermion formalism.
