COLLECTIVE MODES AND CURRENT-ALGEBRAIC SUM-RULES IN NUCLEAR MEDIUM

In-medium sum rules following from the chiral charge algebra of QCD ar e reviewed, and new sum rules are derived. The new sum rules relate th e I-G(J(PC)) = 1(-)(0(++)) excitations (quantum numbers of a(0)(980)) to the scalar and isovector densities, and are non-trivial for the iso spin-asymmetric medium. We present an extensive illustration of the su m rules with the help of quark matter in the Nambu-Jona-Lasinio model. Collective excitations different from the usual meson branches (spin- isospin sound modes) are shown to contribute significantly to the sum rules and to play a crucial role in the limit of vanishing current qua rk masses. (C) 1998 Elsevier Science B.V.