  In this paper the spectrum of quark-antiquark systems, including light mesons
and tachyons, is studied in the true vacuum and in the chiral invariant vacuum.
The mass gap equation for the vacua and the Salpeter-RPA equation for the
mesons are solved for a simple chiral invariant and confining quark model. At
T=0 and in the true vacuum, the scalar and pseudoscalar, or the vector and
axial vector are not degenerate, and in the chiral limit, the pseudoscalar
groundstates are Goldstone bosons. At T=0 the chiral invariant vacuum is an
unstable vacuum, decaying through an infinite number of scalar and pseudoscalar
tachyons. Nevertheless the axialvector and vector remain mesons, with real
masses. To illustrate the chiral restoration, an arbitrary path between the two
vacua is also studied. Different families of light-light and heavy-light
mesons, sensitive to chiral restoration, are also studied. At higher
temperatures the potential must be suppressed, and the chiral symmetry can be
restored without tachyons, but then all mesons have small real masses.
Implications for heavy-ion collisions, in particular for the recent vector
meson spectra measured by the NA60 collaboration, are discussed.
