Pion, σ meson, and diquarks in the two-flavor color-superconducting phase of dense cold quark matter

Abstract

The spectrum of meson and diquark excitations of dense cold quark matter is investigated within the framework of a Nambu--Jona-Lasinio-type model for light quarks of two flavors. It was found that a first-order phase transition occurs when the chemical potential \ensuremath{\mu} exceeds the critical value ${\ensuremath{\mu}}_{c}=350\phantom{\rule{0.3em}{0ex}}\text{MeV}$. Above ${\ensuremath{\mu}}_{c}$, the diquark condensate $\ensuremath{\langle}\mathit{qq}\ensuremath{\rangle}$ forms, breaking the color symmetry of strong interaction. The masses of \ensuremath{\pi} and \ensuremath{\sigma} mesons are shown to grow with the chemical potential \ensuremath{\mu} in the color-superconducting phase, but the mesons themselves become almost stable particles. Moreover, we have found in this phase an abnormal number of three, instead of five, Nambu-Goldstone bosons, together with a color doublet of light stable diquark modes and a color-singlet heavy diquark resonance with mass $~$1100 MeV. In the color-symmetric phase, i.e., for $\ensuremath{\mu}<{\ensuremath{\mu}}_{c}$, a mass splitting of diquarks and antidiquarks is shown to arise if $\ensuremath{\mu}\ensuremath{\ne}0$, contrary to the case of a vanishing chemical potential, in which the masses of antidiquarks and diquarks are degenerate at the value $~$700 MeV.