Photons in gapless colour-flavour-locked quark matter

We calculate the Debye and Meissner masses of a gauge boson in a material consisting of two species of massless fermions that form a condensate of Cooper pairs. We perform the calculation as a function of temperature, for the cases of neutral Cooper pairs and charged Cooper pairs, and for a range of parameters including gapped quasiparticles, and ungapped quasiparticles with both quadratic and linear dispersion relations at low energy. Our results are relevant to the behaviour of photons and gluons in the gapless colour-flavour-locked phase of quark matter. We find that the photon's Meissner mass vanishes, and the Debye mass shows a non-monotonic temperature dependence, and at temperatures of order the pairing gap it drops to a minimum value of order root alpha times the quark chemical potential. We confirm previous claims that at zero temperature an imaginary Meissner mass can arise from a charged gapless condensate, and we find that at finite temperature this can also occur for a gapped condensate.