QCD color superconductivity in compact stars: Color-flavor locked quark star candidate for the gravitational-wave signal GW190814

Insights on the peak in the speed of sound of ultradense matter

“…Rather, we view it as a simple, conservative alternative to other proposals [20, 69, 71-73, 85, 102, 106-108], in the sense that no exotic degrees of freedom are necessary. In particular, our mechanism naturally highlights the role played by superfluid or superconducting gaps in ultradense matter, and it might help explain the physics responsible for the emergence of a non-monotonic speed of sound in previous model calculations [75][76][77][78][79][80]. This could provide valuable guidance for further models of neutron-star matter.…”

“…This mechanism may be relevant for QCD with two colors [62,[89][90][91][92][93][94]. We also note that a diquark gap is often present in models featuring a maximum in the speed of sound [75][76][77][78][79][80]. In fact, in Ref.…”

“…Nevertheless, the nature of the degrees of freedom is not essential to the mechanism and a gap due to the formation of diquarks would do just as well. Implications of diquark condensation to the speed of sound in dense matter were made in [76,78].…”

“…It could also provide a simple explanation for the speed-of-sound peak found in Refs. [75][76][77][78][79][80]. Because of the spontaneous breakdown of the U (1) B symmetry at large µ, this mechanism is expected to display a phase transition.…”

“…In fact, at high temperatures and zero baryon chemical potential holographic [56][57][58] and lattice QCD calculations [59,60] do find that c 2 s ≤ 1/3. Nonetheless, speeds of sound above the conformal result can be found in QCD at large isospin density [61], in two-color QCD [62], in holographic approaches [63][64][65][66], in resummed perturbation theory [67,68], in quarkyonic matter [69][70][71][72][73] and in models of high-density QCD [74][75][76][77][78][79][80][81][82][83][84][85][86][87][88]. At any rate, it is remarkable that questions regarding the maximum speed of sound allowed in nature might be answered, in the near future, by astrophysical observations.…”