We study the speed of sound in strong-interaction matter at zero temperature and in density regimes which are expected to be governed by the presence of a color-superconducting gap. At (very) high densities, our analysis indicates that the speed of sound approaches its asymptotic value associated with the non-interacting quark gas from below, in agreement with first-principles studies which do not take the presence of a color-superconducting gap into account. Towards lower densities, however, the presence of a gap induces an increase of the speed of sound above its asymptotic value. Importantly, even if gap-induced corrections to the pressure may appear small, we find that derivatives of the gap with respect to the chemical potential can still be sizeable and lead to a qualitative change of the density dependence of the speed of sound. Taking into account constraints on the density dependence of the speed of sound at low densities, our general considerations suggest the existence of a maximum in the speed of sound. Interestingly, we also observe that specific properties of the gap can be related to characteristic properties of the speed of sound which are indirectly constrained by observations.

中文翻译：

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致密强相互作用物质中的声速

我们研究零温度和密度范围内强相互作用物质中的声速，预计该速度将受到颜色超导间隙的存在的控制。在（非常）高的密度下，我们的分析表明，声速接近与来自下方的非相互作用夸克气体相关的渐近值，这与第一原理研究一致，该研究没有考虑颜色超导间隙的存在帐户。然而，对于较低的密度，间隙的存在会导致声速增加到其渐近值以上。重要的是，即使间隙引起的压力修正可能看起来很小，我们发现间隙相对于化学势的导数仍然可以相当大，并导致声速的密度依赖性发生质的变化。考虑到低密度下声速对密度依赖性的限制，我们的一般考虑表明声速存在最大值。有趣的是，我们还观察到间隙的特定属性可能与声速的特征属性相关，而声速的特征属性受到观测的间接约束。