  We investigate the heat capacity of liquids through a theoretical approach
based on a quasiparticle description. By interpreting the microscopic dynamics
of particles in liquids in terms of quasiparticles, we suggest a simplified
understanding of the number of degrees of freedom in liquids. A equivalence
between hydrodynamics and U(1) gauge theory, which is newly proposed in the
present paper, develops the quasiparticle description to construct a new
Lagrangian which correctly reproduces the number of modes at the melting points
and at the critical points. The heat capacity evaluated from this Lagrangian
naturally interpolates between these two points, and agrees with the phonon
theory of liquids [Sci. Rep. 2, 421 (2012)].
