Quantum Standard Clocks in the Primordial Trispectrum
  We calculate the primordial trispectrum of curvature perturbation in
quasi-single field inflation, with general sound speeds for both the inflaton
and the massive scalar. Special attention is paid to various soft limits of the
trispectrum, where the shape function shows characteristic oscillatory pattern
(known as the quantum primordial standard clock signal) as a function of the
momentum ratio. Our calculation is greatly simplified by using the "mixed
propagator" developed under a diagrammatic representation of the in-in
formalism.
