  Oscillating massive fields in the primordial universe can be used as Standard
Clocks. The ticks of these oscillations induce features in the density
perturbations, which directly record the time evolution of the scale factor of
the primordial universe, thus if detected, provide a direct evidence for the
inflation scenario or the alternatives. In this paper, we construct a full
inflationary model of primordial Standard Clock and study its predictions on
the density perturbations. This model provides a full realization of several
key features proposed previously. We compare the theoretical predictions from
inflation and alternative scenarios with the Planck 2013 temperature data on
Cosmic Microwave Background (CMB), and identify a statistically marginal but
interesting candidate. We discuss how future CMB temperature and polarization
data, non-Gaussianity analysis and Large Scale Structure data may be used to
further test or constrain the Standard Clock signals.
