  Classically oscillating massive fields can be used as "standard clocks" in
the primordial universe. They generate features in primordial density
perturbations that directly record the scale factor evolution a(t). Detecting
and measuring these "fingerprint" signals is challenging but would provide a
direct evidence for a specific primordial universe paradigm. In this paper,
such a search is performed for the power spectrum of the Cosmic Microwave
Background (CMB) anisotropies using the WMAP7 data. Although a good fit to the
data privileges a scale around k=0.01 Mpc^(-1), we do not find statistical
significance for, neither against, the presence of any feature. We then
forecast the expected constraints a Planck-like CMB experiment can impose on
the fingerprint parameters by using Markov-Chain-Monte-Carlo (MCMC) methods on
mock data. We exhibit a high sensitivity zone for wavenumbers ranging from 0.01
Mpc^(-1) to 0.1 Mpc^(-1) in which fingerprints show up first on the posterior
probability distribution of the wavenumber at which they occur, and then on the
modulation frequency. Within the sensitivity zone, we show that the
inflationary paradigm can be inferred from a single feature generating at least
a 20% modulation of the primordial power spectrum. This minimal value
sensitively depends on the modulation frequency.
