  In this paper, we present the first comprehensive CMB data analysis of
cosmological collider physics. New heavy particles during inflation can leave
imprints in the primordial correlators which are observable in today's
cosmological surveys. This remarkable detection channel provides an unsurpassed
opportunity to probe new physics at extremely high energies. Here we initiate
the search for these relic signals in the cosmic microwave background (CMB)
data from the Planck legacy release. On the theory side, guided by recent
progress from the cosmological bootstrap, we first propose a family of analytic
bispectrum templates that incorporate the distinctive signatures of
cosmological collider physics. Our consideration includes the oscillatory
signals in the squeezed limit, the angular dependence from spinning fields, and
several new shapes from nontrivial sound speed effects. On the observational
side, we apply the recently developed pipeline, CMB Bispectrum Estimator
(CMB-BEST), to efficiently analyze the three-point statistics and search
directly for these new templates in the Planck 2018 temperature and
polarization data. We report stringent CMB constraints on these new templates.
Furthermore, we perform parameter scans to search for the best-fit values with
maximum significance. For a benchmark example of collider templates, we find
$f_{NL}=-91\pm40$ at the $68\%$ confidence level. After accounting for the
look-elsewhere effect, the biggest adjusted significance we get is $1.8\sigma$.
In general, we find no significant evidence of cosmological collider signals in
the Planck data. However, this innovative analysis demonstrates the potential
for discovering new heavy particles during inflation in forthcoming
cosmological surveys.
