  We perform a systematic search for rings around 168 Kepler planet candidates
with sufficient signal-to-noise ratios that are selected from all the
short-cadence data. We fit ringed and ringless models to their lightcurves, and
compare the fitting results to search for the signatures of planetary rings.
First, we identify 29 tentative systems, for which the ringed models exhibit
statistically significant improvement over the ringless models. The lightcurves
of those systems are individually examined, but we are not able to identify any
candidate that indicates evidence for rings. In turn, we find out several
mechanisms of false-positives that would produce ring-like signals, and the
null detection enables us to place upper limits on the size of rings.
Furthermore, assuming the tidal alignment between axes of the planetary rings
and orbits, we conclude that the occurrence rate of rings larger than twice the
planetary radius is less than 15 percent. Even though the majority of our
targets are short-period planets, our null detection provides statistical and
quantitative constraints on largely uncertain theoretical models of origin,
formation, and evolution of planetary rings.
