  Ongoing, extreme-precision Doppler radial velocity surveys seek planets with
masses less than several M$_\oplus$; population-level studies to determine the
distribution of planetary masses, however, remain difficult due to the required
observational time investment, as well as challenges associated with robustly
detecting the lowest mass planets. We outline a novel approach that leverages
extensive, existing RV datasets to constrain masses of exoplanet populations:
stacking periodograms of RV timeseries across many targets. We show that an
exoplanet population may be statistically identifiable in the stacked
periodogram, even when individual planets do not pass the threshold of
detection. We discuss analytical, statistical properties of the stacked
periodogram, perform simulations to demonstrate the efficacy of the method, and
investigate the influence of semi-structured window functions and stellar
activity. Analysis of the Lick-Carnegie Exoplanet Survey data set reveals a
marginally significant ($1.6\sigma$) signal consistent with a population of
exoplanets occupying $3-7$ day periods with typical $K$ between $1.6-5.1$ m
s$^{-1}$. More detailed investigation of signals associated with stellar
activity and yearly systematics may be necessary to confirm this result or
detect other underlying Keplerian contributions.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics