Prospects for kSZ$^2$-Galaxy Cross-Correlations during Reionization
  We explore a new approach for extracting reionization-era contributions to
the kinetic Sunyaev-Zel'dovich (kSZ) effect. Our method utilizes the
cross-power spectrum between filtered and squared maps of the cosmic microwave
background (CMB) and photometric galaxy surveys during the Epoch of
Reionization (EoR). This kSZ$^2$-galaxy cross-power spectrum statistic has been
successfully detected at lower redshifts ($z \lesssim 1.5$). Here we extend
this method to $z \gtrsim 6$ as a potential means to extract signatures of
patchy reionization. We model the expected signal across multiple photometric
redshift bins using semi-numeric simulations of the reionization process. In
principle, the cross-correlation statistic robustly extracts reionization-era
contributions to the kSZ signal, while its redshift evolution yields valuable
information regarding the timing of reionization. Specifically, the model
cross-correlation signal near $\ell \sim 1,000$ peaks during the early stages
of the EoR, when about 20% of the volume of the universe is ionized. Detectible
$\ell$ modes mainly reflect squeezed triangle configurations of the related
bispectrum, quantifying correlations between the galaxy overdensity field on
large scales and the smaller-scale kSZ power. We forecast the prospects for
detecting this signal using future wide-field samples of Lyman-break galaxies
from the Roman Space Telescope and next-generation CMB surveys including the
Simons Observatory, CMB-S4, and CMB-HD. We find that a roughly 13$\sigma$
detection is possible for CMB-HD and Roman after summing over all $\ell$ modes.
We discuss the possibilities for improving this approach and related
statistics, with the aim of moving beyond simple detections to measure the
scale and redshift dependence of the cross-correlation signals.
