The 21 cm-kSZ-kSZ Bispectrum during the Epoch of Reionization
  The high-redshift 21 cm signal from the Epoch of Reionization (EoR) is a
promising observational probe of the early universe. Current- and
next-generation radio interferometers such as the Hydrogen Epoch of
Reionization Array (HERA) and Square Kilometre Array (SKA) are projected to
measure the 21 cm auto power spectrum from the EoR. Another observational
signal of this era is the kinetic Sunyaev-Zel'dovich (kSZ) signal in the cosmic
microwave background (CMB), which will be observed by the upcoming Simons
Observatory (SO) and CMB-S4 experiments. The 21 cm signal and the contribution
to the kSZ from the EoR are expected to be anti-correlated, the former coming
from regions of neutral gas in the intergalactic medium and the latter coming
from ionized regions. However, the naive cross-correlation between the kSZ and
21 cm maps suffers from a cancellation that occurs because ionized regions are
equally likely to be moving toward or away from the observer and so there is no
net correlation with the 21 cm signal. We present here an investigation of the
21 cm-kSZ-kSZ bispectrum, which should not suffer the same cancellation as the
simple two-point cross-correlation. We show that there is a significant and
non-vanishing signal that is sensitive to the reionization history, suggesting
the statistic may be used to confirm or infer the ionization fraction as a
function of redshift. In the absence of foreground contamination, we forecast
that this signal is detectable at high statistical significance with HERA and
SO. The bispectrum we study suffers from the fact that the kSZ signal is
sensitive only to Fourier modes with long-wavelength line-of-sight components,
which are generally lost in the 21 cm data sets owing to foreground
contamination. We discuss possible strategies for alleviating this
contamination, including an alternative four-point statistic that may help
circumvent this issue.
