Detection of Cosmological 21 cm Emission with the Canadian Hydrogen
  Intensity Mapping Experiment
  We present a detection of 21-cm emission from large-scale structure (LSS)
between redshift 0.78 and 1.43 made with the Canadian Hydrogen Intensity
Mapping Experiment (CHIME). Radio observations acquired over 102 nights are
used to construct maps which are foreground filtered and stacked on the angular
and spectral locations of luminous red galaxies (LRG), emission line galaxies
(ELG), and quasars (QSO) from the eBOSS clustering catalogs. We find decisive
evidence for a detection when stacking on all three tracers of LSS, with the
logarithm of the Bayes Factor equal to 18.9 (LRG), 10.8 (ELG), and 56.3 (QSO).
An alternative frequentist interpretation, based on the likelihood-ratio test,
yields a detection significance of $7.1\sigma$ (LRG), $5.7\sigma$ (ELG), and
$11.1\sigma$ (QSO). These are the first 21-cm intensity mapping measurements
made with an interferometer. We constrain the effective clustering amplitude of
neutral hydrogen (HI), defined as $\mathcal{A}_{\rm HI}\equiv
10^{3}\,\Omega_\mathrm{HI}\left(b_\mathrm{HI}+\langle\,f\mu^{2}\rangle\right)$,
where $\Omega_\mathrm{HI}$ is the cosmic abundance of HI, $b_\mathrm{HI}$ is
the linear bias of HI, and $\langle\,f\mu^{2}\rangle=0.552$ encodes the effect
of redshift-space distortions at linear order. We find
$\mathcal{A}_\mathrm{HI}=1.51^{+3.60}_{-0.97}$ for LRGs $(z=0.84)$,
$\mathcal{A}_\mathrm{HI}=6.76^{+9.04}_{-3.79}$ for ELGs $(z=0.96)$, and
$\mathcal{A}_\mathrm{HI}=1.68^{+1.10}_{-0.67}$ for QSOs $(z=1.20)$, with
constraints limited by modeling uncertainties at nonlinear scales. We are also
sensitive to bias in the spectroscopic redshifts of each tracer, and find a
non-zero bias $\Delta\,v= -66 \pm 20 \mathrm{km/s}$ for the QSOs. We split the
QSO catalog into three redshift bins and have a decisive detection in each,
with the upper bin at $z=1.30$ producing the highest redshift 21-cm intensity
mapping measurement thus far.
