PAPER-64 Constraints on Reionization: The 21cm Power Spectrum at z=8.4
  In this paper, we report new limits on 21cm emission from cosmic reionization
based on a 135-day observing campaign with a 64-element deployment of the
Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER)
in South Africa. This work extends the work presented in Parsons et al. (2014)
with more collecting area, a longer observing period, improved redundancy-based
calibration, optimal fringe-rate filtering, and improved power-spectral
analysis using optimal quadratic estimators. The result is a new $2\sigma$
upper limit on $\Delta^{2}(k)$ of (22.4 mK)$^2$ in the range $0.15 < k < 0.5h\
{\rm Mpc}^{-1}$ at $z = 8.4$. This represents a three-fold improvement over the
previous best upper limit. As we discuss in more depth in a forthcoming paper
(Pober et al. 2015, in prep), this upper limit supports and extends previous
evidence against extremely cold reionization scenarios. We conclude with a
discussion of implications for future 21cm reionization experiments, including
the newly funded Hydrogen Epoch of Reionization Array (HERA). $\textbf{The
limits presented in this paper have been retracted: The erratum can be found in
Appendix A.}$
