A Measurement of Cubic-Order Primordial Non-Gaussianity (g_{NL} and
  \tau_{NL}) With WMAP 5-Year Data
  We measure two higher-order power spectra involving weighted cubic and
squared temperature anisotropy maps from WMAP 5-year data to study the
trispectrum generated by primordial non-Gaussianity. Using these measurements
combined with Gaussian and noise simulations, we constrain the cubic order
non-Gaussianity parameters \tau_{NL}, and g_{NL}. With V+W-band data out to
l_{max}=600, we find -7.4 < g_{\rm NL}/10^5 < 8.2 and -0.6 < \tau_{\rm NL}/10^4
< 3.3 improving the previous COBE-based limit on \tau_{\rm NL} < 10^8 nearly
four orders of magnitude with WMAP. We find that the ratio of trispectrum to
bispectrum amplitude as captured by the ratio of \tau_{\rm Nl}/(6f_{\rm
NL}/5)^2 ranges from -3 to 21 at the 95% confidence level.
