The One-Loop Matter Bispectrum in the Effective Field Theory of Large
  Scale Structures
  Given the importance of future large scale structure surveys for delivering
new cosmological information, it is crucial to reliably predict their
observables. The Effective Field Theory of Large Scale Structures (EFTofLSS)
provides a manifestly convergent perturbative scheme to compute the clustering
of dark matter in the weakly nonlinear regime in an expansion in $k/k_{\rm
NL}$, where $k$ is the wavenumber of interest and $k_{\rm NL}$ is the
wavenumber associated to the nonlinear scale. It has been recently shown that
the EFTofLSS matches to $1\%$ level the dark matter power spectrum at redshift
zero up to $k\simeq 0.3 h\,$Mpc$^{-1}$ and $k\simeq 0.6 h\,$Mpc$^{-1}$ at one
and two loops respectively, using only one counterterm that is fit to data.
Similar results have been obtained for the momentum power spectrum at one loop.
This is a remarkable improvement with respect to former analytical techniques.
Here we study the prediction for the equal-time dark matter bispectrum at one
loop. We find that at this order it is sufficient to consider the same
counterterm that was measured in the power spectrum. Without any remaining free
parameter, and in a cosmology for which $k_{\rm NL}$ is smaller than in the
previously considered cases ($\sigma_8=0.9$), we find that the prediction from
the EFTofLSS agrees very well with $N$-body simulations up to $k\simeq 0.25
h\,$Mpc$^{-1}$, given the accuracy of the measurements, which is of order a few
percent at the highest $k$'s of interest. While the fit is very good on average
up to $k\simeq 0.25 h\,$Mpc$^{-1}$, the fit performs slightly worse on
equilateral configurations, in agreement with expectations that for a given
maximum $k$, equilateral triangles are the most nonlinear.
