  We report the first detection of a correlation between gravitational lensing
by large scale structure and the thermal Sunyaev-Zeldovich (tSZ) effect. Using
the mass map from the Canada France Hawaii Telescope Lensing Survey (CFHTLenS)
and a newly-constructed tSZ map from Planck, we measure a non-zero correlation
between the two maps out to one degree angular separation on the sky, with an
overall significance of 6 sigma. The tSZ maps are formed in a manner that
removes primary cosmic microwave background fluctuations and minimizes residual
contamination by galactic and extragalactic dust emission, and by CO line
emission. We perform numerous tests to show that our measurement is immune to
these residual contaminants. The resulting correlation function is consistent
with the existence of a warm baryonic gas tracing the large scale structure
with a bias b_gas. Given the shape of the lensing kernel, our signal
sensitivity peaks at a redshift z~0.4, where half a degree separation on the
sky corresponds to a physical scale of ~10 Mpc. The amplitude of the signal
constrains the product (b_gas/1)(T_e / 0.1 keV)(n_e / 1 m^-3)=2.01\pm 0.31\pm
0.21, at redshift zero. Our study suggests that a substantial fraction of the
"missing" baryons in the universe may reside in a low density warm plasma that
traces dark matter.
