  We report on the detection of gravitational lensing magnification by a
population of galaxy groups, at a significance level of 4.9 sigma. Using X-ray
selected groups in the COSMOS 1.64 deg^2 field, and high-redshift Lyman break
galaxies as sources, we measure a lensing-induced angular cross-correlation
between the samples. After satisfying consistency checks that demonstrate we
have indeed detected a magnification signal, and are not suffering from
contamination by physical overlap of samples, we proceed to implement an
optimally weighted cross-correlation function to further boost the signal to
noise of the measurement. Interpreting this optimally weighted measurement
allows us to study properties of the lensing groups. We model the full
distribution of group masses using a composite-halo approach, considering both
the singular isothermal sphere and Navarro-Frenk-White profiles, and find our
best fit values to be consistent with those recovered using the weak-lensing
shear technique. We argue that future weak-lensing studies will need to
incorporate magnification along with shear, both to reduce residual systematics
and to make full use of all available source information, in an effort to
maximize scientific yield of the observations.
