Lawrence Berkeley National Laboratory

We present the first high-redshift Hubble diagram for Type II-P supernovae (SNe II-P) based upon five events at redshift up toz~;0.3. This diagram was constructed using photometry from the Canada-France-Hawaii Telescope Supernova Legacy Survey and absorption line spectroscopy from the Keck observatory. The method used to measure distances to these supernovae is based on recent work by Hamuy& Pinto (2002) and exploits a correlation between the absolute brightness of SNe II-P and the expansion velocities derived from the minimum of the Fe II 516.9 nm P-Cygni feature observed during the plateau phases. We present three refinements to this method which significantly improve the practicality of measuring the distances ofSNe II-P at cosmologically interesting redshifts. These are an extinction correction measurement based on the V-I colors at day 50, across-correlation measurement for the expansion velocity and theability to extrapolate such velocities accurately over almost the entire plateau phase. We apply this revised method to our dataset of high-redshift SNe II-P and find that the resulting Hubble diagram hasa scatter of only 0.26 magnitudes, thus demonstrating the feasibility of measuring the expansion history, with present facilities, using amethod independent of that based upon supernovae of Type Ia.