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We present ACS, NICMOS, and Keck AO-assisted photometry of 20 Type Iasupernovae SNe Ia from the HST Cluster Supernova Survey. The SNe Ia werediscovered over the redshift interval 0.623 < z < 1.415. Fourteen of these SNeIa pass our strict selection cuts and are used in combination with the world'ssample of SNe Ia to derive the best current constraints on dark energy. Ten ofour new SNe Ia are beyond redshift $z=1$, thereby nearly doubling thestatistical weight of HST-discovered SNe Ia beyond this redshift. Our detailedanalysis corrects for the recently identified correlation between SN Ialuminosity and host galaxy mass and corrects the NICMOS zeropoint at the countrates appropriate for very distant SNe Ia. Adding these supernovae improves thebest combined constraint on the dark energy density \rho_{DE}(z) at redshifts1.0 < z < 1.6 by 18% (including systematic errors). For a LambdaCDM universe,we find \Omega_\Lambda = 0.724 +0.015/-0.016 (68% CL including systematicerrors). For a flat wCDM model, we measure a constant dark energyequation-of-state parameter w = -0.985 +0.071/-0.077 (68% CL). Curvature isconstrained to ~0.7% in the owCDM model and to ~2% in a model in which darkenergy is allowed to vary with parameters w_0 and w_a. Tightening further theconstraints on the time evolution of dark energy will require severalimprovements, including high-quality multi-passband photometry of a sample ofseveral dozen z>1 SNe Ia. We describe how such a sample could be efficientlyobtained by targeting cluster fields with WFC3 on HST.

astrophysical journal/the astrophysical journal

The Hubble Space Telescope Cluster Supernova Survey: V. Improving the  Dark Energy Constraints Above z>1 and Building an Early-Type-Hosted Supernova  Sample