We present griz light curves of 146 spectroscopically confirmed Type Ia
Supernovae (0.03<z<0.65) discovered during the first 1.5 years of the
Pan-STARRS1 Medium Deep Survey. The Pan-STARRS1 natural photometric system is
determined by a combination of on-site measurements of the instrument response
function and observations of spectrophotometric standard stars. We find that
the systematic uncertainties in the photometric system are currently 1.2\%
without accounting for the uncertainty in the HST Calspec definition of the AB
system. A Hubble diagram is constructed with a subset of 113 out of 146 SNe Ia
that pass our light curve quality cuts. The cosmological fit to 310 SNe Ia (113
PS1 SNe Ia + 222 light curves from 197 low-z SNe Ia), using only SNe and
assuming a constant dark energy equation of state and flatness, yields
w=−1.120−0.206+0.360(Stat)−0.291+0.269(Sys).
When combined with BAO+CMB(Planck)+H0, the analysis yields ΩM=0.280−0.012+0.013 and w=−1.166−0.069+0.072 including all
identified systematics (see also Scolnic et al. 2014). The value of w is
inconsistent with the cosmological constant value of −1 at the 2.3σ
level. Tension endures after removing either the BAO or the H0 constraint,
though it is strongest when including the H0 constraint. If we include WMAP9
CMB constraints instead of those from Planck, we find
w=−1.124−0.065+0.083, which diminishes the discord to <2σ. We
cannot conclude whether the tension with flat ΛCDM is a feature of dark
energy, new physics, or a combination of chance and systematic errors. The full
Pan-STARRS1 supernova sample with ∼3 times as many SNe should provide
more conclusive results.Comment: 38 pages, 16 figures, 14 tables, ApJ in pres