  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
$w=-1.120^{+0.360}_{-0.206}\textrm{(Stat)} ^{+0.269}_{-0.291}\textrm{(Sys)}$.
When combined with BAO+CMB(Planck)+$H_0$, the analysis yields $\Omega_{\rm
M}=0.280^{+0.013}_{-0.012}$ and $w=-1.166^{+0.072}_{-0.069}$ 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 2.3$\sigma$
level. Tension endures after removing either the BAO or the $H_0$ constraint,
though it is strongest when including the $H_0$ constraint. If we include WMAP9
CMB constraints instead of those from Planck, we find
$w=-1.124^{+0.083}_{-0.065}$, which diminishes the discord to $<2\sigma$. We
cannot conclude whether the tension with flat $\Lambda$CDM is a feature of dark
energy, new physics, or a combination of chance and systematic errors. The full
Pan-STARRS1 supernova sample with $\sim\!\!$3 times as many SNe should provide
more conclusive results.
