We present griz _ { P 1 } 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 _ { M } = 0.280 ^ { +0.013 } _ { -0.012 } and w = -1.166 ^ { +0.072 } _ { -0.069 } including all identified systematics ( ) .
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 can not 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 .