  We analyze MegaZ-LRG, a photometric-redshift catalogue of Luminous Red
Galaxies (LRGs) based on the imaging data of the Sloan Digital Sky Survey
(SDSS) 4th Data Release. MegaZ-LRG, presented in a companion paper, contains
10^6 photometric redshifts derived with ANNz, an Artificial Neural Network
method, constrained by a spectroscopic sub-sample of 13,000 galaxies obtained
by the 2dF-SDSS LRG and Quasar (2SLAQ) survey. The catalogue spans the redshift
range 0.4 < z < 0.7 with an r.m.s. redshift error ~ 0.03(1+z), covering 5,914
deg^2 to map out a total cosmic volume 2.5 h^-3 Gpc^3. In this study we use the
most reliable 600,000 photometric redshifts to present the first cosmological
parameter fits to galaxy angular power spectra from a photometric redshift
survey. Combining the redshift slices with appropriate covariances, we
determine best-fitting values for the matter and baryon densities of Omega_m h
= 0.195 +/- 0.023 and Omega_b/Omega_m = 0.16 +/- 0.036 (with the Hubble
parameter h = 0.75 and scalar index of primordial fluctuations n = 1 held
fixed). These results are in agreement with and independent of the latest
studies of the Cosmic Microwave Background radiation, and their precision is
comparable to analyses of contemporary spectroscopic-redshift surveys. We
perform an extensive series of tests which conclude that our power spectrum
measurements are robust against potential systematic photometric errors in the
catalogue. We conclude that photometric-redshift surveys are competitive with
spectroscopic surveys for measuring cosmological parameters in the simplest
vanilla models. Future deep imaging surveys have great potential for further
improvement, provided that systematic errors can be controlled.
