  The environmental dependence of galaxy clustering encodes information about
the physical processes governing the growth of cosmic structure. We analyze the
baryon acoustic peak as a function of environment in the galaxy correlation
function of the Baryon Oscillation Spectroscopic Survey CMASS sample. Dividing
the sample into three subsets by smoothed local overdensity, we detect acoustic
peaks in the six separate auto-correlation and cross-correlation functions of
the sub-samples. Fitting models to these correlation functions, calibrated by
mock galaxy and dark matter catalogues, we find that the inferred distance
scale is independent of environment, and consistent with the result of
analyzing the combined sample. The shape of the baryon acoustic feature, and
the accuracy of density-field reconstruction in the Zeldovich approximation,
varies with environment. By up-weighting underdense regions and down-weighting
overdense regions in their contribution to the full-sample correlation
function, by up to 50%, we achieve a fractional improvement of a few per cent
in the precision of baryon acoustic oscillation fits to the CMASS data and mock
catalogues: the scatter in the preferred-scale fits to the ensemble of mocks
improves from 1.45% to 1.34% (pre-reconstruction) and 1.03% to 1.00%
(post-reconstruction). These results are consistent with the notion that the
acoustic peak is sharper in underdense environments.
