Neutron Limit on the Strongly-Coupled Chameleon Field
  The physical origin of the dark energy that causes the accelerated expansion
rate of the universe is one of the major open questions of cosmology. One set
of theories postulates the existence of a self-interacting scalar field for
dark energy coupling to matter. In the chameleon dark energy theory, this
coupling induces a screening mechanism such that the field amplitude is nonzero
in empty space but is greatly suppressed in regions of terrestrial matter
density. However measurements performed under appropriate vacuum conditions can
enable the chameleon field to appear in the apparatus, where it can be
subjected to laboratory experiments. Here we report the most stringent upper
bound on the free neutron-chameleon coupling in the strongly-coupled limit of
the chameleon theory using neutron interferometric techniques. Our experiment
sought the chameleon field through the relative phase shift it would induce
along one of the neutron paths inside a perfect crystal neutron interferometer.
The amplitude of the chameleon field was actively modulated by varying the
millibar pressures inside a dual-chamber aluminum cell. We report a 95%
confidence level upper bound on the neutron-chameleon coupling $\beta$ ranging
from $\beta < 4.7\times 10^6$ for a Ratra-Peebles index of n = 1 in the
nonlinear scalar field potential to $\beta < 2.4\times 10^7$ for n = 6, one
order of magnitude more sensitive than the most recent free neutron limit for
intermediate n. Similar experiments can explore the full parameter range for
chameleon dark energy in the foreseeable future.
