Neutron Limit on the Strongly-Coupled Chameleon Field

TL;DR

This study sets the most stringent laboratory upper bounds on the neutron-chameleon coupling strength in the strongly-coupled chameleon dark energy model using neutron interferometry, advancing constraints on dark energy theories.

Contribution

It provides new experimental upper bounds on neutron-chameleon coupling in the strongly-coupled limit, improving sensitivity over previous measurements.

Findings

Established upper bounds on neutron-chameleon coupling $eta$ for different potential indices.

Demonstrated neutron interferometry as a sensitive probe for chameleon fields.

Improved constraints by an order of magnitude compared to previous limits.

Abstract

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.