Particle physics always conjures up images of very high-energy experiments, particle accelerators etc. A recent study — a table-top experiment with low-energy neutrons — shatters this image and introduces a low-energy technique to probe gravity, dark energy and dark matter.
In a paper published in April, in Physical Review Letters, Jenke et al, led by Hartmut Abele of Technical University of Vienna, Austria, report that ultracold neutrons can be used in “gravity resonance spectroscopy” to yield constraints on the properties of dark energy and dark matter.
These measurements, in fact, disprove the existence of the hypothetical “quintessence” particles which form one explanation for dark energy, at this level of accuracy. But the search for these particles will continue as it is possible that a greater level of accuracy is needed.
In the experiment, conducted at the Institut Laue-Langevin, Grenoble, France, the researchers confined ultracold neutrons between two neutron-mirrors. This system along with the earth’s gravitational field closely mimics the famous “particle in a box” of quantum mechanics. Varying the separation between the walls periodically caused the energy levels of the system to oscillate and the confined neutrons to make transitions, whose values can be measured as well as calculated.
This experiment measures the neutrons’ coupling to gravity very accurately, so that it can actually probe interactions with the very weakly interacting hypothetical particle of dark matter, the axion, and the chameleon field, which is a name for the proposed source of dark energy.
To digress briefly, dark energy is a proposed explanation for the accelerated expansion of the universe and dark matter is needed to describe the rotation curves of galaxies and the large scale structure of the universe. However the properties of these entities have not been understood well at all. The axion is expected to couple to the spin of the neutron and thereby, it would affect the neutrons’ behaviour slightly.
Jenke et al first polarise the neutrons and then pass them through the cavity and then measure their transition frequencies again. The comparison of these experimentally measured values to the theoretical values puts constraints on the coupling constant of the interaction (a measure of the strength of the interaction with the axion field).
“Our experiment now sets much better limits on this strength parameter, so that the ‘allowed’ parameter space (allowed means that it does not contradict observations) gets much smaller,” Says Tobias Jenke of the collaboration in an email.
