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Particles and interactions

Particles and interactions

Ultracold neutrons put a new spin on neutron dipole measurements

27 Oct 2015
Magnetic drum: the spectrometer for the new measurement

A new technique, which is based on magnetic resonance imaging, should allow physicists to carry out more sensitive searches for the neutron’s putative electric dipole moment (EDM). While the neutron EDM is predicted by many extensions of the Standard Model, we are yet to find any evidence for it. The latest work should help researchers to either discover the subtle phenomenon or rule out a number of theories that predict it.

Although the neutron has no net charge, it is composed of charged quarks, and any permanent spatial offset between positively and negatively charged ones could, in theory, give rise to a dipole moment that is sensitive to electric fields. Experiments to look for such an EDM were first carried out in the 1950s and their sensitivity has since improved by more than six orders of magnitude. But physicists continue to push sensitivities ever higher, in the hope of discovering an effect that would violate so-called charge–parity symmetry, thereby explaining how matter came to dominate over antimatter in the early universe.

Precessing axes

In a typical EDM experiment, a batch of ultracold neutrons – at temperatures of 3 mK or less – are trapped in a chamber and polarized so that their spin magnetic moments are all aligned. Ultracold atoms are used as they can only reach a certain height – about 2 m at most – as they have very little kinetic energy. The atoms are then exposed to a magnetic field, causing their spin axes to precess around the field lines with a frequency that depends on the field strength. By then adding an electric field – which alternates between positive and negative values once every few hours – the EDM, should it exist, would couple to the electric field, and this, in turn, would change the neutron’s precession frequency.

To do this, researchers have to ensure that they account for any noise that could mimic an EDM signal. The magnetic field must be extremely stable and any fluctuations monitored precisely – even a car passing close by could have a measurable effect. While the researchers can monitor average field strength using mercury atoms with relative ease, they still cannot account for variations in the magnetic field along the chamber’s vertical axis. Any neutrons located in a region with fluctuations will not precess at the expected frequency, so reducing the strength of the signal from the combined magnetic dipoles. Also, as the neutrons will have a distribution of energies, they will be at different heights throughout the chamber. If this distribution is not known, then it is impossible to say how much of the variation in the dipole signal is due to a neutron EDM, as opposed to a more prosaic variation in the magnetic field.

Flipping spins

To overcome this problem, Philipp Schmidt-Wellenburg of the Paul Scherrer Institute in Switzerland and colleagues have exploited a phenomenon known as “spin echo”. Using a dedicated spallation source at the institute, the researchers loaded spin-polarized neutrons into a 12 cm-high chamber, exposed to a smoothly varying magnetic field that is aligned with the neutrons’ spin along the chamber’s vertical axis. They then applied a brief pulse of a weaker magnetic field that flipped the spin through 90°, causing them to start precessing in another plane. A few tens of seconds later, a second pulse once again flips the spins. By measuring the progress of the neutron spins after another gap of several tens of seconds, the researchers measured the amplitude of the precession frequency.

As expected, the biggest signal – the spin echo – came when the two gaps were of equal length. This resonance occurs because the spins exposed to a higher magnetic field precess quicker and get ahead of the others. But the flipping to the original axis causes a reversal of their positions, thereby refocusing them. By varying the relative duration of the two gaps, Schmidt-Wellenburg and colleagues generate a range of signal strengths at the output. Given that these signal strengths will depend on how many of the neutron spins lag behind or steal a march on the neutrons at different field strengths, a plot of signal strength versus gap length can be used to reconstruct the neutrons’ energy spectrum.

Schmidt-Wellenburg and co-workers will use this technique to maximize the sensitivity of a new EDM experiment they are currently working on, which should be up and running within the next five to 10 years. The current best upper limit on the neutron’s EDM is 3 × 10–26 e cm, which was set by Philip Harris of Sussex University and colleagues in 2006 using an experiment located at the Institut Laue-Langevin in Grenoble, France – Harris’s group has since joined the Swiss-based collaboration. The new experiment is being designed to push that limit down to 5 × 10–28 e cm, which, according to Schmidt-Wellenburg, is as low as the values predicted by many extensions to the Standard Model.

The research is reported in Physical Review Letters.

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