No sign of symmetrons

A high-precision experiment led by Vienna's Technical University (TU Vienna) has set its sights on pinpointing the so-far hypothetical "symmetron fields" using the PF2 ultra-cold neutron source at the Institut Laue-Langevin (ILL) in France. The existence of symmetrons could provide an explanation for mysterious dark energy.

One thing is certain: there's something out there we don't yet know. For years now scientists have been looking for dark matter or dark energy – our current inventory of particles and forces in nature isn't enough to explain major cosmological phenomena, such as why the universe is expanding at an ever faster rate.

New theories for dark energy are constantly being suggested. One of the candidates is the so-called symmetron fields, which are said to pervade space rather like the Higgs field. At the TU Vienna researchers have developed an experiment capable of measuring – with the help of neutrons – extremely small forces. The measurements were taken during a 100-day campaign at the ILL, on its PF2 ultra-cold neutron source. This analysis could have provided pointers to the mysterious symmetrons – but the particles didn't show up. Although this is not the end of the theory, it does exclude the possibility of symmetrons existing across a broad range of parameters, suggesting dark energy is going to have to be explained differently.

The symmetron – a little brother for the Higgs boson?

According to Hartmut Abele, the project's lead scientist, "the symmetron theory would be a particularly elegant explanation for dark matter."

"We already have proof of the Higgs field, and the symmetron field is very closely related." However, as with the Higgs particle, whose mass was not known until the existence of the particle was confirmed, the physical properties of symmetrons cannot be accurately predicted.

As Abele explains, "nobody can say what the mass of a symmetron is, nor how strongly they interact with normal matter. That's why it's so hard to prove their existence experimentally – or their non-existence for that matter." The existence of symmetrons can only be confirmed or refuted within a certain parameter range – symmetrons, in other words, with mass or coupling constants in a specific value range.

Scientists are therefore progressing with caution, from one experiment to the other, testing different parameter ranges. It was already clear that a number of ranges could be excluded. Symmetrons for example with high mass and low coupling constants cannot exist, as they would already have shown up at the atomic level and investigations into the hydrogen atom would have given different results.

Similarly, symmetrons in a certain range with very high coupling constants can also be excluded, as we could already have shown they exist in other experiments using massive pendula.

Using neutrons as force sensors at the ILL neutron source

That said, there was still plenty of scope for admitting the existence of symmetrons, and this is what was investigated with these experiments.

A stream of extremely slow neutrons was shot between two mirror surfaces. The neutrons act as a sensitive force detector as neutrons can be found in two different quantum physical states and the energies of these states depend on the forces exerted on the neutron. If it’s observed that just above the surface of the mirror there is a different force on the neutron than further above, this would strongly suggest the existence of a symmetron field.

Using this method, Mario Pitschmann from TU Vienna, Philippe Brax from the CEA near Paris and Guillaume Pignol from LPSC in Grenoble could see if a symmetron field influenced the neutron. The effect was not, however, proven, despite the extreme accuracy of the measurement.

The precision of the energy difference measurement is about 2x10-15 electron-volts (a figure we owe to a dissertation by Gunther Cronenberg). That is the energy required to lift a single electron in the earth's gravitational field a distance of about 30 micrometres, which is an unimaginably small amount of energy.

The ultra-cold neutrons required for the experiment were generated and delivered by the ILL's PF2 instrument. According to Tobias Jenke, instrument scientist, ILL, "with its unrivalled flux of ultra-cold neutrons, PF2 is practically the only instrument out there for this type of high-precision measurement at extremely low count rates." Jenke played an important part in the development of the TU Vienna experiment. He is now, together with Peter Geltenbort, responsible for the ILL cold neutron source. Austria is a Scientific Member of the Institute and thus has access to its suite of instruments. The experiment is an excellent example of scientific collaboration between Austrian and French researchers.

For the moment things are not looking too bright for the symmetron theory, although it's too early to completely exclude their existence. For Hartmut Abele "we've excluded a broad parameter domain: if there were any symmetrons with properties in this domain we would have found them". To close the remaining loopholes however, science needs even better measurements – or a major discovery providing a completely different solution to the mystery of dark energy.

Instrument:  PF2 - Ultra cold neutron source


Re.: Acoustic Rabi oscillations between gravitational quantum states and impact on symmetron dark energy
Nature Physics, July 2018. DOI: 10.1038/s41567-018-0205-x
Contact: Dr Tobias Jenke, ILL