Looking for symmetrons and dark matter

Looking for symmetrons and dark matter
This collage shows NASA/ESA Hubble Space Telescope images of six different galaxy clusters. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide. © ESA-Hubble CC BY 4.0 via Wikimedia Commons

A high-precision experiment led by Vienna’s Technical University 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), France. The existence of symmetrons could provide an explanation for dark matter.

Scientists have been looking for dark matter for a long time as approximately 80% of the matter in our Universe is dark matter – our current inventory of particles and forces in nature is not 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 like the Higgs field.

At Vienna’s Technical University (TU Vienna) researchers have developed an experiment capable of measuring 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. However, the particles did not 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, meaning dark energy will have to be explained differently.

The symmetron – a little brother for the Higgs boson?

Hartmut Abele, the project’s lead scientist said: “the symmetron theory would be a particularly elegant explanation for dark matter.”

Just like the Higgs particle, the physical properties of symmetrons cannot be accurately predicted.

Abele said: “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.”

Scientists are therefore progressing with caution, testing different parameter ranges. It was already clear that several 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.

Using neutrons as force sensors at the ILL neutron source

However, there is still plenty of scope for 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 and Guillaume Pignol from LPSC, France, 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 2×10-15 electron-volts. That is the energy required to lift a single electron in the earth’s gravitational field, 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.

For the moment things are not looking too bright for the symmetron theory, although it’s too early to completely exclude their existence. Abele concluded: “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 matter.

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