Page 67 - ILLS Annual Report 2018

SCIENTIFIC HIGHLIGHTS
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That said, there is still plenty of room left for symmetrons, and this is what we have investigated in our experiments. We propagated a beam of extremely slow neutrons between two mirror surfaces. The neutrons at a certain height z above the mirror can be found in different quantum physical states [3]. The energies of these states depend on the forces exerted on the neutron, and this is what makes the neutron such a sensitive force detector. If close to the surface of the mirror we observed a different force on the neutron from that for higher separations, this would be a strong pointer to the existence of a symmetron field: in our set-up, the existence of symmetrons with a coupling M would lead to an additional potential in V(z) in addition to gravity, as seen by neutrons with mass m:
This would induce individual shifts in the eigen-energies
of the gravitational quantum states, resulting in changes
in the transition frequencies. As we could not observe
any shifts within our experimental uncertainty, we are consequently able to exclude parts of the parameter space for the existence of symmetron fields. To do so, we use
the predicted shifts of the resonance frequencies for the
Figure 1
Schematic views of the experimental set-up: ultra-cold neutrons pass through the set-up from left to right. In Region (I), they are prepared in the gravitational ground state |1>by passing a slit between a rough surface on the top and a perfectly polished surface on the bottom. Higher states interact with the rough surface and are effectively scattered off the system. In Region (II), transitions between quantum states are induced. The surface for that purpose oscillates with variable frequency and strength. This oscillating boundary condition triggers transitions to higher states, if the resonance condition is met and the oscillation strength is sufficient. Region (III) is identical to Region (I) and only transmits neutrons in the ground state. A highly efficient neutron detector (D) with low background counts the transmitted neutrons.
Practical realisation: the boundary conditions are realised by glass mirrors with rough (1) or perfectly polished (2) surfaces. The rms- roughness of the upper mirror is (0.38±0.17) μm, (see enlargement). The neutrons are detected using a neutron counter (3). All mirrors are mounted on nano-positioning tables (4). An optical system (parts in 5) controls the induced mirror oscillations. A movable system based on capacitive sensors (6) controls and levels steps between the regions. The experiment is shielded by μ-metal against the magnetic field of Earth. Flux-gate magnetic field sensors (7) log residual magnetic fields.
m2 V(z)=mgz+ φ (z)
2M2
transitions |1>→ |3> and |1>→ |4>, add the symmetron
χ - to the local value and perform a full-analysis. We found no positive effect. This lack of effect cannot, however, be proven, despite the extreme accuracy of the measurement.
The ultra-cold neutrons required for the experiment were generated by the ILL’s PF2 instrument. 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 [4, 5]. 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 symmetron theory, although it is too early to completely exclude their existence. We have excluded a broad parameter domain: any symmetrons with properties in this domain would have been revealed by our measurements. 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.
model parameters as fit parameters, fix Earth’s acceleration 2
Figure 2
Capacitive sensors control position of and distance between mirrors.
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