In search of new forces and interactions beyond the Standard Model, an international team of researchers including PRISMA+ The Cluster of Excellence of Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz has now taken a big step forward. The researchers, among them Prof. Dr. Dmitry Budker, use an amplification method based on nuclear magnetic resonance. In her recently published work in scientific advancesthey use their experimental setup to study a particular exotic interaction between spins: a parity-violating interaction mediated by a new hypothetical exchange particle, a so-called Z’ boson, which exists in addition to the Z boson driving the weak interaction conveyed in the standard model. They could not detect this particle in the current setup, but they were able to increase the sensitivity by five orders of magnitude compared to previous measurements. This allows to set limits on the strength of the interaction of the new exchange particle with particles of the Standard Model, which are complementary to astrophysical observations and open up a previously inaccessible area.
Numerous theories predict the existence of exotic interactions beyond the Standard Model. They differ from the four known interactions and are mediated by previously unknown exchange particles. In particular, parity-violating interactions, ie where mirror symmetry is broken, are currently of particular interest. On the one hand because it would immediately point out the special kind of new physics we are dealing with, and on the other hand because its effects are easier to separate from systematic spurious effects that usually do not exhibit mirror symmetry breaking. “In the current article, we examine such an interaction between the spins of electrons and the spins of neutrons mediated by a hypothetical Z’ boson. In a mirrored world, this interaction would lead to a different result; Parity is violated here,” explains Dmitry Budker.
This “result” looks like this: The electron spins within a source are all aligned in one direction, so polarized, and the polarization is continuously modulated, creating an exotic field that can be perceived as a magnetic field and measured with a sensor. In a mirrored world, the exotic field would not point in the same direction as one would expect in a “real” mirror image, but in the opposite direction: the parity of this interaction is violated.
SAPPHIRE – the new jewel in search of new physics
The researchers call their structure, which is based on the two elements rubidium and xenon, “Spin Amplifier for Particle PHysIcs REsearch” – SAPPHIRE for short. They have used this technique in a similar way to search for other exotic interactions and for dark matter fields.
Specifically, in the experimental search for exotic spin-spin interactions, two chambers filled with the vapor of one of the two elements are positioned in close proximity to one another: “In our experiment, we use polarized electron spins from rubidium-87 atoms as the spin source and polarized neutron spins from the noble gas xenon, more precisely the isotope xenon-129, as a spin sensor,” says Dmitry Budker.
The trick is that the special structure and the polarized xenon atoms in the spin sensor initially intensify the field generated in the rubidium source: the effect triggered by a potential exotic field would therefore be 200 times greater. Now the principle of nuclear magnetic resonance comes into play, ie the fact that nuclear spins react to magnetic fields that oscillate at a specific resonance frequency. For this purpose, a small proportion of rubidium-87 atoms are also present in the sensor cell. They in turn act as an extremely sensitive magnetometer to determine the strength of the resonance signal.
The detection of such an exotic field in the right frequency range would then indicate the new interaction that is being sought. Other special experimental details ensure that the structure in the frequency range of interest is particularly sensitive and less sensitive to interference from other magnetic fields, which inevitably also occur in the experiment.
“All in all, this is a fairly complicated setup that has required careful design and calibration. It is very rewarding to work on such challenging and interesting problems with our long-time collaborators from the University of Science and Technology (USTC) in Hefei, China, which hosted the experiment,” reports Dmitry Budker.
After a successful proof-of-principle, the scientists started the first series of measurements to search for the exotic interaction. Although they were still unable to find a corresponding signal after 24 hours of measurements, the five orders of magnitude increase in sensitivity allowed them to set constraints on the strength of the interaction of the new replacement particle with Standard Model particles. Further optimization could even improve the experimental sensitivity to the special exotic interaction by another eight orders of magnitude. This makes it possible to use the highly sensitive SAPPHIRE setup to discover and study new physics with potential Z’ bosons.