Neutrons in search of their mirror twin

Nathalie Ziehl (ETH) and Géza Zsigmond from the Paul Scherrer Institute (PSI) are two of the researchers leading the data analysis work in this experiment performed at the ultracold neutron (UCN) source at PSI. The whole team consisted of 22 scientists, two technicians and the UCN source operators. We asked the researchers whether they would sign off on the ultrashort description (above) of this ultracold experiment…

Yes, this very general formulation sounds correct. The experiment was designed to confirm or exclude the previously claimed anomalies, obtained from experimental data taken at significantly lower precision. We did not yet drastically constrain the parameter space of the theory which would question its validity. But we made an important step constraining the allowed parameter space of neutron to mirror-neutron oscillations.

Could you describe, in your own words, what the idea behind this experiment is and what it would have meant if you had found what you were looking for?

It has been observed since the fifties of the last century that left-right (parity) symmetry is violated in the weak interaction, one of the fundamental interactions in the Standard Model (SM) of particle physics. It was since then proposed that a "mirror" version of the SM particles could exist, thus restoring symmetry on a global level.

The hypothesis of an entire “mirror world” made of “mirror matter” has sparked several speculations on how that would influence phenomena in the visible Universe and if there are portals allowing transitions. Mirror particles would interact with the ordinary ones via gravity but not via the electroweak or strong interactions of the SM.

However, a new type of a very feeble interaction might also be possible. The mentioned feeble interaction would allow for ordinary and mirror particles, which are electrically neutral, to oscillate into each-other according to a given probability distribution in time.

The oscillation probability becomes largest, and can be resonant, if both particle states were energetically “in tune”. This condition can be "tuned" by changing the magnetic field to which the ordinary particles are exposed to.

Various experiments searching for neutron to mirror-neutron oscillations have been performed in the last decades. The ultracold neutron source at the Paul Scherrer Institute provides ideal conditions to study the possibility of such a phenomenon.

Ultracold neutrons (UCNs) can be stored in a vessel and observed for hundreds of seconds and thus a disappearance of neutrons via such a hypothetical oscillation can be measured or excluded with high precision.

One global analysis of several previous experiments with lower sensitivity indicated anomalous signals, hinting at an unexpected disappearance of neutrons. Our experiment was performed with unprecedented high UCN statistics, an improved magnetic field control and precise particle-tracing simulations, and succeeded to exclude these previously claimed anomalies.

In case that in future experiments, with even higher sensitivity, one would discover an anomalous disappearance of neutrons, this would also imply baryon number violation that may help to understand the matter-antimatter asymmetry of the Universe and the nature of Dark Matter.

What did you find, and what do your results tell you?

Our dedicated new experiment successfully excluded previously claimed anormal signals of neutron disappearance that were interpreted as mirror-neutron transitions. We have further relevantly constrained the possible parameter space of the theory, with the best sensitivity worldwide to date.

Can you describe the experimental setup and what makes it special?

In order to achieve a maximal statistical sensitivity for searching for mirror-neutron transitions, we used a 1.5 m3 large neutron storage system and optimised UCN filling, monitoring and emptying/detection after 180 seconds of UCN storage. This allowed us to observe dozens of millions of UCNs per one magnetic-field setting, and to sensitively search for anomalous losses as signature of mirror-neutron transitions. The experimental volume had to be made non-magnetic, of high wall-purity and tight for ultracold neutrons.

The tuning of the total energy level of the neutrons was made possible by a similarly large, magnetic-field coil system, allowing to accurately control the magnetic field values between 0 and about 7 times the size of the Earth’s magnetic field. By such tuning, we were able to scan the parameter space given by the theory, in order to find such transition resonances.

The UCN data taking was complemented by (1) an accurate mapping of the magnetic field in the storage volume, and by (2) a series of Monte Carlo particle-tracing calculations (on ETHZ's EULER and PSI's MERLIN computing clusters) to obtain the theoretical probability of mirror-neutron transitions for every magnetic field setting.

What was the biggest challenge you encountered?

The biggest challenge was the required high temporal stability in the operation parameters of the apparatus: first, the constancy of initial UCN counts from the source which were online monitored, of the final counts after storage. Another big challenge was the stability of the applied magnetic field. This was important, because we were relying the data analysis on identifying tiny changes in the measured UCN detection rate.

How much time did the experiment take (from installation to data taking)?

After more than a year of preparation, all measurements at all magnetic field settings took about two and half months.

How many people were you?

The team was 22 scientists, two technicians, plus the team of the UCN source operation. Two of this team, Ingo Rienäcker and Nathalie Ziehl, were focusing in their PhD works to bring this effort to a very successful end.

What other experiments did yours build on?

This measurement used a dedicated apparatus, specially optimized for this experiment to achieve maximum sensitivity. We used our know-how and working experience from previous projects involving UCN measurements and precision magnetic field generation and control.

What’s next in this field of research?

Prospects for an improved mirror neutron experiment at PSI are being discussed based on the setup presented in this paper. It presents realistic improvement possibilities in UCN statistics, based on the upgrade of the UCN storage vessel coating and size, improved UCN source performance after installation of the new UCN source deuterium insert currently under production, and increased proton beam current.

Paper: Phy. Rev. Lett. paper

Barbara Warmbein