A Touch of Magnetism

This fall at the Paul Scherrer Institute, the construction of a new particle physics experiment will begin to determine the electric dipole of the neutron. It will replace a previous experiment, which has performed the so far most sensitive measurement in recent years and for which data evaluation is still ongoing. The new experiment, co-developed by ETH Ph.D. student Michał Rawlik, can detect almost inconceivably small features of magnetism. A successful outcome of the experiment would help explain why there is so much more matter in the universe than antimatter.

ETH graduate student Michal Rawlik with the small ‘prototype cage’, which serves to neutralize the magnetic fields in its interior.
Image: B. Vogel

Many experiments that are prepared at the Swiss Federal Institute of Technology (ETH) in Zurich are carried out at the Paul Scherrer Institute (PSI) in Villigen (AG) — the same is true of the nEDM experiment. The abbreviation stands for: neutron electric dipole moment. Any day now, the concrete floor will be poured in a laboratory at the PSI, on which the experiment will later be built. The construction alone is expected to take two to three years. It will likely take another three years for the results to be ready. By the end, the participating scientists from 15 different institutes and seven countries hope to clarify the question of whether neutrons – i.e. the electrically neutral particles of the atomic nuclei – have an electric dipole. Neutrons as a whole have no electric charge, since the charge of the up quark is compensated by the charges of the two down quarks. It is possible, however, that within a neutron the positive and negative electric charges are distributed so that they form an electric dipole. The nEDM experiment could provide proof of such an electric dipole.

Physicists have been trying for more than 60 years to use increasingly precise measurements to detect an electric dipole of the neutron. The results available so far give no indication that such a dipole exists. The currently most accurate published measurement of the electric dipole has been performed at the turn of the millennium by the RAL / Sussex / ILL collaboration. “In recent years, at PSI we have measured even more accurately than the colleagues with their previous effort and are now ready for the next step,” says 27-year-old Michał Rawlik. Rawlik comes from Gliwice (Poland). He studied physics in Krakow and then came to Switzerland via a summer school. For three years he has been working on his doctoral dissertation at the ETH Zurich, Hönggerberg Campus. “Our goal is to improve the measurement by another factor of 10,” says Rawlik.

The Unexplained Surplus of Matter

Why do particle physicists go to such great lengths to chase an almost non-measurable electric dipole in an electrically neutral particle? The reason lies in the theoretical considerations of the Russian physicist (and later Nobel Peace Laureate) Andrej Sakharov (1921-1989): If the neutron actually has an electric dipole, the so-called CP symmetry would be violated. This violation would be key to explaining that our visible universe composed of practically only matter, and not antimatter - even though physicists assume that matter and antimatter must have been created at the same rate during the Big Bang. If the nEDM experiment could prove the existence of an electric dipole, particle physics would come a step closer to answering one of the great, unresolved questions of contemporary cosmology.

This is undoubtedly a fascinating perspective. But there is a long way to go. For the proof of an electric dipole requires that the researchers can extremely precisely determine the magnetic field at the place of the measurement. The basic idea of ​​the nEDM experiment is to see if the energy of a neutron is influenced by an externally applied electric field. If this happens, the neutron has an electric dipole (and aligns with an electric field just as a compass needle aligns with the magnetic field of the Earth). “In our experiment, we need a very strong electric field to see if the neutrons are affected,” says Michał Rawlik. “However, the magnetic field must be known exactly because it influences the energy of the neutron via its existing magnetic dipole.” The well-known magnetic field and the so-called Larmor frequency are excellent tools to determine the electric dipole very accurately.

Magnetic Fields Ten Femtotesla Exact

For the first step, ETH physicists with their international colleagues have in recent years repeated the experiment of the RAL / Sussex / ILL collaboration with their 'upgraded' apparatus at PSI. After this fall, a completely new equipment will follow, which will be the size of a 5 x 5 x 5 meter cube. A key challenge is to be able to determine and stabilize the magnetic field very accurately where the measurement takes place. “We work with in one microtesla magnetic field, the Earth's magnetic field is around 50 Microtesla. At the same time we measure magnetic field changes within about ten femtotesla precision. This is smaller than changes produced by current that flow in our brains when we think,” says Rawlik. So that the measurements are not disturbed by surrounding magnetic fields, the experiment is shielded against them by six layers of mu-metal. As an additional shield, Rawlik has designed a ‘magnetic cage’. In the grid construction, wire coils are laid, through which currents are driven that neutralize the magnetic fields inside the cage.

“The construction of the cage and the development of the current control software account for about half of my doctoral thesis,” says Michał Rawlik, who hopes to complete his Ph.D. thesis next year. But even before that happens, researchers from all over the world can benefit from the insights of Zurich’s young scientist. “ETH has recently agreed that I can put my work online as open source software,” says Rawlik, adding, “we are not just among the best in the world to shield the magnetic field; we are also among the best in measuring and controlling the magnetic field.” Michał Rawlik knows that such highly specialized experiments as nEDM can only succeed through scientific teamwork. For example, every component that is used in the experiment is first shipped within the collaboration (www.neutronedm.org) to the National Metrology Institute of Germany in Berlin, where its magnetic compatibility is determined with the upmost precision.

Author: Benedikt Vogel

ETH graduate student Michal Rawlik with the small ‘prototype cage’, which serves to neutralize the magnetic fields in its interior.
ETH graduate student Michal Rawlik with the small ‘prototype cage’, which serves to neutralize the magnetic fields in its interior.Image: B. Vogel

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