Atomic nuclei are rarely perfect spheres. More often, they resemble a slightly squashed ball — the electric charge inside is unevenly distributed, and protons may shift a bit, creating a tiny 'pear-shaped' bulge. This is the Schiff moment — an elusive imprint of forces that go beyond the known Standard Model of particle physics.
A team of physicists led by not specified in the abstract, but the topic is related to symmetry violation used a crystal of yttrium silicate with embedded europium-153 ions. This rare-earth element has a nucleus shaped like an 'octupole pear'—an ideal candidate for searching for the Schiff moment. The crystal, just a few millimeters across, was placed in a strong magnetic field, and the nuclei were made to resonate like tiny tuning forks. Any slight deviation in their behavior could reveal the sought-after asymmetry.
No signal was detected. But the absence itself is a result. The experiment set a new upper limit for the europium Schiff moment: less than 1.7×10⁻⁸ e·fm³. This sensitivity allows probing energies on the order of TeV — where new particles, such as supersymmetric partners of ordinary particles, might hide. The crystal tuning fork remained silent, and this silence speaks louder than words: it rules out an entire class of hypothetical models.
The europium-153 nucleus has an octupole shape, like a pear with several bulges, making it hundreds of times more sensitive to the Schiff moment than spherical nuclei.