The nucleus of deuterium, or the deuteron, can be pictured as a microscopic spinning top. If its charge distribution is not perfectly symmetric, then in an external electric field this top experiences a minuscule tilt of its spin axis. Such a charge imbalance is called an electric dipole moment (EDM)—a measure of how positive and negative charges are separated inside the particle. It is precisely this elusive effect that physicists sought at the COSY facility, a ring-shaped storage accelerator where deuteron beams were accelerated and held by magnetic fields. The team kept a close watch on the spins, the particles' rotation axes, recording any tiny deviations that an EDM might induce.
The result: no measurable tilt was found. But the absence of a signal is also a significant outcome: a new upper limit on the deuteron's EDM has been set — less than 2.5×10⁻¹⁷ e·cm (the product of the elementary charge and a distance, characterizing the dipole moment's magnitude). The search for the electric dipole moment of elementary particles is key to one of cosmology's greatest puzzles. If particles possessed an intrinsic charge asymmetry, it could explain why, after the big bang, matter did not completely annihilate when meeting antimatter, leaving the material that makes up our Universe. Such an asymmetry lies beyond the Standard Model of particle physics, and if found, it would be a long-awaited sign of new physics. The present experiment proved that the method itself, based on precise spin control in a storage ring, works. Next up are even more sensitive measurements, possibly with other particles, that could peer deeper into the nature of fundamental symmetry.
The sensitivity is such that you would notice a razor blade tilted by the thickness of an atom from across a football field.
In Isaac Asimov's 'The Last Question,' the theme of missing antimatter is not addressed, but his science fiction is imbued with a sense of the Universe's unsolved asymmetries.