Positron trapped for months enables magnetic moment test
Physicists at Northwestern University have trapped a single positron for months in a cryogenic vacuum, enabling unprecedented precision measurements of its magnetic moment to test fundamental matter-antimatter symmetry.
Scientists have successfully confined a single positron in a cryogenic vacuum trap for months, overcoming extreme sensitivity challenges. Physicists Gerald Gabrielse and his colleagues at Northwestern University in Illinois conducted the experiment, trapping the antimatter particle using electric and magnetic fields. The achievement enables the first high-precision comparison of matter and antimatter magnetic moments, directly addressing why the universe is dominated by matter.
A single antimatter particle was held in a trap for months. The positron, the antimatter counterpart of the electron with the same mass but opposite charge, is typically annihilated in a fraction of a second. The team used a Penning trap with a 640-micrometer-wide opening, where electric fields on the top and bottom and magnetic fields on the sides created invisible walls to prevent contact. The trap was maintained at 300 millikelvins, just three-tenths of a degree above absolute zero, in an extremely good vacuum.
Experimental methodology
The positron was generated from the decay of sodium-22, a radioactive isotope with a half-life of about two and a half years. The sodium was placed in a special refrigerator to lower the energy of the emitted positrons, which were then further slowed by directing them through a tungsten crystal. Most were annihilated, but a few passed through unscathed or formed positronium before entering the trap. The slowed positrons were directed into the refrigerated chamber, where electromagnetic fields confined a single incoming particle.
Researchers, with key contributions from Tom Myers who developed the trapping solution, then pushed the positron into its lowest possible quantum energy state. This allowed them to keep it trapped indefinitely. They could also push the particle into the next lowest energy state using controlled bursts of microwaves. Tuned microwaves were used to detect energy level jumps and spin flips without disturbing the quantum states.
Scientific significance
The Standard Model predicts the positron's magnetic moment should be exactly opposite to the electron's. The goal is to measure the positron’s magnetic moment with unprecedented precision and compare it with the electron's, which has been measured to a precision of one part in 10 trillion. Any difference would violate fundamental symmetries and could help explain the matter-antimatter asymmetry of the universe.
This research aims to enable precise tests of CPT symmetry, a cornerstone of particle physics. It is part of a long-running effort to explain why the universe is dominated by matter despite theory predicting equal creation of matter and antimatter. The result opens the way to some of the most rigorous tests of particle physics so far. Roni Harnik, a theoretical physicist at Fermi National Accelerator Laboratory, highlighted the significance of the development.
The work is basic research, improving tools for testing fundamental theories rather than leading to a product. Stefan Ulmer at RIKEN noted that trapping an antiparticle is extremely difficult due to its sensitivity to stray particles, light, or vibrations. The researchers previously made the most precise ever measurement of the electron’s magnetic moment. They now intend to measure the positron's magnetic moment to compare with the electron's, enabling rigorous tests of fundamental particle physics theories.
