For decades, antimatter particles have proven difficult to study because they annihilate immediately upon contact with matter. Positrons, the positively charged antimatter twins of electrons, are naturally produced in radioactive decay, cosmic explosions, and energetic particle interactions, but they rarely survive long enough for detailed examination.
The Northwestern team, led by physicist Gerald Gabrielse and spearheaded by researcher Tom Myers, developed an innovative method to slow down and capture a single positron. They used a radioactive isotope of sodium that emits positrons at up to 546 kiloelectronvolts, or about 88 percent of the speed of light. These high-energy particles were directed through a single crystal of tungsten, where interactions with electrons slowed them down. Most positrons annihilate in this process, but a few survive, emerging as positronium: a short-lived bound state of a positron and an electron.
In this particular form of positronium, the electron and positron occupy a highly excited quantum state that keeps them relatively far apart, delaying their inevitable annihilation. The positronium then enters a Penning trap through an opening just 640 micrometers wide, where an electric field strips away the electron, leaving the isolated positron behind.
The trap, cooled to 300 millikelvins (a fraction of a degree above absolute zero) and held in an ultrahigh vacuum, uses electric and magnetic fields to confine the particle without allowing it to touch the trap walls. "We turn on our detector and wait," Gabrielse said. "Eventually we see the electrical signal from a captured positron. We then pull the radioactive source away from the trap to keep any additional positron from loading."
Once captured, the researchers could control the positron's motion with extraordinary precision, cooling it to its lowest quantum energy state and using microwaves to nudge it between energy levels one quantum at a time. They could detect changes in its energy and spin without disturbing the quantum state.
The ultimate goal is to measure the positron's magnetic moment, a property that describes how it behaves like a tiny bar magnet. The electron's magnetic moment has already been measured with astonishing precision. "If the magnitudes are the same, it will be the most precise confirmation of the CPT invariance of the Standard Model, the most fundamental mathematical description of physical reality," Gabrielse said.
The team is still working on the measurement and hopes to report a result early next year. Even if the result agrees perfectly with theory, it would deepen the mystery of why the Standard Model, while making precise predictions, is inconsistent with the observed matter-antimatter asymmetry of the universe.
