In a laboratory at Northwestern University in Evanston, Illinois, Gerald Gabrielse and his team achieved something that seemed impossible: keeping a single positron — the antimatter counterpart of the electron — suspended in a vacuum, cooled to near absolute zero, for months straight. The result, published in Physical Review Letters on October 21, 2026, is not merely a durability record. It is the birth of a new measurement instrument that may finally answer the question that has dogged fundamental physics for eight decades: why is the universe made of matter?
The positron is, by definition, the mirror image of the electron. It has the same mass, the same charge magnitude, but opposite sign — and when it encounters an electron, the two annihilate in an energy burst. This property, far from being a mere laboratory curiosity, is precisely what makes positrons so difficult to study: they disappear upon touching any surface. For decades, physicists could produce positrons in substantial quantities — cobalt-22, for instance, emits one per decay — but manipulating a single particle long enough for precision measurements was considered impractical.
Gabrielse's group circumvented this obstacle with a three-stage architecture. First, positrons emitted from a radioactive sodium-22 source are accelerated through a frigid tungsten crystal lattice. Each collision causes the particle to lose part of its kinetic energy. Upon emerging from the other side, it is significantly slower — and with a reduced probability of annihilation. Second, the particle is guided by electromagnetic fields into an almost perfect vacuum chamber. Finally, the trap's interior is cooled to 300 millikelvins — just 0.3 degrees above absolute zero — until it reaches its quantum ground state.
What was achieved in this state is a "one-positron quantum cyclotron": a single particle that behaves essentially like an artificial atom, with quantized energy levels that researchers can control and observe with precision.
The ultimate goal of the experiment is to measure the positron's magnetic moment — the strength and orientation of the magnetic field the particle generates — with unprecedented precision. The Standard Model of particle physics predicts that the positron's and electron's magnetic moments should have exactly equal values but opposite directions. If measurements reveal any difference, however slight, the Standard Model would be broken — and physicists would have a concrete starting point for reconstructing the theory.
But the motivation goes beyond theoretical vanity. The matter-antimatter asymmetry is, without question, one of the biggest unsolved problems in modern physics. When the universe was born, the Big Bang should have produced equal amounts of matter and antimatter. If this had been true, all matter and all antimatter would have mutually annihilated, and there would be no universe to observe. The fact that something exists — stars, planets, you — means that some subtle difference between the two types of particle allowed matter to "win." Discovering what that difference is remains the holy grail of particle cosmology.
It is important to make clear: the current experiment did not measure the positron's magnetic moment with the precision needed to test the Standard Model. It demonstrated that it can be measured this way. The positron was kept for months, yes, but precision measurements are still to come. The researchers plan to determine two quantities: the energy needed to reverse the positron's spin direction and the energy needed to make it jump from its ground state to its first excited state. The ratio of these two energies is precisely the magnetic moment.
What makes the method promising is its stability. In previous experiments, positrons were produced in plasmas and confined for short periods, preventing long-term measurements. The Northwestern "trap," by contrast, does not touch the particle — it floats at the center of the vacuum, suspended by invisible electromagnetic fields. This eliminates any chance of annihilation through contact and allows the positron to be observed indefinitely, as long as the equipment keeps running.
Northwestern is not the only place studying antimatter. At CERN, the ALPHA experiment produced and analyzed over 24,000 antihydrogen atoms in 2026, measuring their hyperfine structure to 12 significant digits — a hundredfold improvement over previous measurements. In May 2026, the ALPHA collaboration published in Nature the most precise measurement ever obtained for the ground-state hyperfine splitting of antihydrogen. Other experiments at CERN — AEGIS, ASACUSA, BASE, GBAR, and PUMA — study fundamental properties of the antiproton and antihydrogen, testing whether gravity acts differently on antimatter.
The Northwestern method is complementary: instead of studying entire antihydrogen atoms, it isolates the simplest component of antimatter — the positron — and holds it in such a controlled state that first-order quantum measurements become possible. If the precision results confirm or refute the Standard Model's predictions, it will be the first direct evidence of physics beyond it in decades.
The positron has been produced in laboratories since 1932, when Carl Anderson first observed it in cosmic ray tracks on photographic plates. Eighty years later, we still do not know why the universe exists at all. This trapped particle may finally hold the answer.
Sources: Northwestern Now, Scientific American, Nature
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