tl;dr: Physicists at Northwestern used a wall-less trap combining electric and magnetic fields in near-absolute-zero vacuum to isolate a single positron for a month, after slowing the particles via a tungsten crystal lattice. The goal is to precisely measure the positron's magnetic moment and compare it to the electron's (previously measured to 1 part in 10 trillion) to test the Standard Model's symmetry prediction. Any discrepancy could help explain why the universe contains matter rather than antimatter.