To probe deeper into the workings of the universe, physicists accelerate electrons and smash them together. An ordinary electron is like a spinning top that wobbles randomly. For delicate experiments, you need particles whose spins all point in the same direction: this is called polarization. Achieving such a well-ordered beam in a plasma accelerator had long been elusive.
A team of researchers found a workaround. They used a gas of hydrogen halide molecules (compounds like hydrogen fluoride) where electron spins are pre-aligned, like compass needles pointing north. Two laser beams, one ultraviolet and one visible, strike the molecules and create a two-component medium: excited halogen atoms readily lose electrons, while hydrogen protons hold tightly to their polarized electrons. Then plasma takes over, and the desired electrons are captured into an accelerating bunch.
Such a source rivals the best traditional ones, yet it uses a compact plasma accelerator. It's like swapping a bulky factory assembly line for a sleek benchtop tool without sacrificing quality. The result promises new experiments at the intersection of particle physics and the search for deviations from the Standard Model.
Electron spin isn't rotation in the usual sense. It has no size to spin, yet it behaves like a tiny magnet with two orientations.