The end is brutal for electrons hurtling at 99.9999999% of the speed of light through SLAC National Accelerator Laboratory’s two-mile-long beam pipe: a final slam into End Station A. In the late 1960s and early ’70s, such collisions broke apart protons and neutrons to reveal the elementary particles that make them up. The discovery won the experiment’s leader a Nobel Prize. “End Station A is this hallowed ground at SLAC,” said the physicist Timothy Nelson.
Walking to the back of the warehouse, past piles of equipment, Nelson pointed at the skeleton of an old experiment, beyond the point where the tree-trunk-size pipe of the historic accelerator cut off. It’s there, he said, that a soon-to-be-constructed experiment will see — or quickly rule out — one of the most popular new candidates for dark matter.
Almost a century ago, the Swiss astrophysicist Fritz Zwicky described a galaxy cluster that appeared to rotate too fast to be held together by its visible mass. He proposed that invisible matter was lending its gravity to the situation. The evidence has grown and grown, and researchers now believe that 85% of the universe’s matter is hidden. But the mystery of dark matter’s identity has endured.
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