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Proton size still perplexes despite a new measurement

Latest experiment sides with subatomic particle having a smaller girth, but discrepancy remains unexplained

2:00pm, October 5, 2017

SHRIMPIER STILL  A new measurement supports the idea that the proton is even smaller than its already-teeny textbook value. To measure the proton’s radius, scientists used lasers (apparatus shown) to jolt hydrogen atoms into higher energy levels, the spacing of which depends on the proton’s size.

Nonconformists could take a page from the proton’s playbook: The subatomic particle is once again resisting scientists’ attempts to size it up.

Everyone agrees the proton is tiny: Its radius is less than a femtometer, or a trillionth of a millimeter. But scientists still don’t agree on exactly how small it is. A new measurement supports the case for a smaller proton, physicist Lothar Maisenbacher and colleagues report in the Oct. 6 Science. But “in some sense, it deepens the puzzle,” says Maisenbacher, of the Max Planck Institute of Quantum Optics in Garching, Germany.

The researchers peg the proton’s radius at 0.83 femtometers, whereas the textbook value is 0.88 femtometers. That might not seem like a huge difference, but the few-percent discrepancy is stymieing scientists’ attempts to test quantum electrodynamics, the theory of how electrically

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