Two precise experiments have agreed on a proton radius of about 0.84 femtometres, aligning with a surprising 2010 measurement and resolving a long-standing puzzle in particle physics. Researchers used lasers to study electron transitions in hydrogen atoms. The findings, published in Nature and Physical Review Letters, boost confidence in the smaller proton size.
More than 15 years ago, a 2010 experiment using an exotic hydrogen atom revealed the proton might be 4 per cent smaller than previously thought, sparking the 'proton radius puzzle'. A 2019 study further supported this smaller size. Now, complementary experiments led by Dylan Yost at Colorado State University and Lothar Maisenbacher at the Max Planck Institute of Quantum Optics in Germany have confirmed the radius at roughly 0.84 femtometres, or less than 1 million-billionth of a metre. Both teams matched the 2010 result using lasers to measure previously unprobed electron energy transitions in hydrogen atoms, which contain one proton and one electron whose interaction reveals the proton's size. The electromagnetic forces between the particles influence energy states, allowing precise size determination despite challenges like maintaining perfect vacuums and calibrating lasers over years of data analysis. 'When you look at that data, how much money are you willing to bet that the proton radius is what it is? For me personally, right now, with these measurements, the betting odds go significantly up,' Yost said. Maisenbacher added, 'It’s now very, very unlikely that there is still this proton radius puzzle.' Juan Rojo at Vrije University Amsterdam noted the value of diverse methods: 'The proton radius should be a universal property; it should give the same result no matter how you look at it.' These results align with quantum electrodynamics to 0.5 parts per million accuracy, setting no discrepancies for new particles or forces. Yost highlighted potential for tabletop hydrogen experiments to detect light new particles missed by giant colliders.