Physicists think they've resolved the proton size puzzle

A decade-long physics mystery appears solved as new measurements reconcile conflicting data on the proton's fundamental size. | Contesto: cronaca

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• Physicists think they've resolved the proton size puzzle

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An international team of physicists announced this week that they have likely resolved the long-standing "proton radius puzzle," a fundamental discrepancy in measurements of the subatomic particle's size that has challenged the Standard Model of physics for over a decade. The breakthrough, detailed in a series of recent publications, reconciles conflicting results from two different experimental methods, pointing toward a smaller, more precise value for the proton's charge radius and potentially closing a controversial chapter in modern physics. The puzzle began in 2010 when a team using a novel method—measuring the energy levels of an exotic atom called muonic hydrogen, where an electron is replaced by a heavier muon—reported a proton radius significantly smaller than the accepted value. This result, at 0.84087 femtometers, was roughly 4% smaller than the standard 0.8768 femtometers derived from decades of experiments with regular hydrogen and electron scattering. The discrepancy was far larger than the estimated error margins of either technique, suggesting either a profound flaw in understanding quantum electrodynamics (a cornerstone of the Standard Model) or a major, unaccounted-for systematic error in the experiments. For years, the physics community was divided. Some speculated the anomaly could hint at new physics or unknown particles interacting differently with muons than with electrons. Others insisted the discrepancy must stem from experimental issues. The new research, involving highly refined measurements of both ordinary hydrogen and a fresh examination of muonic hydrogen data, now strongly supports the latter explanation. The latest results from precision laser spectroscopy of regular hydrogen atoms align remarkably well with the smaller radius initially found in muonic hydrogen, converging on a value close to 0.848 femtometers. "We believe this is the final nail in the coffin of the proton radius puzzle," stated a lead researcher on one of the collaborating teams. The resolution underscores the extraordinary precision of the muonic hydrogen technique, which is inherently more sensitive to the proton's size because the muon orbits much closer to...

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Categoria: cronaca