In what will be seen as an attempt to extract fresh concessions from Xi Jinping, Trump told reporters there was not a deal in place to reduce any levies on Chinese goods. The Chinese commerce ministry had boosted optimism that 18 months of intensifying protectionism might be coming to an end when it said the two countries had agreed to cancel the tariffs in phases.
But there was immediate pushback from White House officials, who feel more work needs to be done before the two presidents can meet to sign a deal. Although tariffs on imported goods are hitting corporate profits and leading to higher prices for US consumers, Trump said he was happy with the current state of affairs. They want to make a deal … frankly they want to make a deal a lot more than I do.
Randolf Pohl, who led that muonic hydrogen measurement and is now at the Johannes Gutenberg University in Mainz, Germany, has collaborated on other muonic experiments that have confirmed this value. For a while, researchers thought the discrepancy revealed a previously unknown difference in how electrons and muons behave — something that could have upset the established quantum theory of electromagnetic phenomena.
More recently, however, improved spectroscopy experiments using ordinary hydrogen found a shrunken proton , suggesting that muons were not so special after all. The prospect of a revolution in physics began to fade.
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Those efforts culminated with the Science paper 2. After spending eight years perfecting a spectroscopy technique, the team behind that work found a radius of 0. But more-traditional spectroscopy experiments done at Sorbonne University in Paris continued to disagree with this result 4. And no one could explain why the scattering technique had pointed to a larger proton.
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Now, for the first time a scattering experiment has found a smaller proton, too. PRad shoots a beam of electrons at hydrogen molecules, and measures how some of the electrons are deflected. Previous scattering experiments had used higher-energy electron beams, which have limited sensitivity to the proton radius, and then extrapolated to lower electron energies to determine the radius.
That meant they had to make theoretical assumptions that might have skewed the final results. But the lower energies used by PRad circumvent the problem.
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To further improve precision, PRad injected its hydrogen molecules directly into the vacuum pipe that carries the electron beam, rather than keeping it in a metal container as many previous experiments had done. This means there are no electrons hitting metal and confounding the measurement.
Theorists have been going wild coming up with theories to explain an anomalous muon, so the conclusion of the saga will leave many physicists melancholic. But, he adds, experiments designed to push the precision down another order of magnitude could yet find tinier deviation from established theory.
How big is the proton? Particle-size puzzle leaps closer to resolution
Xiong, W. Nature , — Bezginov, N.
Science , — Pohl, R. Fleurbaey, H. Download references.