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A smaller proton



    Physicists are puzzled by a new finding that the radius of the proton radius is smaller than previously believed.

    Protons are one of the three basic building blocks of matter: protons and neutrons form the atomic nucleus, and the nucleus is surrounded by orbiting electrons. The chemical elements are identified by the number of protons in the nucleus. You may have a clear idea about the size of a baseball, but do you know how big a proton is? Using precision laser spectroscopy, scientists have found the proton to be smaller than expected. The long journey to explore the tiny proton started from the very beginning of modern physics.  

    Hydrogen is the simplest of all atoms. Its nucleus consists of a single proton which is orbited by a single electron.  Due to its simplicity, the hydrogen atom is also seen as the “Rosetta stone” of quantum physics, as it has historically often been used as the best object for the investigation of basic questions in physics. While electrons and muons are believed to be point-like particles, the proton is made up of entities called quarks and is therefore an extended object.

    In order to determine the radius of a proton, the researchers replaced the single electron of hydrogen atoms with a negatively-charged muon. Muons are very much like electrons but are 200 times heavier. According to the laws of quantum physics, the muon must therefore travel 200 times closer to the proton than the electron does in an ordinary hydrogen atom. In turn, this means that the characteristics of the muon orbit are much more sensitive to the dimensions of the proton. The muon acts like a satellite monitoring the proton. Due to its close orbit, the muon “feels” the size of the proton and adapts its orbit accordingly. Using a specially-designed laser to determine the characteristics of the muon’s orbit around the proton, physicists can infer the size of the proton with a great deal of accuracy.

    By the 1970s the idea had already been posited to determine the proton radius using muonic hydrogen. However it took more than 40 years between the idea and the realization of such an experiment because many experimental challenges had to be overcome. It has been finally realized in a joint effort in which each team brings its own expertise in the fields of accelerator physics, atomic physics, laser technologies and detectors, says Prof. Yi Wei Liu of National Chiao-Tung University, a partner university of the University System of Taiwan. An international team including Taiwan, Switzerland, Germany, the US, France and Portugal performed the laser spectroscopy on such exotic atoms at the Paul Scherrer Institut near Zurich, Switzerland. The breakthrough happened in the summer of 2009. After 3 months of set up time and three weeks of data taking– day and night - the scientists unambiguously observed a signal on the evening of July 5, 2009. 

    After a long and careful analysis of the signal, the deduced value of the proton radius is ten times more precise at 0.84184 femtometer (1 femtometer = 10−15 meter), but in strong disagreement with the accepted value (0.8768 femtometer). A shrinking proton! It is 4% smaller than expected. This result has shaken the foundation of the modern quantum physics: a chink in the armor. Furthermore, in 2013, a new measurement by the same team has confirmed the smaller proton and made it possible for the first time to determine the magnetic size of the proton. The experimental evidence is solid now.

    According to Prof. Yi Wei Liu, the scientists are still discussing possible reasons for the discrepancy three years after the discovery of the ‘shrunken’ proton. Everything is still under scrutiny: previous high-precision measurements, complicated calculations, maybe at some point even the world's most precise and best-tested fundamental theory itself – quantum electrodynamics – and a new unexplored force between proton-muon? Will this be the twilight for the New Physics? It is being called a “proton-sized puzzle”.


atomic [adj.]  原子的
orbit [n.]軌道運行
Rosetta Stone  [n.]羅塞塔石碑
experimental [adj.]  實驗性的
unambiguously  [adv.]明白地

Source:Taiwan News Online

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