The cover story in the July 8^th issue of Nature, a top international weekly journal of science, was none other than the research work of the collaborative international research team of which NTHU's Associate Professor Yi-Wei Liu (Department of Physics), and doctoral research student Cheng-Yang Kao are members. The team made the monumental discovery that the proton's radius has previously been overestimated by as much as 4%.

As part of the Exotic Hydrogen Atom Project, 32 physicists from three different continents have collaborated for 10 years in the discovery that could have tremendous implications in physics. Professor Liu's AMO (Atomic Molecular and Optical Physics) team, the only participating team from Asia, specializes in precision measurements of basic physical constants.

An exotic hydrogen atom is a hydrogen nucleus orbited by a negatively-charged muon which weighs 200 times more than an electron thus causing it to orbit 200 times closer to the proton which in turn makes it more sensitive to the proton's size measurements. To measure the proton radius, the research team fired muons from PSI's particle accelerator at a cloud of hydrogen. Normally, each hydrogen nucleus consists of a single proton orbited by an electron. In the case of an exotic hydrogen, a muon from the muon beam replaces the electron and orbits around a proton. Using lasers, the team measured relevant muonic energy levels with extremely high accuracy and determined that the proton radius was indeed 4% smaller than previously thought.

The research project took ten years, commencing in 2001. Many repeated tests (in 2002, 2003 and in 2007) to find the accurate measurements had been unsuccessful. It was not until the summer of 2009 that the team began to come up with meaningful signals after three weeks of continuous work. The follow-up lengthy data analysis concluded that the proton radius is 0.84184 femtometers (1 fm = 10^-15 m) --- 4% smaller than 0.8768 fm shown in most standard textbooks. This has caused great excitement in the field of quantum electrodynamics (QED) and calls for the reevaluation of one of the most basic physics constants—the Rydberg constant. Scientists will no doubt be working with its ramifications for years to come.

President Chen expressed that this discovery will likely lead to all physics textbooks to be updated. Professor Liu is very confident that the new data will stand for at least another five years and that all computations based on the old proton size will be re-examined as well. Eventually, new theories will be developed based on this discovery.