After making the cover story of Nature in 2010, an international research team led by Prof. Yi-Wei Liu tackled the puzzle of proton size once again through the transition of another muonic hydrogen atom. The measurement of the size of the proton this time is 0.84087 (39) femtometer; consistent with the prior data but the precision of the measurement was increased to 1.7 folds, and the CODATA differences expanded to seven standard deviations and the proton magnetic radius was also obtained. The research result was published on January 25^th in Science. President Lih J. Chen and the research team held a press conference at National Science Council on March 6^th and stated that Nature and Science are fierce competitors in terms of publishing scientific articles; it is really a rare phenomenon that different stages of the same series of research are published by these two prestigious journals!

Prof. Yi-Wei Liu's atom manipulation laboratory, AMO Research Group, has focused on this program for more than a decade, and they have established a close working relationship with other international research groups. Prof. Liu stated that the last research result, published in 2010 drew much attention globally, and the article was referenced more than 130 times; some researchers who reviewed this article, however, were somewhat skeptical. Luckily, the result of this research can provide more concrete evidences to prove that the size proton may be smaller than we thought!

Proton is consisting of 3 quarks; as a result it is an object with room to extend. The spatial combination manner of quarks with electric nucleus and magnetic property determines the spatial spread of electric nucleus and magnetic property of the proton along with its size. Prof. Liu expressed that the research groups used muons, an electron like elementary particle with 200 folds in mass and shorter life spend, and protons to synthesis muonic hydrogen atoms, and the size of the proton was accurate determined by measuring the energy level of the muonic hydrogen atoms through laser spectroscopy method. Within this exotic atom, the muon with higher mass allows electron to get closer to the proton compare to a normal hydrogen atom, which made the muon heavily influenced by the proton, and thus resulting in the displacement of energy levels.

Currently, physicists around the world are eagerly trying to find the answer to the proton size puzzle and related data obtained from the past experiments by using normal hydrogen atoms, and electron-proton scattering are also being reanalyzed, reviewed and re-examined. Moreover, theoretical physicists from different fields are also trying to explain this inconsistency from different angles using interesting theories as well. However, these hypotheses all need new experiments to verify. "In the future, the new international research team, CREMA of which NTHU's Department of Physics is a member, will make use of existing experimental facilities to perform and improve muonic hydrogen atom experiments, and the research is expected to establish a clearer direction for solving the proton size puzzle," said President Chen at the NSC press conference.