A small proton charge radius from an electron–proton scattering experiment

• Published in: Nature (London) 2019-11, Vol.575 (7781), p.147-150
• Main Authors: Xiong, W., Gasparian, A., Gao, H., Dutta, D., Khandaker, M., Liyanage, N., Pasyuk, E., Peng, C., Bai, X., Ye, L., Gnanvo, K., Gu, C., Levillain, M., Yan, X., Higinbotham, D. W., Meziane, M., Ye, Z., Adhikari, K., Aljawrneh, B., Bhatt, H., Bhetuwal, D., Brock, J., Burkert, V., Carlin, C., Deur, A., Di, D., Dunne, J., Ekanayaka, P., El-Fassi, L., Emmich, B., Gan, L., Glamazdin, O., Kabir, M. L., Karki, A., Keith, C., Kowalski, S., Lagerquist, V., Larin, I., Liu, T., Liyanage, A., Maxwell, J., Meekins, D., Nazeer, S. J., Nelyubin, V., Nguyen, H., Pedroni, R., Perdrisat, C., Pierce, J., Punjabi, V., Shabestari, M., Shahinyan, A., Silwal, R., Stepanyan, S., Subedi, A., Tarasov, V. V., Ton, N., Zhang, Y., Zhao, Z. W.
• Format: Article
• Language: English
• Subjects: 639/766/387/1126; 639/766/419/1131; Atomic properties; Atomic radius; Elastic scattering; Electric charge and distribution; Electric properties; Electrons; Experiments; Humanities and Social Sciences; Hydrogen; Hydrogen atoms; Laboratories; Measurement; multidisciplinary; Physics; Proton scattering; Protons; Scattering; Scattering angle; Science; Science (multidisciplinary); Spectroscopy; Spectrum analysis; Standard deviation; Theory
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-019-1721-2

Elastic electron–proton scattering (e–p) and the spectroscopy of hydrogen atoms are the two methods traditionally used to determine the proton charge radius, r p . In 2010, a new method using muonic hydrogen atoms 1 found a substantial discrepancy compared with previous results 2 , which became known as the ‘proton radius puzzle’. Despite experimental and theoretical efforts, the puzzle remains unresolved. In fact, there is a discrepancy between the two most recent spectroscopic measurements conducted on ordinary hydrogen 3 , 4 . Here we report on the proton charge radius experiment at Jefferson Laboratory (PRad), a high-precision e–p experiment that was established after the discrepancy was identified. We used a magnetic-spectrometer-free method along with a windowless hydrogen gas target, which overcame several limitations of previous e–p experiments and enabled measurements at very small forward-scattering angles. Our result, r p  = 0.831 ± 0.007 stat  ± 0.012 syst  femtometres, is smaller than the most recent high-precision e–p measurement 5 and 2.7 standard deviations smaller than the average of all e–p experimental results 6 . The smaller r p we have now measured supports the value found by two previous muonic hydrogen experiments 1 , 7 . In addition, our finding agrees with the revised value (announced in 2019) for the Rydberg constant 8 —one of the most accurately evaluated fundamental constants in physics. A magnetic-spectrometer-free method for electron–proton scattering data reveals a proton charge radius 2.7 standard deviations smaller than the currently accepted value from electron–proton scattering, yet consistent with other recent experiments.