Proton’s radius is 4% smaller than the previous estimates of 0.88 femtometers

ProtonScientists have reassured that the proton is 4% smaller than the previous research findings.

This research has been published online in the journal Science.

Although the new research has decreased the weight of the proton but the same new research has increased the chances of finding the new particles and/or forces.

Before 2010, there were only two methods for measuring the size of the proton. One was firing the electron beams at protons and measuring the deflection, and the other method was studying the behavior of electrons in hydrogen atoms. Both the methods showed that the radius of proton is nearly 0.88 femtometers, or 0.88 quadrillionths of a meter.

In 2010, scientists developed a new technique to check the proton size in which the muons, instead of electrons, were studied in hydrogen atoms. Muons are the elementary particles with 200 times the mass of the electron. After using this new technique, physicist Aldo Antognini at ETH Zurich and his team published a paper in Nature in which they reported that the proton’s radius is 0.84 femtometers i.e. about 4% less than previous measurements.

Now after more than two years, scientists used the technique and re-examined the size of proton that eliminates the possibility of certain systematic errors and decreases the measurement’s uncertainty by 40%.

“This shows that our experiment is consistent and that there were no mistakes,” Antognini said in a statement.


Antognini, A., Nez, F., Schuhmann, K., Amaro, F., Biraben, F., Cardoso, J., Covita, D., Dax, A., Dhawan, S., Diepold, M., Fernandes, L., Giesen, A., Gouvea, A., Graf, T., Hansch, T., Indelicato, P., Julien, L., Kao, C., Knowles, P., Kottmann, F., Le Bigot, E., Liu, Y., Lopes, J., Ludhova, L., Monteiro, C., Mulhauser, F., Nebel, T., Rabinowitz, P., dos Santos, J., Schaller, L., Schwob, C., Taqqu, D., Veloso, J., Vogelsang, J., & Pohl, R. (2013). Proton Structure from the Measurement of 2S-2P Transition Frequencies of Muonic Hydrogen Science, 339 (6118), 417-420 DOI: 10.1126/science.1230016

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