Until 2010, measurements (backed up by theory) had put the radius of the proton at about 0.87 femtometres, but in that year, experiments at the Max Planck Institute of Quantum Optics in Germany found a 4% discrepancy.
And in 2016, new experiments - measuring the size of the deuteron (one neutron plus one proton) found an 0.8% discrepancy.
Although small in percentage terms, these discrepancies are unexplainable using calculations based on the standard model.
“This independent discrepancy points to experimental or theoretical error or even to physics beyond the standard model.”
Source: Laser spectroscopy of muonic deuterium Science, 12 Aug 2016: Vol. 353, Issue 6300, pp. 669-673
More info: Puzzling out the proton radius puzzle AIP Conf. Proc. 1701, 040014 (2016).
“The discrepancy between the proton charge radius extracted from the muonic hydrogen Lamb shift measurement and the best present value obtained from the elastic scattering experiments, remains unexplained and represents a burning problem of today’s nuclear physics: after more than 50 years of research the radius of a basic constituent of matter is still not understood.”
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