Proton mass measured with unprecedented precision

The particle zoo may house such exotica as K mesons and pentaquarks, but the stuff of our everyday lives is made up of just three beasts: the proton, the electron, and the neutron. The mass of the proton is of interest not just because of the primal nature of the particle but also because the proton mass is used as input in precise determinations of the neutron mass and other quantities. Now a team led by Sven Sturm of the Max Planck Institute for Nuclear Physics has measured the proton mass with a precision of 32 parts per trillion, a threefold improvement on the precision of the currently accepted value. Their result, m_p = 1.007276466583 amu (atomic mass unit), is lower than the accepted value by about 300 ppt.

As with other groups who have precisely measured the proton mass, Sturm and colleagues made use of a Penning trap, which uses electric and magnetic fields to confine ions. With their improved design, the researchers could rapidly switch between trapping protons and ¹²C⁶⁺ and determine their cyclotron frequencies, qB/2πm, as the ions circled within the trap’s magnetic field. The ratio of the frequencies yields the proton mass, given that the mass of the ¹²C atom is defined to be exactly 12 amu and that the correction needed to calculate the ¹²C⁶⁺ ion mass is well known and extraordinarily precise.

The motion of an ion in a Penning trap can be decomposed into faster and slower circular motions perpendicular to the magnetic field and an oscillation along the field axis. Thus the cyclotron frequency must be teased out from observations of the composite trajectory. The greatest source of measurement imprecision is inhomogeneities in the trap’s magnetic field.

The lower value of the proton mass suggested by the Sturm group may be key to resolving two measurements of the helium-3 mass that disagree by more than three standard deviations. Both ³He determinations assumed the currently accepted value of the proton mass. Swapping in the new, lower value determined by Sturm and colleagues cuts the ³He mass discrepancy in half. (F. Heiße et al., Phys. Rev. Lett. 119, 033001, 2017, doi:10.1103/PhysRevLett.119.033001 .)