To determine the mass of a single proton more accurately, the group of physicists from the Max Planck Institute for Nuclear Physics in Heidelberg and RIKEN in Japan performed an important high-precision measurement in a greatly advanced Penning trap system. The sensitive single-particle detectors were partly developed by the RIKEN group, drawing on experience gained with similar traps for antimatter research at CERN’s Antiproton Decelerator while CERN's BASE experiment contributed proton expertise based on 12 years dealing with protons and antiprotons. 

Ernest Rutherford, pioneer in studying the world inside atoms, famously remarked that all science is either physics or stamp collecting. But sometimes physics itself involves dutifully collecting the stats on the world, in the same way that a naturalist might capture and examine butterflies. A precise value for the mass of the proton is one example of the sort of statistic that physicists are eager to collect.

Scientists have been on a quest for a better and better value of the tiny particle's size for decades. The goal with each new measurement is to get closer and closer to the true value of proton’s mass. The properties of the basic building blocks of matter shape a network of fundamental parameters, which are crucial to develop precise quantitative understanding of nature and its symmetries.

The result improves by a factor of three on the precision of the accepted value of the Committee on Data for Science and Technology (CODATA) – which regularly collects and publishes the recommended values of fundamental physical constants – and it also disagrees with its central value at a level of 3.3 standard deviations, which means that the new value is significantly different from the previous result.

The measurements have been carried out in a highly optimized, purpose-built cryogenic Penning-trap setup, dedicated to mass measurements on light ions. Within the trap combination of strong electric and magnetic fields, cooled to 4 degrees Kelvin (- 269.15 °C) is able to store individual protons and highly charged carbon ions. In this trap, the magnetic field forces the particles to move in a circle and by measuring the characteristic frequencies of the trapped particles when they spin around, the mass of the proton follows directly.

While the superconducting magnet and the experiment’s liquid helium cryostat have been re-used, both the trap section as well as the cryogenic electronics and detection circuitry have been newly developed. This was necessary to address the specifically strong requirements on the quality of the trapping fields, set by the low mass and charge of the proton and resulting large motional amplitudes.

In a set of carefully conducted cross-check measurements we have confirmed a series of other literature values and were not able to track any yet uncovered systematic effects imposed by our method. Combined with the independently measured electron mass this measurement yields a factor of 2 more precise proton-electron-mass ratio, too. The striking departure from the accepted value will likely challenge other teams to revisit the proton mass. The discrepancy has already inspired the MPIK-RIKEN team to further improve the precision of their measurement, for instance by storing a third ion in the trap and measuring it simultaneously to eliminate uncertainties originating from magnetic field fluctuations, which are the main source of systematic errors when using the new technique.

Precise measurements like this sharpens our understanding of the way that atoms form molecules and is key to a variety of important calculations. For example, its value influences the Rydberg constant and it is also required for the precise comparison of the masses of the proton and antiproton, in order to perform a stringent test of CPT invariance via a hydrogen anion.

The main systematic limitation of this measurement is given by the residual quadratic magnetic field component combined with the finite axial motion amplitude of the ions. In the next phase of this experiment, the team plans to significantly improve this limitation by compensating the first and second order magnetic inhomogeneities with a dedicated set of in situ superconducting magnetic shims. Additionally, common-mode magnetic field fluctuations will be cancelled by simultaneous phase-sensitive measurements in the RT and MT, allowing for significantly longer measurement times and, correspondingly, a lower statistical uncertainty.

“The methods that will be pioneered in the next step of this experiment will have immediate positive feedback to future BASE measurements, for example in improving the precision in the antiproton-to-proton charge-to-mass ratio.” explains BASE member Andreas Mooser. “We shared knowledge such as know-how on ultra-sensitive proton detectors and the ‘fast shuttling’ method developed by BASE to perform the proton/antiproton charge-to-mass ratio measurement.” explains RIKEN group leader and spokesperson of the AD’s BASE experiment, Stefan Ulmer.

Physicists know that the Standard Model—great for explaining the world of the very small, but useless when it comes to gravity—either unravels somewhere or must be woven into something else. Any difference between a theoretical calculation and an experimental one could indicate physics beyond the Standard Model of particle physics. Both kinds of calculations require the mass of the proton as input, so a better and better measurement of the proton mass will lead to better and better calculations of all sorts—and thereby help us identify any discrepancies between them.