Proton Weight Less Than We Thought, And How That Affects Physics

It is only 30 billionths of one percent, but when you are talking about sub-atomic scales, even that seemingly small a deviation from the accepted value can have much larger repercussions. And that’s how much lighter a single proton’s mass has been found to be by a team of scientists, who used a new method to measure that value for the particle which, along with electrons and neutrons, makes up all the ordinary matter in the universe.

Researchers from the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg, Germany, and Riken in Japan used a device which combines strong electric and magnetic fields to measure the mass of individual protons. Called a Penning trap, it operates at the temperature of 4 degrees Kelvin (452.47 degrees Fahrenheit below zero) and can store single protons and carbon ions. The magnetic field forces the protons to move in circles and measuring the frequency of the protons as they spin allows for the calculation of their mass.

The value found by the researchers is an improvement by a factor of three over the accepted value of the Committee on Data for Science and Technology — an interdisciplinary committee which collects and publishes the recommended values for fundamental constants in science — according to the recent paper they published in the journal Physical Review Letters.

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This is a significant development because proton mass is a fundamental factor in a lot of atomic and particle physics, and a change to its standard value has numerous implications in fields like atomic spectra and quantum electrodynamics (the interaction of light and matter). It also has potential applications in testing the “fundamental symmetry of the Standard Model, the so-called charge, parity and time invariance,” as well as resolving the discrepancy in the mass of tritium, the heaviest isotope of hydrogen.

The single-particle detectors used in the measurements were partly developed by the Riken researchers, some of whom had previously worked on similar traps at CERN’s Antiproton Decelerator. Riken group leader and spokesman for AD’s BASE experiment explained in a statement Wednesday how that experience came in handy.

“The group around Sven Sturm and Klaus Blaum from MPIK Heidelberg that did the measurement has great expertise with carbon, whereas the BASE group contributed proton expertise based on 12 years dealing with protons and antiprotons. We shared knowledge such as know-how on ultra-sensitive proton detectors and the ‘fast shuttling’ method developed by BASE to perform the proton/anti proton charge-to-mass ratio measurement,” he said in the statement on CERN’s website.

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Now that an established fundamental value has been upended by the researchers, they expect other teams of physicists to revisit the subject and challenge their findings, or maybe fine-tune the value further. But the MPIK-Riken researchers conducted a number of supplementary measurements to cross-check and verify their own conclusions.

“It is also planned to tune the magnetic field to even higher homogeneity, which will reduce additional sources of systematic error,” explains BASE member Andreas Mooser. “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.”