Scientists Obtain The Most Accurate Measurements Of Proton Mass To Date

Precise measurements of the mass of a proton find it to be three standard deviations lower than previous estimates.

AsianScientist (July 26, 2017) – Scientists from Germany and Japan have corrected the existing value of the mass of a proton and improved the precision of measurement by a factor of three. Their work was published in Physical Review Letters.

The proton is the nucleus of the hydrogen atom and one of the basic building blocks of all other atomic nuclei. Therefore, the proton’s mass is an important parameter in atomic physics: It is one of the factors that affects how the electrons move around the atomic nucleus, which is reflected in the wavelengths that atoms can absorb and emit again.

By comparing these wavelengths with theoretical predictions, it is possible to test fundamental physical theories. Precise comparisons of the masses of the proton and the antiproton may help in the search for crucial differences between matter and antimatter.

To determine the mass of a single proton more accurately, a group of physicists from the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg and RIKEN in Japan used ultra-sensitive single particle detectors that were partly developed by RIKEN’s Ulmer Fundamental Symmetries Laboratory. They performed a high-precision measurement in a greatly advanced Penning trap system which was designed by Dr. Sven Sturm and Dr. Klaus Blaum from MPIK.

Penning traps are well-proven as suitable ‘weighing scales’ for ions. In such a trap, it is possible to confine, nearly indefinitely, single-charge particles such as a proton by means of electric and magnetic fields. Inside the trap, the particle performs a characteristic periodic motion at a certain oscillation frequency. This frequency can be measured and the mass of the particle can be calculated from it.

To reach the targeted high precision, an elaborate measurement technique was required, involving the carbon isotope 12C with a mass of 12 atomic mass units—the mass standard for atoms.

“First, we stored one proton and one carbon ion (12C6+) each in separate compartments of our Penning trap apparatus. We then transported each of the two ions into the central measurement compartment and measured their motion,” said Sturm.

From the ratio of the two measured values, the group obtained the proton’s mass directly, in atomic units.

“(The measurement compartment) allowed us to measure the proton under identical conditions as the carbon ion, despite the mass of the proton being approximately 12-fold lower and its charge, 6-fold smaller than the carbon ion,” explained Dr. Andreas Mooser, a co-author of the study from RIKEN’s Fundamental Symmetries Laboratory.

The resulting mass of the proton, determined to be 1.007276466583(15)(29) atomic mass units, is three times more precise than the presently accepted value. The numbers in parentheses refer to the statistical and systematic uncertainties, respectively.

Intriguingly, the new value is significantly smaller than the current standard value. Measurements by other authors yielded discrepancies with respect to the mass of the tritium atom, the heaviest hydrogen isotope (T = 3H), and the mass of light helium (3He) compared to the ‘semiheavy’ hydrogen molecule, HD (D = 2H, deuterium, heavy hydrogen).

“Our result contributes to solving this puzzle, since it corrects the proton’s mass in the proper direction,” said Blaum.

Dr. Florian Köhler-Langes, another co-author of the study from MPIK, explained that in the future, the research team will store a third ion in their Penning trap tower. By simultaneously measuring the motion of this reference ion, they will be able to eliminate the uncertainty originating from fluctuations of the magnetic field.

The article can be found at: Heiße et al. (2017) High-Precision Measurement of the Proton’s Atomic Mass.


Source: RIKEN; Photo: Shutterstock.
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