Measuring the mass of atoms is a difficult job and in the last few years, things have gotten a bit harder. Several high-profile, high-precision measurements started to give slightly different values. This had scientists scratching their heads. Is there something wrong in one (or more of the experiments)? Or are we missing something about the physics?

The discrepancy has taken the name of the helium-3 mass puzzle since researchers have been struggling to find a single correct value for the mass of this particular isotope. A paper appearing recently in Physical Review A has once again confirmed such a discrepancy. Researchers at Florida State University (FSU) used a method called penning trap, which uses magnetic fields to measure properties of stable ions (atoms that have lost some of their electrons). They confirmed the findings that started this whole saga a few years ago. 

Back in 2015, the FSU team looked to obtain precise mass measurements of tritium and helium-3. They used their penning trap alongside cyclotron frequency ratios, where mixtures of different ions are accelerated so that heavier ones stay behind and lighter ones come upfront. This approach allowed them to obtain their measurements.

But a follow-up study from a different team at the University of Washington, which used a different deuteron (a proton and neutron bound together) and helium-3, obtained different results. The mass of helium-3 differed by less than a billionth between the two experiments. This might not seem like much but it's big enough in such high-precision experiments.

This was enough to get scientists wondering what was going on. Usually, in this case, the fault lies somewhere in the system. The FSU instrument was tweaked and improved over the last two years and the measurements were taken again.

But the fault might lie in what the “established” values for the proton and deuteron are. For example, last July researchers in Germany were able to obtain the most precise value for the mass of the proton yet. This value was used in the latest paper from FSU which did reduce the discrepancy between the helium-3 mass measurements, although not enough for a single value to be accepted. 

The importance of the helium-3 mass is related to another important measurement of mass, the mass of the electron antineutrino. Measuring this mass is being attempted by an international collaboration through the experiment KATRIN, which uses the decay of tritium (a hydrogen with two extra neutrons) into helium-3. The particular decay releases an electron and an electron antineutrino, so researchers are hoping to work out the mass of the antineutrino in this way.