New Study Finds the Proton to be Surprisingly Light

Imagine hefting a liter of water in your hands. That’s a kilogram of weight. Divide that by a billion, and you’ve got a quantity called a microgram—a thousandth of a single milligram. Divide that amount by a billion, and you’ve got a femtogram—which it’s almost impossible to get an intuitive sense for. But divide by a billion yet again, you’ve got roughly the mass of a single proton—and that’s what scientists have measured with unprecedented precision in a surprising new experiment at the Max Planck Institute in Germany.

Cutting-edge measurements have revealed the proton to be lighter than previously thought.
Image Credit: Gernot Vogt/Max Planck Institute for Nuclear Physics

So how do we measure a quantity that tiny? Hopefully you recognize that the image above has some artistic license to it—a beam balance definitely isn’t going to cut it. To examine the mass of a proton, researchers use a complex arrangement of electromagnetic fields called a Penning trap, the same kind of containment field used to trap and study antimatter without touching it.

A magnetic field curves the trajectory of a charged particle moving through it into a circular path, but since the electric field of the Penning trap keeps the protons roughly in one place, oscillating back and forth along the axis of this orbit, they end up tracing out something that looks more like a helix.

The size of that circle (and how long the proton takes to complete a loop), compared to the strength of the trap’s magnetic field, is what tells us about the charge-to-mass ratio of the proton, and since the proton’s charge is defined very precisely, this allows for a similarly precise calculation of its mass. The system is calibrated by performing the same experiment with a carbon nucleus, which is the reference point for the definition of atomic mass.

This experiment had a record-setting level of precision, and its findings were surprising: the proton is lighter than we expected. It’s a miniscule difference, a few parts in a hundred million, but that still puts the new measurement outside the error bars on the previous figure, an average of several measurements from experiments of different types.

If future experiments with different methods disagree significantly with the new result, it might point to new physics underlying mass and its relationship to charge. For instance, if a trend emerges of the proton continuing to “lose weight”, it might signal variation in the fundamental physical constants that determine things like the charge-to-mass ratio. At present, though, it’s most likely that we’ve simply reached an extraordinary fine value for this mind-bendingly small number.

—Stephen Skolnick

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