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takyon writes:

Your mass just decreased:

[In] recent decades, physicists have combined strong electric and magnetic fields in a device called a Penning trap to measure the proton's mass more and more precisely. In these experiments, an electric field traps the proton while a magnetic field forces it to move in a circle. While it rotates, the proton will vibrate, or oscillate, at a frequency that's related to its mass. Researchers can calculate the proton's mass by measuring these oscillations, and comparing them to those of a reference—typically, the nucleus of a carbon-12 atom, which is defined as 12 atomic mass units.

But no experiment is perfect. Magnetic fields vary in time and space, causing small measurement errors. To reduce the impact of these fluctuations, a group of physicists working in Mainz, Germany, loaded the carbon nucleus and the proton into separate storage traps, then shuttled them quickly into and out of the measurement trap. Although swapping the nucleus and the proton required more than 30 minutes in previous experiments, the German group needed only about 3 minutes—limiting the chances for errors to accumulate. The team also added more motion detectors to their setup, leading to a measurement with an overall precision of 32 parts per trillion.

The researchers found the mass to be 1.007276466583 atomic mass units [DOI: 10.1103/PhysRevLett.119.033001] [DX]. That's roughly 30 billionths of a percent lower than the average value from past experiments—a seemingly tiny difference that is actually significant by three standard deviations, the team reports this week in Physical Review Letters. (By comparison, scientists typically consider two standard deviations enough for an experimental result to be statistically significant.)

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