The mass of an electron appears prominently in many of the fundamental laws that govern the subatomic realm, yet direct measurement has been complicated by the particle’s scrawny mass. Now, a team of physicists has overcome this challenge to produce the most precise electron mass measurement ever made. Instead of trying to measure the mass directly, the researchers bound a single electron to a bare carbon nucleus and placed the resulting atom in a uniform electromagnetic field called a Penning trap (created in an apparatus similar to the one pictured above). Inside the trap, the atom began oscillating in circles with a steady frequency. The team then shot the trapped atom with microwaves, causing the spin of the electron to flip up and down. By comparing the frequency of the atom's circular movements with the frequency of the spin-flipping microwaves, the team used quantum electrodynamics equations to derive the mass of the electron compared with a proton. The team's new measurement is 13 times more precise than previous efforts, with an uncertainty of just 0.03 parts per billion, the researchers report online today in Nature. The group’s precise result will help physicists more accurately calculate the fine-structure constant, an important value in tests of the standard model of particle physics, which shapes our understanding of the basic building blocks of the universe.

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