Physicists have long known that charged particles like electrons will spiral in a magnetic field and give off radiation. But nobody had ever detected the radio waves emanating from a single whirling electron—until now. The striking new technique researchers used to do it might someday help particle physicists answer a question that has vexed them for decades: How much does a ghostly particle called the neutrino weigh?
"This is a great achievement on its own, and we're really looking forward to seeing this technology develop over time," says Guido Drexlin, an astroparticle physicist at the Karlsruhe Institute of Technology in Germany who was not involved in the work.
To understand the experiment, suppose an electron flies horizontally through a vertical magnetic field. It will experience a sideways force that is proportional to both its velocity and the strength of the field. That constant sideways shove will cause the electron to run in circles (see diagram). But that turning will also cause the electron to radiate electromagnetic waves, much as a wet dishcloth will fling off drops of water if you whirl it above your head. Of course, the radiation will sap the electron's energy, so that it will gradually spiral inward.
This effect has been understood for a century. It's used to generate x-ray beams by sending electrons racing around circular particle accelerators known as synchrotrons. Such radiation also emanates from swirling particles in interstellar space. Now, 27 physicists with Project 8, an experiment based at the University of Washington, Seattle, have detected radiation from a single electron. "I thought surely somebody must have done this," says Brent VanDevender, a nuclear physicist and team member from Pacific Northwest National Laboratory in Richland, Washington. "I looked and looked and looked in the literature and couldn't find anything."
To detect the millionth-of-a-nanowatt signal, the Project 8 team needed a source of electrons with a definite energy, a means of collecting the radiation, and ultrasensitive amplifiers to sense the signal. To get the electrons, they started with beads coated with the metal rubidium-83, which undergoes radioactive decay to produce krypton-83 gas. Researchers trapped the gas in a finger-sized cell. Each agitated krypton nucleus then underwent an internal restructuring that caused the atom to kick out an electron with a specific energy.
The electron would circle in the field provided by a superconducting magnet and radiate. Crucially, the cell in which it orbited was a "wave guide," a kind of pipeline designed to carry electromagnetic waves in the right frequency range—25 gigahertz to 27 gigahertz—to a chain of low-noise amplifiers. The team was able to track radiation from a single electron for several milliseconds—long enough to see its frequency gradually increase as the electron spiraled inward, as the researchers report this week in Physical Review Letters.
Particle physicists have long been able to measure the energies of single electrons, say by watching them crash into crystals that give off light in proportion to the electron’s energy. But those techniques generally absorb the electron, VanDevender notes. The new method opens the way to measuring the energy of an electron "nondestructively" without absorbing it.
The Project 8 team hopes to use the technique to measure the mass of the still-mysterious particles known as neutrinos, VanDevender says. They plan to study the tritium nucleus, which contains one proton and two neutrons. It undergoes a process called beta decay, in which one neutron turns into a proton while spitting out a neutrino and an electron. The nearly undetectable neutrino and the electron will share the energy released in the decay, with the split varying randomly from one decay to the next. By measuring the maximum energy of the electrons, researchers can deduce the minimum energy of the neutrinos, and hence the neutrino's mass.
Physicists know that the neutrino mass must be least 50 milli-electron volts (meV), or about 1/10,000,000 the mass of an electron. That's because neutrinos come in three different types, or flavors, depending on how they're generated, and different flavors can morph into one another. Such "neutrino oscillations" are possible only if the different flavors have different masses. At the same time, studies of the evolution of the universe suggest that neutrinos have a mass less than 230 meV. But so far—in spite of decades of effort—direct beta-decay measurements show only that neutrinos weigh less than 2000 meV.
In the immediate future, however, physicists with the Karlsruhe Tritium Neutrino Experiment (KATRIN) plan to make the tritium measurements 10 times more sensitive using more conventional techniques. They should start taking data next year, says Drexlin, co-spokesperson for the KATRIN team. "I don't see [Project 8] as competition to KATRIN but as more of a future possibility to go beyond it," Drexlin says. Still, he notes, many members of the Project 8 team are also members of the KATRIN team, and it's possible that in the future the two techniques might be combined.
*Correction, 22 April, 11:43 a.m.: The diagram has been changed to show the correct orientation of the magnetic field.