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Nuclear physics

Cyclotron radiation from a single electron is measured for the first time

27 Apr 2015
Going in circles: the Project 8 experiment

The cyclotron radiation emitted by a single electron has been measured for the first time by a team of physicists in the US and Germany. The research provides a new and potentially more precise way to study beta decay, which involves the emission of an electron and a neutrino. In particular, it could provide physicists with a much better measurement of neutrino mass, which is crucial for understanding physics beyond the Standard Model.

The Standard Model of particle physics assumes that the mass of neutrinos is zero, but in 1998 the Super-Kamiokande detector in Japan showed conclusively that the particles undergo oscillations and therefore must have mass. Knowing the masses of the three known types of neutrino is crucial to understanding physics beyond the Standard Model, but actually measuring the masses is proving extremely difficult. “Currently, we know more about the mass of the Higgs boson, which was discovered two years ago, than we do about the mass of the neutrino, which was discovered 60 years ago,” says Patrick Huber of Virginia Tech in the US.

Studies of neutrino oscillations tell us only that the average neutrino mass must be at least 0.01 eV/c2, so researchers are also trying to measure the mass using conservation of energy in beta decay. This is a nuclear process that involves the emission of an electron and a neutrino – strictly speaking, an electron antineutrino. Neutrinos are extremely difficult to detect, so physicists instead measure the energy of the electron and use this to calculate the mass of the neutrino.

Upper bound

The best measurements so far give an upper bound on the electron antineutrino mass of 2.05 eV/c2. Scientists are assembling a new detector called KATRIN at Karlsruhe Institute of Technology in Germany. This should measure a neutrino mass as small as 0.2 eV/c2 – which could still leave a 20-fold uncertainty in its value. But KATRIN is the size of a building, and further improvements in measurement accuracy by this method would require an even larger, more expensive spectrometer.

Now, physicists at Pacific Northwest National Laboratory, National Radio Astronomy Observatory, University of California Santa Barbara, University of Washington, the Massachusetts Institute of Technology and Karlsruhe University have set up the Project 8 collaboration, which is taking a different and possibly more elegant approach to measuring neutrino mass. When an electron passes through a magnetic field, its path curves into a circular orbit, and this causes the electron to emit cyclotron radiation at microwave frequencies. The nature of this radiation is dependent on the energy of the electron, and therefore measuring this effect could provide a much more simple and precise technique of measuring the energy than is currently used at KATRIN. The challenge, however, is how to detect the extremely weak femtowatt signal of cyclotron radiation from a single electron.

Now, the Project 8 team has taken an important step in that direction by being the first to detect this cyclotron radiation. Its prototype tabletop apparatus is located at the University of Washington in Seattle, and it uses a centimetre-sized gas cell that is filled with krypton-83 – a gas that undergoes beta decay. In an actual neutrino-mass experiment, the krypton would be replaced with tritium, but this introduces additional technical and safety considerations that will be considered in the future. The cell is placed inside a superconducting coil to generate a magnetic field. Electrons emitted by the beta decay travel in very long circular paths inside the tiny cell, emitting cyclotron microwave radiation, which is then detected by cooled, ultralow-noise detectors.

Small and simple

The researchers measured the energy of single emitted electrons with an accuracy of 30 eV. While this is far too low to obtain a reliable calculation of the neutrino mass, the team is now working to optimize the device to improve its resolution. “The apparatus that we built was very, very small”, says team member Benjamin Monreal of the University of California, Santa Barbara, “And that made the electronics very simple. We’re now preparing the readout designs, the antenna designs, the amplifier designs and the software to try to scale up.”

Huber, who was not involved in the research, is impressed, “They have successfully completed the first, very crucial step,” he says. “From here on, careful engineering and scaling of the device should get them to a point where they can compete with KATRIN.” However, he says, “there are probably more physics experiments that have failed because of ‘mere engineering challenges’ than for any other reason”.

The research is published in Physical Review Letters.

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