About Project 8

A quick summary:

The goal of Project 8 is to measure the mass of the neutrino, which is a fundamental particle (that is, a basic building block of the universe). Neutrinos are incredibly abundant - for every atom in the universe, there are about a billion neutrinos. However, our experience with them is minimal because they barely interact with ordinary matter. In fact, trillions of neutrinos produced by nuclear processes in the sun pass through your body every second, like tiny ghosts.

Neutrinos are strange in another way: they are unusually light, weighing at least a million times less than the next lightest particle, the electron. This points to a special mechanism explaining their mass, one different from how all other particles become massive. Explaining the stand-out neutrino mass would have implications for physicists’ model of the fundamental particles that make up everything. But to help understand the neutrino mass, physicists wish to know just how heavy it is.

It turns out that measuring such a tiny mass from a neutral particle is quite tricky. Instead of trying to capture the neutrino itself, we look at the decay of tritium, which is an isotope of hydrogen. Tritium undergoes beta decay, emitting an electron and a neutrino which have to share the energy released in the decay. Using a new method based radio-frequency detection, we measure the energy of the electron very precisely. Whatever is "missing" must belong to the neutrino. For the highest electron energies, the missing energy amounts to the neutrino's mass.

A cartoon of tritium (hydrogen-3) decay. A neutron turns into a proton, and releases an electron and neutrino.

A cartoon of tritium (hydrogen-3) decay. A neutron turns into a proton, and releases an electron and neutrino. The electron is what we measure in P8, and then deduce the mass of the neutrino.

A plot of the electron spectrum - basically a histogram of electron energies. The blue line is what the spectrum would look like if neutrinos had no mass. The orange line is the effect the neutrino mass has on the spectrum (if it is equal to 1 eV/c^2).

Currently, Project 8 has finished a demonstration of this technique. R&D efforts are underway throughout our collaboration to extend our method to measure more electrons more precisely in the next phases of our experiment. See our publications page for the latest published results. For a smattering of our recent work, head over to the photos page and read the descriptions. If you want a more in-depth technical description, keep reading!

(Text adapted from Talia Weiss)

diving deeper:

As described above, our aim is to measure the absolute neutrino mass using tritium beta decays. This technique, which involves precisely measuring the energies of the beta-decay electrons in the high-energy tail of the spectrum, has a history spanning more than 70 years. The approach to making that measurement taken by the Project 8 collaboration is to use a new method of electron spectroscopy, Cyclotron Radiation Emission Spectroscopy (CRES), which was first demonstrated by the collaboration in 2014.

In Project 8, we adopt the dictum of Arthur Schawlow: "Never measure anything but frequency!" In this case, we take advantage of the motion of charged particles in magnetic fields to make a measurement of their energies. In a constant magnetic field, an electron undergoes cyclotron motion. This occurs at a frequency that depends on the kinetic energy of the electron. Thus, a precise measurement of the frequency amounts to a precise measurement of the energy. In equations, this is denoted as follows:

The behavior described by that equation is shown in the diagram below on the left. Furthermore, we have to trap the electron axially in order to observe it for long enough to measure the cyclotron frequency.

Tritium decay, with electron emitted into a uniform magnetic field. Due to the Lorentz force, it undergoes cyclotron motion and moves in a spiral.

Tritium decay, with electron emitted into a uniform magnetic field. Due to the Lorentz force, it undergoes cyclotron motion and moves in a spiral.

As it spirals, the electron emits electromagnetic radiation. Within the uniform magnetic field, we create a magnetic trap so that the electron bounces back and forth as it spirals, allowing us to observe it for long enough.

As it spirals, the electron emits electromagnetic radiation. Within the uniform magnetic field, we create a magnetic trap so that the electron bounces back and forth as it spirals, allowing us to observe it for long enough.

As one might imagine, the amount of radiation from a single electron spiraling through space is tiny. For a freely-radiating electron undergoing cyclotron motion, the total power radiated is given by the Larmor formula. For a 1 T magnetic field, the amount of power radiated by 18.6 keV electrons is approximately 1 fW at ~26 GHz. The power radiated is given by the following formula:

Once we see this tiny signal from an electron in our detector, the quantity we are interested in is the start frequency. The plot below shows a typical CRES event. On the y-axis is the frequency of the electron's motion, and on the x-axis is time as we observe it moving in our magnetic trap. We call the lines formed in the plot "tracks." Tracks start at the electron's start frequency, which corresponds to that start energy we are looking for. As the electron keeps radiating, it loses energy, which accounts for the upward slope of the tracks. It also sometimes bumps into other particles in the detector, which accounts for the discrete jumps in the track. While the beginning of the track is the most important part, the rest of the track helps us find the event amidst our data.

Advantages of the CRES technique include:

Source = Detector. There is no need to separate the electrons from the radioactive source (a potential energy loss mechanism).

Frequency Measurement. Frequency standards are some of the most accurate measurements we can make to date.

Full Spectrum Sampling. We can in principle measure the entire beta decay spectrum at once, allowing us to leverage greater stability and statistics.

Current Experiment status

Phase I in our science program was to demonstrate the feasibility of CRES on single electrons emitted from a gaseous source, Krypton-83m. In Phase II we have made a measurement of the tritium spectrum using CRES. In Phase III we will demonstrate the ability to make CRES measurements in free space, as well as the ability to produce and trap atomic tritium. The final phase, Phase IV, will use atomic tritium to measure the neutrino mass with a sensitivity of approximately 40 meV.

Currently, efforts are focused on Phase III, with a final paper on Phase II results in the works. We are developing demonstrators of various technologies and working on our detector simulations and event analysis. For most recent progress, see our public talks page.