Neutrino Endpoint

Postcards from the Energy Frontier by Prof Jon Butterworth

There is something very elegant about the KATRIN (Karlsruhe Tritium Neutrino) experiment. 

Here it is, looking like an alien space ship advancing through a German village. That’s not the elegant part though. It made a rather ungainly journey from Deggendorf to Karlsruhe via Hungary, Bulgaria, the Black Sea, the Mediterranean, the English Channel and Leopoldshafen (where this picture was taken). A very long way round.

No, the elegant bit is the simplicity and satisfying historical faithfulness of the underlying principle behind the experiment. It may be trying to pin down the mass of the incredible elusive neutrino, the second-most-abundant but almost undetectable particle in the universe, but the principle behind it is simply conservation of momentum, physics which 15-year-olds  learn for physics GCSE. And it is this same principle which led the great physicist Wolfgang Pauli to postulate the existence of the neutrino in the first place. 

New things in physics are postulated to solve problems. The problem in this case was with the process known as “Beta decay“, where the nuclei of some elements decay into a different element, emitting an electron in the process. Only the electron and the recoiling nucleus are observed, so this was thought to be a “two body” decay. One nucleus decays to two things, the electron and the recoiling nucleus. In such a process, conservation laws apply. Energy can’t be created or destroyed, and momentum is conserved.

A certain amount of energy is released when a particular nucleus decays. For example when Tritium decays to Helium-3. In a two-body decay, that energy goes into the mass and kinetic energy of the electron and the recoiling Helium nucleus. There’s nowhere else it can go.

Tritium decays to Helium-3

And if the Tritium was stationary — that is, had zero momentum — then the momentum of the electron must be exactly the opposite of the momentum of the Helium, so they cancel out and the total momentum remains zero. 

Putting those constraints together, there is only one possible energy and one possible momentum the electron can have.

And that is not what is observed. The electron has a range of energies, always lying below the value that, according to the two-body decay calculation, it should have.

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This is a big problem. So big that Niels Bohr even suggested that maybe we should give up on conservation of energy. However, Wolfgang Pauli had a better idea. Not quite as radical, but still pretty bold. In a famous letter in 1930 he postulated a new, invisible particle, so that:

The continuous beta spectrum would then make sense with the assumption that in beta decay, in addition to the electron, a neutr[ino] is emitted such that the sum of the energies of neutron and electron is constant.

(He may have been a genius but he got the name wrong. The name he originally gave it — neutron — was nicked by James Chadwick two years later in 1932 for the much heavier but also electrically-neutral particle found in the atomic nucleus.)

In other words, if Beta decay is actually a three body decay, there is another degree of freedom. There is somewhere else for the energy to go. Thus the electron can have a range of energies which, as observed, would always be below the “two body” value.

The KATRIN experiment. Image: Karlsruhe IOT

KATRIN measures exactly this process — Tritium Beta decay. And the measurement of interest is to determine the maximum value of the electron energy, the “endpoint” of the electron energy spectrum. Because when the electron has its maximum energy, the neutrino has its minimum. And the minimum energy the neutrino can have is when it is produced at rest, so it has no kinetic energy at all and its energy is just its mass multiplied by the speed of light squared (according to the most famous equation in the world).

The mass of the neutrino is not known, it is an important parameter for the early development of the universe and for the fundamental laws of physics, and KATRIN is trying to measure it (or at least set a maximum value on it) using exactly the same physics which led Pauli to propose the existence of the particle in the first place. And all based on GCSE physics. Albeit with a lot of extremely clever detector physics and engineering to achieve the required precision. The first KATRIN measurement was published last month, and more precision will follow.

Pauli thought the neutrino could never be directly detected. He was wrong there, but that’s another story.

Professor Jon Butterworth is a physics professor at University College London and a researcher on the ATLAS experiment at CERN involved with, amongst other things, the discovery of the Higgs Boson. He is the author of two popular science books Smashing Physics and A Map of the Invisible. Postcards From the Energy Frontier is the successor to Jon’s hugely successful blog for The Guardian, Life and Physics. He is @jonmbutterworth on Twitter.

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