Sensitive German experiment sets new limit on maximum neutrino mass
For the rest of its journey, it floated on the Danube, the Black Sea, the Mediterranean Sea, the Atlantic Ocean, and the Rhine. Such elaborate measures had to be taken because the spectrometer — the core instrument of the experiment — was a 200-tonne affair, making land transport dangerous.
Why make such a massive detector? For that is what it takes to attempt to determine the mass of the hardest-to-detect subatomic particle in the universe: the neutrino. Recently, the KATRIN collaboration published an upper limit on the sum of the masses of the three known neutrino types using 259 days of measurements recorded across five data-taking runs between March 2019 and June 2021.
The collaboration said that this sum couldn't exceed 8.8 x 10-7 times the heft of the electron — a 2x improvement on the previous best constraint. This is a significant feat.
One puzzle after another
Physicists are so keen to study neutrinos because since their discovery in 1938, these particles have confronted them with one puzzle after another about nature. Here are some central questions pertaining to neutrinos' masses that drive research today.
1. How much does a neutrino weigh? Neutrinos come in three types. It has been established, through a phenomenon called particle oscillations, that at least two types of neutrinos have more than zero mass. It was an experimental triumph so intricate with profound theoretical implications that the physicists who led the discovery teams won the 2015 Nobel Prize for physics for making such a seemingly diminutive observation.
Unfortunately, particle oscillations can only measure the differences in the squares of the neutrino masses, not the masses themselves.
Measuring the actual masses is more challenging. This is what sophisticated devices like KATRIN are designed to attempt.
2. A neutrino's mass is so small that in almost every situation it travels nearly at the speed of light (a particle that does travel at the speed of light, the photon, is massless). It is this unbearable lightness that makes their weight difficult to pinpoint in an experiment. Also, physicists don't understand why neutrinos are so light.
3. In the Standard Model — the current best framework scientists have to explain the ways particles interact with each other — there is no way to theoretically confer masses to neutrinos. Said differently, neutrinos are predicted as massless, in conflict with the Nobel-winning oscillation data. This implies the presence of new, hitherto unseen forces and particle species in new Nature – the clearest index yet that something lurks beyond the Standard Model. What is that something?
4. Are neutrinos their own antiparticles? They certainly fit the bill. The antiparticle of a particle type carries opposite charge, so the first criterion for a self-conjugate particle is that it must be electrically neutral — which neutrinos are. As far as physicists can tell, it's also an elementary particle. This is unlike, say, a neutron, which is electrically neutral but composed of charged quarks. As antiquarks are distinct from quarks, an antineutron is distinct from a neutron.
To seal the deal, physicists need to confirm a third requirement: whether the neutrino has a Majorana mass or a Dirac mass. These terms refer to the mechanism by which a neutrino gets its mass: if it follows the Majorana process, then a neutrino would be confirmed to be its own antiparticle. To settle this, physicists are looking for a very delicate natural process called neutrinoless double beta decay: one way that it can occur requires two neutrinos to mutually annihilate themselves.
However, a neutrino is dreadfully hard to catch. Any material used as a detector would be nearly transparent to it. It takes, for instance, a light year's length of metal to stop a single neutrino emitted by the sun. Such elusiveness is why the neutrino took so long to be discovered.
A significant achievement
KATRIN itself closely observes the disintegration of molecular tritium to estimate the neutrino mass. In particular, it focuses on the maximum energies of electrons emitted when tritium decays; these energies carry information on the mass of the neutrino. To set the latest constraint, KATRIN collected data from no fewer than 36 million electrons.
The experiment's feat is also the latest in a long history of similar attempts — beginning in 1991 in Los Alamos in the US and Tokyo, which set a cap on the neutrino mass that was about 20-times weaker than the new KATRIN result.
KATRIN is also not the sole player in the game. For example, observational cosmologists use the fact that neutrinos are key actors in shaping the structure of galaxies to set a tighter upper limit on the sum of the neutrino masses at 1.4 x 10-7 times the electron mass. This limit, however, relies on assumptions about the evolution of the early universe that are hard to test, weakening the validity of the conclusions drawn.
Another kind of experiment that can make a statement on neutrino masses makes use of the neutrinoless double beta decay — but this experiment also assumes neutrinos are self-conjugate at the outset.
The KATRIN result, on the other hand, is robust because it rides on no such assumptions. That is a significant achievement to savour in the face of an opponent as formidable as the neutrino.
Nirmal Raj is an assistant professor of theoretical physics at the Centre for High Energy Physics in the Indian Institute of Science, Bengaluru.
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