The metropolis Deggendorf in Germany is about 350 km by street from Karlsruhe. Yet when the spectrometer of the Karlsruhe Tritium Neutrino Experiment (KATRIN) was constructed in Deggendorf in 2006, it took an 8,600-km detour to Karlsruhe. Of this, solely 7 km was by land, transported on a truck with nice care and police safety.
For the remainder of its journey, it floated on the Danube, the Black Sea, the Mediterranean Sea, the Atlantic Ocean, and the Rhine. Such elaborate measures needed to be taken as a result of the spectrometer — the core instrument of the experiment — was a 200-tonne affair, making land transport harmful.
Why make such a large detector? For that’s what it takes to try to find out the mass of the hardest-to-detect subatomic particle within the universe: the neutrino. Recently, the KATRIN collaboration printed an higher limit on the sum of the plenty of the three identified neutrino varieties utilizing 259 days of measurements recorded throughout 5 data-taking runs between March 2019 and June 2021.
The collaboration mentioned that this sum couldn’t exceed 8.8 x 10-7 occasions the heft of the electron — a 2x enchancment on the earlier greatest constraint. This is a big feat.
One puzzle after one other
Physicists are so eager to review neutrinos as a result of since their discovery in 1938, these particles have confronted them with one puzzle after one other about nature. Here are some central questions pertaining to neutrinos’ plenty that drive analysis in the present day.
1. How a lot does a neutrino weigh? Neutrinos are available in three varieties. It has been established, via a phenomenon referred to as particle oscillations, that at the very least two kinds of neutrinos have greater than zero mass. It was an experimental triumph so intricate with profound theoretical implications that the physicists who led the invention groups received the 2015 Nobel Prize for physics for making such a seemingly diminutive commentary.
Unfortunately, particle oscillations can solely measure the variations within the squares of the neutrino plenty, not the plenty themselves.
Measuring the precise plenty is more difficult. This is what subtle units like KATRIN are designed to try.
2. A neutrino’s mass is so small that in nearly each scenario it travels almost on the pace of sunshine (a particle that does journey on the pace of sunshine, the photon, is massless). It is that this insufferable lightness that makes their weight troublesome to pinpoint in an experiment. Also, physicists don’t perceive why neutrinos are so mild.
3. In the Standard Model — the present greatest framework scientists have to clarify the methods particles work together with one another — there isn’t a method to theoretically confer plenty to neutrinos. Said otherwise, neutrinos are predicted as massless, in battle with the Nobel-winning oscillation knowledge. This implies the presence of new, hitherto unseen forces and particle species in new Nature – the clearest index but that one thing lurks past the Standard Model. What is that one thing?
4. Are neutrinos their very own antiparticles? They actually match the invoice. The antiparticle of a particle kind carries reverse cost, so the primary criterion for a self-conjugate particle is that it should be electrically impartial — which neutrinos are. As far as physicists can inform, it’s additionally an elementary particle. This is not like, say, a neutron, which is electrically impartial however composed of charged quarks. As antiquarks are distinct from quarks, an antineutron is distinct from a neutron.
To seal the deal, physicists want to verify a 3rd requirement: whether or not the neutrino has a Majorana mass or a Dirac mass. These phrases confer with the mechanism by which a neutrino will get its mass: if it follows the Majorana course of, then a neutrino can be confirmed to be its personal antiparticle. To settle this, physicists are on the lookout for a really delicate pure course of referred to as neutrinoless double beta decay: a technique that it may possibly happen requires two neutrinos to mutually annihilate themselves.
However, a neutrino is dreadfully exhausting to catch. Any materials used as a detector can be almost clear to it. It takes, as an example, a lightweight 12 months’s size of steel to cease a single neutrino emitted by the solar. Such elusiveness is why the neutrino took so lengthy to be found.
A major achievement
KATRIN itself intently observes the disintegration of molecular tritium to estimate the neutrino mass. In explicit, it focuses on the maximum energies of electrons emitted when tritium decays; these energies carry info on the mass of the neutrino. To set the newest constraint, KATRIN collected knowledge from no fewer than 36 million electrons.
The experiment’s feat can also be the newest in a protracted historical past of comparable makes an attempt — starting in 1991 in Los Alamos within the US and Tokyo, which set a cap on the neutrino mass that was about 20-times weaker than the new KATRIN end result.
KATRIN can also be not the only participant within the sport. For instance, observational cosmologists use the truth that neutrinos are key actors in shaping the construction of galaxies to set a tighter higher limit on the sum of the neutrino plenty at 1.4 x 10-7 occasions the electron mass. This limit, nonetheless, depends on assumptions concerning the evolution of the early universe which are exhausting to check, weakening the validity of the conclusions drawn.
Another sort of experiment that may make a press release on neutrino plenty makes use of the neutrinoless double beta decay — however this experiment additionally assumes neutrinos are self-conjugate on the outset.
The KATRIN end result, on the opposite hand, is strong as a result of it rides on no such assumptions. That is a big achievement to savour within the face of an opponent as formidable because the neutrino.
Nirmal Raj is an assistant professor of theoretical physics on the Centre for High Energy Physics within the Indian Institute of Science, Bengaluru.
Published – June 11, 2025 05:30 am IST
