An experiment nearly two decades in the making has finally unveiled its measurements of the mass of the universe’s most abundant matter particle: the neutrino.
The neutrino could be the weirdest subatomic particle; though abundant, it requires some of the most sensitive detectors to observe. Scientists have been working for decades to figure out whether neutrinos have mass and if so, what that mass is.
The Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany has now revealed its first result constraining the maximum limit of that mass. The work has implications for our understanding of the entire cosmos, since these particles formed shortly after the Big Bang and helped shape the way structure formed in the early universe.
“You don’t get a lot of chances to measure a cosmological parameter that shaped the evolution of the universe in the laboratory,” Diana Parno, an assistant research professor at Carnegie Mellon University who works on the experiment, told Gizmodo.
Neutrinos come in three flavours: electron, muon and tau, based on how they interact with the corresponding electron, muon, and tau particles. Back in 1957, physicist Bruno Pontecorvo predicted that neutrinos would oscillate between these three different flavours, but this oscillation would require the particle to have mass. Experiments have since proven that oscillation exists, a finding that netted Arthur B. McDonald and Takaaki Kajita the 2015 Nobel Prize.
But figuring their mass out is tricky for various reasons — most importantly, neutrinos only interact with matter via the weak nuclear force, a difficult fundamental force for human-built experiments to access. Then, there’s the weirdness of quantum mechanics; each neutrino flavour is composed of a probabilistic combination of three “mass states.” Due to the weirdness of quantum mechanics, you can measure either the mass state or the flavour of a neutrino, but not both.
Detecting a particle that doesn’t interact with typical sensors required scientists to get creative. The KATRIN experiment begins with 25 grams of a kind of radioactive hydrogen gas, called tritium, stored in a 9 metre container held at cryogenic temperatures — cold enough such that even neon gas is a liquid. These atoms undergo a kind of radioactive decay called beta decay, where one of their neutrons turns into a proton, spitting out an electron and an electron-antineutrino in the process (which would have the same mass as the electron neutrino).
These decay products go into a house-sized detector called a spectrometer that measures the energy of the electrons. The electron and neutrino each carry away some of the energy of the reaction, but how much they take away can vary.
Scientists must look at the spectrum of all the different electron energies, focusing particularly on the electrons that have taken away the maximum energy, whose neutrinos would in turn have taken away the minimum energy. Analysis of the shape of the resulting graphs reveals the maximum energy of any of the neutrino mass states.
The mere fact that oscillation exists sets a lowest possible average mass of the three mass states, less than 0.1 electron volts (eV). After a month of operating and 18 years of planning and construction, KATRIN has now predicted an upper limit of any of the three mass states at 1.1 eV, where an electron weighs around 500,000 eV and a proton weighs nearly a billion.
KATRIN scientists announced the results at the 2019 Topics in Astroparticle and Underground Physics conference in Toyama, Japan, last Friday.
The KATRIN collaboration kicked off in 2001, but “it’s been a long time because it’s a really complicated experiment,” Hamish Robertson, a KATRIN scientist and professor emeritus of physics at the University of Washington, told Gizmodo.
The pressure and temperature of the gas source requires precise control, and there are lots of moving parts. It took years to design and build the enormous spectrometer that rejects unwanted electrons and precisely measures the resulting electrons’ energies.
“It’s fractal at some level,” said Parno. “If you zoom in at any part of the experiment and start asking questions, you get the same level of complexity back again.”
KATRIN is just one of several different strategies to calculate the neutrino’s mass. Just last month, researchers used cosmological data to argue that the sum of the three neutrino masses was at most 0.26 electron volts.
Other experiments hope to calculate the neutrino mass using rare atomic decays. But KATRIN’s findings are valuable because they don’t rely on grand theories of how the universe works, noted Duke University associate physics professor Phillip Barbeau, who was not involved in the study.
This most recent limit on the mass halves the maximum mass determined in other experimental setups and comes from just one month of data. There’s a whole lot more to go, including five years’ worth of data-taking that will further constrain the masses.
Scientists ultimately want to know more than just the maximum mass of the states; they want to know the absolute mass of all three states and how they compare to one another. Solving this problem has implications for understanding the early universe’s behaviour, whether the neutrino is its own antiparticle, and why there’s more matter than antimatter in the universe. Lots of physicists are interested in the result.
“It’s a fundamental parameter,” Kate Scholberg, Duke University professor of physics not involved in the study, told Gizmodo. “If you’re trying to develop overall models of fundamental physics, grand unified theories and that kind of thing, then you want all of the information you can — like the masses of all of the particles.”