## Neutrinos: Getting Lighter by the Minute? Neutrinos, those elusive, ghostly particles that zip through us constantly, have always been shrouded in mystery. We know they’re incredibly light, almost weightless—but just how light? Physicists have been on a relentless quest to pin down this fundamental property, and now, a groundbreaking experiment called KATRIN has thrown down the gauntlet. They’ve just delivered the tightest limit on neutrino mass ever recorded, pushing us closer to unraveling one of the universe’s most profound puzzles. Prepare to enter the fascinating realm of subatomic particles and witness the incredible precision of modern science.
KATRIN: Pushing the Limits of Neutrino Mass
Neutrinos are the enigmatic particles that permeate our universe, flitting through matter with barely a whisper. These neutral and almost massless particles play a crucial role in fundamental processes like the sun’s energy production and the evolution of stars. While we know neutrinos have mass, its exact value remains a tantalizing mystery.
Neutrino Oscillations: The First Hint of Mass:
Observing neutrinos oscillating between different flavors (electron, muon, tau) provided the first evidence that these particles possess some mass. This phenomenon wouldn’t occur if neutrinos were truly massless.
The Quest for a Precise Measurement:
Determining the mass of the neutrino is a formidable challenge. They interact so weakly with matter that detecting them directly is incredibly difficult.
KATRIN: A Giant Leap Towards Clarity
The Karlsruhe Tritium Neutrino experiment (KATRIN) stands as a beacon of hope in this quest. This ambitious project utilizes a groundbreaking technique to measure the mass of the neutrino with unprecedented precision.
Researchers from the Karlsruhe Tritium Neutrino experiment (KATRIN) have announced the most precise upper limit yet on the neutrino’s mass. Thanks to new data and upgraded techniques, the new limit – 0.45 electron volts (eV) at 90% confidence – is half that of the previous tightest constraint, and marks a step toward answering one of particle physics’ longest-standing questions.
Looking for Clues in Electrons
In KATRIN’s case, that means focusing on a process called tritium beta decay, where a tritium nucleus (a proton and two neutrons) decays into a helium-3 nucleus (two protons and one neutron) by releasing an electron and an electron antineutrino. Due to energy conservation, the total energy from the decay is shared between the electron and the antineutrino. The neutrino’s mass determines the balance of the split.
“If the neutrino has even a tiny mass, it slightly lowers the energy that the electron can carry away,” explains Christoph Wiesinger, a physicist at the Technical University of Munich, Germany and a member of the KATRIN collaboration. “By measuring that [electron] spectrum with extreme precision, we can infer how heavy the neutrino is.”
Improvements over Previous Results
The new neutrino mass limit is based on data taken between 2019 and 2021, with 259 days of operations yielding over 36 million electron measurements.
“That’s six times more than the previous result,” Wiesinger says. Other improvements include better temperature control in the tritium source and a new calibration method using a monoenergetic krypton source.
“We were able to reduce background noise rates by a factor of two, which really helped the precision,” he adds.
Keeping Track: Laser System for the Analysis of the Tritium Gas Composition
Improvements to temperature control in this source helped raise the precision of the neutrino mass limit.
What Does This Signify?
At 0.45 eV, the new limit means the neutrino is at least a million times lighter than the electron.
“This is a fundamental number,” Wiesinger says. “It tells us that neutrinos are the lightest known massive particles in the universe, and maybe that their mass has origins beyond the Standard Model.”
Model Independence
Despite the new tighter limit, however, definitive answers about the neutrino’s mass are still some ways off.
“Neutrino oscillation experiments tell us that the lower bound on the neutrino mass is about 0.05 eV,” says Patrick Huber, a theoretical physicist at Virginia Tech, US, who was not involved in the experiment.
“That’s still about 10 times smaller than the new KATRIN limit… For now, this result fits comfortably within what we expect from a Standard Model that includes neutrino mass.”
Expert Analysis
Physicist Björn Lehnert from the Lawrence Berkeley National Laboratory talks to Richard Blaustein about what a new precise measurement of the neutrino means for particle physics
Based at the Lawrence Berkeley National Laboratory in the US, Björn Lehnert is a neutrino physicist who originally did a PhD at the Technische Universität Dresden in Germany on the GERDA experiment.
Following a postdoc at Carleton University in Canada, he moved to California in 2018, where he works on the double-beta-decay experiment LEGEND.
He is also part of the KATRIN collaboration, which today in Nature Physics reports a new upper limit on the mass of the neutrino.
Can you explain what KATRIN (the Karlsruhe Tritium Neutrino Experiment) is designed to do?
KATRIN, which is based at the Karlsruhe Institute for Technology in Germany, was inaugurated in 2018 and is a collaboration between the Czech Republic, Germany, Russia, the UK and US.
It consists of about 130 scientists and is the only experiment that can make direct measurements of neutrino mass.
How do you measure the mass of a neutrino?
Neutrinos are the most abundant – and elusive – particles in the universe and measuring neutrino mass is very difficult.
There are several approaches, some are model dependent in that they are based on assumptions about the universe.
First there is the cosmological approach that considers where neutrinos have influenced the evolution of the universe, specifically in the creation of large-scale structures such as galaxy clusters.
If neutrinos are light, it would favour the formation of smaller-scale structures, while heavier neutrinos disfavours smaller structures.
By measuring the distribution of smaller and larger structures in the universe, it is possible to infer the neutrino’s mass.
Another method is double-beta-decay experiments that search for whether neutrinos are their own antiparticles, so called Majorana particles.
So how does KATRIN measure mass?
KATRIN’s main component is the world’s largest spectrometer – measuring 23 metres long and 10 metres wide – to boast an ultrahigh vacuum.
Tritium – an isotope of hydrogen – undergoes beta decay, producing an electron and an antineutrino.
We then guide the electrons into the spectrometer without changing their energy.
We cannot measure the neutrino directly because it is so weakly interacting, but we can precisely measure the electron’s energy.
As both particles share energy, it is possible to resolve the small influence from the neutrino’s mass by looking at the electrons with the highest energies in the spectrum.
KATRIN has today announced an upper limit for the neutrino mass of 0.8 eV.
What does this signify?
KATRIN started its five-year run in 2019 and this is the first time any lab experiment has produced the required sensitivity to rule out the mass of the neutrino being greater than 0.8 eV (Nature Physics).
That is a real advance as it breaks the “psychological barrier” that we had in not knowing whether the neutrino is heavier than 1 eV.
Importantly, we now know that the neutrino is at least 500,000 times lighter the electron.
Tritium Beta Decay: A Key to Unlocking the Secret
KATRIN focuses on the beta decay of tritium, a radioactive isotope of hydrogen. This decay process releases an electron and an electron antineutrino. Due to energy conservation, the total energy from the decay is shared between the electron and the antineutrino. The neutrino’s mass determines the balance of the split.
“If the neutrino has even a tiny mass, it slightly lowers the energy that the electron can carry away,” explains Christoph Wiesinger, a physicist at the Technical University of Munich, Germany and a member of the KATRIN collaboration. “By measuring that [electron] spectrum with extreme precision, we can infer how heavy the neutrino is.”
The Spectrometer: A Machina of Precision
KATRIN boasts the world’s largest spectrometer, meticulously designed to capture and analyze the electrons emitted during tritium decay. This sophisticated instrument allows scientists to precisely measure the energy spectrum of the electrons.
New Limits and Implications
KATRIN’s latest findings have set a tighter limit on the neutrino’s mass, pushing the boundaries of our understanding.
Refining the Upper Limit
The new limit, 0.45 electron volts (eV), represents a significant improvement over previous measurements. This tighter constraint significantly narrows the possible range of neutrino masses.
Insights into Particle Physics
The precise measurement of neutrino mass has profound implications for our understanding of particle physics. It provides valuable clues about the fundamental building blocks of the universe and the forces that govern them.
Model Independence
Although the new limit is a significant improvement, definitive answers about the neutrino’s mass are still some ways off. Neutrino oscillation experiments tell us that the lower bound on the neutrino mass is about 0.05 eV. That’s still about 10 times smaller than the new KATRIN limit.
Expert Analysis
Patrick Huber, a theoretical physicist at Virginia Tech, US, who was not involved in the experiment, comments: “For now, this result fits comfortably within what we expect from a Standard Model that includes neutrino mass.”
Weighing in
Björn Lehnert, a neutrino physicist at the Lawrence Berkeley National Laboratory, explains the significance of KATRIN’s findings: “KATRIN started its five-year run in 2019 and this is the first time any lab experiment has produced the required sensitivity to rule out the mass of the neutrino being greater than 0.8 eV. That is a real advance as it breaks the ‘psychological barrier’ that we had in not knowing whether the neutrino is heavier than 1 eV.”
Implications and Future Directions
The new limit sets a benchmark for future experiments, which will aim to further refine our understanding of neutrino mass. KATRIN’s findings have significant implications for our understanding of particle physics and the universe as a whole.
Conclusion
The KATRIN experiment has once again tightened the reins on the elusive neutrino mass, pushing the boundaries of our understanding about these fundamental particles. By meticulously measuring the energy spectrum of electron antineutrinos, KATRIN has narrowed the allowed mass range to a mere 0.8 eV/c², a feat that underscores the incredible precision and sophistication of modern physics. This latest result not only refines our theoretical models but also opens new avenues for exploring the nature of dark matter and the very fabric of the universe. The implications of this research extend far beyond the realm of particle physics. Neutrinos, being incredibly light and weakly interacting, offer a unique window into the early universe and the processes that shaped it. Their mass, though minuscule, plays a critical role in understanding the evolution of stars and the formation of galaxies. As KATRIN and other experiments continue to probe the neutrino’s secrets, we inch closer to unraveling the profound mysteries that lie at the heart of our cosmos. The quest to understand the neutrino is a journey into the unknown, a quest that promises to reshape our understanding of the universe and our place within it.
