Neutrinos Are Shrinking, and That's a Good Thing for Physics
On Thursday, researchers unveiled the most precise measurement yet of a neutrino, scaling down the maximum possible mass of the ghostly specks of matter that permeate our universe.
The result, published in the journal Science, does not define the exact mass of a neutrino, just its upper limit. But the finding helps bring physicists closer to figuring out just what is wrong with the so-called Standard Model, their best — albeit incomplete — theory of the laws that rule the subatomic realm. One way physicists know it is not quite accurate is that it suggests that the neutrino should not have any mass at all.
At grander scales, learning more about neutrinos will help cosmologists fill in their ever hazy picture of the universe, including how galaxies clustered together and what influences the expansion of the cosmos since the Big Bang.
'We're looking at trying to understand why we are here,' said John Wilkerson, a physicist at the University of North Carolina, Chapel Hill and an author of the new study. 'And that's something neutrinos may have a key role in.'
Physicists know a few things about neutrinos. They are prolific across the cosmos, created virtually anytime atomic nuclei snap together or rip apart. But they carry no electric charge and are notoriously difficult to detect.
Neutrinos also come in three types, which physicists describe as flavors. And, oddly, they morph from one flavor to another as they move through space and time, a discovery recognized by the Nobel Prize in Physics in 2015. The underlying mechanism that makes these transformations possible, physicists realized, meant that neutrinos must have some mass.
But only just so. Neutrinos are mindboggingly light, and physicists don't know why.
Uncovering the exact values of the mass of neutrinos could lead to 'some kind of portal' to new physics, said Alexey Lokhov, a scientist at the Karlsruhe Institute of Technology in Germany. 'This is, for now, the world's best limit,' he said of his team's measurement.
Dr. Lokhov and his colleagues used the Karlsruhe Tritium Neutrino, or KATRIN, experiment to narrow down the mass of a neutrino. At one end of the 230-foot-long apparatus was a source of tritium, a heavier version of hydrogen with two neutrons in its nucleus. Because tritium is unstable, it decays into helium: One neutron converts into a proton, which spits out an electron in the process. It also spits out an antineutrino, the antimatter twin of a neutrino. The two should have identical mass.
The mass of the original tritium is split among the products of the decay: the helium, electron and antineutrino. Neither neutrinos nor antineutrinos can be directly detected, but a sensor at the other end of the experiment recorded 36 million electrons, over 259 days, shed by the decaying tritium. By measuring the energy of the electron's motion, they could indirectly deduce the maximum mass possible for the antineutrino.
They found that value to be no more than 0.45 electronvolts, in the units of mass used by particle physicists, a million times lighter than an electron.
The upper bound on the mass was measured for only one flavor of neutrino. But Dr. Wilkerson said that nailing down the mass of one makes it possible to calculate the rest.
The latest measurement pushes the possible mass of the neutrino lower than the previous limit set in 2022 by the KATRIN collaboration, of no more than 0.8 electronvolts. It is also nearly twice as precise.
Elise Novitski, a physicist at the University of Washington who was not involved in the work, commended the KATRIN team's careful effort.
'It's really just a tour de force,' she said of the experiment and the discovery. 'I have full confidence in their result.'
The KATRIN team is working on an even tighter boundary on the neutrino mass from 1,000 days of data, which it expects to collect by the end of the year. That will give the physicists even more electrons to measure, leading to a more precise measurement.
Other experiments on the horizon will also contribute to a better understanding of the neutrino's mass, including Project 8 in Seattle and the Deep Underground Neutrino Experiment, spread across two physics facilities in the Midwest.
Astronomers studying the structure of the cosmos at large, thought to be influenced by the vast collection of neutrinos flooding the universe, have their own measurement of the particles' maximum mass. But according to Dr. Wilkerson, the boundaries set by astronomers staring out into the void don't match up with what particle physicists calculate in the lab, as they scrutinize the subatomic world.
'There's something really interesting going on,' he said. 'And the likely solution to that is going to be physics beyond the Standard Model.'
Hashtags
Try Our AI Features
Explore what Daily8 AI can do for you:
Comments
No comments yet...
Related Articles
Yahoo
14 hours ago
- Yahoo
This Particle Isn't Following the Rules of Physics. Maybe the Rules Are Wrong.
Here's what you'll learn when you read this story: For nearly a century, the magnetic anomaly of the fundamental particle known as a muon has served as a means to test theories against experimental reality. Recently, an international collaboration powered by the U.S.-based Fermilab has released its most accurate data on this anomalous magnetic dipole moment, known as g-2 ('gee minus two'). These new results align closely with recent theoretical predictions, and will serve as a benchmark moving forward. The Standard Model of Particle Physics is a remarkable scientific achievement spanning nearly a century, and its predictive power has proven incredibly consistent. However, any scientific model worth its salt also needs to withstand experimental scrutiny, and one of the places those tests are employed is Fermilab. Starting in 2017, an international collaboration of scientists have used data from Fermilab's 50-foot-diameter magnetic ring to measure the wobble of a fundamental particle known as a muon in what is referred to as the lab's 'muon g-2 experiment.' More than 200 times heavier than electrons, muons only survive for a few microseconds, but they have spins that makes them act like tiny magnets. This wobble, or precession, is due to an external magnetic field is called a g-factor, and a century ago, this factor was found to be 2 (hence the name 'muon g-2 experiment'). However, the introduction of quantum field theory complicates this number by bringing strong, weak, and Higgs fields interactions into the equation. This slight deviation from the '2' prediction is known as the muon's anomalous magnetic dipole moment. To better understand this anomaly, Fermilab has consistently released results from its run of experiments that stretches from 2017 to 2023. On June 3, 2025, the muon g-2 experiment finally released its full results, with a precision of roughly 127 parts-per-billion—the most sensitive and accurate measurement of the muon's magnetic anomaly to date. The results of the study were submitted to the journal Physical Review D. 'The anomalous magnetic moment, or g–2, of the muon is important because it provides a sensitive test of the Standard Model of particle physics,' Regina Rameika, the U.S. Department of Energy's Associate Director for the Office of High Energy Physics, said in a press statement. 'This is an exciting result and it is great to see an experiment come to a definitive end with a precision measurement.' Understanding this precise measurement of muon g-2 can help scientists discover new physics, as any deviation between experimental results and theoretical predictions using the Standard Model could point toward unknowns in our understanding of the subatomic world. While experimental physicists work to perfect ways of measuring the magnetic anomaly, theoretical physicists—especially those participating in the Muon Theory Initiative, which released its own update in late May—have largely sorted themselves into two 'camps' when calculating this theoretical prediction, according to Ethan Siegel at Big Think. One camp takes a data-driven approach to Hadronic vacuum polarization and the other uses a computational-based Lattice quantum chromodynamics (QCD) technique. In 2021, it appeared that Fermilab's initial results were much closer to the Lattice QCD computational calculations, dampening (but not eliminating) the possibility of new physics orbiting the muon. Now, with this new calculation in hand, scientists can move forward with renewed confidence in an experimental result that's been a popular test of the Standard Model of Physics for a century. 'As it has been for decades, the magnetic moment of the muon continues to be a stringent benchmark of the Standard Model,' Simon Corrodi, assistant physicist at Argonne National Laboratory and analysis co-coordinator, said in a press statement. 'The new experimental result sheds new light on this fundamental theory and will set the benchmark for any new theoretical calculation to come.' This isn't the end for measuring the muon magnetic anomaly—the Japan Proton Accelerator Research Complex aims to make its own g-2 measurements in the 2030s (though Fermilab says that its initial precision will be worse than their own latest results). Today, this muon g-2 result is a testament of the incredible engineering and multidisciplinary scientific effort required to uncover just a little bit more about our ever-mysterious universe. You Might Also Like The Do's and Don'ts of Using Painter's Tape The Best Portable BBQ Grills for Cooking Anywhere Can a Smart Watch Prolong Your Life?
New York Times
a day ago
- New York Times
The Humanist Who Designed a Deadly Weapon
Times Insider explains who we are and what we do and delivers behind-the-scenes insights into how our journalism comes together. Once, during an interview, I saw him in action as he described a run of knotty calculations he was doing in his head — the kind of math his peers usually worked out on paper or with computers. That gift was surely one reason that Enrico Fermi, a founder of the nuclear age who mentored him at the University of Chicago, called Richard L. Garwin 'the only true genius I have ever met.' It also played to a popular image of Dr. Garwin as slightly robotic, even computerlike, a thinking machine that happened to have legs. Dr. Garwin died last month at 97, leaving behind a legacy of contradictions. In 1951, at age 23, he designed the first hydrogen bomb, the world's deadliest weapon, a planet shaker that could end civilization. He then devoted his life to counteracting the terror. Over four decades of interviews, chats and social interactions, I learned that the man behind the stereotypes was full of surprises, which I wrote about in a recent article. He had a reputation for being cruel to those he saw as less talented. That may have been true in the prime of his professional life. But in person during his later years, Dr. Garwin came across as a gentle academic, a humanist whose life turned out to be rich in benevolent acts. Years ago, Gene Cittadino, a friend of mine who taught science history at New York University, asked me if Dr. Garwin might be willing to speak to his class. After the talk, Gene and several students took him to lunch and were regaled with stories about the presidents he advised. 'He was soft-spoken, sharp as a tack and funny,' Gene recalled. The whiz, he added, 'treated us with respect,' as if we were his colleagues. Want all of The Times? Subscribe.
Yahoo
a day ago
- Yahoo
Astronomers Just Discovered The Biggest Explosions Since The Big Bang
A never-before-seen type of giant space explosion – the biggest bangs since the Big Bang – has been accidentally captured by the Gaia space telescope. From the hearts of distant galaxies, the mapping telescope recorded sudden, extreme increases in brightness – colossal flares of light that lingered far longer than any such flares had been known to previously. These blasts were calculated to release as much energy as 100 Suns would over the course of their combined lifetimes. Analysis of that light revealed something that was both new and familiar at the same time: stars being torn apart by black holes, but on a scale we hadn't observed before. Each star was a large one, at least three times as massive as the Sun; and each black hole was a supermassive beast lurking in the center of the star's host galaxy. Such events are usually known as tidal disruption events, or TDEs. Astrophysicists are calling these new ones 'extreme nuclear transients' – ENTs for short. "We've observed stars getting ripped apart as tidal disruption events for over a decade, but these ENTs are different beasts, reaching brightnesses nearly 10 times more than what we typically see," says astrophysicist Jason Hinkle of the University of Hawaiʻi's Institute for Astronomy (IfA). "Not only are ENTs far brighter than normal tidal disruption events, but they remain luminous for years, far surpassing the energy output of even the brightest known supernova explosions." The rather tame term 'tidal disruption' is used to describe what gravitational forces do to an object that gets too close to a black hole. At a certain point, the power of the external gravitational field surpasses the gravity holding an object together, and it comes apart in a wild scream of light before at least partially falling into the great unknown beyond the black hole's event horizon. There are telescopes trained on the sky to catch these screams, applying a wide field of view to take in as much of the sky as possible, waiting for those unpredictable flares that denote the death throes of an unlucky star. Astronomers have managed to observe a good number of TDEs, and know roughly how they should play out. There's a sudden brightening in a distant galaxy, with a light curve that rises to a rapid peak before gradually fading over the course of weeks to months. Astronomers can then analyze that light to determine properties such as the relative masses of the objects involved. Gaia was a space telescope whose mission was to map the Milky Way in three dimensions. It spent a great deal of time staring at the sky to capture precise parallax measurements of the stars in the Milky Way. On occasion, however, it managed to exceed its mission parameters. When combing through Gaia data, Hinkle and his colleagues found two strange events: Gaia16aaw, a flare recorded in 2016; and Gaia18cdj, which the telescope caught in 2018. Both events bore a strong similarity to an event recorded by the Zwicky Transient Facility in 2020. Because that event was so insanely powerful, and because it was given the designation ZTF20abrbeie, astronomers nicknamed it "Scary Barbie". Hinkle and his team determined that Gaia16aaw and Gaia18cdj are the same kind of event as Scary Barbie, and set about trying to figure out what caused them. They ruled out supernova explosions – the events were at least twice as powerful as any other known transients, and supernovae have an upper brightness limit. A supernova, the team explained, typically releases as much light as the Sun will in its entire, 10-billion-year lifespan. The output of an ENT, however, is comparable to the lifetime output of 100 Suns all rolled together. Rather, the properties of the ENT events, the researchers found, were consistent with TDEs – just massively scaled up. That includes how much energy is expended, and the shape of the light curve as the event brightens and fades. ENTs are incredibly rare – the team calculated that they are around 10 million times less frequent than supernovae – but they represent a fascinating piece of the black hole puzzle. Supermassive black holes are millions to billions of times the mass of the Sun, and we don't have a clear idea of how they grow. ENTs represent one mechanism whereby these giant objects can pack on mass. "ENTs provide a valuable new tool for studying massive black holes in distant galaxies. Because they're so bright, we can see them across vast cosmic distances – and in astronomy, looking far away means looking back in time," says astrophysicist Benjamin Shappee of IfA. "By observing these prolonged flares, we gain insights into black hole growth during a key era known as cosmic noon, when the universe was half its current age [and] when galaxies were happening places – forming stars and feeding their supermassive black holes 10 times more vigorously than they do today." The research has been published in Science Advances. Titan's Atmosphere 'Wobbles Like a Gyroscope' – And No One Knows Why A 'Crazy Idea' About Pluto Was Just Confirmed in a Scientific First A Giant Mouth Has Opened on The Sun And Even It Looks Surprised
