A possible crack in the standard model of particle physics seems to be shrinking, as new data from CERN’s Large Hadron Collider (LHC) contradicts a previous puzzling result that had physicists excited about the possibility of new, exotic physics – but some mysteries remain.
“The standard model survives for the moment,” Josh Bendavid at the Massachusetts Institute of Technology told a packed seminar room at CERN, the particle physics laboratory near Geneva, Switzerland, on 17 September. He was presenting new data on the mass of the W boson, a fundamental particle that is crucial for processes like nuclear decay and setting the mass of the Higgs boson.
Questions about the W boson mass began in 2022, when physicists working with data from the Tevatron collider at Fermilab in Illinois sent shockwaves through the particle physics community. Their value for the W boson mass was starkly different from that predicted by the standard model, our best picture of how the universe’s particles and forces interact, suggesting physicists may have missed something.
But in 2023, researchers at CERN cast doubt on this discrepancy, after they reanalysed old data taken by the ATLAS detector at the LHC. They found a value for the W boson mass that once again agreed with the standard model prediction, dampening hopes for a deviation from known physics.
Now, Bendavid and his colleagues have produced a new value for the W boson mass, using new data from another of the LHC’s detectors, the Compact Muon Solenoid (CMS), and found a value of 80,353 million electronvolts (MeV) which, with an uncertainty of 6 MeV, agrees with the standard model. The tiny uncertainty also makes this the most precise measurement produced at the LHC, said Bendavid.
Ashutosh Kotwal at Duke University in North Carolina, who led the scientific collaboration that produced the Tevatron result, says that it is great to have another measurement of the W boson mass, but as the LHC and Tevatron colliders use different methods to produce the particle, it is harder to compare the results.
“In this fundamental respect of the beams, ATLAS and CMS are identical,” says Kotwal. “What would have been ideal is additional or independent data at the Tevatron.” Unfortunately, the Tevatron shut down in 2011, so there will be no more new data.
All of this means it is too early to tell which W boson mass measurement is correct and that the differences must still be explained. “It doesn’t end with two numbers on the table, it’s the beginning,” says Kotwal. “It’s when we start discussing scientific and technical details about procedures. The truth is out there, there is a W boson mass in the universe. We’re all trying to find it.”
But the term “digital twin” actually came from a NASA employee named John Vickers, who first used it in 2010 as part of a technology road map report for the space agency. Today, perhaps unsurprisingly, Grieves is head of the Digital Twins Institute, and Vickers is still with NASA, as its principal technologist.
Since those early days, technology has advanced, as it is wont to do. The Internet of Things has proliferated, hooking real-world sensors stuck to physical objects into the ethereal internet. Today, those devices number more than 15 billion, compared with mere millions in 2010. Computing power has continued to increase, and the cloud—more popular and powerful than it was in the previous decade—allows the makers of digital twins to scale their models up or down, or create more clones for experimentation, without investing in obscene amounts of hardware. Now, too, digital twins can incorporate artificial intelligence and machine learning to help make sense of the deluge of data points pouring in every second.
Out of those ingredients, Raytheon decided to build its JWST twin for the same reason it also works on defense twins: there was little room for error. “This was a no-fail mission,” says Casey. The twin tracks 800 million data points about its real-world sibling every day, using all those 0s and 1s to create a real-time video that’s easier for humans to monitor than many columns of numbers.
The JWST team uses the twin to monitor the observatory and also to predict the effects of changes like software updates. When testing these, engineers use an offline copy of the twin, upload hypothetical changes, and then watch what happens next. The group also uses an offline version to train operators and to troubleshoot IRL issues—the nature of which Casey declines to identify. “We call them anomalies,” she says.
Science, defense, and beyond
JWST’s digital twin is not the first space-science instrument to have a simulated sibling. A digital twin of the Curiosity rover helped NASA solve the robot’s heat issues. At CERN, the European particle accelerator, digital twins help with detector development and more mundane tasks like monitoring cranes and ventilation systems. The European Space Agency wants to use Earth observation data to create a digital twin of the planet itself.
At the Gran Telescopio Canarias, the world’s largest single-mirror telescope, the scientific team started building a twin about two years ago—before they’d even heard the term. Back then, Luis Rodríguez, head of engineering, came to Romano Corradi, the observatory’s director. “He said that we should start to interconnect things,” says Corradi. They could snag principles from industry, suggested Rodríguez, where machines regularly communicate with each other and with computers, monitor their own states, and automate responses to those states.
The team started adding sensors that relayed information about the telescope and its environment. Understanding the environmental conditions around an observatory is “fundamental in order to operate a telescope,” says Corradi. Is it going to rain, for instance, and how is temperature affecting the scope’s focus?
After they had the sensors feeding data online, they created a 3D model of the telescope that rendered those facts visually. “The advantage is very clear for the workers,” says Rodríguez, referring to those operating the telescope. “It’s more easy to manage the telescope. The telescope in the past was really, really hard because it’s very complex.”
Delicate might not be the first word that springs to mind when you think of the Milky Way. But when Mariangela Lisanti started tinkering with the recipe for our galaxy, she found it surprisingly fragile.
Lisanti, a particle physicist at Princeton University, was simulating what would happen if dark matter – the mysterious stuff thought to account for over 80 per cent of all the matter in the universe – was more exotic than researchers typically assume. She swapped a small fraction of standard dark matter with something more complex. “We thought, we’re only adding 5 per cent, everything will be fine,” she says. “And then we just broke the galaxy.”
There is good reason for such meddling. Since the 1980s, astronomical signs have pointed towards dark matter being a single type of slow-moving particle that doesn’t interact with itself. Particle physicists have gone to great lengths to search for that particle. But decades later, it remains a no-show – perhaps because dark matter isn’t how we have tended to imagine it.
Recently, a series of galactic anomalies has sparked a scramble to explore alternatives. This “complex” dark matter might be as simple as sub-atomic particles that bounce off each other, or as complicated as families of dark particles that form dark atoms, stars and even galaxies. There is a daunting variety of possibilities.
But now, observations of anomalies in our galaxy finally promise to help us narrow down the options. And with…
Delicate might not be the first word that springs to mind when you think of the Milky Way. But when Mariangela Lisanti started tinkering with the recipe for our galaxy, she found it surprisingly fragile.
Lisanti, a particle physicist at Princeton University, was simulating what would happen if dark matter – the mysterious stuff thought to account for over 80 per cent of all the matter in the universe – was more exotic than researchers typically assume. She swapped a small fraction of standard dark matter with something more complex. “We thought, we’re only adding 5 per cent, everything will be fine,” she says. “And then we just broke the galaxy.”
There is good reason for such meddling. Since the 1980s, astronomical signs have pointed towards dark matter being a single type of slow-moving particle that doesn’t interact with itself. Particle physicists have gone to great lengths to search for that particle. But decades later, it remains a no-show – perhaps because dark matter isn’t how we have tended to imagine it.
Recently, a series of galactic anomalies has sparked a scramble to explore alternatives. This “complex” dark matter might be as simple as sub-atomic particles that bounce off each other, or as complicated as families of dark particles that form dark atoms, stars and even galaxies. There is a daunting variety of possibilities.
But now, observations of anomalies in our galaxy finally promise to help us narrow down the options. And with…
Peter Higgs at the Science Museum in London in 2013
Photo by Andy Rain/EPA/Shutterstock
Peter Higgs lived a singular life. He developed a physics theory that stood a chance of radically advancing our understanding of the universe, and lived to see generations of experimentalists chase after and eventually triumphantly corroborate his work in the lab. He died in his home at age 94.
“Without Higgs’s work we wouldn’t understand why there are atoms. Some pretty basic features of our world would not be understandable,” says John Ellis at King’s College London.
Higgs started that work at the University of Edinburgh in the UK in the 1960s. He was thinking about a branch of physics called quantum field theory, and in July of 1964, he took about a week to write a short paper on the topic. Physics Letters accepted the study but rejected Higgs’s more detailed follow-up work just a week later. Even though Physical Review Letters eventually published a revised version of the second paper, it received no fanfare and remained overlooked for years.
Ironically, these papers contained a key ingredient that was sorely lacking from the theory of all particles in the universe: the reason why they have mass.
Almost all known particles need some mass in order to bind to each other and form the structures, like atoms, that comprise our physical world. But physicists understand all particles as excitations of invisible fields that permeate everything – electrons, for example, are excitations of the electromagnetic field – and even the best theories at the time could not explain where these masses come from.
Higgs theorised that particles would acquire mass by interacting with a new type of field. That field had a very special excitation of its own, another particle called the Higgs boson. The Higgs field solved a huge question in theoretical particle physics, and the Higgs boson was a tantalising target that experimentalists could hunt for in order to tie theory to reality.
“If you remove everything from the vacuum, all matter or quantum fluctuations, all electromagnetic stuff, all gravity, you will be left with the Higgs field,” says Frank Close at the University of Oxford. “And we need that just like a goldfish needs water. It stabilises empty space.”
Working independently from Higgs, Belgian physicists Francois Englert and Robert Brout reached the same conclusion, also in 1964.
However, according to Close, who wrote a biography of Higgs in 2022, Higgs did not necessarily set out to write a groundbreaking paper. He simply followed a line of rigorous and often solitary scholarship, which led him to worry deeply about what seemed to be a technical issue that plagued quantum field theory. Other physicists had previously resolved a similar issue in systems with less cosmic implications, such as perfect conductors of electricity. Higgs figured out how to generalise their mathematics to all of particle physics.
But quantum field theory was unfashionable at the time, and when he lectured about his work at prestigious institutions like Harvard University in 1965, Higgs was largely met with scepticism, says Ellis. In 1976, Ellis and two of his colleagues at the CERN particle physics laboratory in Geneva, Switzerland, wrote a paper drawing attention to how the Higgs boson could show up in some experiments at the facility.
“No one really seemed to care, but to us, [the Higgs boson] was extremely important… And I was absolutely sure that the Higgs boson will be found,” says Dimitri Nanopoulos at Texas A&M University, who coauthored the paper. He was a very young researcher at the time, but that study was prescient about the future of particle physics. By 1984, views among physicists had shifted, and they were eager to hunt for the Higgs boson. Leadership at CERN discussed building a new particle collider, in large part to help with the search.
That detector – the Large Hadron Collider (LHC) – found the Higgs boson in 2012. Within the LHC, researchers engineered a careful head-on collision of two incredibly fast protons, a crash capable of producing a Higgs boson. But the boson only lasts for less than a billionth of a billionth of a second before becoming a shower of other particles. Analysis of the collision’s wreckage showed those particles had come from a Higgs boson with such high certainty that the odds of it being a fluke were just 5 in 10 million.
Physicists around the world were rapturous, and Higgs and Englert shared a Nobel prize in physics the next year.
Close and Ellis both say that even before the LHC started to operate, other colliders had obtained less direct evidence vindicating Higgs’s theory, such as very precise measurements of masses of other exotic particles. Higgs was aware of these findings, as he explained to New Scientist in 2012: “I had faith in the theory behind the mechanism as other features of it were being verified in great detail at successive colliders. It would have been very surprising if the remaining piece of the jigsaw wasn’t there.”
Still, the direct search for the Higgs boson at the LHC had a strong influence on particle physics. It bolstered efforts to build new infrastructure like accelerators, and cemented the large collaborations that manage this equipment as a standard approach for conducting scientific research.
Since 2012, the LHC has been upgraded to produce even more energetic collisions, and researchers have set out to answer lingering questions about not only particles, including the Higgs boson itself, but also dark energy and dark matter, the unexplained phenomena that make up most of the universe.
Higgs himself was interested in some of those questions and kept working on them even after he retired in 1996. “The machine at Geneva – which was not designed just to discover the Higgs boson, though sometimes you get that impression – is expected to go on and improve our understanding of the links between particle physics and what happened in the early universe,” he told New Scientist in 2013.
Finding the Higgs boson was the end of one chapter, but not the whole book, says Nanopoulos.
After his retirement, Higgs kept working on his own research. He was particularly interested in supersymmetry, which is a theory that posits the existence of heavy counterparts for every particle that we have detected already. Physicists who share this interest and want to find its experimental signatures hope that the LHC will discover dozens of new particles.
In addition to the Nobel prize, Higgs received several other accolades, including the Paul Dirac Medal and Prize, the Wolf Prize in Physics and the American Physical Society J. J. Sakurai Prize. In 1999, he turned down a knighthood, an act that fit his general rejection of fame. He did not want titles and was embarrassed by the media attention his work garnered over the years, particularly disliking the Higgs boson’s sensational nickname, the “God particle”.
The story of how Higgs even tried to evade the call from the Royal Swedish Academy of Sciences informing him of his Nobel win – by leaving his home without a cell phone – is well-known lore among physicists. Ellis also recalls that Higgs initially turned down the invitation to come to CERN for the official announcement of the discovery of his eponymous boson. But colleagues eventually convinced him to attend the festivities.
Close titled his biography of Higgs “Elusive”, which he says described both the man and the boson. Physicists widely agree that he was one of a kind and respected him for it.
Higgs died in his home in Edinburgh on 8 April after a short illness. He leaves behind two sons, a reinvigorated field of particle-seeking physicists and a clearer understanding of the forces that hold the universe together.
