The Large Hadron Collider’s big success leaves no clear avenue for new physics
At 5 a.m. last Fourth of July, Flip Tanedo rolled out of bed after an hour of repeatedly smacking his alarm clock’s snooze button. Rousting himself at dawn would be worth it, he hoped, because what he was about to hear was likely to have a huge bearing on the course of his career.
Tanedo, a fifth-year theoretical physics Ph.D. candidate at Cornell University, tuned in to a live video feed from Geneva and listened intently as physicists working with the world’s largest particle accelerator discussed a momentous discovery. Data from the Large Hadron Collider revealed what looked very much like the long-sought Higgs boson. The product of a decades-long effort by thousands of physicists, the discovery solidified the leading theory of particle physics, the standard model. The Higgs particle confirmed the existence of a field that permeates the universe, imparting certain subatomic particles with mass while letting photons and other massless particles pass unimpeded.
Even from 4,000 miles away, the excitement was palpable. Two hours earlier, when the discovery was formally announced, hundreds of experimentalists who had sifted through the noise of more than a thousand trillion particle collisions to identify the Higgs entered into sustained applause, about as raucous as particle physicists get. British physicist Peter Higgs, who in 1964 proposed the particle that now bears his name, removed his glasses and wiped away tears.
While Tanedo shared the enthusiasm of his colleagues on the screen, he also had an unsettled feeling. As a theorist his job is to speculate on the inner workings of the universe. Theorists love proposing the existence of new particles and forces, but their theories must be consistent with the findings of past experiments. That makes deviations from the expected like catnip to theorists — opportunities to come up with novel explanations.
But with every new speaker in Geneva, it gradually became clear that there was nothing particularly surprising about this newest addition to the particle zoo. The experimental work seemed to fit perfectly with existing theory. “It wasn’t until a few hours after the talk that I started thinking, ‘OK, what’s next for us?’ ” Tanedo says.
That is the question many theoretical physicists are asking themselves right now. A year after the announcement, the latest analyses confirm a Higgs boson that is as vanilla as Tanedo initially feared.
In confirming the crowning theoretical achievement of 20th century physics, the LHC did exactly what it was designed to do. But Tanedo and other theorists had clung to loftier goals. Although the standard model explains extraordinarily well the particles and forces that dictate much of the world around us, it ignores gravity, and it doesn’t meld with Einstein’s theory of general relativity. And there’s much that the standard model can’t address at all: the dark matter that clumps around, within and between galaxies, for example, and the dark energy that is increasingly stretching the universe apart. In essence, even a complete standard model is incomplete, describing stuff that collectively composes a mere 5 percent of the mass-energy content of the universe. The rest is a mystery.
Scientists had hoped that clues to that mystery — or at least hints about how to start solving it — might emerge from the debris of smashed protons at the LHC. Some expected the machine to detect particles of dark matter; others thought it might find evidence of extra dimensions or of supersymmetry, a popular theory that predicts a menagerie of heavy particles. Ideally, discovering the unexpected within the subatomic shrapnel would allow theoretical physicists to expand the standard model into a stronger theory that more fully explains how the universe works.
Yet as the LHC shuts down for two years of repairs after three years of collisions, it has yet to reveal a single surprise. Adding insult to injury, other intensive physics experiments over the last year have also failed to reveal anything truly exotic. Nature’s secrets, at least for the time being, are frustratingly out of reach. Physicists are now banking on revamped theories and a few peculiar clues that have popped up in a handful of experiments to advance the standard model. “It’s gradually become more and more sobering,” Tanedo says.
All figured out
This is not the first time that the horizons for theoretical advancement in physics have looked a bit hazy and distant. According to physics lore, in 1900 the British physicist Lord Kelvin said: “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.” While there’s debate over whether he actually said it, physicists often use the quote when talking about a difficult juncture in the field.
Kelvin may have felt that between Newton’s laws of gravity and James Maxwell’s equations for electricity and magnetism, scientists pretty much had the laws of physics figured out. Kelvin did, however, mention two small “clouds” — strange phenomena that did not quite mesh with those seemingly ironclad physical laws. One was that some objects seemed to radiate energy in an unexpected way; the other was the unknown composition of the ether, the mysterious substance that was thought to permit light to travel through the universe. Within a few decades, theories explaining those clouds — quantum mechanics and Einstein’s theories of relativity — completely transformed physics and superseded previous, supposedly robust, laws.
Before the LHC collisions began in November 2009, physicists were not quite ready to compare their plight with that of Kelvin — in large part because they still hadn’t seen the Higgs.
By last year’s big July 4 announcement, the LHC had detected a fraction of the hundreds of thousands of Higgs bosons produced in some thousand trillion proton collisions. “The experimentalists did a great job,” says Tim Tait, a theoretical physicist at the University of California, Irvine. “I didn’t think the LHC would discover the Higgs this early.”
But like Tanedo, Tait saw his excitement begin to fade once he digested the details of the discovery. He had hoped that the Higgs boson would be a little different from what the standard model predicts — perhaps it would be more massive, or maybe it would decay into a strange menagerie of particles. But the early results made it clear that the Higgs boson looked just the way the standard model said it would. It had a mass of 125 gigaelectronvolts, give or take a gigaelectronvolt, and decayed into other particles such as W and Z bosons at predictable rates. The Higgs did seem to decay into an unexpectedly high number of photons, but that measurement was far too preliminary to draw any definitive conclusions.
The physicists at the July 4 presentation played it safe, calling the new find a Higgs-like particle, but even they knew they had snagged a Higgs boson with all the characteristics predicted by the standard model. “We were already calling this a Higgs among ourselves,” says Joseph Incandela, spokesman for the CMS detector at the LHC and a physicist at the University of California, Santa Barbara. “With the public we were more conservative.”
Over the past year the evidence has become overwhelming. The CMS detection, which last July stood at the threshold of statistical significance, the five-sigma level of certainty, now sits at about 10 sigma. The mass is a more definitive 125.7 GeV, and the strange photon measurement receded after more analyses.
Furthermore, neither CMS nor the other detectors probing the LHC’s proton collisions have found any hints of new fundamental particles. Combine a bland Higgs particle with the lack of other new findings at the LHC, and suddenly the state of particle physics looks an awful lot like it did during Lord Kelvin’s era. “The extraordinary success of our theories actually makes me kind of uncomfortable,” Tait says.
Supersymmetry a no-show
The long-accepted standard model may be extraordinarily successful, but that is not the case for other theoretical attempts to expand it and resolve its shortcomings. Tanedo has spent much of his time in graduate school working on supersymmetry, a set of theories positing that every fundamental subatomic particle has a heavier sibling called a superpartner.
Some of those massive particles would rarely interact with light or with other particles, making them great candidates for dark matter. Supersymmetry also plugs neatly into string theory, physicists’ best attempt at a theory of everything combining quantum mechanics and general relativity. “Supersymmetry just seems to want to be built into physics,” Tanedo says.
The most compelling supersymmetry theories were proposed in the 1990s, but Tanedo says that the LHC was considered the first machine capable of creating the predicted massive particles (the more energetic the collisions, the more massive the particles you can produce). So, the LHC’s debut seemed the perfect time for an aspiring theorist. Tanedo says physicists openly talked about the LHC finding new particles, extra dimensions and even miniature black holes. “You heard all the promises of a brand new frontier,” Tanedo says. But after three years of LHC collisions, the majority of supersymmetry theories have been thrown in the garbage. Those collisions should have produced at least some superpartner particles, yet none have shown up. “Finding new physics is not as simple as turning on the LHC, as some of us had believed,” Tanedo says. “We still have legitimate hopes and dreams, but people are starting to sweat.”
The only supersymmetric theories that haven’t been entirely ruled out contain a lot of messy work-arounds to jibe with the new LHC results. Theories that predicted four superpartners for the Higgs boson, for example, now predict five. The four-partner theories easily emerge from the mathematical foundations of supersymmetry, while five-partner theories require awkward adjustments that make most theorists uncomfortable. “The results have put a lot of the most glamorous supersymmetry models out of business,” Incandela says. “They’ve forced the theoretical community to really think about what’s going on.”
Physicists associated with the LHC aren’t the only ones worrying. Over the last year, multiple big-budget physics experiments like the Planck satellite (SN: 4/20/13, p. 5) and the Alpha Magnetic Spectrometer (SN: 5/4/13, p. 14) announced results that could have revealed clues about dark energy and dark matter. Each one echoed that of the LHC: Our results are exquisite, they said. The instruments worked as planned. The data provide unprecedented constraints on the standard model.
But no new physics. Such statements were typically followed by some variation of: “Nature is being very stubborn.”
Hope in the dark stuff
The frustration over nature’s stubbornness has Tait looking back to Kelvin for inspiration. Those clouds Kelvin described — seemingly minor, unexplained results in a handful of experiments — eventually led to the two pillars of modern physics. While the LHC has yet to deliver even a wispy cloud of surprise, Tait and other physicists are anxiously following up on a series of unexplained findings that they hope will turn into rain clouds.
Tait has his sights set on tackling the mystery of dark matter. Because dark matter appears to be an actual substance concentrated in and around galaxies, physicists think that it is made up of undiscovered particles. Theorists have proposed various new particles that could make up dark matter, including weakly interacting massive particles (WIMPs) predicted by supersymmetry and low-mass particles called axions.
At the same time, other experiments looking to directly detect dark matter particles have come up with preliminary and often confusing results. In April, physicists announced that silicon crystals in an old iron mine had detected three particles with the characteristics expected for dark matter (SN: 5/18/13, p. 10), but they had a lower mass than most supersymmetry theories would predict.
There is also the strange case of the DAMA experiment, which uses sodium iodide crystals in a cave in Italy to hunt for WIMPs. As early as 2003, DAMA researchers claimed to have definitively detected dark matter; now they say their evidence is overwhelming (SN: 5/10/08, p. 12). Other physicists are skeptical, but Tait says that DAMA’s findings are at least worth investigating. “No one will stand up and say why they don’t believe DAMA,” he says. “Kelvin saw something he didn’t understand and assumed it wasn’t that big of a deal. It may be that DAMA is the thing we don’t understand.”
There are other promising avenues for theorists to ponder, including antimatter and lightweight particles called neutrinos. And there is still the possibility that the LHC will expose clouds of its own. Tait points out that the machine has been running for only three years, and at lower energies than it was originally designed to reach because of concerns over the magnets that steer protons around the ring. “It may be too early to be asking the question of whether we should be seeing new physics by now,” Tait says. “That said, it would have been great to find a new particle.”
The wait for new physics will likely continue at least a few years longer. Once the LHC turns back on in 2015, physicists may gain access to a realm of new, high-mass particles that for now are just out of reach. “I’m guardedly optimistic that in the next run, something will pop up and things will start to come together,” Incandela says. In the meantime, he says he expects theorists to toss out some of their pre-LHC assumptions and formulate new ideas.
As a part of the new generation of theorists, Tanedo may well serve an integral role in devising the framework that will refresh the standard model. Having just earned his Ph.D., he will have to conduct his postdoctoral research without the luxury of new data from the LHC. Nonetheless, he says that the existing clouds, and the possibility of more to come, have him upbeat about the future. “I know some graduates who have taken this with a lot of pessimism,” Tanedo says. “Me, not yet.”
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A. Grant. Cosmic ray detector confirms hints of dark matter. Science News. Vol. 183, May 4, 2013, p. 14. [Go to]
A. Grant. Universe is a teeny bit older than thought. Science News. Vol. 183, April 20, 2013, p. 5. [Go to]