What gives particles mass? For more than half a century physicists have had this enormous question on their minds. In early July, the answer may have come with the discovery of a particle resembling the theoretical Higgs boson, as found in data collected at CERN's Large Hadron Collider (LHC) near Geneva, Switzerland.
On the day of this announcement, Peter Higgs entered the auditorium at the CERN laboratory. A scientist who helped establish the theory nearly 50 years ago and who lent his name to the mechanism, Higgs is an 83-year-old celebrity of the high-energy physics community. He received a standing ovation for his life's work.
To explain his complex theory, scientists often refer to the Higgs particle as a rock star moving through a room packed with adoring fans.
With this analogy, imagine that Higgs, at the moment before entering the crowd, is floating in space 400,000 years after the Big Bang. Here, particles are traveling unhindered in every direction at nearly the speed of light until they run into a big slowdown—when Higgs the rock star loses his speed in the throng of admirers.
As more scientists and reporters in the room notice him, more gather around Higgs. He gains the increasing weight of a celebrity, while others zip through the room, avoiding the slightest nudge. With Higgs' added mass, he grows bigger and finds a connection to other rock stars in the Higgs field.
The Higgs field gives mass to the quarks and gluons—the tiniest known particles—that bind together to create atoms, matter, life and all the things humans see in the universe. And perhaps things we don't so easily see, like dark matter, dark energy and exotic particles yet to be discovered.
"The more famous he is, the more mass he acquires as he goes through the room. More people notice him," says AAAS fellow Howard Gordon, a physicist at Brookhaven National Laboratory (BNL). "So there is something about the Higgs field that couples stronger to high masses."
For Gordon, the search for the Higgs boson has been an uphill climb full of fits and starts. "Isabelle," the particle collider where Gordon first hoped to search for the Higgs, was partially built at BNL. Then in 1983 Isabelle was cancelled. The second particle accelerator, the Superconducting Super Collider (SSC) slated for Texas, didn't pass the first phase of construction before its inflating budget was cut. The third was the Tevatron at Fermi National Accelerator Laboratory (Fermilab) near Chicago, Illinois. Once the world's most powerful accelerator, it shut down last year.
Filling the void left by the SSC was the LHC, where Gordon helped design and build one of the sensitive particle detectors. CERN fired the first protons around the 17-mile LHC ring in 2008—immediately an electrical short destroyed many of its superconducting magnets in a sudden and tragic malfunction. The following year, the machine was back online and successfully smashing protons together and this year set a record for the highest energy ever achieved in accelerating particles: 8 TeV (or 5.3 trillion times the power of a normal flashlight battery). The near-term goal for the LHC is 14 TeV, still 26 TeV short of the SSC design.
Yet with the revelation of a new Higgslike particle, the future of the LHC—and in turn high-energy particle physics—is yet to be decided.
First, however, Gordon and the other LHC scientists must figure out what exactly they've discovered—a challenge heralding a contagious sense of excitement within the physics community that's hardly been seen since the era in which the Higgs theory was founded.
The end of a marathon
Gordon entered the field in 1970, as the first Higgs experiments were being visualized, while at the same time a new theory to unify all of physics was being formed. Called the Standard Model, this theory added up thousands of other theories and experiments in particle physics to establish a new set of the basic building blocks of matter. Over the decades, it successfully predicted the discoveries of several new particles.
Yet the model had problems. It required, for example, a type of symmetry within the masses of a few fundamental particles involved with electroweak force. In the early 1980s, experiments found an unexpected break in that symmetry. The best explanation for the missing mass so far has been that the Higgs is hiding it. For that and many other reasons, physicists are looking at what lies beyond the Standard Model's explanation of the universe.
"It's astonishing to live through a period where you actually recognize new laws of nature and that they suggest things that we might do in the future," says AAAS fellow Chris Quigg, a theoretical physicist at Fermilab.
Quigg began his career the same year as Gordon and in 1984 he co-authored "Supercollider Physics," a seminal work charting the course for accelerators like the Tevatron, the LHC and the aborted SSC.
As high-energy colliders went online, the technology rapidly evolved, allowing scientists to smash particles together at ever-higher energies, narrowing the range where a Higgs boson could be hiding.
Then last December, scientists at Fermilab unveiled new hints in the Higgs search, as collected in data from the Tevatron's highly sensitive detectors, one of which Gordon helped build. Yet the possibility of the Higgs existing in this range did not reach the level of certainty that is de rigueur for particle physics. Following the scent, the LHC collected an unprecedented amount of data over the coming months for the remaining range.
Finally, after a search involving several generations of scientists, Atlas and CMS, two of the detectors studying collisions at the LHC, recorded a bump in the data that fit the unique signature of the Higgs. The evidence had a margin of error of one in 3.5 million, which means the scientists are 99.99995 percent sure they can reproduce that result. On July 4, they announced this discovery.
The LHC will shut down in February. The planned two-year shutdown is to install upgrades on the accelerator and the detectors and to chart a new course for the supercollider.
AAAS fellow Joe Incandela, a physicist at the University of California, Santa Barbara, and spokesperson for CMS, is today looking ahead to a new realm of scientific understanding beyond this landmark discovery. "We've completed one part of the story," he said after the announcement. "We're at the frontier now. We're on the edge of exploration."
The most powerful machine in history
The scientists and graduate students involved with the LHC experiments—more than 8,000 altogether—are far from finished with the machine. According to Gordon, the current goal for the LHC is to run until about 2030.
Planning committees in Europe and the U.S. are now setting the coordinates for the LHC to go where no scientist has explored before. Until this month, only theories could predict what physics lies beyond the Standard Model. Now these scientists are gathering data that will test those theories.
"While I think it's true that discoveries may not be coming at the rate that they were earlier," says Gordon, recalling the era he entered physics, "we're asking more and more questions as we understand things better."
Dark energy, for instance, wasn't even a part of physics vocabulary 15 years ago. Now scientists know it occupies more than 70 percent of the universe. Today the new Higgs-like particle is likewise raising more questions than experimenters can answer.
"The [Higgs] question has been for a long time the most urgent one in particle physics and we're really close to getting an answer to it," says Quigg. With the discovery of this particle, scientists are now "very hungry to find out exactly what it is, how it behaves, how it decays."
Why doesn't the Higgs mass keep growing? The way this boson couples to itself somehow prevents that. More theories beyond the Standard Model are needed. Is it Supersymmetry? This theory that every particle has a superpartner may help explain a type of Higgs boson yet to be discovered, but so far this is not that particle. The answer to what the particle actually is will come in time with more data.
"We know that the Standard Model isn't the whole story and we'll have to work really hard to find physics beyond the Standard Model," says AAAS fellow Joel Butler, manager of the U.S. CMS program at Fermilab. The Illinois laboratory funnels data delivered online from the CMS detector to a room warm and buzzing with the sound of supercomputers that are grinding this raw information through complex algorithms in order to pluck out any new physics.
In the last two years, the four LHC experiments have captured and analyzed about 800 trillion proton collisions. Several planned upgrades over the years will double the accelerator's energy, boost luminosity—in a sense, the number of protons injected into the collisions—and release a river of data for scientists to analyze for decades to come.
"There's a tremendous program if the Higgs exists," says Butler. "It's really not the end of the story. It's the beginning of the story."