Two teams of researchers say they have found a new, fundamental subatomic particle with a handful of traits that are consistent with those predicted for a long-sought Higgs boson – a particle linked to the mechanism that gives mass to other fundamental particles.
The results, presented Wednesday, are preliminary. Still, the data the teams presented drew thunderous applause and a standing ovation from physicists packed into a lecture hall at the European Organization for Nuclear Research in Geneva, where the results were presented.
In December, the same teams unveiled results that hinted they'd found a particle. But this time each team, using independent ways of hunting for Higgs bosons, found their results matched for a few key measurements, each set carrying literally a 1 in a million chance of being wrong.
"As a layman, I'd say we have it," said CERN Director-General Rolf Heuer. As a scientist, however, he added that researchers now have to figure out what "it" is.
The particle's mass, for instance, falls within the range predicted for a Higgs boson that would fit into the so-called standard model – a description physicists have painstakingly built of the fundamental particles that make up matter and the forces that govern their interactions.
But some theories predict more than one Higgs boson. And it's possible an unpredicted Higgs wannabe could display some of the predicted properties, a discovery that would drive theorists back to their white boards.
"One of the most exciting aspects of this observation is that the road remains open for a vast range of 'look-alike' alternatives, where any deviation from the Standard Model would point the way to the existence of other new particles or forces of nature," said Harvey Newman, a physicist at the California Institute of Technology in a prepared statement.
However, key pieces of information that would help distinguish among the possibilities are either missing or incomplete, the researchers acknowledge.
Whatever the outcome, it's clear that the researchers involved and the theorists who predicted the existence of the Higgs bosons in the mid-1960s have entered Nobel Prize territory, to say nothing of new frontiers for exploring the nature of the universe.
"We're reaching into the fabric of the universe at a level we've never [reached] before," says Joe Icandela, a physicist at the University of California at Santa Barbara and the spokesman for one of the two major experiments hunting for the Higgs boson at CERN. "This is not like other ordinary particles. It's a key to the structure of the universe."
If the discovery the teams announced turn out to be the standard-model Higgs, it means "we've completed one part of the story, and we're on the frontier now."
Call it the energy frontier – in this case, energies that approximate levels thought to have existed in the first millionths of a millionth of a second after the big bang, which gave birth to the universe some 13.6 billion years ago.
The teams conducted their experiments using the Large Hadron Collider at CERN, an underground particle accelerator nearly 17 miles in circumference. This demolition-derby track for subatomic particles straddles the French-Swiss border outside of Geneva. It's designed to collide protons with energies comparable to those that existed in the infant universe.
At those energy levels, a standard-model Higgs boson would have existed on its own. But as the universe cooled, the Higgs bosons present at the time would have grown unstable and decayed.
The standard model holds up very well at the low energies in today's universe and for most particle interactions, says University of Wisconsin physicist Wesley Smith, a member of one of the two teams. But when physicists try to crunch the numbers for the standard model at higher energy levels, they get pencil-snapping results – infinities.
That means that at higher energy levels, the standard model "must be wrong," he says.
One solution theorists have proposed to eliminate the nasty infinities is the existence of what Dr. Smith calls much more massive "shadow world" counterparts to the electrons, quarks, and other fundamental particles that standard model describes. These so-called supersymmetry theories in effect cancel out the infinities, making everyone happy.
But no one has found any supersymmetric particles.
"If the Higgs turns out to have some unusual properties, it might indicate that maybe it's not quite the standard-model Higgs, but it's a Higgs you might expect in a supersymmetric model," he says.
In addition to exploring the potential for new physics beyond the standard model, today's discovery – if it's a Higgs particle – would be the first detection of what physicists call a scalar boson.
Bosons are so-called force carriers, associated with the four forces of nature – electromagnetism, the strong force (which binds particles in an atom's nucleus), the weak force (which governs radioactive decay), and gravity. No particle has yet been found for gravity, although one has been proposed: the graviton. The other three – photons, gluons, and the W and Z bosons – have a property known as spin. The graviton is predicted to have spin as well. The Higgs boson is predicted to be the only spinless force carrier.
The Large Hadron Collider (LHC) accelerates two beams of protons traveling in opposite directions until the beams reach the desired collision-energy level. By then, they are traveling within a whisker of the speed of light. Magnets steer the beams through the underground beam lines, then focus them into hair-thin lines in anticipation of looming collisions.
These collisions take place in the hearts of two massive detectors – one for each team, and each with a unique design. This way, if both teams see the same results using independent approaches, both will have more confidence in the outcome.
The energy at the collision point crates new particles, which quickly decay into other particles that the collider's two massive detectors track. By analyzing the types of decay particles, their energies, and their paths through the detectors, researchers can determine the nature of the particle they fleetingly created.
One surprise for the teams came in discovering the particle when the LHC was running at slightly more than half its rated collision energy of 14 trillion electron-volts. The detections were made possible by the sheer number of protons in the beams during the accelerator's 2011 and especially its 2012 runs. Five hundred trillion protons were sacrificed in the production of these results. They yielded only a few tens or dozens of Higgs-like events, says Dr. Icandela.
And every one of those sacrificial protons came from a single, one-liter bottle of hydrogen gas, Dr. Smith adds.
Both teams give essentially the same mass estimate for the Higgs-like particle. Researchers with the CMS detector put the particle's mass at 125.3 billion electron-volts. The group associated with the second detector, dubbed ATLAS, put it at 126.5 billion electron-volts – essentially the same result, given the uncertainties in the data. In addition, both found that when the Higgs-like particles they created decayed, the decay produced at least two of the five combinations of decay particles that theories predict for a standard-model Higgs. Those decay "channels" also turned out to be the best ones to use to reconstruct the Higgs-like particle's mass.
The results – and answer to some of the remaining questions about the particle – should become clearer over the next several months. On Tuesday, CERN officials decided to run the collider for another three months, instead of shutting it down for a long-planned maintenance and upgrade outage. When the collider resumes operation at the end of the outage, engineers say it will be ready to run at its full collision energy.