If it talks like a boson and walks like a boson, what is it?
Yesterday, an international team of physicists announced the discovery of a new particle that looks suspiciously like the long-sought Higgs boson.
Respecting the limits of their data, the physicists are not claiming they discovered an elusive particle that may explain why matter has mass. But they had a hard time quelling their enthusiasm after a multi-decade search that involved billions of dollars, thousands of scientists, and the largest machine on the planet:
The mass question matters because the quest to explain every physical process is a core conceit in physics. The messy part is that 95 percent of the universe’s mass cannot be detected.
This is one of many reasons why mass matters, and understanding its ultimate nature would be a GOOD THING. More broadly, physicists have been mussed about mass for decades: They could not figure out why the universe has it.
Back in 1964, theoretical physicist Peter Higgs and others proposed that an undiscovered subatomic particle, later named the Higgs boson, would explain mass. (Bosons are elementary particles that govern interactions between other particles.)
In yesterday’s epochal announcement, scientists from the European Center for Nuclear Research (CERN) threw down a guarded gauntlet: We think we found it! The data from two giant experiments at CERN are “consistent” with the Higgs boson, without being definitive.
The announcement echoes and amplifies a similar one made last December. Although the “yes, but” nature of the news may be frustrating to non-physicists, the current results satisfy the statistical standard for discovering a new particle, but are not conclusive enough to label it a Higgs.
“We still have a lot of work to do to prove whether this is really the Higgs,” says Sau Lan Wu, a professor of physics at the University of Wisconsin-Madison, who heads a large research group that studies simulated particle collisions and analyzes data from the ATLAS detector.
So while the new results are suggestive, no Nobel prizes are going down quite yet. After all, it’s crooked snooker to change the standards of evidence just because billions have been spent looking for one particular boson.
But because finding the Higgs could mark the triumphant conclusion of a decades-long effort to describe the universe according to the “standard model” of physics, it is certain to interest the Nobel clique in Stockholm. Although the particles in the standard model behave can in baffling ways , every one has been detected — save the Higgs.
The mass of the new particle, about 126 billion electron volts, matches nicely with predictions for the Higgs.
Lacking this one particle, the standard model cannot account for mass, even though mass is ineluctable every time we throw a ball, run a nuclear reactor, or drop an anvil on our foot.
Seriously, finding the Higgs boson would substantiate the Higgs field, a hypothetical structure thought to pervade space and create mass as it interacts with various particles.
To find a Higgs
The Large Hadron Collider (LHC) at CERN, 27 kilometers in circumference, contains two parallel tubes holding particles that are guided by super-conducting magnets and forced to collide inside one of four detectors. Building the project took 10 years and cost north of $6 billion. Each proton can carry energies of 4 trillion electron volts as it moves close to the speed of light.
Because mass and energy are convertible according to Einstein’s famous formula (E = MC2), all this energy can be converted into heavy and/or exotic particles like the Higgs. These particles, less stable than a Hollywood star, immediately decay into other particles, which enter massive, high-tech gizmos that measure their paths and energy levels. That data, in turn, allows physicists to induce the nature and identity of the original heavy particle.
Higgs, the missing link in the standard model, was likely prevalent about a billionth of a second after the Big Bang 13-plus billion years ago. The collisions at the LHC mimic these extreme conditions.
“You have to have this huge amount of energy to make Higgs,” says Wesley Smith, a professor of physics at the University of Wisconsin-Madison, “and still the probability of making the Higgs is so small, you are looking for one collision out of 10 trillion. It’s literally like looking for a needle in a haystack.”
Smith, who played a key role in developing a system that winnows the interesting collisions from trillions of boring ones at the CMS, a gargantuan magnetized detector at the LHC, says that although Higgs bosons are thought to be the source of mass, they do not normally exist. “They enter into all sorts of physical effects, but to make them manifest themselves, to come out of the closet, you need this incredible energy.”
Earlier this week, after the LHC had announced yesterday’s impending announcement , other scientists reported that the Tevatron may have glimpsed the Higgs, although the statistics were weaker. Located near Chicago, the Tevatron was eclipsed by the LHC as the world’s most powerful accelerator, and it shut down at the end of 2011.
What is the sound of many brains collaborating?
Finding the Higgs was a primary justification for building CERN’s giant collider, and excitement was clearly building over the last few weeks, as hundreds of physicists feverishly poked results from Atlas and CMS, the two major particle detectors. “It the equivalent of a cliffhanger, the last episode of the season,” Smith told us last week. “Literally, we all want to see what happens with the rest of the data. It’s never as clear as you want it to be, not past the point of having nagging doubts.”
After culling suspicious events from trillions of collisions, the researchers did “a final, combination analysis, to put it all together and ask ourselves, given all the evidence we have, is there a possible signal?” Smith said. “We’ll fold all the possibilities together and come up with one statistically valid combination that tells us what all the data is telling us.”
Indeed, the signal for a new particle reached the necessary “5 sigma” level, indicating that the chance of error was about one in 3.5 million. But that certainty did not extend to the question of whether it was indeed a Higgs.
And in any case, Smith cautions that statistics can be misleading in physics as in politics. “Statistics are not cut and dried; some of the worst arguments in physics happen not about physics but about statistics.”
I’m positive on the negative!
While the search for the Higgs is usually presented as a quest to plug the last hole in a theoretical contraption called the Standard Model, some physical contrarians hope the boffo boson will not turn up or will prove to be significantly different than expectations.
Either eventuality would add zeal to the search for new theories of matter, mass and energy, keeping that puzzled look on faces of physicists for decades to come.
With any luck, the search could birth monikers that, for sheer clunkiness, eclipse even “Higgs boson.”
“It’s possible we could still find unexpected signals that could imply new physics, that the Standard Model is not the full story,” says Wu, of the Atlas collaboration. “But if we can confirm this is the Higgs, it will be the discovery of the decade.”
CERN plans to crank up the LHC soon, aiming to collect data for a definitive announcement on the Higgs by the end of 2012.
Physicists are a skeptical lot, and you can expect them to continue scrutinizing every bit of data. “We have to figure out, is it Higgs for sure, but we don’t have enough statistics to do those checks now,” says Smith. “So the issue becomes not only, is there an anomaly, but if so, does it behave the way the anomaly we are looking for would behave, and can we rule out other anomalies?”
— David J. Tenenbaum
- The standard model of physics ↩
- The missing Higgs ↩
- The Particle Zoo: Handmade subatomic particle plushies, from standard model and beyond! ↩
- History of the universe ↩
- Profile: Peter Higgs ↩
- The Large Hadron Collider (LHC) ↩
- Building the LHC ↩
- The Higgs boson: Not the answer to everything ↩
- How to measure the mass of the universe ↩
- Universe 101 ↩