UI physicists part of effort to locate Higgs boson particle
In the world of top quarks and leptons, this was big news.
Researchers announced in early December that they may be close to finding the elusive Higgs boson, a crucial missing piece of the standard model for explaining why matter has mass and how the universe's smallest components interact. And UI physicists will play a role in verifying its existence in the coming year.
"It would be very significant. It would explain, we think, the masses of all the fundamental particles of the universe," said UI physicist Tony Liss.
Liss is one of more than a dozen UI researchers involved in the experiments at the European Organization for Nuclear Research's Large Hadron Collider, a 17-mile-long accelerator built under the Swiss-French border. He and UI physicists Steve and Debbie Errede and Mark Neubauer are part of the ATLAS detector project, one of two teams of scientists looking for the Higgs boson.
The work involves smashing protons together at incredible speeds so they spit out even smaller particles, like quarks and, scientists hope, the Higgs boson, to help them better understand the nature of those forces.
"Physicists talk about this like it's second nature. It's very hard to picture," Liss said.
To review: Atoms are made of protons, neutrons and electrons. Protons and neutrons are made of quarks, of which there are six types. The "top quark" was discovered, with help from Liss and other UI physicists, at Fermilab in Batavia in 1995. Leptons include electrons and their heavier cousins, muons, as well as neutrinos.
The elusive Higgs boson is an elementary particle thought to be responsible for giving other subatomic particles their mass (which combines with energy to give an object weight).
Atoms and protons get their mass from the energy that holds them together: remember "E=mc2"? But more fundamental particles, such as quarks and leptons, get their mass from somewhere else, and the Standard Model of particle physics needs the Higgs boson to explain it, Liss said.
The Standard Model is a collection of several theories describing "everything we've observed for the last 50 years, basically," Liss said.
It predicted the existence of certain forces before they were found, such as quarks, and "so far it's been pretty accurate," Liss said. "We need this one last piece."
Scientists announced in early December they had defined a range of likely masses for the Higgs boson (named for British physicist Peter Higgs) but had not found conclusive proof of its existence.
That's because they need more definitive statistical results, Liss and Neubauer said. Physicists search for the particles on "a big background of noise" that can mimic the signal from the particles themselves, Liss said.
"When we're searching for particles like this, there are certain standards for the level of statistical evidence before anybody will say we're sure we've found it," he said. "There are some things about these particular observations that kind of smell right. They were being very cautious. I'll be a little less cautious."
Neubauer, who is more directly involved in analyzing the data on the Higgs boson, called the results "very promising."
"We don't see a signal large enough over the background to claim a discovery," said Neubauer, a fellow with UI's National Center for Supercomputing Applications.
What intrigued him was that both the Atlas group and the other team of scientists, the CMS group, found a hint of a signal at the same mass, 125 Gev, or billion electrovolts, and not anywhere else.
The mass of the Higgs boson is not known, so scientists have to search all masses within the range predicted by the Standard Model, anywhere from 100 to 1,000 GeV, he said. Subsequent experiments suggested the Higgs boson should be at the low end, between 100 and 130 GeV, so the recent findings fit, he said.
"If I had seen either of the experiments alone, it's not that convincing," Neubauer said. "I think put together, the fact that the hint exists at almost the same Higgs boson mass is a fact that I find hard to be a coincidence."
Scientists will spend much of 2012 repeating the experiments to verify the Higgs boson's existence — or prove that it doesn't exist.
"What we really need is more data, more collisions," Neubauer said. "The data we're going to be collecting over 2012 will be definitive."
If it turns out to be true, scientists will then determine whether the particle they've found is the Higgs boson predicted by the Standard Model, or a new particle, in which case there may be "new physics to be discovered," he said. Other physics models predict as many as five Higgs bosons, he said.
A Higgs boson decays almost as soon as its produced. It can decay into two photons, or particles of light; or into two "electroweak" particles, known as Z bosons or W bosons.
The Higgs boson signal was found in the first two scenarios, but not in the "W" mode, which is what Neubauer's group studies.
Neubauer recently spearheaded a project with senior research physicist David Lesny to build a "Tier 2" computing cluster at the UI as part of the Worldwide Large Hadron Collider Computing Grid. Together the Tier 1 and Tier 2 centers around the world process the data produced by the collision experiments. The UI's will begin gearing up in January or February and contribute to the Higgs boson analysis, he said.
In all, four UI professors, eight graduate students and three postdoctoral researchers are involved in the collider experiments.
"The accelerator produces an incredibly rich amount of data. The search for the Higgs boson is just one important part of that," said Liss, who plans to return in January.
Liss' team is looking for some "non-Standard Model behavior" of top quarks, specifically signs that they are decaying. Under the Standard Model, the quark is not supposed to decay, he said.
"If you see something like this, it automatically means there's some new physics going on," Liss said.
The Higgs boson will open up new research avenues.
"One of the things about the top quark that's so interesting about it is that it's very, very heavy. If it gets its mass from the Higgs boson, there should be a very strong interaction between the top quark and the Higgs boson that we'd be able to measure," Liss said. "It's a very difficult thing to do. But once we know what the mass of the Higgs boson is" it will be much easier, he said.