At Particle Lab, a Tantalizing Glimpse Has Physicists Holding Their Breaths

http://www.cnn.com/2011/US/04/08/particle.physics.tevatron/index.html?hpt=T2

CNN...

Is It a New Particle, or Just a Fluke?

By Elizabeth Landau, CNN
April 8, 2011 2:36 p.m. EDT
Click on picture to play video
Discovery of 'new source of energy'?

STORY HIGHLIGHTS

Scientists say they may have found evidence of a particle never observed before
This possible discovery happened at Tevatron at Fermilab in Illinois
There's a 1 in 1,000 chance that it's just a fluke of statistics
Tevatron is scheduled to be shut down at the end of September

(CNN) -- In the search for answers to some of the most mysterious and fundamental questions about the the universe, Europe's $10 billion particle-smashing Large Hadron Collider has been hogging the spotlight in recent years.

Suddenly, this week, physics enthusiasts' eyes turned to Tevatron, a much smaller and less powerful particle accelerator in Batavia, Illinois, that is scheduled to be shut down for good after September. And, depending on what happens with the budget crisis on Capitol Hill, it could be even sooner.

At Tevatron, part of the Fermi National Accelerator Laboratory (Fermilab), scientists said they may have found evidence of a particle never observed before. That would mean a brand new building-block of matter would be added to what physicists know about the universe.

But the keyword is "may" -- there's a 1 in 1,000 chance that it's just a fluke of statistics. In the coming weeks and months, additional data from Tevatron's detectors and the Large Hadron Collider will probably deliver a more definitive answer about whether indeed a new particle has been discovered.

"If it is true, then it's of course very important. But there's a big if," said Csaba Csaki, associate professor of physics at Cornell University, who was not involved in the experiment.

What they're looking for

Research at particle accelerators often addresses big questions that get at the very nature of existence: Why do we -- and everything around us -- have mass?

One theory that has gained a lot of attention is that there is a particle called the Higgs boson that has this power of giving particles mass. For that reason it has been deemed the "God particle" in popular culture.

The possible discovery at Tevatron is not a Higgs-like object, said Rob Roser, a scientist at Fermilab who works with Tevatron's CDF particle detector experiment, where the new data came from.

But that doesn't rule out the potential new particle's involvement in explaining mass, experts said. Physicists Estia Eichten, Kenneth Lane, and Adam Martin said the Tevatron findings may be evidence of a "technicolor" theory involving a brand new force leading to mass. The forces we know about are gravity, electromagnetism and the strong and weak nuclear forces -- "technicolor" would add a fifth.

That is just one explanation, though. More research needs to be done to see if this strange signal at Tevatron comes from a particle with such significance, Roser said.

"It could just be a new particle. It doesn't have to be the end-all of what we know," he said.

How this possible discovery happened

How does Roser's group of some 500 scientists know what to look for? You'll need some funky technical concepts to proceed, so hold on tight:

Tevatron smashes protons (positively-charged components of atoms) and anti-protons (negatively-charged components of anti-matter) into each other at massive energies, creating collisions that generate subatomic particles. Two detectors record what happens at these collisions, so that scientists can analyze the effects.

The weirdness in the data announced this week has to do with things called W bosons and jets. W bosons carry the "weak force," involved in radioactive decay and nuclear fusion. Jets are sprays of other particles, and occur when a quark (another subatomic particle) emerges from a collision. Scientists looked at collision events that produced one W boson and two jets.

Surprisingly, there was a "bump" in the data when they looked at the energies and momenta of these two jets in many instances. It appeared that a brand new particle must have produced them, based on its expected mass. But in order for the bump to be considered a discovery, the data must show no more than about a one in a million chance that it's a fluke. Current data show a 1 in 1,000 chance of being a statistical anomaly.

Fermilab scientists also recently announced a possible discovery of something called "forward-backward top quark asymmetry." That doesn't sound as sexy as "new particle potentially discovered," but physicists are enthusiastic about it, too. These two observations could actually be related, Roser said.

"I'm very excited because we rarely get any hint of possible new physics," Csaki said.

The end of Tevatron, the rise of the Large Hadron Collider

All of this could turn out to be a swan song for Tevatron, which is scheduled to have its last collisions in September. There were talks last year of extending its run until 2014, but that was turned down because of lack of funds. And there's no set budget for 2011, so the current crisis in Washington could result in a speedier shutdown, Roser said.

Still, there's enough data from Tevatron to keep scientists at Fermilab busy for awhile after the accelerator has shut down. At least an additional year is needed to get the physics out of the data set, Roser said.

"This result is not some sort of play to extend our running -- we're resigned to finishing our program properly," Roser said.

Tevatron, built in the 1980s, has the ability to smash particles at energies up to 1 TeV. It was the world's highest-energy particle accelerator until the Large Hadron Collider, underground on the border between France and Switzerland, which began colliding beams in November 2009. Energies at the European machine have gotten up to 3.5 TeV, and will reach the designed goal of 7 TeV in 2014, according to the European Organization for Nuclear Research.

Although the Large Hadron Collider is much more powerful than Tevatron, the U.S. accelerator has the advantage of many more years of data collection, Csaki said. Roser's team's results came from eight years of data, recording trillions of collisions. The European accelerator will be able to collect more data a lot faster, however.

And that power offers hope for thousands of physicists whose theoretical work has yet to be validated.

"There's a generation of physicists like me who grew up without any real new discoveries in particle physics," Csaki said. "We think that the things we have been waiting for for the last two decades likely would happen in the next year or two."

All that ever needs to be said.

Approve....most interesting

http://www.scientificamerican.com/article.cfm?id=fermilab-new-particle&WT.mc...

Scientific American...

U.S. Collider Offers Physicists a Glimpse of a Possible New Particle

The soon-to-be-retired Tevatron collider has uncovered an unexplained signal that could be a previously unknown particle

By John Matson | April 7, 2011

Photo: Physicist Viviana Cavaliere of UIUC

A HINT OF NEW PHYSICS? Viviana Cavaliere of the Tevatron's CDF collaboration explains her group's new result in a talk April 6 at Fermilab. Image: FNAL

Physicists sifting through data generated by the Tevatron particle collider in Illinois have uncovered a signal that neither they nor the long-standing Standard Model of particle physics can explain.

The international team of researchers work with data from CDF, one of the two Tevatron detectors where protons and their antimatter counterparts collide at nearly light speed. The wreckage of those high-energy collisions produces a variety of short-lived particles, which allows physicists a fleeting glimpse into the inner workings of the physical world. The Tevatron, at Fermi National Accelerator Laboratory, is the second-most powerful particle collider in the world after the Large Hadron Collider outside Geneva, Switzerland.

Examining a very specific kind of outcome when protons and antiprotons collide inside the CDF detector, the researchers noticed an unexplained blip in their signal that could be explained by a previously undiscovered elementary particle—but not the Higgs boson, the hotly pursued particle that is theorized to imbue other particles with mass.

The researchers reported their perplexing but unconfirmed new finding in a study posted online April 4 at the physics preprint Web site arXiv.org The researchers have also submitted their results for publication in Physical Review Letters.

The CDF team found that the Tevatron was a bit more prolific than it should be in terms of collisions that yield a heavy elementary particle known as the W boson plus a pair of particulate jets. "What we see is that there a region between 120 and 160 GeV (giga-electron volts) where there is an excess," CDF physicist Viviana Cavaliere of the University of Illinois at Urbana–Champaign explained to a packed Fermilab auditorium April 6. (A giga-, or a billion, electron volts is a unit of particle mass or energy.)

The result is compatible, Cavaliere said, with the collisions producing a W boson plus a hitherto unknown—and even heavier—particle with a mass of about 150 GeV. But that particle appears not to be the Higgs boson, which would be expected to emerge from the collisions alongside a W boson with far less frequency. If CDF has uncovered a new elementary particle, it would be the first such discovery since the tau neutrino was observed at Fermilab in 2000. But in the case of the tau physicists had predicted the particle's existence and had gone out looking for it.

As theorists scramble to figure out just what CDF has found, experimentalists will be working to verify that the detector has found anything at all. The new analysis claims that the data disagree with existing theory to better than three standard deviations, or 3 sigma. Assuming the analysis is correct, that means that there is just a fraction of a 1 percent chance that the effect is a mere statistical glitch. But extraordinary claims demand stronger proof.

"Five sigma is our gold standard," says Brookhaven National Laboratory physicist Sally Dawson, adding that the physics community has seen 3-sigma effects come and go. "If it's true, and if it holds up, it is of course very exciting, because it's completely unexpected," Dawson says. "If it persists, it's very hard to explain theoretically."

"We will learn pretty soon whether it's true or not," says Fermilab theorist Bogdan Dobrescu, who did not contribute to the new study. "This is pretty credible at this stage." If the results hold up, theorists will need to figure out what kind of new particle could fit the bill. "It would be a major breakthrough, especially because this is a particle that no one really predicted to the best of my knowledge," Dobrescu says. "We can try to invent some new particles and see if they have the appropriate properties that we see, but none of the answers are very expected."

The Tevatron, which is slated to shut down for good in the fall, is still collecting data that could strengthen the case for a new particle—or sink it. Cavaliere said that the new analysis began more than a year ago and does not include the latest data from CDF. The team already has already logged a good deal more collisions that await analysis, but Cavaliere cautioned that the expanded data set would not be enough to vault the discovery into the 5-sigma range.

But the physics community will not have to wait long before the new particle gets a reality check. Physicists working with the other detector at the Tevatron, known as DZero, are now replicating the CDF analysis with their own voluminous data set, says Fermilab physicist and DZero co-spokesperson Dmitri Denisov. "We expect we will be able to clarify this topic on a timescale of a few weeks," he says.