Thursday, July 12, 2012

The Higgs That Got Away, the “Bump” that Wouldn’t and Other Loose Ends at Fermilab

It’s possible that the United States could have discovered the Higgs boson first if the Higgs itself was just a little bit lighter or heavier and America’s star particle accelerator wasn’t shut down last year. Instead, the discovery went to CERN, the European physics consortium based in Geneva, Switzerland after America shut off its Tevatron last September.

 “If we would have been more lucky and the Higgs mass would have been at 115 or 160 GeV instead of 125, we could have discovered it before LHC, or at least produce evidence of it,” said Gregorio Bernardi a physicist at Fermilab.

Two days before CERN’s historic announcement, Fermilab released the last Higgs results from the now defunct Tevatron. They had gotten tantalizingly close to seeing the Higgs, but weren’t quite able to announce they had seen hard evidence for it, much less a discovery. They saw a spike in the data right around 125 GeV, the same place where the LHC saw the Higgs, but nowhere near a significance great enough claim the discovery. It effectively set the stage for CERN’s big announcement two days later.

In order to claim a “discovery” in physics one needs to be able to say that the likelihood of their results being random chance be less than one in about a million. On a bell curve used in statistical analysis, this is five standard deviations away from average, also known as a “sigma.” In order to find “evidence,” a three-sigma result is needed.

“We were just shy of three sigma. The odds [that] this signal is just by chance is in the ballpark of a percent or so,” said Dan Hooper, a researcher at Fermilab.

Researchers at Fermilab were racing CERN to be the first to produce evidence that the Higgs boson existed. Fermilab was always the underdog. CERN’s particle accelerator, the Large Hadron Collider, was bigger and more powerful than Fermilab’s Tevatron. Even so in August of 2009, while the LHC was shut down for repairs, Fermilab boldly announced that it had a good chance of finding the Higgs boson before its European rival.

“[The] LHC got indeed worried that Tevatron could come first, and this could have happened if the Higgs would have been at 160 or 115 GeV,” Bernardi said. “However it was never sure the Tevatron would have enough data to claim complete discovery (5 standard deviations), while LHC in the end would have enough data. In the end LHC came first also since the machine performed extremely well in these last two years.”

Once the LHC started up again, it worked better than researchers predicted and was accumulating more data than hoped. In July 2009, CERN announced that it expected to find the elusive boson by the end of 2012, but ultimately only needed seven months.

“Even if [the Tevatron] had been running the whole time, the LHC would have made the discovery first,” Hooper said.

Bernardi agreed, saying that the Tevatron got “rather close, but at least three more years of data would have been needed.”

In an ironic twist, Tevatron detectors are actually better than the LHC at seeing the most common signature of the Higgs boson. The Higgs only lives for a fraction of a second, then breaks into different more stable fundamental particles, most commonly bottom and anti-bottom quarks.

“This is one aspect of the Higgs that the Tevatron is the best game in town,” Hooper said. “At this time the Tevatron is more sensitive to this channel than the LHC is.”

Though the Tevatron is more sensitive to these bottom quarks, the LHC was producing many more of them at a time and more than made up for its lack of sensitivity.

The one last loose end from Fermilab is the fate of a mysterious “bump” in one of its experiments seen in April of last year. Its detector, the CDF, saw a different weak spike in its data, this time around 150 GeV or so. It likely was not the Higgs, its too heavy for that, but what caused the spike is still a mystery.

Because the same bump wasn’t seen in the preliminary LHC results or the Tevatron’s other big experiment DZero, many scientists have written off the signal as the result of some as yet unknown systematic error in the experiment.

“It turned out that this bump was probably a combination of statistics and systematic effects,” Bernardi said. “Once [DZero] and the LHC collaborations showed they were not seeing this bump, and that they were possible experimental effects to explain this, this matter was put to rest.”

However not all researchers are so convinced. Hooper maintains that there still are some lingering questions about the bump. What confounded the researchers is how the bump stubbornly wouldn’t disappear from the CDF’s data. The CDF team reran their experiment, and found that the significance of the bump jumped from a 3.2-sigma event, up to 4.1-sigma result, just under the certainty needed to declare a discovery.

“I think its still an open question, but there’s a lot of skepticism among my colleagues about that particular signal,” Hooper said. He added that now the LHC has found the Higgs, one of the other areas for it to look more carefully into is this mysterious bump.

1 comment:

  1. A documentary team filmed during this time in the "race to find the Higgs before CERN" The PBS documentary is really worth watching.