This morning, physicists representing the Large Hadron Collider's two largest experiments — ATLAS and CMS — announced that both teams have independently uncovered signals that point to the appearance of the Higgs boson. That's the long-sought sub-atomic particle thought to endow all other particles with mass.
So have CERN physicists found the Higgs boson? Not yet — but they're closer than they've ever been before. And most exciting of all: the teams' results have never been more similar.
The announcement came in the form of two presentations — the first by ATLAS's Fabiola Gianotti, the second by CMS's Guido Tonelli. According to Giaotti, the most recent ATLAS findings indicate that if the Standard Model Higgs boson exists, it most likely has a mass in the range of 115—130 gigaelectronvolts (GeV). And as it turns out, CMS's latest findings were very similar; according to Tonelli, if the Higgs exists, it is almost certainly no heavier than than 127 GeV.
Most exciting, however, was the announcement that each team had uncovered, independently, evidence for the existence of a lightweight Higgs in the region of 125 GeV (about 125 times the mass of a proton); ATLAS saw a hint of the Higgs at 126 GeV, while CMS saw one at 124 GeV. According to New Scientist, this is the first time both experiments have seen a signal at nearly the same mass.
"We're very competitive, but once I see they're coming with results, I'm happy," Tonelli says. "Their results are important for us. They're obtained in a completely independent manner."
But these hints of a lightweight Higgs still lack the degree of certainty required to make a decisive announcement as to whether or not the particle actually exists. The 126 GeV signal detected by ATLAS has a statistical significance of 2.3 sigma, meaning that there's a roughly 2 percent chance that the signal is the result of a random fluctuation.
Those might sound like pretty good odds to most of us, but to claim a discovery in particle physics requires a 5 sigma signal — indicating a less than 1 in one million chance that the results are due to a background fluctuation. The 124 GeV signal detected by CMS has a statistical significance of just 1.9 sigma. Combining the results of the experimental datasets (which will be no mean task — remember, these results were arrived at from different angles) could ratchet the statistical significant up into the 3.7—3.9 sigma range.
So what's the ultimate takeaway? I think Giaotta summarized the situation nicely:
"I think it would be extremely kind of the Higgs boson to be here," she told the audience gathered at this morning's seminar. "But it is too early" to draw any final conclusions, she said. "More studies and more data are needed...I don't know what the conclusions will be."
What physicists at CERN do know, however, is that 2012 is shaping up to be an exciting year for particle physics.
"We need more study and more data," explained Gianotti. "Given the outstanding performance of the LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012."
"The window for the Higgs mass gets smaller and smaller," echoed CERN director-general Rolf-Dieter Heuer at the end of this morning's seminar. "But it is still alive."
Do you have questions about this morning's announcement? We want to hear them! In tomorrow's "Ask a Physicist" column, our resident physics expert Dr. Dave Golderg will address your questions about the Higgs Boson and the Standard Model. Feel free to post your questions and comments over here, or email Dave at email@example.com.