L. Taylor / T. McCauley / CMS / CERN
This graphic records proton collision events in the Large Hadron Collider's Compact Muon Solenoid in which four high-energy electrons (shown as red towers) are observed. The event shows characteristics expected from the decay of a Higgs boson but is also consistent with background processes.
The latest results from the Large Hadron Collider serve as a reality check for expectations that radical scientific discoveries are just around the corner. A month ago, folks were buzzing about prospects that the elusive Higgs boson might soon be found. This week, they're talking about how the Higgs boson, as well as other exotic ideas such as supersymmetry and superstring theory, might be merely a will o' the wisp.
Reservations about the imminent revolution in particle physics cropped up in the wake of last week's Lepton Photon conference in Mumbai, India. Some observers speculated that fresh results could confirm an anomalous "bump" in earlier data from the LHC's two main detectors, ATLAS and the Compact Muon Solenoid.
Such a bump could suggest the mass-energy level where the Higgs boson was lurking. Detecting the Higgs boson, also known as the "God Particle," has been the main goal of the $10 billion particle collider on the French-Swiss border. Physicists are anxious to see it because it would be the last fundamental particle predicted by the Standard Model, one of physics' most successful theories. The Higgs mechanism could explain why some particles have mass while others don't.
But instead of confirming the earlier bump in their data, researchers at Europe's CERN particle physics lab reported last week that "the significance of those fluctuations has slightly decreased." That led some observers to suggest that the Higgs boson "likely doesn't exist."
In fact, it's still too early to render a verdict. "Variations up and down on significance are to be expected," Fermilab physicist Don Lincoln, author of a book on the LHC titled "The Quantum Frontier," told me in an email today. "The two conferences are only a month apart, and things don't change hugely between them."
So far, the most significant findings from the LHC are those that have virtually ruled out broad areas of the mass-energy spectrum where the Higgs might have been detected — mostly in the range between 145 billion and 466 billion electron volts, with 95 percent certainty. There's a better chance of finding the Higgs at lower masses, below 145 billion electron volts, but that's going to be a trickier challenge for the high-powered LHC.
Sergio Bertolucci, CERN's research director, put an optimistic spin on the non-findings, declaring that "these are exciting times for particle physics."
"Discoveries are almost assured within the next 12 months," he said. "If the Higgs exists, the LHC experiments will soon find it. If it does not, its absence will point the way to new physics."
So long, supersymmetry?
That new physics could theoretically include supersymmetry and string theory, weird concepts that propose the existence of whole classes of yet-to-be-discovered particles (or "sparticles"). Such concepts represent a departure from the Standard Model, and for that reason physicists are looking closely for any anomalies that would open the way to new physics.
For those physicists, the latest data from the LHCb detector — which is particularly sensitive to matter-antimatter anomalies in the decay of B-mesons — might represent a bit of a letdown. Researchers reported in Mumbai that their measurements were "in agreement with the Standard Model prediction," although they said "there is still room for a new physics contribution."
A spokesperson for the LHCb experiment, Tara Shears of Liverpool University, told the BBC that her team's results "put supersymmetry on the spot." Other physicists are starting to wonder whether the concept will have to be discarded in favor of other exotic ideas, such as a fifth fundamental force known as technicolor.
For physicists, non-discoveries can be as valuable as discoveries. But if CERN's big machine doesn't produce some breakthrough physics, it's likely to be more difficult to sell taxpayers and politicians on the next big machine.
Years ago, CERN theoretical physicist John Ellis told me it "might be a little bit difficult to explain to our politicians, that here they gave us 10 billion of whatever, your favorite currency unit, and we didn't find the Higgs boson." Ellis and his colleagues don't have to provide that explanation just yet, but stay tuned. A year from now, physicists will either be struggling to explain the weird phenomena they're seeing ... or struggling to explain the absence of weird phenomena.
Update for 2 p.m. ET Sept. 1: The BBC's Pallab Ghosh revives hopes for the Higgs in a report saying that the "Higgs particle could be found by Christmas." Those expectations are based on a Quantum Diaries blog posting by University of Wisconsin researcher Richard Ruiz, who notes there's a chance that the LHC will collect enough data by the end of this year to make statistical judgments about whether the Standard Model's version of the Higgs exists or not over a broad range of possible masses.
Guido Tonelli, spokesman for the LHC's Compact Muon Solenoid experiment, told the BBC, "We could discover the Standard Model version of the Higgs boson or exclude it earlier than expected. Could we discover it by Christmas? In principle, yes."
There are several caveats: First, the forecast assumes that data will keep flowing from the LHC at its current better-than-predicted rate. Second, researchers are shifting their focus to regions of the mass spectrum where the results are more difficult to interpret, and therefore physicists may require more data than they originally expected. And third, the projections apply only to the kind of Higgs particle predicted by the Standard Model. A non-standard Higgs boson could still escape the net.
Fermilab's Lincoln says updated results from the LHC are due to be announced in mid-November at the Hadron Collider Physics symposium in Paris, so the situation may become clearer at that time. Stay tuned ... maybe we'll have something more definitive by Thanksgiving.
More about the LHC:
- Got a computer? Help out the atom smashers
- What's a hadron? Take a tour of the particle zoo
- Interactive: Inside the Big Bang Machine
- Special report on the Large Hadron Collider
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