LHC firing on all cylinders, but no Higgs boson yet
The Large Hadron Collider particle accelerator has moved rapidly from tentative operation to scientific results, researchers say at a Paris conference.
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It took 100 years to discover all the physics particles that have thus far been observed. It took the Large Hadron Collider four months.
That's the report Monday from the International Conference on High-Energy Physics in Paris, where LHC researchers announced they've successfully retraced the steps of earlier particle accelerators. The final step was the likely view of a super-heavy and short-lived particle called the top quark, first seen in 1995 at Fermilab's Tevatron accelerator near Chicago.
"They have re-found all the known particles in the standard model," the successful but ultimately insufficient explanation that physicists use to catalog fundamental particles, said Rolf Heuer, director general of the CERN laboratory in Geneva that houses the massive particle accelerator. "The experiments have shown they are ready for new physics once new physics will appear," he told reporters at the conference.
The elusive Higgs boson, a predicted but as yet unobserved particle thought to imbue other particles with mass, remains unseen--but at the present stage of getting the LHC up to speed, that was no surprise. Still, it's clear the LHC, despite its troubled start-up and the fact that it's not expected to run at full power for years, has begun carrying the baton for particle physics.
"This is a very exciting meeting in which we're starting to see a transition in our field. For quite a few decades, our ability to study nature on the smallest distance scales was occurring at Fermilab," said Mel Shochet, a Tevatron researcher, professor at the University of Chicago, and chairman of the U.S. Energy Department's High Energy Physics Advisory Panel. "Here at this meeting we're now seeing the first results at even higher energies [from the LHC]. It's very impressive the detectors are understood in such detail this early in the program."
The LHC is designed to probe the frontiers of the new physics beyond the standard model. One big one is "supersymmetry"--the idea that all the particles in the standard model have companion "sparticles" on the other side of a supersymmetric mirror. Some of those particles could account for dark matter--the invisible and thus far not directly detectable mass that pervades the universe. A collection of at least five different Higgs boson types also would bridge into supersymmetry. The LHC also is designed to probe the differences between matter and antimatter that over the history of the universe have led to an imbalance that permits galaxies, stars, planets, and people to exist without vanishing in an explosive release of gamma-ray light. And the LHC will attempt to peer back closer to the time of the Big Bang, when energy levels were so high that quarks weren't confined to forming particles in atomic nuclei as they are today.
The LHC may be ascendant, but don't write off the Tevatron. It can accelerate particles to an energy level of 1 tera-electron volt, considerably lower than the LHC's 7TeV current level and 14TeV planned energy. That's not been enough so far to find where the Higgs boson is, but it's been enough to rule out some areas where it isn't.
The standard model predicts a certain energy range over which Higgs bosons could be found. At the Paris conference, Tevatron researchers announced they narrowed that range. Previously 15 percent had been ruled out, and now it's up to 25 percent, Shochet said.
These high-end accelerators work by smashing speeding particles into each other and seeing what other particles are produced. In the LHC's case, two beams of protons travel nearly at the speed of light in opposite directions around an underground ring. At four areas around the 27km circumference, the beams converge so packets of protons sail through each other like two very high-speed handfuls of sand thrown through the air.
Only sometimes do particles actually collide, given the spaces in between the protons, but LHC operators are working to maximize that likelihood by increasing what's known as luminance. They're taking measures such as putting more protons in each packet, putting more packets in the ring at the same time, and focusing the packets into a narrower, more concentrated beam.
Through these measures, the physicists announced at the Paris show that they've increased luminance by a factor of 1,000 since the LHC's inital 7TeV run in March--two beams at 3.5TeV each. Increasing the luminosity is important because it increases the likelihood that rarities such as top quarks or Higgs bosons will be spotted.
LHC operators want to further increase luminosity by another factor of 500 to 1,000 this year, Heuer said.
The researchers also detailed what they expect to happen after this year on the way to a full 14TeV energy level: a 15-month shutdown for all of 2012 and the first quarter of 2013. The shutdown, partly to upgrade detectors and partly to address shortcomings uncovered during a major 2008 incident at the LHC, had been planned earlier, but the LHC staff have solidified it.
Futher year-long shutdowns are planned in 2016 and 2020. CERN plans to run the LHC through about 2030, Heuer said.
And physicists are looking beyond the LHC. Next up likely will be a linear, rather than circular, collider.
"In order to complement the results from the LHC, an e+/e- collider will be needed," Heuer said, referring to electrons and their positively charged antimatter comrades, positrons.
Two efforts are under way to design a linear accelerator, the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). The LHC's results will help determine what next steps are taken, Heuer and Shochat said.
Meanwhile, the LHC will fill in the gaps. Among the innumerable LHC researchers is a new generation of particle physicsts.
"At the experiments of the LHC, more that 2,500 students are now producing their Ph.D. theses," Heuer said. "These young guys are like piranhas. They are going onto this data and producing results."