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CERN juggles subatomic mysteries

ATLAS Collaboration / CERN

A computer graphic shows a cross-section of the particle tracks generated on Sunday by one of the last proton collisions in the Large Hadron Collider's ATLAS detector before it was shut down for the switchover to lead-ion collisions.

The Large Hadron Collider has been turned off for a scheduled switchover, but researchers are continuing their quest at Europe's CERN particle-physics center to unravel some of the world's top scientific mysteries — including whether or not the Higgs boson really exists, and whether or not neutrinos can really travel faster than light.

In a news release, CERN declared that the world's most powerful particle collider largely surpassed its observational objectives "for the second year running." The metric for success is known as the inverse femtobarn, which is equal to about 70 trillion particle collisions. At the beginning of this year's run, the LHC's goal was to produce 1 inverse femtobarn during 2011, but instead it delivered almost six inverse femtobarns to each of the two main detectors, ATLAS and CMS. In comparison, Fermilab produced 10 inverse femtobarns in the course of a decade.

"At the end of this year's proton running, the LHC is reaching cruising speed," Steve Myers, CERN's director for accelerators and technology, said in today's news release.

Where's the Higgs hiding?
So far, researchers at the LHC have ruled out wide swaths of the energy spectrum as potential hiding places for the Higgs boson, the so-called "God particle" that is the last big missing piece in the Standard Model of particle physics. Detection of the Higgs would be the biggest prize in the particle hunt. But if the Higgs doesn't match physicists' expectations, they might have to try a whole new approach for solving the subatomic puzzle. (And some of them are actually looking forward to that prospect.)

Nature News quotes University of Padua physicist Tommaso Dorigo, a member of the CMS team, as saying he's "willing to bet a few bucks" that the Higgs is lurking around the energy level of 120 billion electron volts, one of the regions that hasn't yet been ruled out. Other physicists have said they'll have enough data by the end of next year to determine whether or not the Standard Model Higgs exists. Some have even suggested they'll know by Christmas, based on an analysis of the data already gathered.

On that score, CMS spokesperson Guido Tonelli dangled an intriguing teaser in today's release: "As we speak, hundreds of young scientists are still analyzing the huge amount of data accumulated so far; we'll soon have new results and, maybe, something important to say on the Standard Model Higgs Boson."

Little big bangs ahead
While the data-crunchers huddle over the numbers, the collider itself is being prepared for four weeks' worth of lead-ion collisions. Such heavy-ion smash-ups are aimed at re-creating the conditions that existed just an instant after the big bang, when the whole universe is thought to have consisted of a primordial soup known as quark-gluon plasma.

During previous lead-ion runs, researchers were able to produce small dollops of the soup, but this time around, they want to probe internal structure of the ions in greater detail. To do that, they'll experiment with smashing protons and lead ions together, which sounds a bit like the Reese's peanut-butter cup of particle physics. ("You got your protons in my lead ions!")

"Smashing lead ions together allows us to produce and study tiny pieces of primordial soup, but as any cook will tell you, to understand a recipe fully, it's vital to understand the ingredients," said Paolo Giubellino, spokesperson for the ALICE ion-smashing experiment, "and in the case of quark-gluon plasma, this is what proton-lead ion collisions will bring."

About those neutrinos...
The faster-than-light neutrino study involves a different research collaboration that uses facilities at CERN on the French-Swiss border, as well as at Italy's Gran Sasso underground observatory, more than 450 miles away. The physicists behind the OPERA experiment created a worldwide stir in September when they announced that they clocked bunches of neutrinos traveling from CERN to Gran Sasso at a speed beyond what was thought to be the cosmic speed limit.

OPERA's collaborators called upon the physics community to help them understand how this could have happened, or where they went wrong, and since then they've gotten lots of suggestions. Scores of papers have been submitted to the ArXiv.org preprint website, proposing possible explanations as well as potential flaws in the experiment. One concern has been that the experiment didn't account properly for relativistic effects such as gravitational time dilation. Another concern is that the pulses of neutrinos were so long that it'd be easy to mismeasure the travel time. 

Now the BBC has picked up on reports that the OPERA experiment will be rerun, this time with short bursts of neutrinos rather than a long pulse.  The BBC quoted CERN's director of research, Sergio Bertolucci, as saying that "this will allow OPERA to repeat the measurement, removing some of the possible systematics."

Rutgers physicist Matt Strassler, who was among those concerned about the length of the neutrino pulses, said in a blog post that rerunning the experiment with shorter pulses was the "obvious thing to do."

"It's like sending a series of loud and isolated clicks instead of a long blast on a horn; in the latter case you have to figure out exactly when the horn starts and stops, but in the former you just hear each click and then it's already over," he wrote.

Strassler quoted Japanese physicist Mitsuhiro Nkamura as saying the cross-check could be completed in just a few weeks. "So this is very good news," Strassler said. Stay tuned for another dose of weirdness ... or a dose of reality.

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