jueves, 5 de julio de 2012

New Particle Resembling Long-Sought Higgs Boson Uncovered at Large Hadron Collider

The CERN collider, the most powerful atom smasher in history, appears to have fulfilled its primary quest
Higgs hunt audienceGATHER ROUND: Dozens of students and physicists gathered at Columbia University's Low Library early Wednesday morning to get the latest news on the Higgs boson.Image: John Matson

NEW YORK—The city that never sleeps was mostly asleep. The bars were closed. But at 4:45 A.M., inside a library on Columbia University's Manhattan campus, Michael Tuts was getting ready to pop the champagne.
The physicist had good reason to celebrate. The massive team of scientists of which he is a part—3,000 researchers working on the ATLAS experiment at Europe's Large Hadron Collider—had just announced the discovery of a new particle. The particle looks an awful lot like the long-sought, and long-hypothetical, Higgs boson, most famous for explaining why elementary particles, such as quarks, have mass. A competing, comparably sized experiment, known as CMS, had arrived at a very similar finding at the collider facility.
Both research teams announced their results during a morning seminar at CERN, the European laboratory for particle physics that operates the Large Hadron Collider, or LHC. But the morning start in Geneva meant that U.S. physicists and other curious observers were tuning in to the announcement during the predawn hours. Tuts and his Columbia colleagues decided to host a viewing party at the campus library, with a live video feed from CERN as well as coffee, cookies, soft drinks and chips. About 50 people, many of them students, turned up for the event, which began around 2:30 A.M.
Unlike some past announcements centered on the Higgs in the past few years, which have produced as much ambiguity and confusion as anything else, this one did not disappoint. ATLAS physicists said that their most recent data reveal the presence of an unknown particle with a mass of about 126.5 GeV, or 126.5 billion electron-volts. An electron-volt is a physicist’s unit of mass or energy; for comparison, the proton has a mass of about 1 GeV. The CMS collaboration found evidence for a new particle with a mass of 125.3 GeV.
Crucially, both teams' findings appear exceptionally robust. In physics terms, evidence for a new particle requires a “3-sigma” measurement, corresponding to a 1-in-740 chance that a random fluke could explain the observations, and a claim of discovery requires a 5-sigma effect, or a 1-in–3.5 million shot that the observations are due to chance. In December representatives of the two experiments had announced what one called “intriguing, tantalizing hints” of something brewing in the collider data. But those hints fell short of the 3-sigma level. The new ATLAS finding met not just that level of significance but cleared the gold standard 5-sigma threshold, and CMS very nearly did as well, with a 4.9-sigma finding.
"This is the payoff," Tuts said after the two teams had announced their latest analyses in the Higgs hunt. "This is what you do it for." Peter Higgs himself, who was in Geneva for the seminar along with other eminent physicists who developed the theory, sounded a similar note after the ATLAS and CMS teams had unveiled their conclusions. "For me, it's really an incredible thing that it's happened in my lifetime," Higgs said to the audience at CERN. He was among a half-dozen physicists who in the 1960s proposed what is now known as the Higgs mechanism, hypothesizing the existence of a field permeating all of space, along with an associated particle. The field imparts particles with mass by exerting a sort of drag on them, slowing them down much like a human being slows down when she tries to walk through water instead of air.
The newfound particle fits the bill for the Higgs boson, but the researchers cautioned that more work is needed to compare the properties of the particle to those predicted for the Higgs. After all, the LHC’s detectors cannot identify the Higgs directly. The LHC accelerates protons to unprecedented energies of four trillion electron-volts (4 TeV) before colliding a clockwise-traveling proton beam with a counterclockwise beam. From the smash-up new particles emerge, some of them existing for just an instant before decaying to other particles.
In the case of the Higgs, physicists can only infer its existence and its properties from the more mundane particles it decays to produce—say, gamma-ray photons or pairs of electrons. The new particle has the right mass to be the Higgs and broadly decays as predicted, although a few ambiguities remain. Fortunately, more data are right around the corner. "We have only recorded one third of the data expected in 2012," ATLAS spokesperson Fabiola Gianotti of CERN said during her presentation. "This is just the beginning. There is more to come."
Both Gianotti and CMS spokesperson Joe Incandela of the University of California, Santa Barbara, were greeted by large outbursts of applause when they displayed the slides outlining the results of their Higgs search.
"There aren’t many discoveries like this," Columbia physicist Brian Cole told the group assembled on campus for the early-morning viewing party. "This trumps, I would say, everything in my physics career.…So I hope you all remember this for the rest of your lives."
Just before sunrise, four Columbia undergraduates made their way out of the library and back across campus. Only two of them were studying physics, they said—one was focusing on chemistry and another on math. But they all agreed that it had been worth staying up late to see history in the making.