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Jeudi, 17 Novembre 2011 20:04

LHC May Have Found Crack in Modern Physics

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LHC May Have Found Crack in Modern Physics

By Jon Cartwright, ScienceNOW

In late 2008, a few onlookers believed that the Large Hadron Collider (LHC) would bring the end of the world. Three years later, our planet remains intact, but the European particle smasher may have made its first crack in modern physics.

If this crack turns out to be real, it might help explain an enduring mystery of the universe: why there’s lots of normal matter, but hardly any of the opposite—antimatter. “If it holds up, it’s exciting,” says particle physicist Robert Roser of the Fermi National Accelerator Laboratory in Batavia, Illinois.

LHC May Have Found Crack in Modern PhysicsTo understand why physicists are excited, look around: We’re surrounded by stuff. That might seem obvious, but scientists have long wondered why there’s anything at all. Accepted theories suggest that the big bang should have produced equal amounts of matter and antimatter, which would have soon annihilated each other. Clearly, the balance tipped in favor of normal matter, allowing the creation of everything we see today—but how, no one’s sure.

Most probably, theorists say, the properties of matter and antimatter aren’t quite symmetrical. Technically, this disparity is known as charge-parity (CP) violation, and it should crop up when particles naturally decay: either normal particles would decay more often than their antiparticles do or vice versa. According to the accepted theory of elementary particles, the standard model, there should be a low level of CP violation but not enough to explain the prevalence of normal matter. So experiments have been trying to find cases in which CP violation is higher.

That’s where LHCb, one of six detectors at the LHC, may have been successful. It has been tracing the paths of particles known as D0 mesons, which, along with their antiparticles, can decay into pairs of either pions or kaons. By tallying these pions and kaons, the LHCb physicists have calculated the relative decay rates between the D0 particles and antiparticles. The result, revealed at a meeting in Paris this week, is startling: the rates differ by 0.8%.

On the face of it, this level of CP violation is at least eight times as high as the standard model allows, so it could help explain why there is still “stuff” in the universe. But there’s a caveat: It’s not precise enough. For true discoveries, physicists demand a statistical certainty of at least five sigma, which means there should be less than one chance in 3 million of the result’s being a random blip in the data. Currently, the LHCb team has a certainty of three sigma, so there’s about one chance in 100 the result is a fluke.

Matthew Charles, a physicist at the University of Oxford in the United Kingdom and spokesperson for the 700-strong LHCb collaboration, is naturally cautious. “The next step will be to analyze the remaining data taken in 2011,” he says. “The sample we’ve used so far is only about 60 percent of what we’ve recorded, so the remainder will improve our precision quite a bit and will give us a strong clue as to whether the result will hold up.” For that analysis, the public will have to wait until next year.

Particle physicist Paul Harrison of the University of Warwick in the United Kingdom, who works on other LHCb studies, isn’t getting his hopes up. “I’m not betting my pension on this result standing the test of further data,” he says. He thinks the uncertainty is simply too big. “Since we are measuring hundreds of different things at the LHC, then every so often one of them will give a three-sigma effect like this at random.”

There are reasons to be positive, though. Last year, the CDF collaboration based at Fermilab reported a similar difference between the D0 decay rates of 0.46 percent. At the time, the result was thought likely to be a blip because CDF’s statistical uncertainty was fairly big, but taken together with the LHCb result, it might be seen to carry more weight. And CDF, like LHCb, still has more data to trawl through.

“We are now obviously very motivated to extend our analysis to our full data sample and see if we can get an independent confirmation of the LHCb result,” says Giovanni Punzi of the University of Pisa in Italy, a spokesperson for the CDF collaboration.

This story provided by ScienceNOW, the daily online news service of the journal Science.

Image: Peter Ginter/CERN


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