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每日推荐阅读:Neutrinos found to switch to elusive 'tau' flavour

2015-6-17 18:03| 发布者: 山姆大叔| 查看: 487| 评论: 2|原作者: 山姆大叔

摘要 : Using a beam shot through the Earth's crust, physicists have found the first direct proof of a metamorphosis between two of the three known types of neutrinos — those known as ‘muon’ and ‘tau’ fl ...


Using a beam shot through the Earth's crust, physicists have found the first direct proof of a metamorphosis between two of the three known types of neutrinos — those known as ‘muon’ and ‘tau’ flavours of the elementary particles.

The experiment, OPERA (Oscillation Project with Emulsion-tracking Apparatus) at the Gran Sasso underground lab in central Italy, made headlines in 2012 after it announced that it had clocked neutrinos travelling faster than light, in apparent violation of Einstein’s special theory of relativity. But although that claim later proved to be an embarrassment, when researchers discovered multiple potential sources of error in their measurements, the OPERA collaboration has now achieved its original goal of observing the switch in flavours.

“It was an extremely difficult measurement that no one had done before,” says Marco Pallavicini, a neutrino physicist at the University of Genoa who is not a member of the OPERA collaboration.

There are three known types, or 'flavours', of neutrino: electron, muon and tau. The particles'  names allude to the fact that on the rare occasions when neutrinos interact with protons or neutrons, they variously produce electrons, muons or tau leptons.

Scientists had long suspected that neutrinos could change 'flavour,' transforming from one type to another. Several previous experiments that used known sources of particular type of neutrino have detected fewer neutrinos than would be expected if the particles did not change flavour.

In July 2013, the T2K experiment in Japan saw the first direct evidence of the appearance of a different flavour — rather than just the disappearance of the original one. It detected electron neutrinos in a beam originally made of muon neutrinos.

Hidden target
Between 2008 and 2012, CERN, the particle physics lab near Geneva, Switzerland, aimed a beam of muon neutrinos at the base of the Gran Sasso massif, 730 kilometres to the southeast. The Italian lab is carved inside the rock there, off a highway tunnel.

By the time the neutrinos arrived at Gran Sasso, some of the muon neutrinos had turned into tau neutrinos. When these hit lead targets inside the OPERA detector, they produced tau leptons, the latest results show.

The leptons decay in just a trillionth of a second, says Giovanni De Lellis, a physicist at the University of Naples who took over as OPERA spokesperson in 2012 after a no-confidence vote resulted in the resignation of his predecessor. “Even though it travels at nearly the speed of light, it only runs for less than a millimetre,” he says.

OPERA detected the short-lived particles with an array of 150,000 'bricks', each containing 57 stacked emulsion plates and weighing 8 kilograms. This set-up has 110,000 square metres of surface area, so researchers set up an automated system to search the plates for microscopic streaks that would signal the brief presence of tau leptons.

In partial results announced last year2, the OPERA collaboration counted four likely tau lepton sightings, not quite enough to claim a discovery according to the stringent criteria of particle physics. But the physicists have now found a fifth such event, enough for De Lellis to declare the experiment successful on 15 June.

“The result could not be taken for granted,” he says. Once the CERN beam was shut off, De Lellis and his team were limited to existing data, and finding five events took a bit of luck, he admits. “It could have been six — or four, or three.”


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引用 嗄嗉 2015-6-18 17:07
引用 山姆大叔 2015-6-18 18:05

这两天都是nature的题源,今天换了economist,试试看读起来怎么样

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