【每日阅读训练第三期——速度越障2系列】【2-7】科技-particle physics-neutrino

babybearmm

neutrino这词，说实话，我这个外行原本不知道。可最近半年来这词实在太火了，尤其是最近的Science / Nature 几乎每期都少不了它的靓影…..所以今天咱就一起来认识一下吧，看看某个由爱因斯坦提出的、咱在中学物理课上学过的公理，到底有没有被推翻呢？
 

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The neutrino and its friends

Neutrinos are one of the fundamental particles which make up the universe. They are also one of the least understood.

Neutrinos are similar to the more familiar electron, with one crucial difference: neutrinos do not carry electric charge. Because neutrinos are electrically neutral, they are not affected by the electromagnetic forces which act on electrons.Neutrinos are affected only by a "weak" sub-atomic force of much shorter range than electromagnetism, and are therefore able to pass through great distances in matter without being affected by it. If neutrinos have mass,they also interact gravitationally with other massive particles, but gravity is by far the weakest of the four known forces.

Three types of neutrinos are known; there is strong evidence that no additional neutrinos exist, unless their properties are unexpectedly very different from the known types. Each type or "flavor" of neutrino is related to a charged particle (which gives the corresponding neutrino its name).  Hence, the "electron neutrino" is associated with the electron, and two other neutrinos are associated with heavier versions of the electron called the muon and the tau (elementary particles are frequently labelled with Greek letters, to confuse the layman).

Looking for Neutrinos, Nature's Ghost Particles
To study some of the most elusive particles, physicists have built detectors in abandoned mines, tunnels and Antarctic ice

By Ann Finkbeiner
Smithsonian magazine,     November 2010

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We’re awash in neutrinos. They’re among the lightest of the two dozen or so known subatomic particles and they come from all directions: from the Big Bang that began the universe, from exploding stars and, most of all, from the sun.They come straight through the earth at nearly the speed of light, all the time, day and night, in enormous numbers. About 100 trillion neutrinos pass through our bodies every second.

The problem for physicists is that neutrinos are impossible to see and difficult to detect. Any instrument designed to do so may feel solid to the touch, but to neutrinos, even stainless steel is mostly empty space, as wide open as a solar system is to a comet. What’s more, neutrinos, unlike most subatomic particles, have no electric charge—they’re neutral, hence the name—so scientists can’t use electric or magnetic forces to capture them. Physicists call them “ghost particles.”

To capture these elusive entities, physicists have conducted some extraordinarily ambitious experiments. So that neutrinos aren’t confused with cosmic rays (subatomic particles from outer space that do not penetrate the earth), detectors are installed deep underground. Enormous ones have been placed in gold and nickel mines, in tunnels beneath mountains, in the ocean and in Antarctic ice. These strangely beautiful devices are monuments to humankind’s resolve to learn about the universe.

It’s unclear what practical applications will come from studying neutrinos.“We don’t know where it’s going to lead,” says Boris Kayser, a theoretical physicist at Fermilab in Batavia, Illinois.

Physicists study neutrinos in part because neutrinos are such odd characters: they seem to break the rules that describe nature at its most fundamental. And if physicists are ever going to fulfill their hopes of developing a coherent theory of reality that explains the basics of nature without exception, they are going to have to account for the behavior of neutrinos.
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In addition, neutrinos intrigue scientists because the particles are messengers from the outer reaches of the universe, created by violently exploding galaxies and other mysterious phenomena. “Neutrinos may be able to tell us things that the more humdrum particles can’t,” says Kayser.

Physicists imagined neutrinos long before they ever found any. In 1930, they created the concept to balance an equation that was not adding up. When the nucleus of a radioactive atom disintegrates, the energy of the particles it emits must equal the energy it originally contained. But in fact, scientists observed, the nucleus was losing more energy than detectors were picking up. Soto account for that extra energy the physicist Wolfgang Pauli conceived an extra, invisible particle emitted by the nucleus. “I have done something very bad today by proposing a particle that cannot be detected,”?Pauli wrote in his journal. “It is something no theorist should ever do.”

Experimentalists began looking for it anyway. At a nuclear weapons laboratory in South Carolina in the mid-1950s, they stationed two large water tanks outside a nuclear reactor that, according to their equations, should have been making ten trillion neutrinos a second. The detector was tiny by today’s standards, but it still managed to spot neutrinos—three an hour. The scientists had established that the proposed neutrino was in fact real; study of the elusive particle accelerated.

A decade later, the field scaled up when another group of physicists installed a detector in the Homestake gold mine, in Lead, South Dakota, 4,850 feet underground. In this experiment the scientists set out to observe neutrinos by monitoring what happens on the rare occasion when a neutrino collides with a chlorine atom and creates radioactive argon, which is readily detectable. At the core of the experiment was a tank filled with 600 tons of a chlorine-rich liquid, perchloroethylene, a fluid used in dry-cleaning. Every few months, the scientists would flush the tank and extract about 15 argon atoms, evidence of15 neutrinos. The monitoring continued for more than 30 years.
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Hoping to detect neutrinos in larger numbers, scientists in Japan led an experiment 3,300 feet underground in a zinc mine. Super-Kamiokande, or Super-K as it is known, began operating in 1996. The detector consists of50,000 tons of water in a domed tank whose walls are covered with 13,000 light sensors. The sensors detect the occasional blue flash (too faint for our eyes to see) made when a neutrino collides with an atom in the water and creates an electron. And by tracing the exact path the electron traveled in the water,physicists could infer the source, in space, of the colliding neutrino. Most,they found, came from the sun. The measurements were sufficiently sensitive that Super-K could track the sun’s path across the sky and, from nearly a mile below the surface of the earth, watch day turn into night. “It’s really an exciting thing,” says Janet Conrad, a physicist at the Massachusetts Institute of Technology. The particle tracks can be compiled to create “a beautiful image,the picture of the sun in neutrinos.”

But the Homestake and Super-K experiments didn’t detect as many neutrinos as physicists expected. Research at the Sudbury Neutrino Observatory (SNO, pronounced “snow”) determined why. Installed in a 6,800-foot-deep nickel mine in Ontario, SNO?contains1,100 tons of “heavy water,” which has an unusual form of hydrogen that reacts relatively easily with neutrinos. The fluid is in a tank suspended inside a huge acrylic ball that is itself held inside a geodesic superstructure, which absorbs vibrations and on which are hung 9,456 light sensors—the whole thing looking like a 30-foot-tall Christmas tree ornament.

Scientists working at SNO discovered in 2001 that a neutrino can spontaneously switch among three different identities—or as physicists say, it oscillates among three flavors. The discovery had startling implications. For one thing, it showed that previous experiments had detected far fewer neutrinos than predicted because the instruments were?tuned to just one neutrino flavor—the kind that creates an electron—and were missing the ones that switched. For another, the finding toppled physicists’ belief that a neutrino,like a photon, has no mass. (Oscillating among flavors is something that only particles with mass are able to do.)
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How much mass do neutrinos have? To find out, physicists are building KATRIN—the Karlsruhe Tritium Neutrino Experiment. KATRIN’s business end boasts a 200-ton device called a spectrometer that will measure the mass of atoms before and after they decay radioactively—thereby revealing how much mass the neutrino carries off. Technicians built the spectrometer about 250 miles from Karls­ruhe, Germany, where the experiment will operate; the device was too large for the region’s narrow roads, so it was put on a boat on the Danube River and floated past Vienna, Budapest and Belgrade, into the Black Sea,through the Aegean and the Mediterranean, around Spain, through the English Channel, to Rotterdam and into the Rhine, then south to the river port of Leopoldshafen, Germany. There it was offloaded onto a truck and squeaked through town to its destination, two months and 5,600 miles later. It is scheduled to start collecting data in 2012.

Physicists and astronomers interested in the information that neutrinos from outer space might carry about supernovas or colliding galaxies have set up neutrino “telescopes.” One, called IceCube, is inside an ice field in Antarctica. When completed, in 2011, it will consist of more than 5,000 blue-light sensors (see diagram above). The sensors are aimed not at the sky, as you might expect, but toward the ground, to detect neutrinos from the sun and outer space that are coming through the planet from the north.The earth blocks cosmic rays, but most neutrinos zip through the8,000-mile-wide planet as if it weren’t there.

A long-distance neutrino experiment is taking place under several Midwestern states. A high-energy accelerator, which generates subatomic particles, shoots beams of neutrinos and related particles as much as six miles deep, beneath northern Illinois, across Wisconsin and into Minnesota.The particles start at Fermilab, as part of an experiment called the Main Injector Neutrino Oscillation Search (MINOS). In less than three-thousandths of a second, they hit a detector in the Soudan iron mine, 450 miles away. The data the scientists have gathered complicates their picture of this infinitesimal world: it now appears that exotic forms of neutrinos, so-called anti-neutrinos,may not follow the same rules of oscillation as other neutrinos.

“What’s cool,” says Conrad, “is that it’s not what we expected.”

When it comes to neutrinos, very little is.

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Read more: http://www.smithsonianmag.com/science-nature/Looking-for-Neutrinos-Natures-Ghost-Particles.html#ixzz1qDbnzgbg


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Neutrinos not faster than light
ICARUS experiment contradicts controversial claim.
16 March 2012 Corrected:

The ICARUS detector in Gran  Sasso, Italy, has confirmed that neutrinos travel no faster than the speed of light.

Neutrinos obey nature's speed limit, according to new results from an Italian experiment. The finding, posted to the preprint server arXiv.org, contradicts a rival claim that neutrinos could travel faster than the speed of light.

Neutrinos are tiny, electrically neutral particles produced in nuclear reactions. Last September, an experiment called OPERA turned up evidence that neutrinos travel faster than the speed of light (see 'Particles break light speed limit'). Located beneath the Gran Sasso mountain in central Italy, OPERA detected neutrinos sent from CERN, Europe's premier particle-physics laboratory near Geneva, Switzerland. According to the group's findings, neutrinos made the 731-kilometre journey 60 nanoseconds faster than predicted if they had travelled at light speed.

The announcement made international headlines, but physicists were deeply sceptical. The axiom that nothing travels faster than light was first formulated by Albert Einstein and is a cornerstone of modern physics. OPERA defended its announcement, saying that it could find no flaw in its measurement.

Now another experiment located just a few metres from OPERA has clocked neutrinos travelling at roughly the speed of light, and no faster. Known as ICARUS, the rival monitored a beam of neutrinos sent from CERN in late October and early November of last year. The neutrinos were packed into pulses just 3nanoseconds long. That meant that the timing could be measured far more accurately than the original OPERA measurement, which used 10-microsecondpulses.

“Our results are in agreement with what Einstein would like to have,” says Carlo Rubbia, the spokesperson for ICARUS and a Nobel prizewinning physicist at CERN. Neutrinos measured by the experiment arrived within just 4 nanoseconds of the time that light travelling through a vacuum would take to cover the distance, well within the experimental margin of error.

Because the pulses from CERN were so short, ICARUS measured only seven neutrinos during the late autumn run, but Rubbia says that the relatively low number does not matter. “How many times do you have to say 'zero' to make sure it's zero?” he asks.

The findings are yet another blow to OPERA, which was already under intense scrutiny from the wider experimental community. Almost as soon as the announcement was made, physicists began trying to poke holes in the OPERA analysis, and on 23 February researchers from within the OPERA team announced that they had uncovered possible timing problems with their original measurements (see 'Timing glitches dog neutrino claim'). Those problems could have led to the60-nanosecond discrepancy.

Dario Autiero, a physicist at the Institute of Nuclear Physics in Lyon, France,and physics coordinator for OPERA, welcomes the latest result. He notes that OPERA continued to detect faster-than-light neutrinos in October and November,when the shorter pulses were used. The team continues to search for possible sources of error, he says.

For some, the new measurements settle the matter once and for all. “The OPERA case is now conclusively closed,” says Adam Falkowski, a theoretical physicist at the University of Paris-South in Orsay, France.But Rubbia says that he is still awaiting further measurements set to be made later in the spring by OPERA, ICARUS and two other experiments inside Gran Sasso.

“Had we found 60 nanoseconds, I would have sent a bottle of champagne to OPERA,” Rubbia says. But as it stands, he suspects he will be toasting Einstein. “It's quite a relief, because I'm a conservative character,” he says.
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Source: Nature doi:10.1038/nature.2012.10249
http://www.nature.com/news/neutrinos-not-faster-than-light-1.10249?WT.ec_id=NEWS-20120320

Recommended listening:
http://blog.ted.com/2008/04/29/brian_cox/        
An intriguing talk - particle physics, astronomy, and a physicist’s perception of our universe

半阙

SF~~~

58’
1’59
1’45
1’45
1’55
之前有看过中微子的介绍。。。。


越障：
1、现在的一个O实验发现：中微子比光速快
2、这是对爱因斯坦“光速是最快的“言论的挑战，做实验的这个人坚持说没有测量误差。
3、各种质疑就来了。最后有人实验证明O实验有60（不知道什么单位）的误差。
4、在等O的新的measurement.

phoebe0624

文章真有趣呐！
速度
48''
1'09''
1'15''
1'23''
1'27''

越障

Neutrinos move fast than light?

Recently,OPERA research found that the Neutrinos move a little faster than light. The researchers set the experiment in October and November.
Many physicists do not agree with this statement, because Albert Einstein  improved the light fast theory. The theory is the foundation of physics.
A another research called ICARUS have another experiment to improve that there are some problems in the OPERA research's experiment. The researchers believe that OPERA did not record time accurately. They claim that Neutrinos have roughly speed as same as light.
Some physicists cast doubt on the ICARUS's wider experiment circumstance.
Still need more experiments.

双色鹿

1. 1min9s
2. 1min27s
3. 1min47s
4. 1min35
5. 1min40s
越障3min51s

Neutrino速度没有光速快
之前OPERA做了个实验，说明neutrino速度超过了光速，打破了爱因斯坦的定理，此定理是所有物理学的基础，且OPERA说自己没有错误
后来ICARUS又做了个实验，方法和OPERA类似，但是一个条件由10变3使得结果会更精确，结论是neutrino速度没有光速快，爱因斯坦的理论是对的
OPERA后来找到了自己的误差点所在，而且说会继续找自己实验的错误

kevin405hu

1'38
2'05
2'11
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2'21

3'58

dengly

占座

abjure

吐血了


1:34
1:48
2:29
2:35
2:35

Crystaljoy

~~~~(>_<)~~~~ 科技文

1:48
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2:10
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3:39

kaitlynyl

速度：只有1按时读完，其余均超过1min
1.neutrino's characteristics.There's only three types of neutrino.
2.We are awashed in the neutrino.The scientists find it difficult to do research because neutrino is impossible to be seen and difficult to be detected.They study neutrino in part.reasons.
3.Why neutrino intrigue scientists.THe scientists imagined neutrino long before they found it.STh about calculating an equation.A decade later,they found the method to detect it.
4.They want to detect neutrino in large numbers.THe detecting process and results.
5.The mass of neutrino.They found anti neutrino then felt cool.
越障：哈哈感谢baby，今天很轻松。今天补了两天作业，正好让我有时间补完。
MI:n travels no faster than light.
The process of last experiment.
After the anouncement takes out,physicists were sceptical.But  the OPERA said their measurement  was right.
No other team has this result .Some opinions of other people.
The result is wrong.The team continues to search for sources of error.The opinion of a people,who felt a relief.

搞G战士DB

速度：'47   1'21   1'34  1'45   1'29
越障：4'47
中微子的速度没有光快。一个叫OPERA的实验得出了中微子的速度快于光速的结论。一些科学家表示怀疑。在距离OPERA不远的地方一个叫I什么的实验证明中微子的速度慢于光速。但O声称他们的实验没有错误。然后好像是发现了哪儿出现了误差，什么单位从3变成了10，使得实验更加可靠。O声称会继续进行实验，如果找出60什么单位，就开香槟酒什么的……
一个月倒计时，今天起开始写越障回忆，欢迎各位NN拍砖