雅思阅读 168: Chasing the Ghost Particles of the Cosmos（追逐宇宙中的幽灵粒子）

改编自 Nobel Prize / University of Tokyo ICRR / Super-Kamiokande。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://www.nobelprize.org/prizes/physics/2015/popular-information/

Reading Passage

A. Of all the particles that fill the universe, few are as numerous and as aloof as the neutrino. Billions of them stream through every human body every second — cast off by nuclear reactions in the Sun, by exploding stars and by cosmic rays striking the upper atmosphere — and almost none of them stop to interact with the atoms they pass through. So rarely does a neutrino touch ordinary matter that a typical one could sail through a light-year of lead without being deflected. It was this near-untouchability that earned neutrinos the nickname "ghost particles", and it is what made them, for decades, almost impossible to study. To catch one, physicists had to build instruments of improbable scale: enormous tanks of ultrapure water buried deep underground, shielded from the ordinary cosmic noise that would otherwise swamp the faint, rare signals they hunt. The reward for the effort turned out to be far larger than anyone expected, because these shy particles carried in their behaviour evidence that the reigning theory of physics was incomplete. The detectors themselves are monuments to patience: a human passing a few light-years from one would not notice it, and a single neutrino can travel through a light-year of lead before its chance of an interaction approaches certainty. Waiting for such a particle therefore means building a vast target of pure water, burying it under a mountain of rock to screen out the cosmic-ray noise, and then sitting in the dark, listening for the faint flash when, once in a blue moon, a neutrino actually stops by.

B. The first concrete puzzle came from the Sun. Theorists calculated how many electron neutrinos the Sun ought to produce as it burns its nuclear fuel, and early detectors — vast tanks deep in mines — began to count them. To everyone's surprise, only about one-third of the expected number arrived on Earth. Either the solar models were wrong, or something happened to the missing neutrinos on their eight-minute journey. The discrepancy became known as the solar neutrino problem, and it nagged at physics for decades. A second, related puzzle turned up in "atmospheric" neutrinos, the kind produced when cosmic rays crash into the atmosphere overhead. Detectors on the ground could tell roughly how many should rain down from the sky above, and also how many should arrive having travelled all the way through the Earth from the atmosphere on the opposite side. In theory the two counts should match.

C. In 1998 an experiment called Super-Kamiokande, set in a zinc-mine cavern deep under a Japanese mountain, reported that they did not. The detector, a towering stainless-steel tank filled with 50,000 tonnes of ultra-pure water and lined with thousands of light-sensitive detectors, observed that muon-type neutrinos arriving from directly overhead — which had travelled only a few dozen kilometres from the atmosphere — appeared in roughly the expected numbers. But muon neutrinos coming from the other side of the Earth, which had traversed thousands of kilometres of rock, arrived at barely half that figure. Takaaki Kajita, working with the Super-Kamiokande team, concluded that the disappearing particles had not vanished at all; they had changed. During their longer journey, a proportion of the muon neutrinos had metamorphosed into a third type, the tau neutrino, which the detector was not set up to count. The logic was elegant. Neutrinos arriving from straight overhead had travelled only a short distance and arrived as expected; those that crossed the whole Earth had had time, en route, to become something else. The longer the journey, the more neutrino flavour was lost — exactly the signature of oscillation.

D. That metamorphosis, named neutrino oscillation, has a consequence that shook particle physics. Mathematically, a particle can only oscillate between types if it possesses mass. The Standard Model, the long-standing theory that describes known particles, had been written on the assumption that neutrinos were completely massless. The disappearing neutrinos therefore implied that the Standard Model, for all its successes, was missing something fundamental: neutrinos must carry at least a tiny mass. This was among the first solid evidence that the neat picture of particle physics built over the twentieth century did not tell the whole story. The solar neutrino puzzle fell into place at the same time: the Sun does produce the predicted number of electron neutrinos, but some of them convert into the other types during their flight, which is why detectors tuned to only one flavour found too few. The same maths also implied that the three neutrino flavours are not pure, distinct particles but mixtures, each one a blend of underlying masses that wobble into one another as they travel.

E. Independent experiments soon confirmed the picture. At the Sudbury Neutrino Observatory in Canada, researchers could count all neutrino types at once and showed that the total arriving from the Sun matched theoretical predictions exactly, while the electron-type fraction alone was low — precisely the oscillation signature. For this work, Kajita and the Canadian experiment's Arthur McDonald shared the 2015 Nobel Prize in Physics, following an earlier 2002 Nobel for Masatoshi Koshiba, whose pioneering detector had opened the field. Later, accelerator experiments such as K2K and T2K produced neutrinos artificially and watched them oscillate over controlled distances, pinning down further details. These human-made beams were crucial because they let physicists vary the distance and the starting flavour at will, turning a mysterious shortage into a measurable, repeatable laboratory effect. What began as a frustrating accounting shortage — too few particles arriving — ended by forcing a quiet revolution, revealing that even the lightest, most ghostly inhabitants of the cosmos are heavier, and stranger, than anyone had dared to assume. Every second, that quiet fact passes through the reader's body without a single touch.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. The solar mystery — neutrinos that failed to arrive ii. How neutrinos are produced in the laboratory iii. Half as many neutrinos from below — and the idea of transformation iv. The engineering of a lead shield v. Why oscillation means the Standard Model is incomplete vi. The history of the atomic bomb vii. Independent confirmation and the Nobel recognition

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. Why are neutrinos called "ghost particles"? A. They emit a faint glow. B. They almost never interact with ordinary matter. C. They appear only at night. D. They are invisible to all detectors.

6. What did early solar-neutrino detectors find? A. Exactly the predicted number of electron neutrinos. B. Only about one-third of the expected number. C. No neutrinos at all. D. More neutrinos than predicted.

7. What did Super-Kamiokande observe about muon neutrinos from below? A. They arrived in exactly the expected number. B. They arrived at about half the expected number. C. They arrived twice as often. D. They were indistinguishable from light.

8. Why does neutrino oscillation imply neutrinos have mass? A. Oscillation between types is mathematically possible only if the particle has mass. B. Heavy particles always travel faster. C. Oscillation requires a magnetic field. D. The Standard Model proves it.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. Neutrinos produced by the Sun take about eight minutes to reach Earth.
10. Super-Kamiokande is located in an observatory on the surface of a mountain.
11. The Standard Model originally assumed that neutrinos had no mass.
12. The Sudbury Neutrino Observatory counted only electron-type neutrinos.
13. Kajita and McDonald shared the 2015 Nobel Prize in Physics.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

Muon neutrinos travelling through the Earth sometimes change into the unobserved (14) __________ neutrino. This phenomenon, called oscillation, requires neutrinos to carry a tiny (15) __________.

答案与解析

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