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The unknown pervades the universe. That which people can see, with the aid of various sorts of telescope, accounts for just 4% of the total mass. The rest, however, must exist. Without it, galaxies would not survive and the universe would not be gently expanding, as witnessed by astronomers. What exactly constitutes this dark matter and dark energy remains mysterious, but physicists have recently uncovered some more clues, about the former, at least. One possible explanation for dark matter is a group of subatomic particles called neutrinos. Neutrinos are thought to be the most abundant particles in the universe. According to the Standard Model, the most successful description of particle physics to date, neutrinos come in three varieties, called "flavors". Again, according to the Standard Model, they are point-like, electrically neutral and massless. But in recent years, this view has been challenged, as physicists realized that neutrinos might have mass. The first strong evidence came in 1998, when researchers at an experiment, based in Japan, showed that muon neutrinos produced by cosmic rays hitting the upper atmosphere had gone missing by the time they should have reached an underground detector. Its operators suspect that the missing muon neutrinos had changed flavor, becoming electron neutrinos or-more likely-tau neutrinos. Theo- ry suggests that this process, called oscillation, can happen only if neutrinos have mass. Over the coming months and years, researchers hope to produce the most accurate measurements yet. The researchers created a beam of muon neutrinos first. On the other side of the target sat a particle detector that monitored the number of muon neutrinos leaving. The neutrinos then travelled 750km (450 miles) through the Earth to a detector in a former iron mine in Soudan, Minnesota. Researchers then were able to confirm that a significant number of muon neutrinos had disappeared-that is, they had changed flavor. While their mass is so small that neutrinos cannot be the sole constituent of dark matter, they have an advantage in that they are at least known to exist. The same cannot be said for sure of another possible form of dark matter being studied by a group of physicists in Italy. If the result continues to withstand scrutiny, it would appear to be evidence for an exotic new sort of fundamental particle, known as an axion, which could also be a type of dark matter. What aspect about neutrinos has been questioned in recent years
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The unknown pervades the universe. That which people can see, with the aid of various sorts of telescope, accounts for just 4% of the total mass. The rest, however, must exist. Without it, galaxies would not survive and the universe would not be gently expanding, as witnessed by astronomers. What exactly constitutes this dark matter and dark energy remains mysterious, but physicists have recently uncovered some more clues, about the former, at least. One possible explanation for dark matter is a group of subatomic particles called neutrinos. Neutrinos are thought to be the most abundant particles in the universe. According to the Standard Model, the most successful description of particle physics to date, neutrinos come in three varieties, called "flavors". Again, according to the Standard Model, they are point-like, electrically neutral and massless. But in recent years, this view has been challenged, as physicists realized that neutrinos might have mass. The first strong evidence came in 1998, when researchers at an experiment, based in Japan, showed that muon neutrinos produced by cosmic rays hitting the upper atmosphere had gone missing by the time they should have reached an underground detector. Its operators suspect that the missing muon neutrinos had changed flavor, becoming electron neutrinos or-more likely-tau neutrinos. Theo- ry suggests that this process, called oscillation, can happen only if neutrinos have mass. Over the coming months and years, researchers hope to produce the most accurate measurements yet. The researchers created a beam of muon neutrinos first. On the other side of the target sat a particle detector that monitored the number of muon neutrinos leaving. The neutrinos then travelled 750km (450 miles) through the Earth to a detector in a former iron mine in Soudan, Minnesota. Researchers then were able to confirm that a significant number of muon neutrinos had disappeared-that is, they had changed flavor. While their mass is so small that neutrinos cannot be the sole constituent of dark matter, they have an advantage in that they are at least known to exist. The same cannot be said for sure of another possible form of dark matter being studied by a group of physicists in Italy. If the result continues to withstand scrutiny, it would appear to be evidence for an exotic new sort of fundamental particle, known as an axion, which could also be a type of dark matter. What does the word "pervades"(Line 1, Para. 1) mean
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Most worthwhile careers require some kind of specialized training. Ideally, therefore, the choice of an (1) should be made even before the choice of a curriculum in high school. Actually, (2) , most people make several job choices during their working lives, (3) because of economic and industrial changes and partly to improve (4) position. The "one perfect job" does not exist. Young people should (5) enter into a broad flexible training program that will (6) them for a field of work rather than for a single (7) .Unfortunately many young people have to make career plans (8) benefit of help from a competent vocational counselor or psychologist. Knowing (9) about the occupational world, or themselves for that matter, they choose their lifework on a hit-or-miss (10) .Some drift from job to job. Others (11) to work in which they are unhappy and for which they are not fitted.One common mistake is choosing an occupation for (12) real or imagined prestige. Too many high school students-or their parents for them-choose the professional field, (13) both the relatively small proportion of workers in the professions and the extremely high educational and personal (14) . The imagined or real prestige of a profession or a "white collar" job is (15) good reason for choosing it as life’s work. (16) , these occupations are not always well paid. Since a large proportion of jobs are in mechanical and manual work, the (17) of young people should give serious (18) to these fields.Before making an occupational choice, a person should have a general idea of what he wants (19) life and how hard he is willing to work to get it. Some people desire social prestige, others intellectual satisfaction. Some want security, others are willing to take (20) for financial gain. Each occupational choice has its demands as well as its rewards. 1（）
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