PrepTest 157, Section 2, Question 7
The mu mesons generated by cosmic rays just(107 more words not shown)
Which one of the following(5 more words not shown)
- Atake longer to(5 more words not shown)
- Bare quite difficult(5 more words not shown)
- Care much less(4 more words not shown)
- Ddecay more slowly(6 more words not shown)
- Eare probably not(4 more words not shown)
Reading the argument
We don't need to know what a mu meson is. We need to track how long it lasts.
The laboratory ones are almost at rest, and they decay very quickly. In fact, their typical lifetime is much shorter than the time needed for the fast-moving ones to travel from outside the atmosphere to our detectors. If the traveling ones decayed at that same rate, very few would survive the trip.
What we’re looking for
But we detect far more than that prediction allows—about a hundred times as many. Something is wrong with applying the laboratory decay rate to the fast-moving particles. The natural completion is that the fast-moving ones decay more slowly, so more of them last long enough to reach the detectors. Follow that comparison rather than getting stuck on the unfamiliar scientific vocabulary.
Working the answers
- A
A longer trip wouldn't explain why so many particles survive to be detected. If they were decaying at the laboratory rate, extra travel time would give them even more opportunity to decay before arriving. The argument points toward a difference in how long the particles last, not a longer journey.
- B
Detection is part of the evidence, but the passage doesn't establish that the equipment has difficulty detecting these particles. In fact, we detect far more than the laboratory decay rate would predict. A claim about difficult detection doesn't supply the missing explanation for that large count.
- C
We haven't been given an earlier estimate of the overall number of particles to revise downward. The comparison is between the number actually detected and the much smaller number expected if the particles decayed at the laboratory rate. Calling them less numerous doesn't explain that contrast.
- DCorrect
The fast-moving mu mesons decay more slowly than the ones almost at rest. That allows many more to remain intact during the trip to Earth. The conclusion addresses the precise assumption that produced the inaccurate prediction: that both groups would decay at the same rate.
- E
The passage identifies cosmic rays as the source of these particles. It gives us no reason to reject that starting point. The surprise concerns their survival during the trip, so changing their origin doesn't follow from the comparison.
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