What does “setting limits” mean in an experiment?
Experimental physicists, such as those working on CDMS, frequently are faced with the situation of quantifying what it means when they have not yet found what they are searching for. Sometimes the result is that no events have been found, but more often it is the case that events have been seen which are consistent with expected sources of backgrounds. In either case, there are mathematical formulae for calculating “90% confidence level upper limits” on the rate of the signal which hasn’t been clearly detected. Such a limit implies that if you could do the same experiment 100 times, the result would be the same (or fewer) events detected in 90 of those experiments. What is dark matter? Judging by the way galaxies rotate, scientists have known for 70 years that the matter we can see does not provide enough gravitational pull to hold the galaxies together. There must exist some form of matter that does not emit or reflect light. According to the Wilkinson Microwave Anisotropy Probe, or W
