I did say in a previous reply that I would get the answer to you in a day’s time. Here it is, although I guarantee you will not like or accept it :-
You may be talking about the Frish and Smith experiment. Here is a summary of it.
The muons coming down through the atmosphere may not be going just under light speed, but somewhat faster. In this way the muons will travel a greater distance before decaying. E.g. a muon going at double light speed will travel 1200m before decaying, a muon going at 10c will go 6000m, etc.
This possibility would likely strike a relativity supporter as absurd. So much so, that they would probably not even consider it in the first place. One reason for this has to do with conditioned thinking. Another (more scientific) reason though is because the speed of the muons has already been measured and seen to be less than light. But is that really the case?
Now it is true to say that, once in the scintillator we can measure the muon’s speed because we have a distance and a time. But keep in mind that the muons at this point have been slowed down from their original assumed speed of 0.995c and are now going much slower. Based on the fact that some of them decay within the cylinder (in 2 microseconds) and the cylinder looks 60cm deep means they must now be going 300,000m/s, or 1000 times slower than light at that point. The scintillator can’t be used to measure the original speed, only the slowed-down speed. So the real question is how is the original 0.995c speed determined?
The muon strikes the iron with a certain amount of kinetic energy. As it passes through, this energy gets steadily converted to potential energy according to the equation:-
Energy(potential) = force * distance
If the muon comes to a stop, this means the original kinetic energy of the muon has been fully converted. Kinetic energy is given by the equation:-
E(kinetic) = ½ mass * velocity^2
If the stopping distance is known, the initial velocity can be determined. IE how fast the muon was going when it hit the iron.
The ½ m v^2 equation however is for classical mechanics. There is another equation that is used in relativity. It is:-
E – E0 = m *c^2 – m0 * c^2
What is the difference between these two equations? For low velocities there is basically no difference. But for high velocities, the velocity calculated by the classical formula can allow v>c. Whereas the relativistic formula ensures v<c. The assumption therefore is that the muons entering the iron block are travelling at less than light speed, so ensuring that the final answer is also less than lightspeed.