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What does the 5 sigma mean?

5 sigma is a measure of how confident scientists feel their results are. If experiments show results to a 5 sigma confidence level, that means if the results were due to chance and the experiment was repeated 3.5 million times then it would be expected to see the strength of conclusion in the result no more than once.

One analogy is trying to find an odd dice in a set 60, just by looking at a summary of all the rolls. Imagine you have one dice that had a 5 on every face in your set of 60. After each roll of the 60 dice, you are told how many 5’s were rolled. On average you would expect ten 5’s. But as the rolls of the dice are random you might see perhaps nine 5’s the first time or twelve 5’s the second time. As you continue to roll the dice, you can measure the spread of results around your expected result of ten 5’s. One way to measure the spread is the standard deviation, or sigma. The more you roll, the smaller the standard deviation gets. After enough rolls, you might get to a point where the average number of 5’s is 11 and your standard deviation is 0.2. You expected a result of 10 but your findings are higher by 5 sigmas. So you can be 99.9999% sure you have a dodgy dice in your set.

In particle accelerators, scientists look at the particles produced by the collisions to work out what happened. Lots of particles are produced at the collisions but only some come from important reactions that scientists are looking for, e.g.a Higgs Boson decaying into two photons at a specific energy. But there are many other processes that can produce two photons at the right energy. The LHC looks at millions of particle collisions and counts the number of times two photons at the right energy are produced and compares this to the number predicted by current. Similar to the dice when there was an excess number of 5’s, the LHC looks for an excess number of times two photons are produced; with the excess number being produced by the Higgs Boson. Once the excess reaches a 5 sigma level, the Higgs is considered discovered.

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