Triggering is an essential part of the Large Hadron Collider [LHC]; there are so many collisions happening each second at the LHC, compared to the number that the experiments can afford to store for later study, that the data about most of the collisions (99.999%) have to be thrown away immediately, completely and permanently within a second after the collisions occur. The automated filter, partly hardware and partly software, that is programmed to make the decision as to what to keep and what to discard is called “the trigger”. This all sounds crazy, but it’s necessary, and it works. Usually.
Let me give you one very simple example of how things can go wrong, and how the ATLAS and CMS experiments [the two general purpose experiments at the LHC] attempted to address the problem. Before you read this, you may want to read my last post, which gives an overview of what I’ll be talking about in this one.