From ATLAS Blog at CERN: “Needle in a haystack”


Vivek Jain, Indiana University
March 16, 2012

Vivek Jain

“The LHC is designed to collide bunches of protons every 25 ns, i.e., at a 40 MHz rate (40 million/second). In each of these collisions, something happens. Since there is no way we can collect data at this rate, we try to pick only the interesting events, which occur very infrequently; however, this is easier said than done. Experiments like ATLAS employ a very sophisticated filtering system to keep only those events that we are interested in. This is called the trigger system, and it works because the interesting events have unique signatures that can be used to distinguish them from the uninteresting ones.

The ATLAS Trigger and Data Acquisition System

The ATLAS trigger system is a combination of electronic circuit boards and software running on hundreds of computers and is designed to reduce the 40 MHz collision rate to a manageable 200-400 events per second. Each event is expected to be around 1 Mbyte (for comparison, this post corresponds to about 4-5 kilobytes), so you can see that we are dealing with a lot of data. And, all this has to be done in real time. In a previous post, Regina Caputo gave an overview of triggers. Here I expand on that.”

See the full blog post here.