In a previous post, Regina gave an overview of triggers. Let me add to that and give some numbers.
When the LHC is operating at design parameters, we will have collisions every 25 ns, i.e., at a 40 MHz rate (40 million/second). Obviously, we can’t collect data at the rate, so we pick the interesting events, which occur infrequently. A trigger is designed to reject the uninteresting events and keep the interesting ones; your proverbial “needle in the haystack”, as you will see below. The ATLAS trigger system is designed to collect about 200 events per second, where the amount of data collected for each event is expected to be around 1 Mbyte (for comparison, this post corresponds to about 4-5 kilobytes).
Before I get to the numbers of events that we will collect, let me first explain a couple of concepts; cross-section of a particular process and luminosity. Cross-section is jargon; basically, it gives you an estimate of the probability of a certain kind of event happening. Luminosity is a measure of the “intensity” of the beam. The number of events that we collect of a given type is given by the product of Luminosity and Cross-section.
One common kind of interaction is when two protons “glance” off each other, without really breaking up; these are called Elastic Collisions”. Then you have protons colliding and breaking up, and producing “garden-variety” stuff, e.g., pions, kaons, protons, charm quarks, bottom quarks, etc; these are labelled Inelastic Collisions. The sum of all these processes is the “total cross-section”, and is about 100 millibarns, i.e., 1/10th of barn; the concept of a “barn” probably derives from the expression “something is as easy as hitting the side of a barn”! So, a cross-section of 100 millibarns implies a very, very large probability1; for 7 TeV collisions, this cross-section decreases by about 10-20%, i.e., not much.
In contrast, the cross-section for producing a Higgs boson (with mass = 150 GeV, i.e., 150 times the mass of a proton) is 30 picobarns (30*10-12 barns), i.e., approximately 3 billion times less than the “total cross-section” (at 7 TeV, the Higgs cross-section decreases by a factor of four). The cross-section for producing top quarks is about 800 picobarns (at 7 TeV, this is down by a factor of eight). So, you can see the need for a good trigger!
The design parameters imply a luminosity of 1034, i.e., looking head-on at the beam there are 1034 protons/square cm/second. So, taking the product of cross-section and luminosity, we estimate that we will get approximately 109 “junk events”/second and 0.3 Higgs events/second! Of course, there are other interesting events that we would like to collect, e.g., those containing top quarks will come at a rate of 8 Hz. We also record some of the “garden-variety” events, because they are very useful in understanding how the detector is working. So, this is what the trigger does, separate what we want from what we don’t want, and do it all in “real time”.
As mentioned above, we plan to write to disk approximately 200 events per second. If we run the accelerator continuously for a year, we will collect 6*1015 bytes of data, i.e., 6 petabytes; this will fill about 38,000 IPods (ones with 160 GB of storage)! Each event is then passed through the reconstruction software, which will add to its size. We have come up with ways to handle all this data; I can talk about that in a later post.
–Vivek Jain, Indiana University
p.s. For fun facts about ATLAS, check out the new ATLAS pop-up book that is coming out soon! If you are on Facebook, go here. You can also see a video of this book.
1In standard units, 1 barn = 10-24 cm2
