I’m beginning to think that the phrase ‘combined TileCal/Level-one trigger tests’ is synonymous with the phrase ‘TileCal is experiencing cooling failures’. The TileCal cooling system is designed to keep the electronics at an acceptable operating temperature. If the cooling fails, then the power to the system must be shut down to prevent damage to the electronics. It is not that TileCal experiences cooling failures often, but it seem we always experience cooling failures when doing combined tests with the level-one. And no cooling means no power which means no combined testing for you today. Seriously, you can set your clock by it. The first time it happens, you’re frustrated. The second time you’re laughing. The third time you’re suspicious. The fourth time, you’re convinced you are the butt of some cosmic joke.
I am exaggerating of course. Usually we recover from the failure rather quickly and are able to continue the tests. But the correlation is uncanny….
The level-one trigger is like ATLAS’ spam filter. The beam will collide inside ATLAS approximately 40 million times per second. We can’t possibly store all of that data to disk, nor would we want to. Most of those 40 million per second events aren’t very interesting. Rather I should say, they aren’t AS interesting. They are ‘old physics’, physics we have studied before. We are interested in ‘new physics’, physics we have never seen before. Here is an example:
This is a simulation of what a supersymmetry (SUSY) event might look like in the detector. (Much more complicated then the cosmic data we are taking now.) SUSY, like other theories of new physics, predicts certain types of events that will be produced at the LHC. Typically these events involve lots of particles, with lots of energy, flying everywhere as seen in the picture. The goal of the level-one trigger is to sift through those 40 million events per second, find the interesting one like SUSY and ditch the not-so-interesting ones. And it has to cut the event rate down by a factor of 500, meaning for every event that the level-one accepts, it has rejected 500 events. There are additional layers to the trigger. Once an event passes the level-one, it must also pass the ‘high level triggers’ before being written to disk. The final rate of events being stored for analysis is approximately 100-200 events per second. Imagine that. For every email in your inbox, there are 200,000 deleted as spam.
The Tile Calorimeter as well as the electromagnetic calorimeter (called the Liquid Argon Calorimeter) plays a critical role in the level-one trigger decision. In this picture, TileCal is the orange sections (the upper and lower row of orange) and the liquid argon is the gray sections (plus the two orange sections in the center on the left and right). The level-one makes a decision based on the amount of energy deposited in small regions of the calorimeters. SUSY events are predicted to have very large energy deposits. The purpose of combined tests between TileCal and the level-one is to calibrate the electronics’ signal in voltage and convert that to the amount of energy deposited in the calorimeter. If the electronics aren’t working properly or the calibration is incorrect, the trigger might delete as spam the very events that you are interested in studying. The calibration is a long process and will take many months but it is crucial to get right. Because once the events are rejected, we can’t get them back. There will be more tests on Monday. The cooling gods allowing of course.
