The world of particle physics is waiting for the beam injection into the LHC scheduled for late 2009. In the mean time, the Collaborations working at the experiments sitting around the 27 km long ring are tuning their tools. How do we work in absence of beam ? We detect very energetic particles coming from the sky!
Cosmic rays are mainly protons with large energy (orders of magnitude larger than the energy of particles produced in accelerators) originating on the Sun or during to-be-understood events in the far Universe. When the cosmic ray enters the atmosphere, it interacts with oxygen molecules and creates a shower of particles. These particles, called pions and kaons, eventually decay into muons. Muons are what ATLAS can see as they travel the 100 m of rock under which our detector is installed.
From detection point of view, the main difference between signals produced by the muons from the sky and the particles from the beam interaction is the nature of the particle itself (during collisions we’ll produce electrons, photons, jets of hadrons, along with muons) and its direction (cosmic rays are vertical from top down, while collision particles will stem from the interaction point and fly outwards).
Nevertheless, large collection of cosmic rays are crucial for detector commissioning, calibration, and alignment. Besides testing almost all 100 million channels constituting the ATLAS detector, the cooling systems, the read-out electronics, etc … the so called “cosmic run” (period of time dedicated to record cosmic events) is designed to verify how robust the data acquisition system is. Let me step back for some necessary introduction.
When LHC is operating, bunches of protons will collide 40 million times a second (each bunch contains 100000000000 protons). We expect an average of 1 billion collisions a second in the detector. Not all of these are interesting events and not all of these could possibly be recorded! ATLAS designed a smart system for selecting interesting events. The selection system is the trigger system. It is a three stage system, where the first so called “level” is hardware, while the second and the third ones are applications running on ~500 and ~2000 dual-pc processors. The reduction rate leads to have 100000 events a second surviving the first level of trigger and 3000 and 200 the second and the third one, respectively. The data needs to be reconstructed (all patterns of signal combined together to understand what type of physics process occurred in a given event) as soon as the data are accepted by the full system.
TRIUMF ATLAS team members with one of the control stations for the TRIUMF Tier 1 (Vancouver, Canda).
The “Tier 0”, a parallel computer network with 100,000 CPUs is set up to immediately store and manage the raw data (1s and 0s of binary code) produced by ATLAS. The Tier 0 then distributes data via fiber optic lines to 11 “Tier 1” sites across North America, Asia and Europe. “To put this into perspective, an average household computer with a very good connection may be able to download data at a rate of one or two megabytes per second (if you are very lucky! I get 500 kilobytes/second). So, LHC engineers have designed a new kind of data handling method that can store and distribute petabytes (million-gigabytes) of data to LHC collaborators worldwide (without getting old whilst waiting for a download).” (Ian O’Neill)
The cosmic run provides a full practice for the entire system. Data from the detectors are accepted by the trigger system and assembled online. The quality of the data recorded is assessed by detailed monitoring systems capable of reporting failure modes on a short time scale (collision data are very precious! this is the reason why dozens of experts are sitting in the ATLAS Control room and in remote sites 24/7!). The beginning of the offline processing of the data at the Tier 0 starts not later than 1 hour after data is recorded. The ATLAS Collaboration, for the second time in 2009, turned the full detector on and recorded millions of cosmic events in almost 2 weeks of data taking. The activity was intence (and tense!), but the detector performed well. ATLAS is ready for beam … and physics!
