Scientists on the ALICE experiment at the Large Hadron Collider just completed the installation of a crucial component for tracking high-energy particle jets. Without it, physicists would be lacking critical tools to select which events out of billions to store and analyze.
Engineers and physicists around the world worked intensively over five years to complete the electromagnetic calorimeter, or EMCal. The United States, supported by the Department of Energy’s Nuclear Physics Office, contributed 70 percent of the project costs. Scientists installed the last two pieces of the 80-ton device on Jan. 18.
The EMCal’s heft comes from its many sheets of lead absorbers, which it needs to stop particles coming from collisions in the detector in order to measure their energy. “The calorimeter measures the energy of individual photons and electrons,” said ALICE physicist Peter Jacobs. "It's a sort of particle thermometer."
The ALICE detector’s calorimeter was specifically designed to study the most complex collisions at the LHC, those created using beams of heavy ions. These collisions recreate big-bang-like conditions and produce events with many more particles than the Large Hadron Collider’s usual collisions using beams of protons.
CERN typically smashes lead ions together each November. These collisions produce a goopy mixture, known as the quark-gluon plasma, in the center of ALICE. Occasionally, a very energetic quark or gluon, called a jet, will also be created in the collision. When this happens, the QGP gets in its way, and that interaction is important for researchers seeking to understand material which first existed in the earliest moments of the universe. The EMCal allows ALICE to select and record the rare events containing such jets, and to measure their properties precisely.
A second arm of the EMCal will be added to ALICE during the long LHC shutdown in 2013.
The two pieces of the EMCal scientists installed this year were small; they add only about 10 percent to the calorimeter’s overall coverage, Jacobs said. However, all the small parts do add up -- every new measurement gets us a little closer to the heart of the matter.