... ALICE forges ahead with further detectors

Following the installation of the HMPID, the project has progressed swiftly with further detectors being lowered into the ALICE cavern. The first supermodule of the ALICE transition radiation detector was successfully installed on 10 October.

In the final configuration, 18 supermodules that make up the transition radiation detector will cylindrically surround the large time projection chamber in the central barrel of the ALICE experiment. Each supermodule is about 7 metre long and consists of 30 drift chambers in six layers. The construction of the modules is a collaboration between five institutes in Germany (Universities of Frankfurt and Heidelberg and Gesellschaft fuer Schwerionenforschung mbH in Darmstadt), Romania (NIPNE Bucharest) and Russia (JINR Dubna) with radiators (See 'Did you know?' section) produced at the University of Muenster, Germany.

During the summer, the drift chambers for the first supermodule were equipped with readout electronics and inserted into the supermodule hull at the University of Heidelberg. After transportation to CERN on 27 September, the module was tested on the surface using cosmic rays before it was lowered into the ALICE cavern on 9 October. The final installation took place one day later.

The aim of the ALICE experiment is to study the nature of quark-gluon plasma, considered a fundamental key to understanding the basic structure of ordinary matter. Quarks are held together by gluons with a bond so strong that they are incredibly difficult to separate under normal circumstances. The LHC will accelerate heavy ions close to the speed of light and collide them head-on to form extremely hot and dense fireballs in which quarks and gluons are freed for a fleetingly short instant. The ALICE detectors will observe up to 20000 particle tracks created in these fireballs. The task set for the Transition Radiation Detector (TRD) is to identify, from this huge background of particles, the high-energy electron pairs that are generated. These electron pairs can be the decay products of certain signatures (quarkonia, open charm, open beauty), which can be used as probes to study quark-gluon plasma.

In some sense this task is like looking for a needle in a haystack. The transition radiation detector will provide a trigger decision to identify the rare events containing high energy electron pairs in real time. The trigger system consists of readout electronics right on the detector and other dedicated electronics off-detector. Within 6 microseconds of each collision, the detector signals have to be digitised and the data of all channels (1.2 million for the whole TRD) have to be processed. All of the particle tracks crossing the TRD need to be reconstructed in order to find the desired electrons. For this purpose, highly integrated full custom readout chips have been developed at the University of Heidelberg. In total, 15000 CPUs right on-detector are employed in each of the 18 supermodules of the TRD to serve this function.

Did you know?

Transition radiation is produced by fast charged particles as they cross boundaries between materials with different dielectric constants. Its origin is connected to the fact that the electric field of the particle is different in each material. In simple terms, the particle has to 'shake off' this difference when it crosses the boundaries. Since the emission of transition radiation is connected to particle velocity, it is often used as a signature to identify electrons. Electrons are lightweight; they therefore travel much faster than other particles that have the same momentum.

The transition radiation photons produced by electrons have wavelengths in the X-ray range. However, the number of photons produced per boundary crossing is very small. Thus, in typical transition radiation detectors, either stacks of foils of very precise thickness and foil separation or fibre or foam materials are used to provide several hundred boundaries. The radiators are placed directly in front of a particle detector, such as the ALICE TRD drift chambers, in order to detect the transition radiation photons.