Successful restart of the ALICE detector — Stable beam following extensive upgrades


Extensive upgrades have been made over the past two years to the ALICE experiment at the European Research Center CERN in Geneva. GSI plays a major role in the construction and operation of the giant detector which now returned to operation and delivered first data in test collisions.

During the so-called second long shutdown (LS2), the CERN accelerator LHC (Large Hadron Collider) underwent extensive upgrades and can now collide lead nuclei at rates of up to 50 kilohertz in the ALICE detector. To fully exploit this potential, the measurement setup also had to be improved. For this purpose, the Time Projection Chamber TPC could be renewed and reinstalled at the ALICE detector. A new Muon Forward Tracker was also installed. In May, the largest pixel detector ever built — the Inner Tracking System ITS — took the seat of its predecessor between the beam pipe and the TPC. The final piece of the puzzle, the Fast Interaction Trigger FIT, was installed in July.

The TPC in particular represents a real innovation: The previous TPC readout chambers could process a maximum of three kilohertz. The new chambers use so-called GEM technology (Gas Electron Multiplier) and can read out data continuously — in contrast to the previous technology, which was based on multi-wire proportional chambers. The changed method is the only option to process the LHC's new high collision rates. As a consequence, this also required new software systems for data acquisition, calibration, reconstruction and analysis.

GSI has been involved in the development of new measurement instruments, in particular in the design and construction of the ALICE TPC, and in the ALICE scientific program from the very beginning. Also this time, GSI contributed significantly to the development of the new readout chambers. A substantial part of the chambers was built in collaboration between the ALICE research department and the detector laboratory at GSI. Staff from both GSI departments also assisted in the insertion of the chambers on site at CERN. Likewise, GSI’s IT department made key contributions to the new software systems. The GSI computer center remains an integral part of the computer network for data analysis of the ALICE experiment. The expertise from the upgrades is also relevant for the future operation of FAIR. For example, continuous data streams will also be read out at the Compressed Baryonic Matter (CBM) experiment.

The work on ALICE was part of a Helmholtz-wide initiative, including, next to GSI, also the Karlsruhe Institute of Technology (KIT) and the Deutsches Elektronen-Synchrotron (DESY): a large investment fund of the Helmholtz Association was devoted to upgrades of ALICE as well as the two other experiments ATLAS and CMS for the “Full Exploitation of the Large Hadron Collider”.

ALICE is one of the four large experiments at CERN's LHC collider and in particular investigates heavy ion collisions of lead atom nuclei. When the nuclei collide with unimaginable energy, conditions like those prevailing in the first moments of the universe are created. During the collisions, a so-called quark-gluon plasma is formed for a very short time — a state of matter that existed in the universe shortly after the Big Bang. This plasma transforms back into normal matter within fractions of a second. The particles produced in the process provide information about the properties of the quark-gluon plasma. Thus, the measurements can look into the birth of the cosmos and reveal information about the basic building blocks of matter and their interactions. (CP)

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