First traces from ALICE – Niels Bohr Institute - University of Copenhagen

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16 June 2008

First traces from ALICE

The very first traces have been measured in the large detector in the ALICE experiment at CERN. Danish researchers from the Niels Bohr Institute at the University of Copenhagen have carried out the test experiments on the world’s largest atomic accelerator in CERN, the European Research Centre near Geneva in Switzerland.

The traces have been measured in the large detector, Time Projection Chamber (TPC), which is an ultra-modern electronic, tree-dimensional track chamber for measuring the particles generating when atomic nuclei are accelerated and colliding with a tremendous force. The detector is five meters long and five meters in diameter, it is hollow and filled with a gas that produces ionization when particles are passing through, and in this way it is possible to read the traces after the particles.

The traces measured now come from laser beams, which are sent into the detector where they hit 336 one millimeter big mirrors that are placed along the internal side of the detector with very high accuracy. The laser beams simulate particle traces, and they are used as a known signal for calibrating the detector. The trick is that you know the laser beams very accurately, and consequently they can be used as a reference for measurements and for adjusting the electronics and software of the detector according to them.

“We were very happy when we succeeded in measuring the first traces. It is an important step demonstrating that the detector system works.” says Børge Svane Nielsen, who is a particle physicist at the Niels Bohr Institute and technical coordinator of the Danish research project.

The laser calibration system, which consists of the laser radiation, the entire optics, electronics and the software are the Danish contribution to the TPC-detector, which is an international cooperation between many research groups.

Dramatic new discoveries
In late July the 27 km long Large Hadron Collider (LHC) accelerator will be started, and already a few weeks after starting up the researchers expect the first collisions between protons. During the autumn the energy will be increased to 10 Tera Electron Volt – it is approximately 10 times more than what has previously been possible.

The LHC accelerator and the four experiments, including the gigantic ALICE experiment, are expected to open the way for new dramatic discoveries in physics, among others about the primordial soup (the so-called qvark-gluon-plasma) that is the condition in which the universe is assumed to have been immediately after the Big Bang 13.7 billion years ago.