Ultrarelativistic Heavy Ion Collisions

Simulation of the time evolution of a collision between two Lorentz contracted heavy nuclei
Simulation of the time evolution of a collision between two Lorentz contracted heavy nuclei

According to our current knowledge, protons and neutrons - the constituents of the atomic nucleus are composed of smaller particles, the so-called quarks. The carrier of the forces between these constituents are the gluons. Up to now quarks have not been observed as free particles. According to nowadays accepted theories, at times shortly after the big bang space was occupied by freely roaming quarks and gluons, the so-called quark-gluon plasma. During the expansion and cooling of the universe, the basic building blocks of our matter (protons and neutrons) were created by combining quarks and gluons. Today the temperature and density of this phase transition can be computed quite precisely in extensive QCD lattice calculations. In addition, there exists an experimental program at CERN and other facilities for several years. In this program physicists try to study the properties of quark matter, which is presumably produced for a very short period in ultrarelativistic heavy ion collisions.

Our research group is continuing the work of the group of Prof. Dr. R. Santo, who participated in the first experiments of this kind at the SPS accelerator at CERN. The measurement of direct photons in these experiments yielded a significant part of our current knowledge of high-energy, dense nuclear matter.

At present, the new heavy ion collider RHIC at Brookhaven National Laboratory, USA, produces collisions of heavy ions, which are an order of magnitude more energetic than those at the CERN SPS. The experimental program has been running since the year 2000, and has already provided data from colliding gold nuclei, from deuteron-gold, as well as proton-proton collisions. Our research group is participating in the PHENIX-experiment, one of the two big experiments (400 members, 12 countries) at RHIC. We have built and are in charge of a lead glass array consisting of about 10,000 individual detector modules. With this calorimeter spectra of neutral pi-mesons and photons can be measured up to high transverse momenta. These measurements provide important input for the investigation of highly excited hadronic or partonic matter, i.e. the quark-gluon plasma. One of the most important outcomes of the first years was the measurement of jet suppression at high transverse momentum, which is widely regarded as caused by the formation of partonic matter during central collisions of heavy ions. This jet suppression was measured by PHENIX for charged and neutral pions, the latter mainly by our group using the above mentioned lead glass calorimeter.

Schematic layout of the PHENIX-Experiment at the RHIC (BNL, USA)
Schematic layout of the PHENIX-Experiment at the Relativistic Heavy Ion Collider RHIC (Brookhaven National Laboratory, USA)

Members of our group are frequently visiting RHIC to participate in beam times and are also taking part in data analyses for their diploma and doctoral theses.

Another huge increase in available energy compared to RHIC is planned with the Large Hadron Collider (LHC). The machine is scheduled for operation in 2007 at the European center for high energy physics, CERN. In addition to the planned experiments concentrating on proton-proton collision, there is also a dedicated heavy ion experiment, called ALICE. One of the central detectors of ALICE is a big Transition Radiation Detector (TRD), which we are planning and building in close collaboration with other European institutes and universities.

Transition radiation x-rays are produced by the passage of highly relativistic charged particles through layers of material with different indices of refraction. Transition radiation detectors are unique tools for separating high energy electrons and positrons from charged pions. Unlike pions, electron and positrons are not subject to the strong force. This makes them ideal probes to study the hottest and densest phase of such collisions. In ALICE the TRD will be used to study the production of J/Psi and Y-particles. In addition, the detector is used as a fast second level trigger system for charged particles with high momentum.

The TRD, a six-layered barrel, shown as component 4 of the ALICE setup, consists of 540 individual modules and covers at total area of about 740 m2. Our group is involved in the detector development, particularly in the design and test of the radiators, in which the transition radiation is produced. The group is responsible for developing and constructing the radiators of the entire detector. To this end we have set up a test facility for automatic x-ray transmission measurements of various materials. We regularly perform test measurements to determine the particle identification properties at various facilities like GSI, Darmstadt and CERN, Geneva.

Along with the development of the detector hardware goes the development of special software packages for the TRD for electron-pion separation and momentum determination.

Cut view of the ALICE experiment at the CERN LHC (ALICE)
Cut view of the ALICE experiment at the CERN LHC (ALICE)

During operation ALICE will generate about 2PB (1015 bytes) of data per year. This unprecedented data volume of an experiment needs a completely revised analysis strategy. Therefore, ALICE participates in the design and use of software for the so-called LHC Computing Grid (LCG). The LCG is a system of world-wide distributed computing resources. Our group is participating in the development of framework software for operating such Grids as well as utilizing them for the purpose of simulations and future data analysis.

Recently, the Wissenschaftsrat (the scientific advisory body to the federal government) has recommended a heavy ion collider for construction at GSI, Darmstadt. This collider will allow to study highly compressed baryonic matter. However, the beam rates anticipated there exceed the capabilities of existing detector technologies by about an order of magnitude. As part of a joint European effort, funded by the EU, our group will develop novel gas detectors with small cell sizes and high rate capability. In collaboration with other institutes the aim is to also design special low noise, low power, and high speed readout electronics. Aside from their use in particle physics experiments these detectors also bear an enormous potential for use in medical imaging with x-rays.