A Look at the Cosmic Primeval Soup

Since March 2010, it operates according to routine – the Large Hadron Collider LHC in Geneva, the strongest accelerator of all times. Usually, it uses hydrogen nuclei (protons) to achieve new energy records. Yet as of autumn 2010, the 27 kilometre large ring is to collide the nuclei of lead atoms for the first time. This is when the bell tolls for ALICE: The office building big, 10,000 tons heavy particle detector is specialised on analysing the energy rich collisions of the fast lead nuclei down to the tiniest detail. Physicists from the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt are involved in this to a major degree. They had a leading part in developing two of the altogether 18 subdetectors of ALICE.

By way of the lead experiments at the LHC, the physicists intend to create a state of matter as it must have been shortly after the Big Bang 13.7 billion years ago – the quark gluon plasma. This is the experts’ name for a kind of cosmic primeval soup consisting of quarks, the building blocks of protons and neutrons, as well as of gluons, “glue particles” holding together the quarks within the atomic nucleus. Immediately after its birth, the young cosmos must have consisted of this extremely hot primeval soup – if only for a few microseconds. Then it further expanded and got increasingly cold, so that the matter stars and planets are made of could originate.

The LHC can artificially produce quark gluon plasma by colliding lead nuclei at record energy, and thus can reconstruct those conditions of the early universe. Yet this “laboratory primeval soup” is likely to exist only for approximately 10-23 seconds. After that, it explodes into thousands of particles flying in all directions. ALICE will measure all these particles in as much detail as possible, so that the physicists can reconstruct the quark gluon plasma – a precise gigantic camera for highly energetic collisions.

ALICE is comprised of 18 highly complex subsystems. Two of these, the GSI built in a leading role in cooperation with the universities in Heidelberg, Frankfurt, Darmstadt and Münster. The Time Projection Chamber (TPC) measures the traces of the charged particles produced in a collision. It is of cylindrical shape, some five metres long and thick and filled with a special gas. The particles chasing through the chamber after a collision ionise this gas. This results in the creation of veritable traces of electrons, which are pulled to the cylinder caps by an electric field. There, sensors register the electrons and then a sophisticated software can reconstruct all traces – in 3D. “The TPC is like a big, three-dimensional digital camera with an extremely high number of pixels”, explains GSI physicist Dr. Ana Marin. “It is the largest of its kind world-wide.”

The Clou: The traces of these polarised particles are bent by a strong magnetic field. “By measuring the bending of the traces, we can deduce the particles’ impulse”, says Marin. “This is an important value for reconstructing the plasma.” The chamber furthermore ascertains the energy deposition of the particles in the chamber’s gas. From this, the researchers can deduce which particle type has caused which trace. A demanding task: The researchers reckon with up to 10,000 charged particles flitting through the detector after a collision.

The second component the GSI experts have considerably contributed to, is the Transition Radiation Detector. It helps the researchers to identify the electrons originating in the collision. It also contributes towards excluding physically uninteresting collisions, which then do not even have to be stored. This considerably relieves ALICE’s readout electronics.

The LHC is going to collide the lead nuclei with around 30 times the energy than smaller accelerators such as RHIC in the USA. “In the older experiments the most important thing was to verify by way indication that quark gluon plasma existed at all”, explains Ana Marin. “With the LHC, we expect the plasma to last longer and achieve a greater volume. This enables us to examine the plasma’s characteristics for the first time in greater detail.” No less than 1,500 physicists from all over the world are involved in ALICE. Ultimately, they hope to find new details about the universe’s early stages.

ALICE has already performed first measurements with proton collision and the first results are published. “The detector functions superbly”, says Ana Marin. “Everything functions as it should.” Yet the real excitement starts in autumn. That is when for a whole month lead nuclei are to circle the Geneva ring for the very first time. “And that”, says Marin, “then is ALICE’s baptism of fire.”