Work in this field is currently being undertaken at the new Relativistic Heavy Ion Collider (RHIC), which is situated at the Brookhaven National Laboratory in the United States. Members of the group are collaborating on the STAR experiment, which is one of four experiments at RHIC. STAR started to acquire data in the summer of 2000 when gold ions were first collided. During this initial run, the collision energy was 7 times greater than studied by earlier experiments at CERN. In 2001, the collision energy was increased to its design value, nearly 12 times greater than that achieved at CERN with similar ion species. At these unprecedented high energies, we aim to study the nature of the strong interaction in the limit of high density and high temperature, the properties of the quark-gluon plasma and its transition back to normal hadronic matter. Over the next few years RHIC will study heavy-ion collisions between a variety of nuclear species over a range of energies in an attempt to locate the critical energy density at which this transition takes place. RHIC will also pursue a programme of colliding polarised protons, in order to solve the intriguing puzzle of where the nucleon gets its spin.

Details

When two heavy nuclei are collided at very high energy, considerable compression and heating of the nuclear matter will take place. As a result of this heating and compression, it is predicted that nuclear matter will "melt" into its constituent quarks and gluons, a state of matter known as the quark-gluon plasma, or QGP. This state of matter would have existed in the first moments after the Big Bang, and may possibly be found today in the core of dense stellar object such as neutron stars.

Under normal conditions quarks are confined to hadrons by their strong mutual attraction, which is mediated by gluons. Their interaction is governed by Quantum Chromodynamics (QCD), which is an asymptotically free theory. This means that the interaction between quarks becomes weaker, the smaller the separation of the quarks. Additionally, long range interactions become screened in a high density medium, due to the well known process of Debye screening. By colliding heavy nuclei at high energy it is hoped that the density of produced particles will be so great that quarks will become deconfined over a volume comparable with the size of the colliding nuclei.

The process described above corresponds to a transition between two distinct phases of nuclear matter, from a colour insulating (quarks confined) state, to a colour conducting (quark deconfined) state, and back ! The plasma phase is very short-lived and upon expansion and cooling will hadronise into a variety of baryons, anti-baryons and mesons. The experimental challenge is to confirm the fleeting existence of the deconfined phase from these final state hadrons and subsequently to study the properties of this new state of matter.

Attempts to create the QGP started with experiments at CERN and at the Brookhaven National Laboratory in the mid-1980s. The group has been involved in experiments at CERN since the inception of its heavy-ion programme in 1986. The first CERN experiments were carried out with beams of rather light nuclei, such as oxygen and sulphur. Truly heavy-ion collisions only became possible in 1994 with the introduction of a Pb ion source. During this phase of the CERN programme, the group has collaborated on the NA49 experiment, a pioneering, large acceptance hadronic spectrometer, which uses Time Projection Chambers to take a 3-dimensional electronic picture of each collision. The Birmingham group are specialists in the reconstruction and analysis of strange particles, (hadrons containing one or more strange quarks), the production of which is predicted to be sensitive to the creation of the QGP.

The group is currently involved in the STAR experiment at the Relativistic Heavy Ion Collider (RHIC), which is situated at the the Brookhaven National Laboratory in the United States. RHIC is a new facility that was commissioned in the spring and early summer of 2000. It is now the premier facility for the study of hot and dense nuclear matter, providing collisions of nuclei ranging is mass from protons to gold, and at energies up to 10 times that achievable at CERN. Tentative evidence for the creation of the QGP was the widely reported outcome of the CERN programme following the Quark Matter conference in 1999. The goal of experiments at RHIC is to confirm these findings and at these higher energies exploit new experimental probes to study the state of matter produced in the most violent collisions.

A reconstructed Au+Au collision in the STAR TPC at a CMS energy of 130 GeV

STAR, like NA49, is a large acceptance hadron spectrometer. Its main tracking device is also a Time Projection Chamber (TPC). Arrays of silicon detectors and novel radial drift TPCs are also used to increase the acceptance of the experiment and its sensitivity to strange hadrons, that decay within a few centimetres of the primary collision.

Students working in this area are involved in research at the interface between nuclear and particle physics, which has relevance in other fields too, such as cosmology and astrophysics. They participate in experiments, involving large international collaborations, which are at the forefront of their field.