NA49 experiment

Background

According to the Standard Model (SM), quarks can only exists in combinations of two and three as hadrons, and a single quark cannot be alone in a vacuum. Quarks experience the strong interaction, mediated by gluon exchange, whereas hadrons experience the nuclear force, described by the complicated phenomenon of hadronic interaction. Quark Matter is the name given to the state at which quarks are deconfined from a hadron volume. Searching for Quark Matter tests the SM, in particular the strong interaction, which is predicted by the lattice gauge theory. Following the Big Bang, the Universe is supposed to have consisted of Quark Matter, and the investigation into this state could provide data for astrophysical studies.

Particle theory predicts that heating normal nuclear matter above a critical value (similarly with density also) will result in a deconfined quark-gluon matter. To produce this state, fixed target experiments are used. A thin metal foil target is bombarded with a beam of heavy nuclei accelerated close to the speed of light. Immediately after the collision, a hot and dense state of quark-gluon matter may be created, which will drive an explosive expansion. At this point the density and temperature decreases and hadrons are emitted from the matter, which can be detected by detectors.

Experimental setup

Four large-volume time projection chambers (TPC) were used for the NA49 experiment, for tracking and for particle identification. The first two TPCs were inside dipole magnets with superconducting coils, used to determine particle momentum from the bending of charged-particle trajectories. The other two TPCs were placed behind the magnets to deduce the ionisation energy loss (dE/dx) and particle velocity. The experiment also used an adapted large calorimeter from previous SPS experiments, which was able to measure the transverse energy of hadrons emitted from the collision. Time of flight (ToF) measurements were made by two scintillation counter walls, with a time resolution of 60 ps. Front end electronics were used to read out the TPCs.

The beam used was from the SPS and consisted of the isotope 208Pb, a heavy, dense nuclear species, with an energy of 33 TeV. The target used in the experiment were thin lead foils, resulting in a Pb+Pb nuclear collision when the beam was directed at it.

Results

The energy density created in the collisions of the NA49 experiment was determined to be larger than the critical value, and therefore high enough to probe into the quark-gluon matter. This was determined to be 3 GeV per cubic femtometre, which showed agreement with lattice QCD. Furthermore, the experiment was also able to determine a 'freeze-out' temperature of 120 MeV, the temperature at which collisions among the produced hadrons stop.

More results were used to determine the parton-hadron phase transition which agrees with the lattice QCD prediction. The results indicate that the nature of the phase transformation occurs with no large latent heat jump, which is subject to theoretical discussions.

References

References

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