Photon and Meson Production in Ultra-Relativistic Nucleus-Nucleus Collisions

Abstract

In high-energy collisions of heavy nuclei the interaction of baryons and their parton constituents leads to abundant production of mesons, so that a hadronic system is created with a particle density much larger than the mere baryonic superposition density of 2γρ ₀. Here ρ ₀ is the normal nuclear density of 0.17/fm³ and γ the center-of-mass Lorentz factor with a value γ ≈ 10 at the SPS energy of 200 GeV/nucleon. Collisions among the constituents of the dense hadronic system lead to thermal excitation. Eventually, a physical situation similar to the big-bang scenario might thus be established in the laboratory. This allows to investigate the existence and the nature of the phase transition to quark matter¹ which is predicted by QCD lattice calculations^{2, 3, 4, 5} in hadronic-matter systems at high energy density. The study of the thermodynamic behaviour of a strongly-interacting-matter system will provide an interesting test of the confinement property of QCD. The interaction of elementary quark and gluon constituents is weakend by colour screening at very short distances. Sufficiently hot and dense matter should therefore become a gas of noninteracting quarks and gluons which move quasifreely in a deconfined but overall colour neutral environment given by the dimensions of the dense-matter volume. Numerical simulations of statistical QCD in the non-perturbative sector of QCD are very time-consuming calculations and still suffer from severe approximations. These refer to the quark-masses and the limited lattice size. The order of the phase transition is still not firmly predicted. Calculations with infinitly large or zero quark masses yield a first-order phase transition, while finite quark masses (m _u,d = 10 MeV, m _s = 200 MeV) seem to indicate a second-order phase change⁵.