B-3 Vaporization – 0 Introduction

1. B-3 Vaporization – 0 Introduction • Generalities • A central collision at relativistic energies • Hadrons • Hadron creation • Strangeness production (1) • Anisotropy of the fireball • Source temperature • The quark-gluon plasma • The ‘bag’ model • Lattice Quantum Chromo Dynamics • How to create a plasma • In a heavy ion collision • Colliders • Low-mass dileptons • Charmonium suppression • Direct photons • Strangeness production (2) • Experiments

2. B-3 Vaporization – 1 Generalities Definition: state of nuclear matter in central collisions of heavy nuclei at relativistic energies. It is characterized by the emission of nucleons, other hadrons, and mesons. Major interest: Exploration of the phase diagram of nuclear matter towards the phase transition from the quark-gluon plasma to the hadron gas. Limitations: • Complex dynamics • Final state interactions • Small system size • Small life time

3. B-3 Vaporization – 2 A central reaction at relativistic energies Au+Au at 2 AGeV target projectile t (fm/c) 10 20 30 0 expansion fragmentation freeze-out initial conditions v ~ 0.95 c compression r ~ 2.5-3 r0 particle production

4. B-3 Vaporization – 3 Hadrons Hadrons: particles that interact by the strong interaction Baryons:  massive particles  3 quarks  fermions: half integer spin constrained by the Pauli principle p, n, L, S, D, X, W Mesons:  intermediate mass particles  q-anti q  bosons: integer spin can not be constrained by the Pauli principle p, K, h, r, w, f, D, J/y, B, Y

5. B-3 Vaporization – 4 Hadron creation t K- K+ d p p- p+ GEANT simulation for Ni+Ni at 1.93 AGeV Complex production mechanisms data from the FOPI detector

6. B-3 Vaporization – 5 Strangeness production (1) K+ = us The evolution of strangeness production can up to now only be tested with kaons and antikaons. One observes a dependence of the strangeness production on the number of nucleons of the system and the centrality of the reaction. There is no indication of any saturation that would signal the population of a certain state. It seems in agreement with transport model calculations where the reaction times are found to be insufficient to achieve strangeness equilibration. number of participants P.Senger et al., J. Phys. G 25(1999) R59

7. B-3 Vaporization – 6 Anisotropy of the fireball Fireball: participant region of the reaction Au+Au at 11 AGeV  collective longitudinal expansion = flow isotropically emitting thermal source data N. Herrmann, Nucl. Phys. A 685 (2001) 354c

8. B-3 Vaporization – 7 Source temperature The thermodynamic temperature at the freeze-out stage can be determined from particle ratios. Chemical freeze-out happens whenever the average energy per hadron falls below 1 GeV. Despite the time scale and the dynamics involved, it seems that the system reaches a quasi-equilibrated state. baryon chemical potential N. Herrmann, Nucl. Phys. A 685 (2001) 354c

9. B-3 Vaporization – 8 The quark-gluon plasma The quark-gluon plasma is observed if the density reaches 5 to 10 times r0 and/or T> 150 MeV. The number of hadrons per volume unit is such that the hadrons lose their identity. The quarks are not belonging anymore to one particular hadron because the confinement forces are decreasing due to the presence of numerous intermediate quarks and anti-quarks.

10. B-3 Vaporization – 9 The ‘bag’ model Schematically, the quarks are placed in a bag where reigns the perturbative QCD vacuum: a vacuum really ‘empty’, i.e. where the quark condensate is zero = a vacuum where the quarks do not interact. They interact only between themselves, and then have weak masses (only few MeV for u and d flavors). The quarks are maintained in the bag due to the outside pressure which represents the ‘true’ vacuum. As a consequence, for a nucleon, this is the action of this non perturbative vacuum that confers to the quarks an effective mass of about 300 MeV. When the system reaches TC, the internal pressure becomes strong enough to compensate the pressure due to the non perturbative vacuum and become a stable plasma. PPQG = Pp TC = (90/34p2)1/4 B1/4 The TC values which are obtained via this naïve approach are close to the ones predicted by the lattice QCD calculations. ‘empty’ (perturbative) vacuum bag ‘true’ (non perturbative) vacuum pressure B: energy density QCD: Quantum Chromo Dynamics

11. B-3 Vaporization – 10 Lattice Quantum Chromo Dynamics These calculations allow to describe exactly the thermodynamical states of a quark and gluon system in interaction inside the QCD non perturbative domain around T ~ 100-300 MeV and m ~ 0. Early universe (t < 10-5 s) = QGP chiral symmetry quark condensation qL qL SUL3 SUR3 X TC qR qR T  TC spontaneous break-up of the chiral symmetry qR qL qR qR qL qL qR qL

12. B-3 Vaporization – 11 How to create a plasma Two ways to create a plasma: 1. Increase the density while keeping T=0 One fills the energy levels of the system with “existing quarks” (u,d) which leads to an increase of the density r and of the chemical potential m. m is the energy necessary to add a quark to the system and corresponds to the Fermi energy EF when T=0. It is representative of the difference between the number of quarks and antiquarks present in the system. with V: volume and Z: partition function 2. “Warm” it up while r=0 The energy density increases only because of an addition of thermal energy that is used to create quark-antiquark pairs. The system fills up with matter and anti-matter in equal proportions. Consequently, the chemical potential and the baryonic density remains zero. In the contrary, the temperature increases and the system goes from a mesonic gas phase to a hot plasma phase when T becomes higher than TC.

13. B-3 Vaporization – 12 In a heavy ion collision The plasma that one hopes to create in a heavy ion collision is in between the two situations. The created system is characterized in the same time by a non zero baryonic density (because of the addition and the compression of the initial nucleons) and by a non zero temperature (coming from the energy dissipation of the incident nuclei during the nucleon-nucleon interactions). • Temporal evolution of a central nucleus- • nucleus collision at ultra relativistic • energies: • Liberation of quarks and gluons due to the high energy deposited in the overlap region of the two nuclei. • Equilibration of quarks and gluons • Crossing of the phase boundary and hadronization • Freeze-out T QGP mixed phase TC hadron gas energy density e Therefore interesting experimental information is contained in the study of the distributions of (mostly charged) hadrons at freeze-out. Specific probes of QGP: 1. direct photons 4. charmonium suppression 2. low-mass dileptons 5. jet-quenching 3. strangeness 6. fluctuations

14. B-3 Vaporization – 13 Colliders sNN: maximum nucleon-nucleon center-of-mass energy in a collider: Ecm = 2Einc = sNN extrapolations! Central nucleus-nucleus collisions Normal Pb nucleus: e0 = 0.15 GeV/fm3 n0 = 0.16 fm-3

15. B-3 Vaporization – 14 Low-mass dileptons h ee w eep0 r/w ee f ee h ee p0 eeg The properties of the vector mesons should change when produced in dense matter, due to medium effects. In particular, near the phase transition to the quark-gluon plasma, chiral symmetry should partially restored. As a consequence, vector mesons should become indistinguishable from their chiral partners, inducing changes in the masses and decay widths of the mesons. The present measurements are not accurate enough to clearly distinguish between a change in the mass of the r meson (signaling the restoration of chiral symmetry) and a broadening due to conventional hadronic interactions. mee C. Lourenco, Nucl. Phys. A 685(2001)384c

16. B-3 Vaporization – 15 Charmonium suppression The formation of a deconfined medium should induced a considerable suppression of the charmonium rate partially due to the breaking of the c-anti c bound by scattering with energetic (deconfined) gluons.  J/Y suppression yield of Drell-Yan dimuons “normal J/Y absorption line” (absorption expected in normal nuclear matter) production rate NA50 data transverse energy peripheral central C. Lourenco, Nucl. Phys. A 685(2001)384c

17. B-3 Vaporization – 16 Direct photons The direct photons are likely to escape from the system directly after production without further interactions, unlike the hadrons. Thus, the photons carry information on their emitting source from throughout the entire collision history, including the hot and dense phase. First measurement of direct photons in the WA98 experiment The excess of measured photons in comparison to the background expected from hadronic decays suggests a modification of the prompt photon production in nucleus-nucleus collisions, or additional contributions from pre-equilibrium or thermal photon emission.  stringent test for different reaction scenarios, including those with quark-gluon plasma formation pT-dependent systematical errors T. Peitzmann et al., Nucl. Phys. A 685 (2001) 399c

18. B-3 Vaporization – 17 Strangeness production (2) pBe pPb PbPb pBe pPb PbPb The multistrange particles and antiparticles are expected to provide a sensitive observable to identify quark matter formation since, in a QGP scenario, the enhancement is expected to increase with the strangeness content of the particle (statistical hadronization). In a purely hadronic scenario (i.e. no QGP), it is not expected, since multistrange hadron production is hindered with respect to singly strange production by high thresholds and low cross-sections. WA97 experiment H. Helstrup et al., Nucl. Phys. A 685 (2001) 407c Strong evidence of the production of deconfined matter in central Pb+Pb collisions at SPS energies (momentum: 158 A GeV/c).

19. B-3 Vaporization – 18 Experiments WA98

20. B-3 Vaporization – 20 Experiments in progress