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

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

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

B-3 Vaporization – 3 Hadrons Hadrons: particles that interact by the strong interaction Mesons:  intermediate mass particles  q-anti q  bosons: integer spin can not be constrained by the Pauli principle , K, , , , , D, J/ , B, Y Baryons:  massive particles  3 quarks  fermions: half integer spin constrained by the Pauli principle p, n, 

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

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

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

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

B-3 Vaporization – 8 The quark-gluon plasma The quark-gluon plasma is observed if the density reaches 5 to 10 times  0 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.

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. B: energy density QCD: Quantum Chromo Dynamics bag pressure ‘empty’ (perturbative) vacuum ‘true’ (non perturbative) vacuum When the system reaches T C, the internal pressure becomes strong enough to compensate the pressure due to the non perturbative vacuum and become a stable plasma. P PQG = P  T C = (90/34  2 ) 1/4 B 1/4 The T C values which are obtained via this naïve approach are close to the ones predicted by the lattice QCD calculations.

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 ~ MeV and  ~ 0. TCTC quark condensation Early universe (t < s) = QGP  chiral symmetry SU L 3 SU R 3 X qLqL qLqL qRqR qRqR T  TC  spontaneous break-up of the chiral symmetry qLqL qRqR qRqR qLqL qLqL qRqR qRqR qLqL

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  and of the chemical potential .  is the energy necessary to add a quark to the system and corresponds to the Fermi energy E F 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  =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 T C.

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). TCTC T energy density  hadron gas QGP mixed phase Temporal evolution of a central nucleus- nucleus collision at ultra relativistic energies: 1.Liberation of quarks and gluons due to the high energy deposited in the overlap region of the two nuclei. 2.Equilibration of quarks and gluons 3.Crossing of the phase boundary and hadronization 4.Freeze-out 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 photons4. charmonium suppression 2. low-mass dileptons5. jet-quenching 3. strangeness6. fluctuations

B-3 Vaporization – 13 Colliders MachineAGSSPSRHICLHC  s NN (GeV) dE T /d  (GeV) ? dN b-anti b /d  ~ 0?  (GeV/fm 3 ) ? n baryon (fm -3 ) ? Central nucleus-nucleus collisions  s NN : maximum nucleon-nucleon center-of-mass energy in a collider: E cm = 2E inc =  s NN Normal Pb nucleus:  0 = 0.15 GeV/fm 3 n 0 = 0.16 fm -3 extrapolations!

B-3 Vaporization – 14 Low-mass dileptons 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  meson (signaling the restoration of chiral symmetry) and a broadening due to conventional hadronic interactions.   ee    ee      ee    ee   ee   ee  m ee C. Lourenco, Nucl. Phys. A 685(2001)384c

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/  suppression C. Lourenco, Nucl. Phys. A 685(2001)384c transverse energy production rate yield of Drell-Yan dimuons “normal J/  absorption line” (absorption expected in normal nuclear matter) peripheralcentral NA50 data

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. p T -dependent systematical errors 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 T. Peitzmann et al., Nucl. Phys. A 685 (2001) 399c

B-3 Vaporization – 17 Strangeness production (2) pBepPbPbPb pBepPbPbPb 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).

B-3 Vaporization – 18 Experiments WA98

B-3 Vaporization – 20 Experiments