B-3 Vaporization – 0Introduction

• 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

3020100initial conditions

v ~ 0.95 ccompression ~ 2.5-3 0

particle production

expansionfragmentation

freeze-out

Au+Au at 2 AGeV

B-3 Vaporization – 3Hadrons

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 – 4Hadron creation

Complex production mechanisms

- +

K-K+

p

d

t

data from the FOPI detector

GEANT simulation for Ni+Ni at 1.93

AGeV

B-3 Vaporization – 5Strangeness 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 – 6Anisotropy 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 – 7Source 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 – 9The ‘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 TC, the internal pressure becomes strong enough to compensate the pressure due to the non perturbative vacuum and become a stable plasma.

PPQG = PTC = (90/342)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.

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 ~ 0.

TC

quark condensation

Early universe (t < 10-5 s) = QGP

chiral symmetry

SUL3 SUR

3X

qL

qL

qR qR

T TC

spontaneous break-up of the chiral symmetry

qL

qR

qR qL

qL

qRqR

qL

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 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 =0The 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.

lnT Z

V

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).

TC

T

energy density

hadron gas

QGPmixed phase

Temporal evolution of a central nucleus-nucleus collision at ultra relativisticenergies:

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 gluons3. Crossing of the phase boundary and

hadronization4. 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 photons 4. charmonium suppression2. low-mass dileptons 5. jet-quenching3. strangeness 6. fluctuations

B-3 Vaporization – 13Colliders

Machine AGS SPS RHIC LHC

sNN (GeV) 4.9 17.3 200 5500

dET/d (GeV) 192 363 625 1800?

dN b-anti b/d 170 100 25 ~ 0?

(GeV/fm3) 1.2 2.4 4.1 11.6?

nbaryon (fm-3) 1.1 0.65 0.17 ?

Central nucleus-nucleus collisions

sNN: maximum nucleon-nucleon center-of-mass energy

in a collider: Ecm = 2Einc = sNN

Normal Pb nucleus: 0 = 0.15 GeV/fm3

n0 = 0.16 fm-3

extrapolations!

B-3 Vaporization – 14Low-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 themeson (signaling the restoration of chiral symmetry) and a broadening due to conventional hadronic interactions.

ee

ee

ee

e

e

ee

ee

mee

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)

peripheral central

NA50 data

B-3 Vaporization – 16Direct 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.

pT-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 – 17Strangeness 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. H

els

trup

et a

l., Nu

cl. P

hy

s. A

68

5 (2

00

1) 4

07

c

Strong evidence of the production of deconfined matter in central Pb+Pb collisions at SPS energies (momentum: 158 A GeV/c).

B-3 Vaporization – 18Experiments

WA98

B-3 Vaporization – 20Experiments