B3 Vaporization 0

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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
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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
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B-3 Vaporization 2 A central reaction at
relativistic energies
AuAu at 2 AGeV
target
projectile
t (fm/c)
20
10
30
0
expansion fragmentation freeze-out
initial conditions v 0.95 c

• compression
• r 2.5-3 r0
• particle production

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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 p, K, h,
r, w, f, D, J/y, B, Y
Baryons ? massive particles ? 3 quarks ?
fermions half integer spin constrained by
the Pauli principle p, n, L, S, D, X, W
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B-3 Vaporization 4 Hadron creation
GEANT simulation for NiNi at 1.93 AGeV
Complex production mechanisms
data from the FOPI detector
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B-3 Vaporization 5 Strangeness production (1)
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
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B-3 Vaporization 6 Anisotropy of the fireball
Fireball participant region of the reaction
AuAu at 11 AGeV
? collective longitudinal expansion flow
isotropically emitting thermal source
data
N. Herrmann, Nucl. Phys. A 685 (2001) 354c
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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
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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 Tgt 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.
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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
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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 lt 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
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B-3 Vaporization 11 How to create a plasma
Two ways to create a plasma 1. Increase the
density while keeping T0 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 T0. 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
r0 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.
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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
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B-3 Vaporization 13 Colliders
extrapolations!
Central nucleus-nucleus collisions
Normal Pb nucleus e0 0.15 GeV/fm3 n0
0.16 fm-3
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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 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
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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
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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
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B-3 Vaporization 17 Strangeness production (2)
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 PbPb collisions at SPS
energies (momentum 158 A GeV/c).
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B-3 Vaporization 18 Experiments
WA98
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B-3 Vaporization 20 Experiments
in progress