Spring Semester 2012 Farid Ould-­‐Saada Heavy ion collisions and quark-gluon plasma
•  Section 5.5: quark gluon plasma
•  CERN Summer student lecture program
2010  Carlos Lourenço, CERN
•  ALICE and Heavy Ions at LHC  Trine
Tveter
•  The ALICE experiment Henrik Qvigstad,
CERN visit 2012

Confinement   In QCD at normal energy-­‐densities  q and g confined within hadrons (confinement not understood)   At extremely high E-­‐densities QCD predicts de-­‐confinement of q an g across volume larger compared to that of a hadron    new state of matter: quark-­‐gluon plasma ▪  first µs after Big-­‐Bang? Neutron stars? ▪  Lattice QCD  transition energy~160-­‐190 MeV  1012K 
Stages in formation of q-­‐g plasma and subsequent hadron emission (a)  2 heavy nuclei collide at HE
(b)  interact via colour field
(c)  de-confinement and plasma formation and possible
radiation of photons and lepton-pairs
(d)  As plasma cools, hadrons condensate and are
emitted
5/29/12
F. Ould-Saada
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RHIC gold-­‐gold ion collisions at 200GeV per nucleon Head-­‐on collisions  several 1000 final state particles produced 
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  View of a 200 GeV gold-­‐gold interaction in STAR detector at RHIC   ALICE, ATLAS, CMS data at LHC, CERN at some TeV/nucleon  q-­‐g-­‐plasma signatures: 
  Copious production of strange particles in excess of pp collisions    suppression – open charm (D-­‐mesons) favoured 
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Under which conditions can a q-­‐g plasma be made? What are the rules governing evolution and transition to and from this kind of matter? Crucial role in understanding basic nature of confinement! 5/29/12
F. Ould-Saada
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The following slides (except the last four) are from lectures
given by Carlos Lourenço
At Summer student lectures, CERN 2010
http://indico.cern.ch/scripts/SSLPdisplay.py?stdate=2010-07-05&nbweeks=7
From High-Energy Heavy-Ion Collisions to Quark Matter
Carlos Lourenço, CERN
CERN, July 2010
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Quarks, Gluons and the Strong Interaction
A proton is a composite object
made of quarks...
and gluons
No one has ever seen a free quark;
QCD is a “confining gauge theory”
“Confining”
V(r)
r
“Coulomb”
Hadrons = particles composed of quarks and gluons
baryons: protons, neutrons, etc : 3 quarks
mesons: pions, kaons, J/ψ, etc : 1 quark + 1 antiquark
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A very very long time ago... quarks and gluons were “free”.
As the universe cooled down, they got confined into hadrons
and have remained imprisoned ever since...
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The QCD phase transition
QCD calculations indicate that, at a critical temperature around 170 MeV, strongly
interacting matter undergoes a phase transition to a new state where the quarks
and gluons are no longer confined in hadrons
We can create a system of deconfined quarks and gluons
•  by heating
•  by compression
Lattice QCD calculations
Hadronic
Nuclear
Matter
Matter
Quark
Gluon
Plasma
(confined)!
deconfined
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The phase diagram of water
solid
liquid
vapor
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Temperature
Tc
Early universe
The phase diagram of QCD, today
quark-gluon plasma
hadron gas
nucleon gas
nuclei
ρ0
net baryon density
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Temperature
The true phase diagram is substantially more complex
ρ0"
baryon density
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How do we study bulk QCD matter ?
•  We collide heavy nuclei at very high energies
 producing a large volume of hot and compressed baryonic matter
Pb
Pb
p
π-
π+
12
The time evolution of the QCD matter produced in HI collisions
soft physics
regime
hard (high-pT) probes
The “fireball” evolution:
•  Starts with a “pre-equilibrium state”
•  Forms a QGP phase (if T is larger than Tc)
•  At chemical freeze-out, Tch, hadrons stop being produced
•  At kinetic freeze-out, Tfo, hadrons stop scattering
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Two labs to recreate the Big-Bang
AGS : 1986 – 2000
•  Si and Au beams ; √s ~ 5 GeV
•  only hadronic variables
RHIC : 2000 – ?
•  Au beams ; up to √s = 200 GeV
•  4 experiments (only two remain)
SPS : 1986 – 2003
•  O, S, In, Pb beams ; √s ~ 20 GeV
•  hadrons, photons and dileptons
LHC : 2010 – ?
•  Pb beams ; up to √s = 5500 GeV
•  ALICE, CMS and ATLAS
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One Pb-Pb collision seen by the NA49 TPCs
at the CERN SPS (fixed target)
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One Au-Au collision seen by the STAR TPC
Momentum determined by track
curvature in magnetic field…
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What is the question ?
We want to study the nature of Quantum Chromo-Dynamics under the extreme
conditions which occurred in the earliest stages of the evolution of the Universe
We collide high-energy heavy nuclei, to produce hot and dense strongly
interacting matter, over extended volumes and lasting a finite time;
but the produced system evolves (expands) very fast...
How can we “observe” the properties of the QCD matter we create in this way ?
How can these “observations” be related to the predicted transition to a phase
where colour is deconfined ?
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Another challenge: creating and calibrating the probes
The “probes” must be produced together with the system they probe !
They must be created very early in the collision evolution, so that they exist
before the QGP might be formed :
⇒ jets, quarkonia states
We must have “trivial” probes, not affected by the dense QCD matter, to serve as
baseline reference :
⇒ photons, Drell-Yan dimuons
We must have “trivial” collision systems, to understand how the probes are affected
in the absence of “new physics” :
⇒ pp, p-nucleus, light ion collisions
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“Tomography” of the QCD matter produced in HI collisions
Tomography in medical imaging :
The measured absorption of a calibrated probe
gives the 3-D density profile of the medium.
Tomography in heavy-ion collisions :
- Jet suppression gives the density profile of the matter
- Quarkonia suppression gives the state
(hadronic or partonic) of the matter
Note: “heavy ion physics” is much more than jet and quarkonium suppression…
These are just two QGP signatures… easier to explain than the others 
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What is jet quenching ?
In pp, expect two
back-to-back jets
In the QGP, expect
mono jets...
The “away-side” jet
gets absorbed by
the dense QCD
medium
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Dense matter absorbs quarks and gluons… but not photons
Quarks and gluons interact in the
medium and lose energy
High energy photons shine
through the dense QCD medium
photon
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Jet quenching: setting the stage with pp collisions
Azimuthal correlations show
strong back-to-back peaks
Azimuthal Angular Correlations
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Jet quenching: discovery mode with central Au-Au collisions
Azimuthal correlations show
that the “away-side jet” has
disappeared...
if we only detect high-pT
particles, pT > 2 GeV/c
Azimuthal Angular Correlations
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Jet quenching: where is the energy gone?
The “away-side” energy is no
longer collimated into jets but
distributed over many soft
particles…
seen when the pT threshold
is lowered to 200 MeV/c
Azimuthal Angular Correlations
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The “centrality” of a nucleus-nucleus collision
The signals of QGP formation should show up in the most “central” nuclear collisions
Geometrical parameters of the collision centrality :
•  b = impact parameter
distance between centers of colliding nuclei,
perpendicular to the beam-axis
•  Number of participant nucleons: Npart
•  Number of binary nucleon-nucleon collisions: Ncoll
None of them is directly measured !
Must be derived from measured variables, through models
b
participants
spectators
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Peripheral collision
26/49
Mid-central collision
27/49
Central collision
28/49
Jet suppression in heavy-ion collisions at RHIC
RHIC
pp
d-Au
a ta
d
e
c
n
refere
cess
o
r
p
e
photons referenc
central
Au-Au
Two-particle azimuthal correlations show
back-to-back jets in pp and d-Au collisions;
the jet opposite to the high-pT trigger particle
“disappears” in central Au-Au collisions
The pion yield in central Au-Au is
5 times lower than expected from
pp collisions but the photons are
not affected by the dense medium
Interpretation: the produced hard partons (our probe) are “anomalously absorbed”
by the dense colored medium created in central Au+Au collisions at RHIC energies
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The little bang: a summary by the Scientific American
QGP
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Back to the future: LHC experiments will study QGP properties
The search for evidence of QGP
formation and the study of its
properties will start at the LHC in
November, when Pb ions will
collide at much higher energies
than ever before
RHIC
SPS
LHC
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LHC: the Large Heavy-ion Collider
CMS
ALICE
ATLAS
Details in the next lectures…
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Observation of a centrality-­‐dependent dijet asymmetry in lead-­‐lead collisions at √sNN = 2.76 TeV with the ATLAS detector at the LHC http://prl.aps.org/abstract/PRL/v105/i25/e252303 5/29/12
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F. Ould-Saada
5/29/12
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  ALICE and Heavy Ions at LHC  Trine Tveter   The ALICE experiment Henrik Qvigstad, CERN FYS3510 visit 2012