From High-Energy Heavy-Ion Collisions to Quark Matter Episode I : Let the force be with you
Carlos Lourenço, CERN CERN, August, 2007 1/41 The fundamental forces and the building blocks of Nature
Gravity • one “charge” (mass) • force decreases with distance
m1 m2
Electromagnetism (QED) Atom • two “charges” (+/-) • force decreases with distance
+ -
+ +
2/41 Atomic nuclei and the strong “nuclear” force
The nuclei are composed of: quark • protons (positive electric charge) • neutrons (no electric charge)
They do not blow up thanks to the neutron “strong nuclear force” • overcomes electrical repulsion • determines nuclear reactions
• results from the more fundamental colour force (QCD) → acts on the colour charge of quarks (and gluons!) → it is the least well understood force in Nature
proton
3/41 Confinement: a crucial feature of QCD
We can extract an electron from an atom by providing energy electron
nucleus
neutral atom
But we cannot get free quarks out of hadrons: “colour” confinement !
quark-antiquark pair created from vacuum quark
Strong colour field “white” π0 “white” proton 2 EnergyE grows = mc with separation! (confined quarks) (confined quarks) “white” proton
4/41 Quarks, Gluons and the Strong Interaction
A proton is a composite object No one has ever seen a free quark; made of quarks... QCD is a “confining gauge theory” and gluons
V(r) kr “Confining”
r 4 α − s 3 r “Coulomb”
5/41 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...
6/41 The QCD phase transition
Quantum Chromo Dynamics (QCD) is the theory that describes the interactions between quarks and gluons [Nobel Prize 2004]
QCD calculations indicate that, at a critical temperature, Tc 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
hadrons
quarks and gluons
How hot is a medium of T ~ 170 MeV?
7/41 Temperature at the centre of the Sun ~ 15 000 000 K
Temperature of the matter created in heavy ion collisions T ≈ 170 MeV ⇒ more than 100 000 times hotter !!!
8/41 The phase diagram of water
9/41 The phase diagram of QCD
quark-gluon plasma Early universe Early Tc
nucleon gas nuclei
ρ0 net baryon density
10/41 The first QCD Phase Diagram
N. Cabibbo and G. Parisi, Phys. Lett. B59 (1975) 67
11/41 How do we study bulk QCD matter?
• We must heat and compress a large volume of QCD matter • It can be done in the lab by colliding heavy nuclei at very high energies
Pb208
Pb208 π+ p π−
• When 2 nuclei of 208 nucleons collide, each “participating nucleon” interacts around 4 or 5 times, on average!
•At √s = 20 GeV, around 2200 hadrons are produced in central Pb-Pb collisions! (to be compared to the 8 or so produced in pp collisions)
12/41 The time evolution of the matter produced in HI collisions
soft physics regime
hard (high-pT) probes
The “fireball” expands through several phases: • Pre-equilibrium state
• Quark-gluon plasma phase, T>Tc
•At chemical freeze-out, Tch, hadrons stop being produced
•At kinetic freeze-out, Tfo, hadrons stop scattering 13/41 Kinetic freeze-out
Chemical freeze-out
14/41 Two labs to recreate the Big-Bang
• AGS : 1986 - 2000 • Si and Au beams ; up to 14.6 A GeV • only hadronic variables
• RHIC : 2000 - ? • Au beams ; up to sqrt(s) = 200 GeV • 4 experiments
• SPS : 1986 - 2003 • O, S and Pb beams ; up to 200 A GeV • hadrons, photons and dileptons
• LHC : 2008 - ? • Pb beams ; up to √s = 5.5 TeV • ALICE, CMS and ATLAS
15/41 The CERN SPS heavy ion physics program
• 1986 : Oxygen at 60 and 200 GeV/nucleon Since 1986, many SPS experiments • 1987 – 1992 : Sulphur at 200 GeV/nucleon studied high-energy nuclear collisions • 1994 – 2002 : Lead from 20 to 158 GeV/nucleon to probe high density QCD matter • 2003 : Indium at 158 GeV/nucleon • and p-A collisions: reference baseline
dimuons 2004 In hadrons NA60 multistrange dielectrons
2000 photons NA57 hadrons hadrons NA49 strangelets Pb NA50 WA98 WA97 CERES NA52 1994 NA44 dimuons 1992 hadrons WA93 WA94 Helios-3 S NA35 NA36 NA38 O WA80 WA85 Helios-2 1986
16/41 One Pb-Pb collision seen by the NA49 TPCs at the CERN SPS (fixed target)
17/41 The Relativistic Heavy Ion Collider (RHIC)
1 km PHOBOS RHIC BRAHMS PHENIX STAR
AGS
TANDEMS
18/41 The RHIC experiments
• Successfully taking data since year 2000 • Au+Au collisions at √s = 200 GeV complemented by data collected at lower energies and with lighter nuclei • Polarized pp collisions at 500 GeV also underway (spin program)
STAR 19/41 One Au-Au collision seen by the STAR TPC
Momentum determined by track curvature in magnetic field…
20/41 21/41 Reminder: what’s the idea?
We would like to understand the nature of Quantum Chromo-Dynamics (QCD) under the kind of extreme conditions which occurred in the earliest stages of the evolution of the Universe
We do experiments in the laboratory, colliding 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 (and chiral symmetry is approximately restored)?
22/41 “Seeing” what the atoms are made of
The first exploration of subatomic structure was undertaken by Rutherford, in 1909, using Au atoms as targets and α particles as probes
Interpretation: The positive charge is concentrated in a tiny volume with respect to the atomic dimensions
23/41 Seeing what the nucleons are made of
The deep inelastic scattering experiments made at SLAC in the 1960s established the quark-parton model and our modern view of particle physics
proton p electron 2
p1
The angular distribution of the scattered electrons is determined by the distribution of charge inside the proton
Constant form factor ⇒ scattering on point-like constituents of the protons ⇒ quarks
1990 Nobel Prize in Physics
24/41 “Seeing” the QCD matter formed in heavy-ion collisions
We also study the QCD matter produced in HI collisions by seeing how it affects well understood probes, as a function of the temperature of the system (centrality of the collisions)
Matter under study
Probe QGP ? Calibrated “probe source” Calibrated “probe meter”
Calibrated heat source
25/41 Challenge: find the good probes of QCD matter
The good QCD matter probes should be:
vacuum Well understood in “pp collisions”
Only slightly affected by the hadronic hadronic matter, in a very well understood way, matter which can be “accounted for”
QGP Strongly affected by the deconfined QCD medium...
Jets and heavy quarkonia (J/ψ, ψ’, χc, Υ, Υ’, etc) are good QCD matter probes !
26/41 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 are there before the matter to be probed: ⇒ hard probes (jets, quarkonia, ...)
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, d-Au, light ions
27/41 “Tomography” of the produced QCD matter
Tomography in medical imaging: Uses a calibrated probe, and a well understood interaction, to derive the 3-D density profile of the medium from the absorption profile of the probe.
“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
28/41 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
29/41 The quarks and gluons get stuck
High energy quarks and gluons created in the collision are expected to be absorbed while trying to escape through the deconfined QCD matter
30/41 Jet quenching: setting the stage with pp collisions
Azimuthal correlations show strong back-to-back peaks
Azimuthal Angular Correlations
31/41 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
32/41 The “centrality” of a nucleus-nucleus collision b = impact parameter distance between colliding nuclei, perpendicular to the beam-axis large b: peripheral collisions b small b: central collisions not measured ! must be derived from measured variables, through models
participants Quantitative measures of the collision centrality:
• Number of participant nucleons: Npart
• Number of binary nucleon-nucleon collisions: Ncoll • Multiplicity density of charged particles at
mid-pseudorapidity: dNch/dη (η=0)
• Forward hadronic energy: EZDC • Transverse energy: E T spectators ... among others 33/41 Peripheral Event
STAR 34/41 Mid-Central Event
STAR 35/41 Central Event
STAR 36/41 Jet suppression in heavy-ion collisions at RHIC
RHIC pp ta ocess nce da nce pr d-Au refere photons refere
central Au-Au
Two-particle azimuthal correlations show The photons are not affected by back-to-back jets in pp and d-Au collisions; the dense medium they cross the jet opposite to the high-pT trigger particle “disappears” in central Au-Au collisions
Interpretation: the produced hard partons (our probe) are “anomalously absorbed” by the dense colored medium created in central Au+Au collisions at RHIC energies
37/41 The photons shine through the dense QCD matter
High energy photons created in the collision are expected to traverse the hot and dense QCD plasma without stopping
38/41 There are many “signatures” of the QGP
Direct photons Disoriented Chiral Condensates Fluctuations in 〈pT〉, Nch Jet quenching Medium effects on hadrons mentioned in today’s lecture Particle interferometry (HBT) Particle ratios Quarkonia suppression Radial and elliptic flow mentioned in tomorrow’s lecture Spectra ⇒〈pT〉, dN/dy, dET/dy Strangeness enhancement Thermal dileptons
and many others...
For more, see for example the proceedings of the most important conferences of the field: “Quark Matter”, “Strangeness in Quark Matter”, “Hard Probes”, etc. (several per year!)
39/41 Does it matter?
Connecting Quarks with the Cosmos, a report listing the 11 most important science questions for the new century
Executive Summary
We are at a special moment in our journey to understand the universe and the physical laws that govern it. More than ever before astronomical discoveries are driving the frontiers of elementary particle physics, and more than ever before our knowledge of the elementary particles is driving progress in understanding the universe and its contents. The Committee on the Physics of the Universe was convened in recognition of the deep connections that exist between quarks and the cosmos.
40/41 11 Science Questions for the New Century
1. What is Dark Matter? 2. What is the nature of Dark Energy? 3. How did the Universe begin? 4. Did Einstein have the last word on Gravity? 5. What are the masses of the neutrinos, and how have they shaped the evolution of the Universe? 6. How do cosmic accelerators work and what are they accelerating? 7. Are protons unstable? 8. What are the New States of Matter at exceedingly high density and temperature? 9. Are there additional space-time dimensions? 10.How were the elements from Iron to Uranium made? 11.Is a new theory of matter and light needed at the highest energies?
It seems that the study of Quark Matter... matters
41/41