1/41 From High-Energy Heavy-Ion Collisions to Quark Matter Episode I : Let the force be with you Carlos Lourenço, CERN CERN, August, 2007

2/41 The fundamental forces and the building blocks of Nature Gravity one “charge” (mass) force decreases with distance m1m1 m2m2 Electromagnetism (QED) two “charges” (+/-) force decreases with distance Atom

3/41 Atomic nuclei and the strong “nuclear” force The nuclei are composed of: protons (positive electric charge) neutrons (no electric charge) They do not blow up thanks to the “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 neutron quark

4/41 Confinement: a crucial feature of QCD Strong colour field Energy grows with separation! E = mc 2 We can extract an electron from an atom by providing energy “white” proton nucleus electron quark quark-antiquark pair created from vacuum “white” proton (confined quarks) “white”  0 (confined quarks) But we cannot get free quarks out of hadrons: “colour” confinement ! neutral atom

5/41 No one has ever seen a free quark; QCD is a “confining gauge theory” A proton is a composite object made of quarks... and gluons Quarks, Gluons and the Strong Interaction r V(r) “Coulomb” “Confining”

6/41 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...

7/41 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, T c 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 quarks and gluons hadrons How hot is a medium of T ~ 170 MeV?

8/41 Temperature at the centre of the Sun ~ K Temperature of the matter created in heavy ion collisions T  170 MeV  more than times hotter !!!

9/41 The phase diagram of water solidliquidvapor cappuccino

10/41 The phase diagram of QCD Temperature net baryon density Early universe nuclei nucleon gas hadron gas quark-gluon plasma TcTc 00

11/41 N. Cabibbo and G. Parisi, Phys. Lett. B59 (1975) 67 The first QCD Phase Diagram

12/41 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 Pb 208 ++  p 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)

13/41 hard (high-p T ) probes soft physics regime The time evolution of the matter produced in HI collisions The “fireball” expands through several phases: Pre-equilibrium state Quark-gluon plasma phase, T>T c At chemical freeze-out, T ch, hadrons stop being produced At kinetic freeze-out, T fo, hadrons stop scattering

14/41 Chemical freeze-out Kinetic freeze-out

15/41 AGS : Si and Au beams ; up to 14.6 A GeV only hadronic variables RHIC : ? Au beams ; up to sqrt(s) = 200 GeV 4 experiments SPS : O, S and Pb beams ; up to 200 A GeV hadrons, photons and dileptons LHC : ? Pb beams ; up to  s = 5.5 TeV ALICE, CMS and ATLAS Two labs to recreate the Big-Bang

16/41 NA35 NA36 NA49 Helios-2 Helios-3 NA44 CERES NA38 NA50 NA60 WA98 WA85 WA97 NA57 WA94 O Pb multistrange photons hadrons dimuons dielectrons hadrons strangeletshadrons dimuons WA80 WA93 NA52 In S Since 1986, many SPS experiments studied high-energy nuclear collisions to probe high density QCD matter 1986 : Oxygen at 60 and 200 GeV/nucleon 1987 – 1992 : Sulphur at 200 GeV/nucleon 1994 – 2002 : Lead from 20 to 158 GeV/nucleon 2003 : Indium at 158 GeV/nucleon and p-A collisions: reference baseline The CERN SPS heavy ion physics program

17/41 One Pb-Pb collision seen by the NA49 TPCs at the CERN SPS (fixed target)

18/41 The Relativistic Heavy Ion Collider (RHIC) RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS 1 km

19/41 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 The RHIC experiments

20/41 One Au-Au collision seen by the STAR TPC Momentum determined by track curvature in magnetic field…

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22/41 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)? Reminder: what’s the idea?

23/41 The first exploration of subatomic structure was undertaken by Rutherford, in 1909, using Au atoms as targets and  particles as probes “Seeing” what the atoms are made of Interpretation: The positive charge is concentrated in a tiny volume with respect to the atomic dimensions

24/41 The deep inelastic scattering experiments made at SLAC in the 1960s established the quark-parton model and our modern view of particle physics electron proton p1p1 p2p2 Seeing what the nucleons are made of Constant form factor  scattering on point-like constituents of the protons  quarks 1990 Nobel Prize in Physics The angular distribution of the scattered electrons is determined by the distribution of charge inside the proton

25/41 QGP ? 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) Calibrated “probe source” Matter under study Calibrated “probe meter” Calibrated heat source Probe “Seeing” the QCD matter formed in heavy-ion collisions

26/41 vacuum QGP hadronic matter The good QCD matter probes should be: Well understood in “pp collisions” Only slightly affected by the hadronic matter, in a very well understood way, which can be “accounted for” Strongly affected by the deconfined QCD medium... Jets and heavy quarkonia (J/ ,  ’,  c, ,  ’, etc) are good QCD matter probes ! Challenge: find the good probes of QCD matter

27/41 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

28/41 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 “Tomography” of the produced QCD matter

29/41 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

30/41 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

31/41 Jet quenching: setting the stage with pp collisions Azimuthal correlations show strong back-to-back peaks Azimuthal Angular Correlations

32/41 Jet quenching: discovery mode with central Au-Au collisions Azimuthal correlations show that the “away-side jet” has disappeared... if we only detect high-p T particles, p T > 2 GeV/c Azimuthal Angular Correlations

33/41 Quantitative measures of the collision centrality: Number of participant nucleons: N part Number of binary nucleon-nucleon collisions: N coll Multiplicity density of charged particles at mid-pseudorapidity: dN ch /d  (  =0) Forward hadronic energy: E ZDC Transverse energy: E T... among others The “centrality” of a nucleus-nucleus collision b b = impact parameter distance between colliding nuclei, perpendicular to the beam-axis large b: peripheral collisions small b: central collisions not measured ! must be derived from measured variables, through models participants spectators

34/41 STAR Peripheral Event

35/41 STAR Mid-Central Event

36/41 Central Event STAR

37/41 Jet suppression in heavy-ion collisions at RHIC RHIC pp d-Au central Au-Au Two-particle azimuthal correlations show back-to-back jets in pp and d-Au collisions; the jet opposite to the high-p T 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 reference data The photons are not affected by the dense medium they cross photons reference process

38/41 High energy photons created in the collision are expected to traverse the hot and dense QCD plasma without stopping The photons shine through the dense QCD matter

39/41 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!) There are many “signatures” of the QGP Direct photons Disoriented Chiral Condensates Fluctuations in  p T , N ch Jet quenching Medium effects on hadrons Particle interferometry (HBT) Particle ratios Quarkonia suppression Radial and elliptic flow Spectra   p T , dN/dy, dE T /dy Strangeness enhancement Thermal dileptons and many others... mentioned in today’s lecture mentioned in tomorrow’s lecture

40/41 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. Does it matter?

41/41 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