1. 1 Part 1: Free the quarks! Part 2: Measuring dimuons in heavy-ion collisions Part 3: “The dog that didn’t bark” and other scenes from the particle zoo From High-Energy Heavy-Ion Collisions to Deconfined Quark Matter Carlos Lourenço, CERN PH-EP Part 1: Free the quarks!

2. 2 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. 3 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. 4 Analogies and differences Strong colour field Energy grows with separation! E = mc 2 to study the structure of an atom… “white” proton …we can split it into its constituents nucleus electron quark quark-antiquark pair created from vacuum “white” proton (confined quarks) “white”  0 (confined quarks) Confinement: fundamental & crucial (but not well understood!) feature of strong force neutral atom

5. 5 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. 6 A very long time ago... quarks and gluons lived free and happily bouncing of each other in a plasma state As the universe cooled down, they got confined and have remained imprisoned ever since...

7. 7 Creating a state of deconfined quarks and gluons To understand the strong force and the phenomenon of confinement: we must create and study a system of deconfined quarks (and gluons) Nuclear Matter (confined) Hadronic Matter (confined) Quark Gluon Plasma deconfined ! by heating by compression  deconfined colour matter ! Lattice QCD calculations

8. 8 Expectations from Lattice QCD calculations  /T 4 ~ number of degrees of freedom hadronic matter: few d.o.f. deconfined QCD matter: many d.o.f. QCD lattice calculations indicate that, above a critical temperature, T c, or energy density,  c, strongly interacting matter undergoes a phase transition to a new state where the quarks and gluons are no longer confined in hadrons How hot is a medium of T ~ 173 MeV?

9. 9 Temperature at the center of the Sun ~ 15 000 000 K Temperature of the matter created in heavy ion collisions T c  173 MeV ~ 2 000 000 000 000 K... it’s pretty hot!

10. 10 N. Cabibbo and G. Parisi, Phys. Lett. B59 (1975) 67 The first QCD Phase Diagram Curious “warnings” in the paper:

11. 11 The true phase diagram of QCD (?) Temperature 00 baryon density is

12. 12 The phase diagram of water

13. 13 Can we explore the phase diagram of nuclear matter ?  We think so ! by colliding nuclei in the lab by varying the nuclei size (A) and colliding energy (  s) by studying spectra and correlations of the produced particles  Requirements system must be at equilibrium (for a very short time)  system must be dense and large Exploring the Phases of Nuclear Matter Can we find and explore the Quark Gluon Plasma ?  We hope so ! by colliding large nuclei at very high energies  How high ? QCD calculations on the lattice predict: Critical temperature:T c  173 MeV Critical energy density:6  normal nuclear matter

14. 14 Bulk QCD matter We must heat and compress a large volume of QCD matter Maybe achievable by colliding heavy nuclei (Au, Pb) at high energies Thousands of particles produced in each collision White: hadrons; colored: quarks and gluons

15. 15 Simulation of a high-energy heavy-ion collision

16. 16 Chemical freeze-out (at T ch  T c ): end of inelastic scatterings; no new particles (except from decays) Kinetic freeze-out (at T fo  T ch ): end of elastic scatterings; kinematical distributions stop changing hard (high-p T ) probes soft physics regime The time evolution of the matter produced in HI collisions

17. 17 Chemical freeze-out Kinetic freeze-out

18. 18 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 Two labs to recreate the Big-Bang

19. 19 NA35 NA36 NA49 Helios-2 Helios-3 NA44 CERES NA38 NA50 NA60 WA98 WA85 WA97 NA57 WA94 O Pb multistrange photons hadrons dimuons dielectrons 1986 1994 2000 hadrons strangeletshadrons dimuons 1992 2004 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

20. 20 One Pb-Pb collision seen by the NA49 TPCs at the CERN SPS (fixed target)

21. 21 The Relativistic Heavy Ion Collider (RHIC) RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS 1 km

22. 22 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

23. 23 One Au-Au collision seen by the STAR TPC Momentum determined by track curvature in magnetic field…

24. 24

25. 25 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?

26. 26 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 1908 Nobel Prize in Chemistry

27. 27 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

28. 28 incoming electron scattered electron Can we do the same kind of “deep inelastic” experiments to see what kind of matter we produce in high-energy nuclear collisions ? QGP ? No... Our problem is much more difficult to solve Seeing the QCD matter formed in heavy-ion collisions

29. 29 Calibrated “LASER” Matter under study Calibrated “light meter” Calibrated Heat Source What’s the Matter? Study how the measured “probe” is affected by the matter it traverses, as a function of the temperature of the system Probe

30. 30 Challenge: find the good probes vacuum QGP hadronic matter Good probes: Almost not affected by the hadronic matter or affected in a very well understood way (which can be “subtracted” or “corrected for”) Fully suppressed by the QGP deconfined medium... well, almost fully suppressed (otherwise it would be too easy)

31. 31 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 (the QGP) is formed: hard probes (jets, quarkonia,...) We must have “trivial” probes, not affected by the dense QCD matter, to serve as baseline reference for the interesting probes: 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

32. 32 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

33. 33 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

34. 34 Calibrated “LASER” Matter under study Calibrated “light meter” Calibrated Heat Source What’s the Matter? Study how the measured “probe” is affected by the matter it traverses, as a function of the temperature of the system Probe

35. 35 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

36. 36 Peripheral Event STAR

37. 37 Mid-Central Event STAR

38. 38 Central Event STAR

39. 39 Some experiments use two completely independent “centrality variables”, such as 1) beam spectators energy: E ZDC 2) multiplicity of produced tracks: N ch target projectile central collisions peripheral collisions central peripheral Centrality variables

40. 40 New physics at the SPS: charmonium suppression The most “head-on” collisions between heavy nuclei show a significantly suppressed J/  yield when compared with the baseline defined by proton-nucleus interactions

41. 41 New physics at RHIC: “jet” quenching The photons are not affected by the dense medium they cross The high-p T hadrons in central Au+Au collisions are strongly suppressed with respect to the expected scaling from pp collisions

42. 42 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 the previous slides

43. 43 The report Connecting Quarks with the Cosmos, from the NRC, lists the 11 most important questions to be addressed in 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?

44. 44 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? 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