From HighEnergy HeavyIon Collisions to Deconfined Quark Matter
- Slides: 44
From High-Energy Heavy-Ion Collisions to Deconfined Quark Matter 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 Carlos Lourenço, CERN PH-EP
The fundamental forces and the building blocks of Nature Gravity • one “charge” (mass) • force decreases with distance m 1 m 2 Electromagnetism (QED) • two “charges” (+/-) • force decreases with distance + - + + Atom
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 quark neutron • 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
Analogies and differences to study the structure of an atom… electron …we can split it into its constituents nucleus neutral atom Confinement: fundamental & crucial (but not well understood!) feature of strong force quark-antiquark pair created from vacuum quark “white” proton (confined quarks) Strong colour field 2 E = mc Energy grows with “white” protonseparation! “white” 0 (confined quarks)
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” 5
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. . .
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) • by heating • by compression deconfined colour matter ! Lattice QCD calculations Hadronic Nuclear Matter Quark Gluon Plasma (confined)! deconfined
Expectations from Lattice QCD calculations /T 4 ~ number of degrees of freedom deconfined QCD matter: many d. o. f. hadronic matter: few d. o. f. QCD lattice calculations indicate that, above a critical temperature, Tc, 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 Me. V?
Temperature at the center of the Sun ~ 15 000 K Temperature of the matter created in heavy ion collisions Tc 173 Me. V ~ 2 000 000 K. . . it’s pretty hot!
The first QCD Phase Diagram N. Cabibbo and G. Parisi, Phys. Lett. B 59 (1975) 67 Curious “warnings” in the paper:
The true phase diagram of QCD (? ) Temperature is r 0 baryon density
The phase diagram of water
Exploring the Phases of Nuclear Matter Can we explore the phase diagram of nuclear matter ? Ø 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 Ø Requirements • system must be at equilibrium (for a very short time) system must be dense and large Can we find and explore the Quark Gluon Plasma ? Ø We hope so ! • by colliding large nuclei at very high energies Ø How high ? • QCD calculations on the lattice predict: • Critical temperature: Tc 173 Me. V • Critical energy density: 6 normal nuclear matter 13
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
Simulation of a high-energy heavy-ion collision
The time evolution of the matter produced in HI collisions soft physics regime hard (high-p. T) probes • Chemical freeze-out (at Tch Tc): end of inelastic scatterings; no new particles (except from decays) • Kinetic freeze-out (at Tfo Tch): end of elastic scatterings; kinematical distributions stop changing 16
Kinetic freeze-out Chemical freeze-out
Two labs to recreate the Big-Bang • AGS : 1986 - 2000 • Si and Au beams ; up to 14. 6 A Ge. V • only hadronic variables • RHIC : 2000 - ? • Au beams ; up to sqrt(s) = 200 Ge. V • 4 experiments • SPS : 1986 - 2003 • O, S and Pb beams ; up to 200 A Ge. V • hadrons, photons and dileptons • LHC : 2008 - ? • Pb beams ; up to s = 5. 5 Te. V • ALICE, CMS and ATLAS 18
The CERN SPS heavy ion physics program Since 1986, many SPS experiments studied high-energy nuclear collisions to probe high density QCD matter • 1986 : Oxygen at 60 and 200 Ge. V/nucleon • 1987 – 1992 : Sulphur at 200 Ge. V/nucleon • 1994 – 2002 : Lead from 20 to 158 Ge. V/nucleon • 2003 : Indium at 158 Ge. V/nucleon • and p-A collisions: reference baseline dimuons 2004 In multistrange 2000 Pb photons hadrons NA 57 WA 98 WA 97 1994 1986 dielectrons NA 49 hadrons CERES WA 93 WA 80 WA 94 WA 85 strangelets NA 50 NA 52 dimuons NA 44 1992 S O NA 60 hadrons Helios-3 NA 35 Helios-2 NA 36 NA 38
One Pb-Pb collision seen by the NA 49 TPCs at the CERN SPS (fixed target)
The Relativistic Heavy Ion Collider (RHIC) PHOBOS PHENIX 1 km RHIC STAR AGS TANDEMS BRAHMS
The RHIC experiments • Successfully taking data since year 2000 • Au+Au collisions at s = 200 Ge. V complemented by data collected at lower energies and with lighter nuclei • Polarized pp collisions at 500 Ge. V also underway (spin program) STAR 22
One Au-Au collision seen by the STAR TPC Momentum determined by track curvature in magnetic field… 23
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? 25
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 a particles as probes Interpretation: The positive charge is concentrated in a tiny volume with respect to the atomic dimensions 1908 Nobel Prize in Chemistry 26
Seeing what the nucleons are made of The deep inelastic scattering experiments made at SLAC in the 1960 s established the quark-parton model and our modern view of particle physics proton p 2 electron p 1 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 27
Seeing the QCD matter formed in heavy-ion collisions Can we do the same kind of “deep inelastic” experiments to see what kind of matter we produce in high-energy nuclear collisions ? scattered electron incoming electron QGP ? No. . . Our problem is much more difficult to solve 28
What’s the Matter? Matter under study Calibrated “light meter” Calibrated “LASER” Probe Calibrated Heat Source Study how the measured “probe” is affected by the matter it traverses, as a function of the temperature of the system 29
Challenge: find the good probes vacuum Good probes: hadronic matter QGP 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)
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
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 32
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 33
What’s the Matter? Matter under study Calibrated “light meter” Calibrated “LASER” Probe Calibrated Heat Source Study how the measured “probe” is affected by the matter it traverses, as a function of the temperature of the system 34
The “centrality” of a nucleus-nucleus collision b = impact parameter distance between colliding nuclei, perpendicular to the beam-axis b large b: peripheral collisions small b: central collisions not measured! must be derived from measured variables, through models Quantitative measures of the collision centrality: participants • Number of participant nucleons: Npart • Number of binary nucleon-nucleon collisions: Ncoll • Multiplicity density of charged particles at mid-pseudorapidity: d. Nch/dh (h=0) • Forward hadronic energy: EZDC • Transverse energy: ET. . . among others spectators 35
Peripheral Event STAR 36
Mid-Central Event STAR 37
Central Event STAR 38
Centrality variables Some experiments use two completely independent “centrality variables”, such as 1) beam spectators energy: EZDC 2) multiplicity of produced tracks: Nch peripheral central peripheral collisions central collisions projectile target
New physics at the SPS: charmonium suppression The most “head-on” collisions between heavy nuclei show a significantly suppressed J/y yield when compared with the baseline defined by proton-nucleus interactions
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
There are many “signatures” of the QGP Direct photons Disoriented Chiral Condensates Fluctuations in p. T , Nch Jet quenching Medium effects on hadrons Particle interferometry (HBT) Particle ratios Quarkonia suppression Radial and elliptic flow Spectra p. T , d. N/dy, d. ET/dy Strangeness enhancement Thermal dileptons mentioned in the previous slides 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!) 42
Does it matter? 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. 43
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
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