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11 Physics Questions for the New Century

11 Physics Questions for the New Century. What is dark matter? What is dark energy? How were the heavy elements from iron to uranium made? Do neutrinos have mass? Where do ultra-energy particles come from?

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11 Physics Questions for the New Century

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  1. 11 Physics Questions for the New Century • What is dark matter? • What is dark energy? • How were the heavy elements from iron to uranium made? • Do neutrinos have mass? • Where do ultra-energy particles come from? • Is a new theory of light and matter needed to explain what happens at very high energies and temperatures? • Are there new states of matter at ultrahigh temperatures and densities? • Are protons unstable? • What is gravity? • Are there additional dimensions? • How did the Universe begin? The February 2002 issue of Discover magazine based its cover story on the recent 105-page public draft of the National Research Council Committee on Physics of the Universe report, Connecting Quarks with the Cosmos

  2. Probing hot and dense matter with hard probes • Why we do it • How we do it • What we found … • … and why we are excited about it Thomas Ullrich

  3. Conventional Wisdom – the Standard Model The Structure of Matter 6 Quarks: up, down, strange, charm, bottom, top 6 Leptons: electron, muon, tau + 3 neutrinos Hadrons: Mesons qq Baryons qqq

  4. q1 q2 Conventional Wisdom – the Standard Model • Properties governed by • QuantumChromoDynamics (QCD) • Forces between quarks: exchange of gluons • Confinement: • at large distances coupling between quarks is strong  Free quarks not observed in nature • Asymptotic Freedom: • at short distance the coupling is weak (quasi-free)

  5. The Phase Diagram of QCD Early universe quark-gluon plasma critical point ? Tc Temperature colour superconductor hadron gas nucleon gas nuclei CFL r0 Neutron stars vacuum baryon density

  6. Exploring the Phases of Nuclear Matter • 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 correlation of the • produced particles • Requirements • system must be at equilibrium (for a short time) •  system must be dense and large

  7. Exploring the Phases of Nuclear Matter • Can we find and explore the Quark Gluon Plasma ? • We hope so! • by colliding large nuclei at the highest possible • energy • How high? • Numeric QCD calculation (lattice QCD) tells us: • Critical temperature: Tc 2 · 1012 K • Critical energy density: 6 normal matter

  8. Exploring the Phases of Nuclear Matter

  9. Exploring the Phases of Nuclear Matter • Evolution of a Collision: • Pre-Equilibrium • Plasma-phase • Phase Transition • Hadron Gas • Freeze-out System is hottest and thus best studied in the central region

  10. PHOBOS BRAHMS 12:00 o’clock BRAHMS PHOBOS RHIC 2:00 o’clock PHENIX 10:00 o’clock STAR RHIC PHENIX 8:00 o’clock 4:00 o’clock STAR 6:00 o’clock AGS 9 GeV/u Q = +79 U-line BAF (NASA) m g-2 LINAC BOOSTER HEP/NP AGS TANDEMS 1 MeV/u Q = +32 TANDEMS Relativistic Heavy Ion Collider (RHIC) • 2 concentric rings of 1740 superconducting magnets • 3.8 km circumference • counter-rotating beams of ions from p to Au • max center-of-mass energy: AuAu 200 GeV, pp 500 GeV RHIC Runs Run I: Au+Au at s = 130 GeV Run II: Au+Au and pp at s = 200 GeV

  11. The STAR Collaboration • 451 Collaborators (294 authors) • 45 Institutions • 9 Countries: • Brazil, China, England, France, Germany, India, Poland, Russia, US

  12. Time Projection Chamber (TPC) • 3-dim CCD camera with 70 M pixels • worlds biggest TPC (diameter =4 m, length = 4 m) • measures trajectory of tracks • TPC + magnetic field  momentum (p) of tracks • momentum + energy loss in gas  identify particles (p,K,p) • designed to handle huge multiplicity of tracks (~ 5000) The STAR Experiment • STAR: Solenoidal Tracker at RHIC • multipurpose detector system for hadronic measurements • large coverage (geometrical acceptance) • tracking of charged particles in high multiplicity environment • measure correlations of observables • study of hard processes (jet physics)

  13. The STAR TPC Simulation and animation by Gene Van Buren, movie by Jeff Mitchell.

  14. STAR Peripheral Event From real-time Level 3 display. color code  energy loss

  15. STAR Mid-CentralEvent From real-time Level 3 display.

  16. STAR Central Event From real-time Level 3 display.

  17. The STAR Experiment Year: 2001 Year: 2002 Year: 2003 Year: 2004

  18. Units in (Nuclear) High-Energy Physics • Energy: 1 eV = 1.6 · 10-19 J • 1 eV ~ 40  energy of molecules in air • Particle masses: E = mc2 (usually c  1  E = m) • proton (p) ~ 1 GeV/c2 = 1.7 · 10-27 kg • Characteristic Momentum: ~ 1 GeV/c • velocity ~ speed of light  momentum more useful • Particle sizes: proton ~ 1 fm = 10-15 m • diameter of Au nuclei ~ 14 fm • Characteristic time ~ 1 fm/c = 3.3 · 10-24 s • as many fm/c in a second as seconds in 10 thousand trillion years • collision  freeze-out ~ 10-20 fm/c

  19. participants spectators Commonly Used Terms in Heavy-Ion Physics Centrality: central (head-on) collision peripheral (grazing shot) • Number of Participants (Npart): • Number of nucleons participating in the collision • p+p collisions = 2 • central Au+Au ~ 350 (Au = 79 p + 118 n = 197) • Used to describe geometry of a collision

  20. p pT q pz beam axis Commonly Used Terms in Heavy-Ion Physics Transverse Momentum (pT): Momentum p can be decomposed in a transversepTand a longitudinal component pz pT is generated in collision (pz present in entrance channel) • to describe particle spectra one usually plots: # of tracks as a function of pT

  21. What have we learned at RHIC after 2 Runs? • Majority of results so far on “soft” physics • soft ~ pT < 2 GeV/c • ~99% of all particles (bulk matter)

  22. Results on “Soft” Physics • Particle production per participant is large • Total Nch ~ 5000 (Au+Au s = 200 GeV)  ~ 20 in p+p • Nch/Nparticipant-pair ~ 4 (central region)  ~2.5 in p+p •  A+A is not a simple superposition of p+p • Energy density is high ~ 4-5 GeV/fm3 (model dependent) • System exhibits collective behavior (flow) •  strong internal pressure • The system appears to freezes-out very fast • explosive expansion • Large system: at freeze-out  2  size of nuclei

  23. The Missing Pieces … • Are the conditions created in a Au+Au collision really sufficient to create a QGP? • Does the system equilibrate? • What is the initial Temperature? • What is the Quark & Gluon density? To answer these questions we need to study the system early in the collision

  24. Can we do the same at RHIC? scattered electron incoming electron The Ideal Experiment The first exploration of subatomic structure was undertaken by Rutherford at Manchester in 1909 using Au atoms as targets and a particles as probes. NO QGP But we can get close ………

  25. Fast Partons (Quarks & Gluons) Traversing Matter • Jets: • high-pT parton produced in a hard (high momentum transfer) scattering process • no single quarks  partons fragments into many correlated particles • emitted in a cone • leading particles: particles in cone with highest momentum • Calculable in QCD (at high-pt) • created early in the collision hadrons quark quark hadrons leading particle

  26. vacuum QGP Probing Soft Physics with Hard Scattering • Au+Au Collisions: • Before high-pt partons hadronize and form jets they interact with the medium •  decreases their momentum •  fewer high pt particles •  “jet quenching” Goal: Search for features of high-pT spectra in Au+Au which do not show up in p+p

  27. STAR Au+Au jet event Jets in Heavy Ion Collisions at RHIC Jet event in e+e-collision Can We See Jets in Au+Au?

  28. Df Df beam axis Two Particle Azimuthal Correlations at High-pT Strong and direct evidence for hard scattering and parton fragmentation (jets) at RHIC • Per Event select high-pT particle (pT >4 GeV/c) • Calculate Df for other high-pT particles (pT > 2 GeV/c) within a small polar angle range • Calculate Df for all high-pT particles outside the range • Short range correlations: particles in jet cone + background • Long range correlations: background • Difference: particles in jet cone front view (xy) side view (zy)

  29. High-pT Spectra in 130 GeV Au+Au Collisions • Easy to measure but hard to correct • Luckily we have 2 experiments at RHIC studying high-pT  cross-checks between PHENIX & STAR pT data from PHENIX + STAR agree well over 6 orders of magnitude

  30. In Detail: High-pT Spectra from STAR Basic Idea: peripheral collisions are p+p like  no suppression central collisions hot and dense matter  suppression

  31. How to Compare Peripheral with Central Collisions? • Particle Production viahard processesshould scale • withNbin, the number of underlying binary nucleon- • nucleon collisions • (assuming no “collective” or nuclear effects) • Example Au+Au: • 5% central: Nbin ~ 1000 (Npart ~ 350) • Peripheral (60-80%): Nbin ~ 20 (Npart ~ 20)

  32. Expectations • We measure: Yield(pT) in central and peripheral • Create Ratio: • If no “effects”: • R < 1 in regime of soft physics • R = 1 at high-pT where hard scattering dominates Suppression: • R < 1 at high-pT

  33. Central/Peripheral Normalized by Nbin suppression suppression

  34. Comparsion of p+p with Au+Au Collisions • p-p data available over wide range of s from various experiments, but not for 130 GeV • Use interpolation verified by models and QCD calculations Comparison:

  35. Comparison Au+Au/p+p at Lower Energies • Pb+Pb collisions at s ~ 20 GeV at CERN/SPS RAA  parton energy loss (if any) is overwhelmed by initial state soft multiple scattering (Cronin effect) Crossing at ~ 1.5 GeV/c Transverse Momentum (GeV/c)

  36. Comparison Au+Au/p+p at RHIC (STAR)

  37. The Discovery of “Jet Quenching”? • There is more evidence not presented here (e.g. azimuthal anistropy) but also more questions: • What is interacting in the medium: partons or fragmenting hadrons? • Are there nuclear effects which change the distributions of quarks and gluons in the nuclei • and many more …. • Some claim that, others are more cautious • (If you ask me YES) • Lots of theoretical efforts • Example: E. Wang & X.N. Wang: • Energy loss in “cold” nuclear matter ~ 0.5 GeV/fm • Treatment of expanding media is important! • Energy loss at RHIC equivalent to an energy loss in static medium of ~ 7.3 GeV/fm • Initial density is about 15 times that in cold matter • To Do List: • More statistics to extend pT range • Measure p+p at RHIC with lots of statistics • better reference data for A+A • Measure p+A at RHIC with lots of statistics • study nuclear effects in ‘cold’ matter • Measure A+A with varying A (Si, Ca, Fe, …) Yes, we are greedy … At high-pT: extend measurement by 1 GeV/c needs factor of 3 more events to reach same # of entries as in previous bin

  38. Summary • A first in Relativistic Heavy-Ion Collisions: Jets • Major discovery: suppression of high-pT particles • PHENIX and STAR in agreement • Data are consistent with jet quenching • but data do not yet offer enough standard of proof • alternative explanation need to be studied • pT range still too small (< 6 GeV/c), barely out of soft regime • Are the data consistent with the formation of a new state of matter? • yes, was predicted, was observed • still, needs a closer look (we are working on it)

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