What is a heavy ion? Accelerator terminology: Any ion with A>4, Anything heavier than α-particle

Transcription

1 Outline Introduction to Relativistic Heavy Ion Collisions and Heavy Ion Colliders. Production of particles with high transverse momentum. Collective Elliptic Flow Global Observables Particle Physics with Nuclei: Ultra-Peripheral Collisions

2 What is a heavy ion? Accelerator terminology: Any ion with A>4, Anything heavier than α-particle This talk: truly heavy ions ions with A 200 ( 197 Au, 208 Pb)

3 Measure of collision energy: center-of-mass energy per nucleon-nucleon collision, s nn.

4 The maximum s nn of truly heavy ions has increased dramatically during the last 25 years: pp/pp Ions

5 Heavy-Ion Laboratories Two main, high-energy laboratories: Brookhaven (Long Island, New York) AGS, RHIC CERN (Geneve, Switzerland/France) SPS(fixed target), LHC Other laboratories: LBL (Lawrence Berkeley Lab, California) Bevalac (Betatron) GSI (Darmstadt, Germany) FAIR JINR (Dubna, Russia) Synchrophasotron

6 Heavy-Ion History Birth of the field: O accelerated at AGS and SPS Fixed target experiments Truly heavy ions at AGS and SPS, Au and Pb. SPS: NAXX or WAXX (NA49, WA98, NA57, ) AGS: EXXX (E802, E872, ) First collisions at RHIC (Au+Au) First Heavy-Ion Collider Experiments: STAR, PHENIX, PHOBOS, BRAHMS

7 The Relativistic Heavy Ion Collider (RHIC) Collider for heavy nuclei and (polarized) protons at Brookhaven National Laboratory. Au+Au s = 200 A s = 500 A GeV 200 GeV so far) 1.3 km

8 Inside the RHIC tunnel

9 A large system of accelerators Relativistic Heavy-Ion Collider at Brookhaven National Laboratory

10 Example of a large system of accelerators First step: Ion source + Tandem Van de Graaff E = 1 MeV/nucleon, Q=+32e (32 of 79 Au electrons removed)

11 Example of a large system of accelerators Second step: Transfer Line Tandem Van de Graaff Booster.

12 Example of a large system of accelerators Third step: Booster, increase in E: 1 MeV/nucleon 95 MeV/nucleon. Followed by stripping foil, removes 77 of the 79 electrons from Au atom.

13 Example of a large system of accelerators Fourth step: AGS (Alternating Gradient Synchrotron) increase in E: 95 MeV/nucleon 10.8 GeV/nucleon. Stripping foil removes the remaining two electrons.

14 Example of a large system of accelerators Final step: RHIC, increase in E: 10.8 GeV/nucleon 100 GeV/nucleon in two beams. The beams are focussed for collisions at 4 (6) interaction points.

15 At each interaction point there is an experiment

16 Some RHIC parameters The particles travel in bunches around the ring, 60 bunches/ring, bunch length: 10cm Particles/bunch: Bunch crossing frequency 10 MHz Time between bunch crossings 100 nsec. Transverse size of the beam: ~ μm Au+Au p+p Luminosity: cm -2 s cm -2 s -1 Total cross section: 11 barn 42 mb Interaction rate: 2,2 khz 420 khz

17 Pre-accelerators for the LHC

18 Experiments at RHIC STAR

19 The STAR Detector Main detector: Time Projection Chamber (TPC), 4m long, 4m diameter, η <1.5 Other components: * Forward TPCs * Electromagnetic Calorimeter * Silicon Vertex Tracker

20 The PHENIX Detector Two central tracking arms: η <0.35, 2 Δϕ=90 Drift Chambers, MWPCs, RICH, ToF, EmCal Two muon arms: 1.2< η <2.3, Tracking: Cathode Strip Chambers, MuId: Streamer tubes

21 The BRAHMS Detector 2 Movable Spectrometer arms (TPCs for tracking). Particle Id for η <3. The PHOBOS Detector Charged Particle Tracking (Si Pad detectors) for η <5.5, but Particle Id only near mid-rapidity.

22 System and energies studied so far First run in June 2000: s = 130 A GeV Second run July Jan. 2002: 200 A GeV 200 GeV Third Run Jan May 2003: 200 A GeV 200 GeV Fourth Run Jan May 2004: 200 A GeV 63 A GeV (short) 200 GeV Fifth Run Jan June 2005: 200 A GeV 63 A GeV 200 GeV Quark Matter 2001 Quark Matter 2002 White Papers, [Nucl. Phys. A Vol 757 (2005)] Quark Matter 2004 Quark Matter 2005

23 The main goal of RHIC is to study hot and dense nuclear matter The nuclear phase diagram

24 THE PHASE TRANSITION ε/t 4 T/T C Fritiof Karsch et al. Hagedorn 1965: Density of hadronic resonances: ρ(m) exp(m/t H ) Thermodynamic system of hadrons w/ T>T H impossible T H 160 MeV ( m π Λ QCD )

25 What is ε? Average energy per unit volume: ε = 1 V E dn de de 0 What is dn/de? Statistical mechanics Phase space density of particles: g = E / kt ( πh) 1 V d n 3 dp e ± 1 + for fermions - for bosons g - degeneracy

26 d 3 n/dp 3 dn/de (dp 3 = 4πp 2 dp 4πE 2 de) 1 V dn de = g 2π 2 1 E 2 ( ) 3 E/ kt hc e ± 1 For (massless) bosons ( - ): ε = 1 g E dn de = 1 2 V de 2 π e 0 0 E For (massless) fermions ( + ): g E dn ε = 1 de 1 E 2 3 V = de 2π hc e E/ de = 1 ( 3 ) 3 E/ kt hc 30 ( hc) 3 de = + 1 g π (kt) ( ) kt 3 30 ( hc) g π 2 4 (kt) 4

27 In natural units: ε = g 2 π 30 (kt) ( hc) 4 3 = g 2 π 30 T 4 and ε T 4 = (g b g f ) 2 π 30 Consider a system with kt=150 MeV (chemical potential μ=0 no net baryon number) Hadrons: Pions, g b =3 (spin=0, no color, isospin=1) ε/t 4 ~ 3 (π 2 /30) 1.0 Quarks and Gluons: u,d or u,d,s g f = 24 or 36 (spin=1/2, 3 colors, q and q, 2 or 3 flavors) gluons g b =16 (8 color comb., spin=1) ε/t 4 ~ 37 (π 2 /30) 12.2 or 47½ (π 2 /30) 15.6

28 THE PHASE TRANSITION Fritiof Karsch et al. For a quark-gluon plasma: kt C = 150 MeV ε > 1.0 GeV /fm 3 kt C = 170 MeV ε > 1.7 GeV /fm 3 ε = g b g f 2 π 30 (kt) ( hc) 4 3 Compare normal nuclear matter: ε 0.16 GeV/fm 3

29 Measuring ε and T directly is difficult

30 Not a static source The system expands and cools down Most particles emitted at the late stages of the collisions

31 A more general strategy to understand heavy-ion collisions: Use heavy ions (Au+Au) to produce a new state of matter (the Quark Gluon Plasma). Use p+p, d+au results as benchmarks to disentangle the effects of the produced state.

32 The first remarkable result from RHIC: 1. Suppression of particles with high-p T, Jet-quenching

33 Jet-quenching and high-p T suppression p T or p : Transverse momentum Before: p // = +100 GeV/c, p = 0 p // = 100 GeV/c, p = 0 After: //

34 p T distribution in p+p collisions Intersecting Storage Ring (ISR), CERN ~1973, s = GeV p+p π 0 + anything

35 p T distribution in p+p collisions at RHIC STAR p+p h +/- + X PHENIX p+p π 0 + X

36 p T distribution in Au+Au collisions at RHIC Au+Au π 0 + X compared with scaled p+p π 0 + X central N coll = 975 ± 94 T AA = 22.8 ± 1.6/mb Scaled with N coll from nuclear geometry, Glauber Model (in the classical limit).

37 The collisions are characterized by their centrality or impact parameter. Two measures: Np : Number of participating nucleons N coll : Number of binary (nucleonnucleon) collisions

38 Nuclear Geometry I For a particle going through some (uniform) material: L <N coll > = σ n L σ: cross section n: number density of target; particles/volume proton+nucleus collision: b r r < NColl( b) >= σ nn ρ( b, z) dz r T ( b) A T A (b) Nuclear Thickness Function

39 Nuclear Geometry II nucleus+nucleus collision: A b b A bb Consider a narrow tube with area d 2 b at impact parameter b : Incoming particles (A): N A r = ( b ρ A, z ) dz r d 2 b ' r r r ( ) (, ) (, ) d 2 d < NColl > = ρ ba za dza σ nn ρ bb zb dzb b ' r r b r b r T A ( b') T ( b') 2 B + 2 A A B r b A r b r = b' 2 r b B r b r = + b' 2

40 Nuclear Geometry III nucleus+nucleus collision: A b b A bb B r r r b r b r r 2 < NColl( b) >= σ nn TA( b') TB ( + b') d b' σ 2 2 where T AB r r r b r b r r ( ) ( ') ( b ') d 2 b = TA b TB + b ' 2 2 is the nuclear overlap function nn T AB r ( b)

41 Finally, have to relate b to something we can measure: For each centrality bin, <N p > and <N coll > are calculated. Centrality <N coll > <N p > 0 10% 955±94 325± % 603±59 235± % 374±40 167±5

42 p T distribution in Au+Au collisions at RHIC Au+Au π 0 + X compared with scaled p+p π 0 + X central N coll = 975 ± 94 T AA = 22.8 ± 1.6/mb Scaled with N coll from nuclear geometry, Glauber Model. N 1 EVT 2 π 0 d N r d σ pp π AA =< TAB( b) > dp dy dp dy T High-pT suppression 2 T 0

43 p T distribution in Au+Au collisions at RHIC Quantify the suppression using the R AA measure: R AA 2 π 0 (1/ N EVT ) d N AA / ( pt ) = r 2 π 0 < T ( b) > d σ AB pp dp / T dp dy T dy Expectation in absence of medium effects

44 The experimental result: Charged hadrons (not identified)

45 Centrality Dependence

46 Can A+A collisions be understood from parton+parton or nucleon-nucleon interactions? Not entirely, a dense medium is created in the collisions. The produced particle lose energy (gluon bremsstrahlung) as they traverse it. Prediction: Bjorken (1982), Gyulassy & Wang (1992),

47 The high-p T suppression is unique to RHIC not seen at the SPS s = 17 GeV (but the Croonin enhancement is absent) NA49

48 NO suppression in d+au PHENIX BRAHMS STAR Phobos

49 AuAu Run4 p T reach for π 0 s governed by statistics PHENIX Preliminary PHENIX Preliminary

50 The suppression remains also at p T = 20 GeV

51 A consequence of pion suppression and transverse flow/expansion: p/π > 1! Central Au+Au Central Cu+Cu Peripheral Au+Au Not consistent with only QCD+fragmentation function.