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The Strange Physics Occurring at RHIC

The Strange Physics Occurring at RHIC. Why do we do this research?. To explore the phase diagram of nuclear matter. How: By colliding nuclei in lab. By varying energy (√s) and size (A). By studying spectra and particle correlations. Rajagopal and Wilczek,

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The Strange Physics Occurring at RHIC

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  1. The Strange Physics Occurring at RHIC

  2. Why do we do this research? To explore the phase diagram of nuclear matter How: • By colliding nuclei in lab. • By varying energy (√s) and size (A). • By studying spectra and particle correlations. Rajagopal and Wilczek, hep-ph/-0011333 To probe properties of dense nuclear matter How: • By colliding most massive and highest energy nuclei. • By comparing to more elementary systems. • Through high pT studies

  3. Lattice QCD calculations G. Schierholz et al., Confinement 2003 Action density in 3 quark system in full QCD H. Ichie et al., hep-lat/0212036 • Coincident transitions: deconfinement and chiral symmetry restoration • Recently extended to mB> 0, order still unclear (1st, 2nd, crossover ?) TC≈ 170 MeV F. Karsch, hep-ph/0103314

  4. A theoretical view of the collision Tc – Critical temperature for transition to QGP Tch– Chemical freeze-out (Tch Tc) : inelastic scattering stops Tfo – Kinetic freeze-out (Tfo Tch) : elastic scattering stops

  5. 2 concentric rings of 1740 superconducting magnets 3.8 km circumference counter-rotating beams of ions from p to Au RHIC @ Brookhaven National Lab. Relativistic Heavy Ion Collider h Long Island Previous Runs: • Au+Au @ sNN=130 GeV & 200 GeV • p+p @ sNN =200 GeV • d+Au @ sNN =200 GeV Present Run: • Au-Au sNN=200 GeV

  6. Geometry of heavy-ion collisions Preliminary sNN = 200 GeV participants Uncorrected spectators Particle production scales with increasing centrality peripheral (grazing shot) central (head-on) collision Number participants (Npart): number of nucleons in overlap region Number binary collisions (Nbin): number of equivalent inelastic nucleon-nucleon collisions Nbin ≥ Npart

  7. Particle creation and distributions 19.6 GeV 130 GeV 200 GeV PHOBOS Preliminary dNch/dh Central Peripheral h Total multiplicity per participant pair scales with Npart Not just a superposition of p-p To get much further need PID

  8. STAR is a large acceptance detector preliminary preliminary X STAR Preliminary K0s f STAR Preliminary STAR Preliminary K* L STAR Preliminary W K Preliminary

  9. Strangeness enhancement General arguments for enhancement: 1. Lower energy threshold TQGP > TC ~ ms = 150 MeV Note that strangeness is conserved in the strong interaction 2. Larger production cross-section 3. Pauli blocking (finite chemical potential) Strange particles with charged decay modes Enhancement is expected to be more pronounced for multi-strange baryons and their anti-particles Arguments still valid but now use Strange particles for MUCH MORE

  10. Strangeness enhancement? • Canonical (small system): Computed taking into account energy to create companion to ensure conservation of strangeness. Quantum Numbers conserved exactly. • Grand Canonical limit (large system): Just account for creation of particle itself. The rest of the system acts as a reservoir and “picks up the slack”. Quantum Numbers conserved on average via chemical potential Phase space suppression of strangeness in small system/low temperature canonical suppression • increases with strangeness • decreases with volume • ~ observed enhancements [Hamieh et al.: Phys. Lett. B486 (2000) 61]

  11. Correlation volume Grand Canonical description is only valid in a system in equilibrium that is large. BUT being large is not a sufficient condition for being GC! • if A+A were just superposition of p+p STILL need to treat CANONICALLY System must know it is large... • Must know that an Ω+ generated here can be compensated by, say, an Ω- on the other side of the fireball! • what counts is the correlation volume How does the system KNOW its big? • Not from hadronic transport: no time • One natural explanation: returning from deconfined state

  12. Grand canonical applicable at RHIC? [Tounsi & Redlich: hep-ph/0111159] 130 GeV See drop in “enhancement” at higher energy Enhancement values as ~predicted by model Correlation volume not well modeled by Npart System is in G.C. state for most central data

  13. A theoretical view of the collision 1 Chemical freezeout (Tch  Tc) : inelastic scattering stops

  14. Models to evaluate Tch and B Statistical Thermal Model F. Becattini; P. Braun-Munzinger, J. Stachel, D. Magestro J.Rafelski PLB(1991)333; J.Sollfrank et al. PRC59(1999)1637 Assume: • Ideal hadron resonance gas • thermally and chemically equilibrated fireball at hadro-chemical freeze-out Recipe: • GRAND CANONICAL ensemble to describe partition function  density of particles of species i • fixed by constraints: Volume V, , strangeness chemical potentialS,isospin • input: measured particle ratios • output: temperature T and baryo-chemical potential B Particle density of each particle: Qi : 1 for u and d, -1 for u and d si : 1 for s, -1 for s gi:spin-isospin freedom mi : particle mass Tch : Chemical freeze-out temperature mq : light-quark chemical potential ms : strangeness chemical potential gs : strangeness saturation factor Compare particle ratios to experimental data

  15. Thermal model fit to data • Particle ratios well described: Tch = 160  5 MeV mB = 24  5 MeV ms = 1.4 1.4 MeV gs = 0.99 0.07 Data – Fit (s) Ratio Created a Large System in Local Chemical Equilibrium

  16. Tch systematics Hagedorn (1964): • if the resonance mass spectrum grows exponentially (and this seems to be the case) • there is a maximum possible temperature for a system of hadrons Blue – Exp. fit Tc= 158 MeV r(m) (GeV-1) filled: AA open: elementary Green - 1411 states of 1967 Red – 4627 states of 1996 m [Satz: Nucl.Phys. A715 (2003) 3c] Seems he was correct – can’t seem to get above Tch ~170MeV

  17. A theoretical view of the collision 2 Chemical freezeout (Tch ) ~ 170 MeV Time between Tch and Tfo

  18. Thermal model reproduced data Created a Large System in Local Chemical Equilibrium Data – Fit (s) Ratio Do resonances destroy the hypothesis? Used in fit

  19. Resonances and survival probability  K* lost  K K*   K* K K K measured Initial yield established at chemical freeze-out Decays in fireball mean daughter tracks can rescatter destroying part of signal Rescattering also causes regeneration which partially compensates Two effects compete – Dominance depends on decay products and lifetime  lost K* K measured Kinetic freeze-out Chemical freeze-out time Ratio to “stable” particle reveals information on behaviour and timescale between chemical and kinetic freeze-out

  20. Resonance ratios Life time [fm/c] :  (1020) = 40 L(1520) = 13 K(892) = 4 ++ = 1.7 r = 1.3 Thermal model [1]: Tch = 177 MeV mB = 29 MeV UrQMD [2] Nch [1] P. Braun-Munzinger et.al., PLB 518(2001) 41 D.Magestro, private communication [2] Marcus Bleicher and Jörg Aichelin Phys. Lett. B530 (2002) 81-87. M. Bleicher, private communication Need >4fm between Tch and Tfo Small centrality dependence: little difference in lifetime!

  21. A theoretical view of the collision 3 1 2 Chemical freezeout (Tch ) ~ 170 MeV Time between Tch and Tfo 4fm Kinetic freeze-out (Tfo Tch): elastic scattering stops

  22. purely thermal source light 1/mT dN/dmT heavy m explosive source light T,b T 1/mT dN/dmT heavy m Kinetic freeze-out and radial flow Want to look at how energy distributed in system. Look in transverse direction so not confused by longitudinal expansion Slope = 1/T Look at p or m= (p2 + m2 )distribution A thermal distribution gives a linear distribution dN/dm e-(m/T) If there is radial flow dN/dm- Shape depends on mass and size of flow Heavier particles show curvature

  23. Radial flow and hydro dynamical model bs R Shape of the m spectrum depends on particle mass Two Parameters: Tfo and b p,K,p fit E.Schnedermann et al, PRC48 (1993) 2462 r =s(r/R)n Tfo ~ 90  10 MeV, <  > = 0.59 ± 0.05c

  24. Flow of multi-strange baryons Au+Au sNN=200 GeV STAR Preliminary  68.3% CL 95.5% CL 99.7% CL • , K, p: Common thermal freeze-out at Tfo ~ 90 MeV <> ~ 0.60 c • : Shows different thermal freeze-out behavior: Tfo ~ 160 MeV <> ~ 0.45 c Higher temperature Lower transverse flow Probe earlier stage of collision? But: Already some radial flow! Tfo ~ Tch Instantaneous Freeze-out of multi-strange particles? Early Collective Motion?

  25. A theoretical view of the collision 4 3 1 2 Chemical freezeout (Tch ) ~ 170 MeV Time between Tch and Tfo  4fm Kinetic freeze-out (Tfo) ~ 90 MeV (light particles) Very Early Times

  26. Early collective motion Look at “Elliptic” Flow SPS, RHIC AGS Almond shape overlap region in coordinate space Anisotropy in momentum space Interactions v2: 2nd harmonic Fourier coefficient in dN/d with respect to the reaction plane

  27. v2 of strange particles Equal Energy Density lines P. Kolb, J. Sollfrank, and U. Heinz Seems to saturate at v2~20% for p~3.0 GeV/c  v2(p) follows  evolution  v2(p) consistent with  and  v2(p) • Multi-strange particles show sizeable elliptic flow! • Reach hydro. limit Hydro: P. Huovinen et al.

  28. Why high p physics at RHIC? schematic view of jet production jet production in quark matter hadrons hadrons leading particle leading particle q q q q hadrons leading particle Early production in parton-parton scatterings with large Q2. Direct probes of partonic phases of the reaction New penetrating probe at RHIC • attenuation or absorption of jets “jet quenching” • suppression of high phadrons • modification of angular correlation • changes of particle composition

  29. Collisions of small with large nuclei quantify all cold nuclear effects. Small + Large distinguishes all initial and final state effects. The control experiment – d-Au No Medium! Proton/deuteron- nucleus collision Medium? Nucleus-nucleus collision

  30. Jet suppression Hard scatter back-to-back jet – Angular correlation at 0 and p Central Au-Au backwards jet suppressed d-Au backwards jet is visible Jet suppression is a final state effect

  31. Energy loss creates anisotropy? y x Jet Propagation STAR Preliminary Energy loss results in anisotropy due to different “length” of matter passed through by parton depending on location of hard scattering Hypothesis seems verified

  32. Identified particle correlations STAR Au+Au 5% ptrig > 2.5 GeV/c 2.5 GeV/c <passoc< ptrig 1/Ntrigger*dN/d(∆Φ) ∆Φ (radians) Fig. 3 1/NtriggerdN/d(∆Φ) ∆Φ (radians) Fig. 5 Why: To gain insight on possible different fragmentation function of different parton. To probe further differences in mesons and baryons at high p Correlation for K0s, L and L, in both cases, there is an absence of a ‘back-to-back’ partner correlation. Need more statistics for further studies

  33. Nuclear modification factor N-N cross section <Nbinary>/sinelp+p If no “effects”: R < 1 in regime of soft physics R = 1 at high-p where hard scattering dominates “Hard” Physics - Scales with Nbin: Number of binary collisions number of equivalent inelastic nucleon-nucleon collisions Nuclear Modification Factor: Can replace p-p with peripheral Rcp

  34. Suppression of identified particles Two groups (2<p <6GeV/c): - K0s, K, K*, f  mesons - L, X, W baryons Mass or meson/baryon effect? PHENIX: PRL 91, 172301 L L show different behaviour to K Suppression ofK sets in at lower p Rcp K Come together again at p ~ 6 GeV? “standard” fragmentation? Clearly not mass dependence

  35. d-Au control experiment Enhancement is the well known “Cronin Effect” Au + Au, RAA << 1; d+Au, RdAu > 1 RAA results confirm there are final state effects

  36. Parton coalescence and medium p • When slope exponential: coalescence wins • When slope power law: fragmentation wins recombining partons: p1+p2=ph Fries et al. QM2004 fragmenting parton: ph = z p, z<1 • Recombination p(baryons)> p(mesons)> p(quarks) (coalescence from thermal quark distribution ...) • Pushes soft physics for baryons out to 4-5 GeV/c • Reduces effect of jet quenching Do soft and hard partons recombine or just soft+soft ? Explore correlations with leading baryons and mesons

  37. v2 and coalescence model STAR Preliminary Hadronization via quark coalescence: v2 of a hadron at a given p is the partonic v2 at p/n scaled by the # of quarks (n). Au+Au sNN=200 GeV MinBias 0-80% • Works for K0s,  &  • v2s ~ v2u,d ~ 7% D. Molnar, S.A. Voloshin Phys. Rev. Lett. 91, 092301 (2003) V. Greco, C.M. Ko, P. Levai Phys. Rev. C68, 034904 (2003) R.J. Fries, B. Muller, C. Nonaka, S.A. Bass Phys. Rev. C68, 044902 (2003) Z. Lin, C.M. Ko Phys. Rev. Lett. 89, 202302 (2002)

  38. Exotica searches (pentaquarks) Rotational excitations include Particle Data Group 1986 reviewing evidence for exotic baryons states “…The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this areamake it likely that it will be another 15 years before the issue is decided. “ PDG dropped the discussion on pentaquark searches after 1988. Constituent quark model of the1960s has been very successful in describing known baryons as 3-quark states QCD and quark model do not forbid composites of more quarks But early searches were unsuccessful and finally given up Minimum quark content is 4 quarks and 1 antiquark “Exotic” pentaquarks are those where the antiquark has a different flavor than the other 4 quarks Quantum numbers cannot be defined by 3 quarks alone. Chiral Soliton Model: Ns and Ds rotational states of same soliton field The mass splittings are predicted to be equally spaced Diakonov et al. Z phys A 359 (1997) 305

  39. Early evidence for pentaquark’s Q+ results : Highest? Significance (CLAS) = 7.8 (hep-ex/0311046) X5 results : NA49: X-- (1860)  X-p- X0 (1860)  X- p+ Width limits are experimental resolution Counts significance=5.6 Need strong confirmation of second member of anti-decuplet Mass (Xp) GeV/c2

  40. RHIC - ideal place for pentaquarks B/B ratio ~ 1 should see anti-pentaquark If form QGP should coalesce into pentaquarks? Look at Q+ K0s + p STAR Preliminary p-p Q+ /event (stat. model calc.) 0.5 – 1.5 1.5 Million events  0.8 – 2.3 M Efficiency 3%  25 – 70 K Branching Ratio 50%  10 –25 K BR 50% from K0s  5 –18 K BG in mass range/event 2 BG in sample  3 M Significance  = Signal/√(2 X BG+Signal)2-7 s STAR Preliminary d-Au Similar calc. for p-p 0.25-3 s d-Au1-16 s No Clear Signal Yet.

  41. Q-n + K- Other pentaquarks at RHIC PHENIX Preliminary d-Au Au-Au Minbias Possible peaks need more investigation

  42. Q+ at the AGS Really need to determine properties spin, parity etc Use AGS Kaon beam (D. Ashery, E. Piasetzky, R. Chrien, P.Pile) Q+ K+ d Q+ p ( Ethresh= 400 MeV) K+ p  Q+p+ (Ethresh = 760 MeV) d K+ p Why: • Large production cross-section compared to electro magnetic processes (Liu and Ko) 104:1 • Only measure single particle mtm to determine mass • Angular distribution determines spin

  43. Determining spin and parity K+ d Q+ p Intrinsic parity- + +? + K+ p Q+ p+ Intrinsic parity- + +? - Parity Conserved  1= (-1)DL n1n2 DL = If – Ii DL = Odd DL = Even K+ d Q+ p spin 0 1 ½(?) ½ K+ p Q+ p+ spin 0 1 ½(?) 0 DL = 0 DL = 1 DL determines the decay angular distribution Determination of spin and parity will help select between theories Correlated quark & Chiral soliton models predicts Jpc=½+ (p-wave) Quark model naïve expectation is Jpc=½− (s-wave)

  44. Summary 0 1 2 3 4 5 6 7 8 9 10 11 12 GeV/c Different physics for different scales Hydro ReCo pQCD Strange particles are useful probes for each scale All evidence suggest RHIC creates a hot and dense medium with partonic degrees of freedom Only just beginning to understand the rich physics of RHIC

  45. Extra Slides

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