Helen Caines - Yale University March 2004 STAR The Strange Physics Occurring at RHIC

Helen Caines – March 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. How: ♦ By colliding most massive and highest energy nuclei. ♦ By comparing to more elementary systems. ♦ Through high p T studies To probe properties of dense nuclear matter Rajagopal and Wilczek, hep-ph/

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

Helen Caines – March A theoretical view of the collision T c – Critical temperature for transition to QGP T ch – Chemical freeze-out ( T ch  T c ) : inelastic scattering stops T fo – Kinetic freeze-out ( T fo  T ch ) : elastic scattering stops

Helen Caines – March Brookhaven National Lab. h Long Island Relativistic Heavy Ion Collider Previous Runs: ♦  s NN =130 GeV & 200 GeV ♦  s NN =200 GeV ♦  s NN =200 GeV Present Run: ♦ Au-Au  s NN =200 GeV 2 concentric rings of 1740 superconducting magnets 3.8 km circumference counter-rotating beams of ions from p to Au

Helen Caines – March Number binary collisions (N bin ): number of equivalent inelastic nucleon-nucleon collisions Geometry of heavy-ion collisions Preliminary  s NN = 200 GeV Uncorrected peripheral (grazing shot) central (head-on) collision spectators Particle production scales with increasing centrality N bin ≥ N part participants Number participants (N part ): number of nucleons in overlap region

Helen Caines – March Particle creation and distributions dN ch /dh  19.6 GeV130 GeV200 GeV PHOBOS Preliminary Central Peripheral Total multiplicity per participant pair scales with N part Not just a superposition of p-p To get much further need PID

Helen Caines – March STAR is a large acceptance detector STAR Prelimin ary K  K* STAR Preliminary preliminary K0sK0s   Preliminary STAR Preliminary  preliminary

Helen Caines – March Strangeness enhancement General arguments for enhancement: 1. Lower energy threshold T QGP > T C ~ m s = 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

Helen Caines – March 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 canonical suppression  increases with strangeness  decreases with volume  ~ observed enhancements [Hamieh et al.: Phys. Lett. B486 (2000) 61] ♦ Phase space suppression of strangeness in small system/low temperature

Helen Caines – March 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 l One natural explanation: returning from deconfined state

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

Helen Caines – March A theoretical view of the collision 1 Chemical freezeout (T ch  T c ) : inelastic scattering stops

Helen Caines – March Models to evaluate T ch and  B Compare particle ratios to experimental data Q i : 1 for u and d, -1 for  u and  d s i : 1 for s, -1 for  s g i : spin-isospin freedom m i : particle mass T ch : Chemical freeze-out temperature  q : light-quark chemical potential  s : strangeness chemical potential  s : strangeness saturation factor Particle density of each particle: 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

Helen Caines – March Thermal model fit to data ♦Particle ratios well described: T ch = 160  5 MeV  B = 24  5 MeV  s = 1.4  1.4 MeV  s = 0.99  0.07 Data – Fit (s) Ratio Created a Large System in Local Chemical Equilibrium

Helen Caines – March T ch 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 [Satz: Nucl.Phys. A715 (2003) 3c] filled: AA open: elementary Blue – Exp. fit T c = 158 MeV r(m) (GeV -1 ) Green states of 1967 Red – 4627 states of 1996 Seems he was correct – can’t seem to get above T ch ~170MeV m

Helen Caines – March A theoretical view of the collision Chemical freezeout (T ch ) ~ 170 MeV Time between T ch and T fo 2

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

Helen Caines – March Resonances and survival probability Chemical freeze- out Kinetic freeze- out measured lost  K K  K*K* 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 time Ratio to “stable” particle reveals information on behaviour and timescale between chemical and kinetic freeze-out K*K*  K

Helen Caines – March Resonance ratios Thermal model [1]: T ch = 177 MeV  B = 29 MeV [1] P. Braun-Munzinger et.al., PLB 518(2001) 41 D.Magestro, private communication [2] Marcus Bleicher and Jörg Aichelin Phys. Lett. B530 (2002) M. Bleicher, private communication Need >4fm between T ch and T fo UrQMD [2] Life time [fm/c] :  (1020) = 40  (1520) = 13 K(892) = 4  ++ = 1.7  = 1.3 Small centrality dependence: little difference in lifetime! N ch

Helen Caines – March A theoretical view of the collision 3 1 Chemical freezeout (T ch ) ~ 170 MeV Time between T ch and T fo  4fm Kinetic freeze-out (T fo  T ch ): elastic scattering stops 2

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

Helen Caines – March Radial flow and hydro dynamical model T fo ~ 90  10 MeV, = 0.59 ± 0.05c R  s s E.Schnedermann et al, PRC48 (1993) 2462 Shape of the m  spectrum depends on particle mass Two Parameters: T fo and   r =  s (r/R) n p,K,p fit

Helen Caines – March Flow of multi-strange baryons ♦ , K, p: Common thermal freeze- out at T fo ~ 90 MeV ~ 0.60 c ♦  : Shows different thermal freeze- out behavior: T fo ~ 160 MeV ~ 0.45 c But: Already some radial flow! T fo ~ T ch Instantaneous Freeze-out of multi-strange particles? Early Collective Motion? Higher temperature Lower transverse flow Probe earlier stage of collision? Au+Au  s NN =200 GeV STAR Preliminary  68.3% CL  95.5% CL  99.7% CL

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

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

Helen Caines – March v 2 of strange particles ♦ Multi-strange particles show sizeable elliptic flow! ♦ Reach hydro. limit ♦ Seems to saturate at v 2 ~20% for p  ~3.0 GeV/c ♦  v 2 (p  ) follows  evolution ♦  v 2 (p  ) consistent with  and  v 2 (p  ) Hydro: P. Huovinen et al. Equal Energy Density lines P. Kolb, J. Sollfrank, and U. Heinz

Helen Caines – March Why high p  physics at RHIC? q q hadrons leading particle leading particle schematic view of jet production hadrons q q leading particle jet production in quark matter 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 Early production in parton-parton scatterings with large Q 2. Direct probes of partonic phases of the reaction

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

Helen Caines – March Jet suppression Hard scatter back-to-back jet – Angular correlation at  and  ♦ Central Au-Au backwards jet suppressed ♦ d-Au backwards jet is visible Jet suppression is a final state effect

Helen Caines – March Energy loss creates anisotropy? Jet Propagation y x 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

Helen Caines – March Identified particle correlations 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 K 0 s,  and , in both cases, there is an absence of a ‘back-to- back’ partner correlation. Need more statistics for further studies Fig. 3 ∆Φ (radians) 1/N trigger *dN/d(∆Φ) ∆Φ (radians) 1/N trigger dN/d(∆Φ) Fig. 5 STAR Au+Au 5% p  trig > 2.5 GeV/c 2.5 GeV/c <p  assoc < p  trig

Helen Caines – March Nuclear modification factor “Hard” Physics - Scales with N bin : Number of binary collisions number of equivalent inelastic nucleon-nucleon collisions /  inel p+p N-N cross section Nuclear Modification Factor: If no “effects”: R < 1 in regime of soft physics R = 1 at high-p  where hard scattering dominates Can replace p-p with peripheral R cp

Helen Caines – March Suppression of identified particles Two groups (2<p  <6GeV/c): - K 0 s, K , K*,   mesons -   baryons R cp Clearly not mass dependence Come together again at p  ~ 6 GeV? “standard” fragmentation?  show different behaviour to K Suppression of K sets in at lower p   K Mass or meson/baryon effect? PHENIX: PRL 91,

Helen Caines – March d-Au control experiment Au + Au, R AA 1 R AA results confirm there are final state effects Enhancement is the well known “Cronin Effect”

Helen Caines – March Parton coalescence and medium p  ♦ 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 recombining partons: p 1 +p 2 =p h ♦ When slope exponential: coalescence wins ♦ When slope power law: fragmentation wins Fries et al. QM2004 fragmenting parton: p h = z p, z<1

Helen Caines – March v 2 and coalescence model Hadronization via quark coalescence: v 2 of a hadron at a given p  is the partonic v 2 at p  /n scaled by the # of quarks (n). ♦ Works for K 0 s,  &  ♦ v 2 s ~ v 2 u,d ~ 7% D. Molnar, S.A. Voloshin Phys. Rev. Lett. 91, (2003) V. Greco, C.M. Ko, P. Levai Phys. Rev. C68, (2003) R.J. Fries, B. Muller, C. Nonaka, S.A. Bass Phys. Rev. C68, (2003) Z. Lin, C.M. Ko Phys. Rev. Lett. 89, (2002) Au+Au  s NN =200 GeV STAR Preliminary MinBias 0-80%

Helen Caines – March Exotica searches (pentaquarks) ♦ 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 Chiral Soliton Model: Ns and  s rotational states of same soliton field ♦ 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. 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 area make it likely that it will be another 15 years before the issue is decided. “ PDG dropped the discussion on pentaquark searches after The mass splittings are predicted to be equally spaced Rotational excitations include Diakonov et al. Z phys A 359 (1997) 305

Helen Caines – March Early evidence for pentaquark’s  + results : Highest? Significance (CLAS) = 7.8 (hep-ex/ )  5 results : NA49:  -- (1860)   -  -  0 (1860)   -  + Width limits are experimental resolution Mass (Xp) GeV/c 2 Counts significance=5.6 Need strong confirmation of second member of anti-decuplet

Helen Caines – March RHIC - ideal place for pentaquarks No Clear Signal Yet.  B/B ratio ~ 1 should see anti-pentaquark If form QGP should coalesce into pentaquarks? Look at  +  K 0 s +   + /event (stat. model calc.)  0.5 – Million events  0.8 – 2.3 M Efficiency 3%  25 – 70 K Branching Ratio 50%  10 – 25 K BR 50% from K 0 s  5 – 18 K BG in mass range/event  2 BG in sample  3 M Significance  = Signal/√(2 X BG+Signal)  2-7  Similar calc. for p-p   d-Au  1-16  p-p d-Au STAR Preliminary

Helen Caines – March Other pentaquarks at RHIC  +   n+K -  +  p+K 0           p                p   Au-Au Minbias Possible peaks need more investigation  -   n + K - PHENIX Preliminary d-Au

Helen Caines – March  + at the AGS K + d   + p ( E thresh = 400 MeV) K + p   +  + (E thresh = 760 MeV) Use AGS Kaon beam (D. Ashery, E. Piasetzky, R. Chrien, P.Pile) Why: ♦ Large production cross-section compared to electro magnetic processes (Liu and Ko) 10 4 :1 ♦ Only measure single particle mtm to determine mass ♦ Angular distribution determines spin K+K+ d ++ p Really need to determine properties spin, parity etc

Helen Caines – March Determining spin and parity K + d   + p Intrinsic parity - + +? + K + p   +  + Intrinsic parity - + +? - Parity Conserved  1= (-1)  L n 1 n 2  L = I f – I i  L = Odd  L = Even K + d   + p spin 0 1 ½(?) ½ K + p   +  + spin 0 1 ½(?) 0  L = 1  L = 0  L determines the decay angular distribution Determination of spin and parity will help select between theories Correlated quark & Chiral soliton models predicts J pc =½+ (p-wave) Quark model naïve expectation is J pc =½− (s-wave)

Helen Caines – March Summary GeV/c pQCD ReCo Hydro Different physics for different scales 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

Helen Caines – March Extra Slides