Modern Nuclear Physics with STAR @ RHIC: Recreating the Creation of the Universe

1. Modern Nuclear Physics with STAR @ RHIC:Recreating the Creation of the Universe Rene Bellwied Wayne State University (bellwied@physics.wayne.edu) • Lecture 1: Why and How ? • Lecture 2: Bulk plasma matter ? (soft particle production) • Lecture 3: Probing the plasma (via hard probes)

2. What is our mission ? • Discover the QGP • Find transition behavior between an excited hadronic gas and another phase • Characterize the states of matter • Do we have a hot dense partonic phase and how long does it live ? • Characterize medium in terms of density, temperature and time • Is the medium equilibrated (thermal, chemical)

3. The idea of two phase transitions Deconfinement The quarks and gluons deconfine because energy or parton density gets too high (best visualized in the bag model). Chiral symmetry restoration Massive hadrons in the hadron gas are massless partons in the plasma. Mass breaks chiral symmetry, therefore it has to be restored in the plasma What is the mechanism of hadronization ? How do hadrons obtain their mass ? (link to LHC and HERA physics)

4. What do we measure in a collider experiment ? • particles come from the vertex. They have to traverse certain detectors but should not change their properties when traversing the inner detectors • DETECT but don’t DEFLECT !!! • inner detectors have to be very thin (low radiation length): easy with gas (TPC), challenge with solid state materials (Silicon). • Measurements: - momentum and charge via high resolution tracking in SVT and TPC in magnetic field (and FTPC) - PID via dE/dx in SVT and TPC and time of flight in TOF and Cerenkov light in RICH - PID of decay particles via impact parameter from SVT and TPC • particles should stop in the outermost detector • Outer detector has to be thick and of high radiation length (e.g. Pb/Scint calorimeter) • Measurements: - deposited energy for event and specific particles - e/h separation via shower profile - photon via shower profile

5. What do we have to check ? • If there was a transition to a different phase, then this phase could only last very shortly. The only evidence we have to check is the collision debris. • Check the make-up of the debris: • which particles have been formed ? • how many of them ? • are they emitted statistically (Boltzmann distribution) ? • what are their kinematics (speed, momentum, angular distributions) ? • are they correlated in coordinate or momentum space ? • do they move collectively ? • do some of them ‘melt’ ?

6. Signatures of the QGP phase For more detail see for example: J. Harris and B. Müller, Annu, Rev. Nucl. Part. Sci. 1996 46:71-107 (http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.nucl.46.1.71) Phase transitions are signaled thermodynamically by a ‘step function’ when plotting temperature vs. entropy (i.e. # of degrees of freedom). The temperature (or energy) is used to increase the number of degrees of freedom rather than heat the existing form of matter. In the simplest approximation the number of degrees of freedom should scale with the particle multiplicity. At the step some signatures drop and some signatures rise

7. Peripheral Au + Au STAR Preliminary Central Au + Au ? Evidence: Some particles are suppressed If things are produced in pairs then one might make it out and the other one not. If things require the fusion of very heavy rare quarks they might be suppressed in a dense medium • If the phase is very dense (QGP) than certain particles get absorbed

8. Evidence: Some particles are enhanced • Remember dark matter ? Well, we didn’t find clumps of it yet, but we found increased production of strange quark particles

9. How do we know what happened ? • We have to compare to a system that did definitely not go through a phase transition (a reference collision) • Two options: • A proton-proton collision compared to a Gold-Gold collision does not generate a big enough volume to generate a plasma phase • A peripheral Gold-Gold collision compared to a central one does not generate enough energy and volume to generate a plasma phase

10. y=-1 y=1 y=2.2 y=3.7 Rapidity y = ln (E+pz/E-pz) = lorentz invariant ‘velocity’ Transverse momentum pt = sqrt (px2+py2) Kinematic variables of choice y = -6 0 +6

11. 0.) Global observablesA.) particle productionB.) particle spectraC.) particle flowD.) particle correlations

12. Quarks and gluons are studied on a discrete space-time lattice Solves the problem of divergences in pQCD calculations (which arise due to loop diagrams) 0.5 4.5 15 35 75 GeV/fm3 e/T4 Lattice Results Tc(Nf=2)=1738 MeV Tc(Nf=3)=1548 MeV T/Tc (F. Karsch, hep-lat/9909006) • There are two order parameters Lattice QCD T = 150-200 MeV e ~ 0.6-1.8 GeV/fm3

13. Assessing the Initial Energy Density: Calorimetry Bjorken-Formula for Energy Density: PRD 27, 140 (1983) – watch out for typo (factor 2) Time it takes to thermalize system (t0 ~ 1 fm/c) ~6.5 fm pR2 Central Au+Au (Pb+Pb) Collisions: 17 GeV: eBJ  3.2 GeV/fm3 130 GeV: eBJ 4.6 GeV/fm3 200 GeV: eBJ  5.0 GeV/fm3 Note: t0 (RHIC) < t0 (SPS) commonly use 1 fm/c in both cases

14. Assessing the Initial Energy Density: Tracking Bjorken-Formula for Energy Density: Gives interestingly always slightly smaller values than with calorimetry (~15% in NA49 and STAR).

15. eBj~ 23.0 GeV/fm3 eBj~ 4.6 GeV/fm3 Lattice ec The Problem with eBJ • eBJ is not necessarily a “thermalized” energy density • no direct relation to lattice value • requires boost invariance • t0 is not well defined and model dependent • usually 1fm/c taken for SPS • 0.2 – 0.6 fm/c at RHIC ? • system performs work p·dV  ereal > eBJ • from simple thermodynamic assumptions  roughly factor 2

16. Boost invariance based on rapidity distributions

17. So what is e now ? • At RHIC energies, central Au+Au collisions: • From Bjorken estimates via ET and Nch: e > 5 GeV/fm3 • From energy loss of high-pT particles: e≈ 15 GeV/fm3 • From Hydromodels with thermalization: ecenter≈ 25 GeV/fm3 • All are rough estimates and model dependent (EOS, t0, ... ?) , no information about thermalization or deconfinement. Methods not completely comparable • But are without doubt good enough to support that e >> eC≈ 1 GeV/fm3

18. How do we use hadrons ? • Discovery probes: • CERN: Strangeness enhancement/equilibration • RHIC: Elliptic flow • RHIC: Hadronic jet quenching • Characterization probes: • Chemical and kinetic properties • HBT and resonance production for timescales • Fluctuations for dynamic behavior

19. Particle Identification in STAR

20. Basic Idea of Statistical Hadronic Models • Assume thermally (constant Tch) and chemically (constant ni) equilibrated system • Given Tch and  's (+ system size), ni's can be calculated in a grand canonical ensemble Chemical freeze-out (yields & ratios) • inelastic interactions cease • particle abundances fixed (except maybe resonances) Thermal freeze-out (shapes of pT,mT spectra): • elastic interactions cease • particle dynamics fixed

21. Particle production:Statistical models do well We get a chemical freeze-out temperature and a baryochemical potential out of the fit

22. Ratios that constrain model parameters

23. Statistical Hadronic Models : Misconceptions • Model says nothing about how system reaches chemical equilibrium • Model says nothing about when system reaches chemical equilibrium • Model makes no predictions of dynamical quantities • Some models use a strangeness suppression factor,others not • Model does not make assumptions about a partonic phase; However the model findings can complement other studies of the phase diagram (e.g. Lattice-QCD)

24. Thermalization in Elementary Collisions ? Seems to work rather well ?! Beccatini, Heinz, Z.Phys. C76 (1997) 269

25. Thermalization in Elementary Collisions ? • Is a process which leads to multiparticle production thermal? • Any mechanism for producing hadrons which evenly populates the free particle phase space will mimic a microcanonical ensemble. • Relative probability to find a given number of particles is given by the ratio of the phase-space volumes Pn/Pn’ = fn(E)/fn’(E)  given by statistics only. Difference between MCE and CE vanishes as the size of the system N increases. This type of “thermal” behavior requires no rescattering and no interactions. The collisions simply serve as a mechanism to populate phase space without ever reaching thermal or chemical equilibrium In RHI we are looking for large collective effects.

26. Statistics  Thermodynamics p+p Ensemble of events constitutes a statistical ensemble T and µ are simply Lagrange multipliers “Phase Space Dominance” A+A • We can talk about pressure • T and µ are more than Lagrange multipliers

27. Are thermal models boring ? Good success with thermal models in e+e-, pp, and AA collisions. Thermal models generally make tell us nothing about QGP, but (e.g. PBM et al., nucl-th/0112051): Elementary particle collisions: canonical description, i.e. local quantum number conservation (e.g.strangeness) over small volume. Just Lagrange multipliers, not indicators of thermalization. Heavy ion collisions: grand-canonical description, i.e. percolation of strangeness over large volumes, most likely in deconfined phase if chemical freeze-out is close to phase boundary.

28. T systematics • it looks like Hagedorn was right! • 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 • indeed, we don’t seem to be able to get a system of hadrons with a temperature beyond Tmax ~ 170 MeV! [Satz: Nucl.Phys. A715 (2003) 3c] filled: AA open: elementary

29. Does the thermal model always work ? Data – Fit (s) Ratio • Particle ratios well described by Tch = 16010 MeV, mB = 24 5 MeV • Resonance ratios change from pp to Au+Au  Hadronic Re-scatterings!

30. Strange resonances in medium Short life time [fm/c] K* < *< (1520) <  4 < 6 < 13 < 40 Rescattering vs. Regeneration ? Medium effects on resonance and their decay products before (inelastic) and after chemical freeze out (elastic). Red: before chemical freeze out Blue: after chemical freeze out

31. ResonanceProduction in p+p and Au+Au Life time [fm/c] :  (1020) = 40 L(1520) = 13 K(892) = 4 ++ = 1.7 Thermal model [1]: T = 177 MeV mB = 29 MeV UrQMD [2] [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 Rescattering and regeneration is needed !

32. Resonance yields consistent with a hadronic re-scattering stage • Generation/suppression according to x-sections p p D p Preliminary r/p p p D L* D/p More D K Chemical freeze-out f/K f Ok p p r p p Less K* K*/K p r K* Less L* L*/L K K f 0.1 0.2 0.3 K

33. Strangeness: Two historic QGP predictions • restoration of csymmetry -> increased production of s • mass of strange quark in QGP expected to go back to current value (mS ~ 150 MeV ~ Tc) • copious production of ss pairs, mostly by gg fusion [Rafelski: Phys. Rep. 88 (1982) 331] [Rafelski-Müller: P. R. Lett. 48 (1982) 1066] • deconfinement  stronger effect for multi-strange • by using uncorrelated s quarks produced in independent partonic reactions, faster and more copious than in hadronic phase • strangeness enhancement increasing with strangeness content [Koch, Müller & Rafelski: Phys. Rep. 142 (1986) 167] Strangeness production depends strongly on baryon density (i.e. stopping vs. transparency, finite baryo-chemical potential)

34. Strangeness enhancement in B/B ratios • Baryon over antibaryon production can be a QGP signature as long as the baryochemical potential is high (Rafelski & Koch, Z.Phys. 1988) • With diminishing baryochemical potential (increasing transparency) the ratios approach unity with or without QGP, and thus only probe the net baryon density at RHIC.

35. STAR p+p 200 GeV BRAHMS, PRL nucl-ex/0207006 New RHIC data of baryon ratios • The ratios for pp and AA at 130 and 200 GeV are almost indistinguishable. The baryochemical potentials drop from SPS to RHIC by almost an order of magnitude to ~50 MeV at 130 GeV and ~20 MeV at 200 GeV.

36. Lines of constant lS I. Increase instrange/non-strangeparticle ratios PBM et al., hep-ph/0106066 total II. Maximum isreached mesons III. Ratios decrease (Strange baryonsaffected more stronglythan strange mesons) See P.Senger’s talk baryons hidden strangeness mesons <E>/<N> = 1 GeV Peaks at 30 A GeV in AA collisions due to strong mB dependence Strangeness enhancement:Wroblewski factor evolution Wroblewski factor dependent on T and mB dominated by Kaons

37. K/p K+/K- [GeV] Strangeness enhancement • K/p – the benchmark for abundant strangeness production:

38. The SPS ‘discovery plot’ (WA97/NA57)Unusual strangeness enhancement N(wounded) N(wounded)

39. The switch from canonical to grand-canonical(Tounsi,Redlich, hep-ph/0111159, hep-ph/0209284) The strangeness enhancement factors at the SPS (WA97) can be explained not as an enhancement in AA but a suppression in pp. The pp phase space for particle production is small. The volume is small and the volume term will dominate the ensemble (canonical (local)). The grand-canonical approach works for central AA collisions, but because the enhancements are quoted relative to pp they are due to a canonical suppression of strangeness in pp.

40. Grandcanonical prediction Strangeness enhancement factors at RHIC No Npart-scaling in Au-Au at RHIC -> lack of Npart scaling = no thermalization ? Alternatives: no strangeness saturation in peripheral collisions (gs = 1) non-thermal jet contributions rise with centrality

41. Identified particle spectra : p, p, K-,+, p-,+, K0s and L

42. BRAHMS: 10% central PHOBOS: 10% PHENIX: 5% STAR: 5% Identified Particle Spectra for Au-Au @ 200 GeV • The spectral shape gives us: • Kinetic freeze-out temperatures • Transverse flow • The stronger the flow the less appropriate are simple exponential fits: • Hydrodynamic models (e.g. Heinz et al., Shuryak et al.) • Hydro-like parameters (Blastwave) • Blastwave parameterization e.g.: • Ref. : E.Schnedermann et al, PRC48 (1993) 2462 Explains: spectra, flow & HBT

43. “Thermal” Spectra Invariant spectrum of particles radiated by a thermal source: where: mT= (m2+pT2)½transverse mass (Note: requires knowledge of mass) m = b mb + s ms grand canonical chem. potential T temperature of source Neglect quantum statistics (small effect) and integrating over rapidity gives: R. Hagedorn, Supplemento al Nuovo Cimento Vol. III, No.2 (1965) At mid-rapidityE = mT cosh y = mTand hence: “Boltzmann”

44. “Thermal” Spectra (flow aside) • Describes many spectra well over several orders of magnitude with almost uniform slope 1/T • usually fails at low-pT • ( flow) • most certainly will fail • at high-pT • ( power-law) N.B. Constituent quark and parton recombination models yield exponential spectra with partons following a pQCD power-law distribution. (Biro, Müller, hep-ph/0309052)  T is not related to actual “temperature” but reflects pQCD parameter p0 and n.

45. light 1/mT dN/dmT heavy mT explosive source light purely thermal source T,b 1/mT dN/dmT heavy T mT “Thermal” spectra and radial expansion (flow) • Different spectral shapes for particles of differing mass strong collective radial flow • Spectral shape is determined by more than a simple T • at a minimum T, bT

46. Thermal + Flow: “Traditional” Approach Assume commonflow pattern and common temperature Tth 1. Fit Data  T 2. Plot T(m) Tth, bT • is the transverse expansion velocity. With respect to T use kinetic energy term ½ m b2 This yields a common thermal freezeout temperature and a common b.

47. Hydrodynamics in High-Density Scenarios • Assumes local thermal equilibrium (zero mean-free-path limit) and solves equations of motion for fluid elements (not particles) • Equations given by continuity, conservation laws, and Equation of State (EOS) • EOS relates quantities like pressure, temperature, chemical potential, volume = direct access to underlying physics Kolb, Sollfrank & Heinz, hep-ph/0006129

48. Hydromodels can describe mT (pT) spectra • Good agreement with hydrodynamic prediction at RHIC & SPS (2d only) • RHIC: Tth~ 100 MeV,  bT  ~ 0.55 c

49. Blastwave: a hydrodynamic inspired description of spectra Spectrum of longitudinal and transverse boosted thermal source: bs R Ref. : Schnedermann, Sollfrank & Heinz, PRC48 (1993) 2462 Static Freeze-out picture, No dynamical evolution to freezeout

50. The Blastwave Function • Increasing T has similar effect on a spectrum as • increasing bs • Flow profile (n) matters at lower mT! • Need high quality data down to low-mT