Heavy Ions at the SP[b]S What Is There Left to Do? SPSC Special Workshop Villars s/Ollon • • • • September 22 – 28, 2004 Superdense (and -hot) QCD.
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Heavy Ions at the SP[b]S What Is There Left to Do? SPSC Special Workshop Villars s/Ollon • • • • September 22 – 28, 2004 Superdense (and -hot) QCD Matter What We Already Know (SPS & RHIC) What We Would Like to Know Possible goals of Future HI Expts at the SPS QCD Phase Diagram Early universe RHIC, LHC T QGP Critical endpoint Quark-Gluon Plasma (QGP) Meson Dominated HG Hadronic matter Chiral symmetry restored Baryon Dominated HG Chiral symmetry broken Nuclei Color superconductor Neutron stars B From hadrons to QGP 30 gDOFT 2 4 QGP = quark-gluon plasma 0 QCD equation of state from lattice QCD Hadron gas 0 QCD EOS for B 0 S. Ejiri et al. (Bielefeld-Swansea) Nf=2 Baryon number is “unthawed” for T/Tc = 0.9 – 1.1 Evidence that the carriers of baryon number (quarks) become light and color d.o.f. is freed. The perturbative QGP At T > 2Tc the QGP looks perturbative (neglect mq): 15 s 1 4 2 2 16 50 s 21 4 4 T 1 N T F 30 21 30 2 s 3 2 2 2 1 2 1 2 q ( T 2 q ) q Expansion in s does not converge, rather one must include interactions into the particle modes (“quasiparticles”) as basis for the expansion. All perturbatively calculable terms (up to s3 ln s) have been calculated, agreeing well with LGT equation of state down to near Tc. Quasiparticles in the QGP Physical excitation modes at high T are not elementary quarks and gluons, but “dressed” quarks and gluons: T Compton scattering on a thermal gluon! (k , ) Propagator of transversely polarized gluons 1 1 k k D(k , )1 2 k 2 ( gT )2 1 ln 2 2 k k Effective mass of gluon: 1 gT 3 1 k mG* gT 2 k 0 mG* (screening) (massive mode) Color Screening in Quenched LGT O. Kaczmarek et al. Rapid disappearance of color screening near Tc suggests binding of dynamical gluons into “glueballs”. Drop in (T) too fast to be described by dynamical glue mass Singlet potential T=0 coupling of static quarks sQGP = a Plasma of Bound States ? Lattice simulations (quenched!) of spectral functions (Asakawa et al., Datta et al.) indicate persistence of (quasi-)bound states above Tc Very speculative scenario! (cc ) Shuryak & Zahed Signatures of the QCD Phase Change • • • • • • • • • Enhancement of s-quark production Dissolution of Ψ, bound states Disappearance of light hadrons (ρ0) Effects of “latent heat” in (E,T) relation Thermal g radiation Event-by-event fluctuations Large energy loss of fast partons Bulk hadronization Collective vacuum excitations What We Know Already Classical QGP Signatures seen at SP[b]S The SP[b]S “Panorama” The Novel Discoveries of RHIC The Three “Classic” Signals Strangeness Enhancement… …probes chiral symmetry restoration and deconfinement Mass (MeV) NA57 data 1000000 Q CD mass 100000 Higgs mass (sss) 10000 QCD mass disappears (qss) 1000 100 (qqs) 10 1 u d s Flavor c b t Charmonium Suppression …probes color deconfinement ? Y’ NA50 Low-Mass Lepton-Pair Excess CERES Collision broadening of the r resonance in the baryon-rich hadronic medium Three Legs Are a Bit Shaky… …a 4th Leg Adds Stability ! Thermodynamics near Tc 30 gDOFT 2 4 T van Hove E Additional energy is used up to liberate new degrees of freedom (color!) in transition region Hadrons at the Boiling Point RHIC SPS AGS Plateau is an indicator of latent heat of phase transformation. K/ excitation function Sensitive to B The “Horn”... K+ K- K/ Excitation Function II K e( u s mK ) / T K e( s u mK ) / T Measures effective K mass K K K K e 2 mK / T RHIC …a Plateau! SPS …becomes Part of… The “Horn”... AGS The Villars Panorama e+e- Photons Chemistry J/Y HBT Thorsten Renk hep-ph/0403239 Spectra The SP[b]S Panorama The Model Framework ( x, y, z, t ) r, ,s 12 ln tt zz , t 2 z 2 R s 12 ln E pz E pz s 12 ln tz tz s(r,s , ) FR (r, ) FL (s , ) Partial stopping: front ( 0 ) 0.60 with 0 1fm/c front ( f ) 1.45 with f 15fm/c Compare with Pb = 2.9 at freeze-out The Cooling Curve High initial temperature due to large degree of stopping! 0 1fm/c T0 300 MeV QGP 6.5fm/c LGT and RG EOS Had 8.5fm/c T f 100 MeV QGP R rms 8.55fm f Had f 0.57 Hadron Spectra Transverse spectra fix vf┴ Resonance decays Rapidity distribution determines ffront Hadro-Chemistry Statistical hadronization No “free” parameters: Tc from LGT, B and S from conservation laws. Lepton pairs …are sensitive to in-medium modifications of r-meson and the duration of the hadronic phase, contribution from QGP visible above 1 GeV invariant mass. QGP Outshines Hadron Gas! … because it is so much hotter. Photon spectrum is sensitive to the initial temperature T0 Almost all observed direct photons are emitted by the QGP Charmonium Suppression Ionization by thermal gluons The RHIC Discoveries Jet Quenching Baryon Excess Recombination ? Large Elliptic Flow Jet Quenching High-energy parton loses energy by rescattering in dense, hot medium. q q Radiative energy loss: dE / dx r L kT 2 L q q Scattering centers = color charges g Can be described as medium effect on parton fragmentation: z D p h ( z , Q 2 ) D p h ( z , Q 2 ) D p h , Q2 1 E / E Suppression of high-pT hadrons PHENIX Data: Identified 0 ? Away side jet Same side jet STAR The Baryon “Puzzle” What makes baryons different from mesons ? Hadronization mechanisms q q q q q q q q Recombination Fragmentation Baryon Meson 1 Baryon 1 Meson pM 2 pQ pB 3 pQ Recombination vs. Fragmentation 2 dN M P u d dxw ( R , xP ) w ( R ,(1 x ) P ) ( x ) M d 3P (2 )3 , Recombination: E Fragmentation: dN h P u dz 1 E 3 d w ( r , z P )D h ( z ) 3 3 d P (2 ) 0 z Recombination… w (r , xP ) w (r , (1 x) P ) exp P u / T Meson w (r , xP ) w (r , x ' P ) wg (r , (1 x x ') P ) exp P u / T Baryon 1 …always wins over fragmentation for an exponential spectrum (z<1): exp( P u / T ) exp( P u / zT ) … but loses at large pT, where the spectrum is a power law ~ (pT) -b Recombination vs. Fragmentation II Fries, Bass, Nonaka, BM RAA T = 170 MeV v┴ = 0.55c Greco, Ko, Levai The Partons Flow! Recombination model suggests that hadronic flow reflects partonic flow (n = number of valence quarks): part v had n v 2 2 pThad npTpart Provides measurement of partonic v2 !? Was Landau right ? STAR + PHENIX Central plateau or peak of a Gaussian? BRAHMS (P. Steinberg- PHOBOS) y2 dN K s1/ 4 exp dy 2 L 2L with L = ln(g) PHOBOS Implications of RHIC Results • Early equilibration & development of pressure • Highly dissipative medium • Very high density of (colored) scattering centers at early times (dNG/dy dNh/dy). • Collectivity at the parton level • Hadrons at RHIC are not “hotter” than at SPS • Physics at RHIC is not boost invariant What We Would Like to Know Focus on 3 issues: What are the initial conditions ? What kind of material is the “QGP” ? Where is the QCD critical point ? GSI/FAIR ??? e+e- Photons Chemistry DONE J/Y HBT Mostly Done Spectra The SP[b]S Panorama High pT Photons - From the “depth of time” Initial stopping Coherent bremsstrahlung 2-photon HBT interferometry Discriminates against 0 decays and makes it possible to identify “direct” photons at lower pT. Goal for RHIC II, but physics case at SPS may be stronger! Proposed, but dropped, at RHIC What kind of Stuff are the Comovers ? … and how do they disrupt J/Y production ? • Absorption by hadrons via charm exchange • Momentum broadening • Color screening • Ionization by (thermal) gluons The J/Y Story - Continued Color screening may not suffice to dissolve J/Y at SPS. Dissociation by mesons in HG or gluons in QGP. Much theoretical progress (Rapp et al., Duraes et al., Maiani et al.,…): dis(QGP) ~ 1-1.5 mb dis(HG) 0.5 mb (T<Tc) Rapp et al. T=160 MeV is not enough ! Maiani et al. Charm Enhancement… …could upset the picture Needed Charm Measurements • • • • • Open charm yield needs to be measured in Pb+Pb Does charm quarks thermalize and flow? What is the contribution from recombination? Are baryons with charm enhanced? What is the contribution from c to J/Y ? • Ancillary result: Low-mass l+l- spectrum. Quark recombination ? Strong evidence at RHIC for formation of energetic (few GeV) hadrons by quark recombination: Suggests valence quarks remain independent until hadronization, but there is no evidence for presence of dynamical gluons. Is a similar picture valid at SPS energies? L, X, W enhancement suggests it. q q Recombination Baryon 1 Meson q q q q q Fragmentation Baryon Meson 1 q Elliptic Flow at SPS – Pions vs. Protons Is there a similar scaling law at SPS energies? NA49 Requires particle ID with good statistics to higher pT The QCD Critical End Point If it’s there, we need to find it… The QCD Critical End Point Philipsen & Forcrand Fodor & Katz Crossover [Tc(m),c(m)] First order P.T. Smaller quark mass Critical quark masses mic(B=0) B=360 MeV decr. m QCD Critical End Point II Fodor & Katz: Tc 160 MeV, Bc 360 MeV; Ejiri et al.: Bc 420 MeV Quark number susceptibility E/A ~ 30-40 GeV Philipsen & Forcrand: Bc very sensitive to mq Ejiri et al. Equation of State Near the CEP C. Nonaka (Duke) & M. Asakawa s(T , B ) 12 [1 tanh Sc (T , B )]sH (T , B ) Using 3-dim Ising model critical exponents and scaling hydro. 12 [1 tanh Sc (T , B )]sQ (T , B ) Hydrodynamics Near the CEP Focusing effect of critical end point makes it “easy to find” ! (Tc,c) = (155, 368) MeV Standard EOS with 1st order phase transition gives completely different evolution at phase boundary! Isentropic trajectories Dynamical Critical Fluctuations Dynamical critical enhancement is almost gone at kinetic freezeout ! • Fluctuations coupled to critical modes diverge in equilibrium at CEP, Critical enhancement survives if probe decouples early. • but remain finite in dynamical scenarios. Proposed observables: pT, N Np Strangeness !!! Nonaka & Asakawa L/Ltotal T [MeV] Why Look at Strangeness fluctuations? Because mseff is very sensitive to light quark condensate! ‘t Hooft interaction: LI [det f (qR qL ) det f (qLqR )] qq sR/L sL/R qq Dynamical strange quark mass fluctuates with light quark condensate, but strange hadron abundances freeze out close to Tc. Strange baryons would also be sensitive to critical baryon number fluctuations, but E-by-E fluctuations may be hard to measure. Hadron Abundance Fluctuations (K+K-)/+- would be better! Hatta & Stephanov suggest d(p+-p-)2/(p++p-). Conclusions • SPS-HI panorama is coming into focus. • SPS is unique facility to explore critical end point of QCD phase diagram. • Critical measurements: – Open (and hidden) charm hadrons – Good statistics for ID’d hadrons at pT 4-5 GeV/c – E-by-E fluctuations for observables with sensitivity to critical fluctuations and early freeze-out in dependence of system size – Coherent bremsstrahlung, gg HBT Time for Questions