Relativistic Heavy Ions: the UK perspective

1. STAR Relativistic Heavy Ions:the UK perspective Peter G. Jones University of Birmingham, UK NuPECC Meeting, University of Glasgow, 3-4 October 2008

2. T0 ≈ 4-5 Tc (LHC) T0 ≤ 2Tc (RHIC) 250 200 CERN-SPS 150 BNL-AGS 100 GSI-SIS 50 chemical freeze-out curve 0 The nuclear phase diagram early universe Location of critical point uncertain: F. Karsch, BNL Workshop, 9-10 March 2006. Z. Fodor, S. Katz, JHEP 0203 (2002) 014, 0404 (2004) 050 C. R. Alton et al., Phys. Rev. D71 (2005) 054508 R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005) 114014 Chemical Temperature Tch [MeV] quark-gluon plasma critical point? Lattice QCD deconfinement chiral restoration hadron gas neutron stars atomic nuclei 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV]

3. UK participation • Involved since the inception of the CERN Heavy Ion programme 16O, 32S 208Pb 208Pb, 197Au 208Pb WA85 WA94 WA97 NA57 ALICE J. Kinson D. Evans G.T. Jones O. Villalobos-Baillie I. Bloodworth P. Jovanovic A. Jusko R. Lietava P. Norman (1999) M. Thompson (1999) R. Clarke (2004) P. Bacon (2005) S. Bull (2005) J. Kinson D. Evans G.T. Jones O. Villalobos-Baillie A. Bhasin P. Jovanovic A. Jusko R. Lietava R. Platt (2007) D. Tapia Takaki (2008) H. Scott J. Kinson J.N. Carney O. Villalobos-Baillie M.F. Votruba R. Lietava A. Kirk D. Evans (1992) J.P. Davies (1995) A.C. Bayes (1995) M. Venables (1997) ALICE D. Evans P.G. Jones C. Lazzeroni G.T. Jones O. Villalobos-Baillie L. Barnby R. Lietava M. Bombara A. Jusko M. Krivda Z. Matthews S. Navin R. Kour P. Petrov A. Palaha NA36 NA49 STAR J.M. Nelson P.G. Jones L. Barnby M. Lamont (2002) J. Adams (2005) L. Gaillard (2008) A. Timmins (2008) T. Burton E. Elhahuli J.M. Nelson R. Zybert P.G. Jones (1992) E.G. Judd (1993) J.M. Nelson R. Zybert P.G. Jones H. Caines (1996) L. Hill (1997) T. Yates (1998) L. Barnby (1999) R. Barton (2001) 1987 1994 1999 2008

4. Strangeness at the CERN-SPS • Strangeness enhancement as a signature of QGP formation • If T > TC ≈ ms, expect copious thermal s-quark production. • Gluon fusion shown to dominate over light quark annihilation. • Enhancement is measured relative to proton-proton collisions. NA35/NA49 WA97 NA57

5. CERN-AA CERN-pp RHIC–AA s strangeness Strangeness saturation factor net-baryon density B Chemical freezeout temperature Tch net-strangeness density S= 0 Statistical/thermal models • Hadrons are produced statistically – enhancement explained? STAR

6. Soft versus Hard QCD • The advantage of high energy colliders , K, N, … , K, N, …  f Hadron gas s = 1? (H) QGP Light-cone trajectory s = 0.4 (Q) Parton formation and thermalisation 0 = q z Soft process e.g. strangeness Hard process e.g. jets, charm A A Soft processes occur over the lifetime of the system. Hard processes occur at early times and serve as a “standard candle”.

7. X.-N. Wang and M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480 key prediction: jets are quenched Leading hadron Fragmentation radiated gluons pTOT pT pL heavy nucleus High pT particle production • High pT jets are well described by perturbative QCD Jet of high pT hadrons – initial state Parton distribution functions – pQCD calculable Hard scattering cross-section – final state Fragmentation function

8. binary collisions High-pT hadrons in A+A collisions Central STAR: Phys. Rev. Lett. 89 (2002) 202301 STAR Central Peripheral Peripheral scale factor p+p reference

9. 8 < pT(trigger) < 15 GeV/c Trigger particle Associated (near-side) Df Associated (away-side) STAR: Phys. Rev. Lett. 97 (2006) 162301 Measuring jets by two-particle correlations STAR

10. Away side broadening or quenching? • Measure “jet” yields as a function of zT = pT(assoc)/pT(trig) STAR: Phys. Rev. Lett. 97 (2006) 162301 STAR Near-side Away-side || < 0.63 || < 0.63 Suppression by factor 4-5 in central Au+Au. No suppression

11. 2-d ( correlations |h| ~ 1 h ~ 0 Trigger particle Trigger particle Df Dh

12. (Armesto et al, PRL 93, (2004); Eur. Phys. J. C 38 461) In vacuo (pp) fragmentation flowing medium anisotropic shape static medium broadening 2-d ( correlations d+Au Au+Au Dh Df Away-side Dh Df Away-side Near-side Near-side Disappearance of away-side correlation = jet quenching. Modification of near-side correlation = coupling of jet to the medium?

13. yield,) Extracting near-side “jet” yields Au+Au 20-30% 3 < pT,trig. < 4 GeV/c and pT,assoc. > 2 GeV/c STAR   1 Ridge yield  0 -1 Jet yield -2 0 2  () Npart Birmingham analysis: particle-type composition of the jet/ridge. Strange particles now being used as a diagnostic tool.

14. HMPID PID (RICH) @ high pt TOF PID TRD Electron ID PMD g multiplicity TPC Tracking, dEdx ITS Low pt tracking Vertexing MUON m-pairs PHOS g,p0 ALICE at the LHC Access to a wide range of observables in one experiment!

15. UK–ALICE • Birmingham’s role in ALICE • The ALICE central trigger system. • Only major subsystem which is the responsibility of a single university group. • Strong involvement in the science (Physics Performance Reports). • Now one of the largest university groups in ALICE. • ALICE trigger • Up to 60 inputs (every 25 ns) • 24 L0 – 1.6 s (100 ns decision time) • 24 L1 – 6 s • 12 L2 – 90 s • 50 trigger classes / 6 detector clusters • Pb-Pb collisions: 8 kHz interaction rate • p-p collisions: 200 kHz interaction rate David Evans / ALICE trigger

16. c b ALICE - Key Physics • Study QCD on its natural (energy) scale T > TC ≈ QCD. • Explore quark and gluon dynamics in a hot medium. • Hot topics: • Collective behaviour – sQGP. • Opacity to jets – gluon density. • Heavy flavour production – Debye screening. • Some new theoretical developments: • AdS/CFT correspondance • Connection between string theory and ... • … strongly-coupled gauge theories. • Provides an alternative to (lattice) QCD. • Some (limited) success so far. l+ jets p l– g* c p g b K p p

17. New ideas in Hadronization David d'Enterria (CERN) David Evans (Birmingham) Nick Evans (Southampton) Nigel Glover (IPPP) Peter Jones (Birmingham) Frank Krauss (IPPP) Kasper Peeters (MPI) Marija Zamaklar (Durham)

18. p + p 0 + X ALICE – pp physics • ALICE has a competitive programme of pp physics • Precision measurements of inelastic cross-sections. • Particle production as a function of pT. • Test of QCD calculations. • Study of diffractive events. • Probes nucleon structure. • Advantages of ALICE • Low transverse momentum coverage. • Particle tracking. • Particle identification. • More speculative … • Multiplicity: pp (LHC) = CuCu (RHIC) • QGP in pp collisions? STAR

19. UK–ALICE Physics • First physics • Multiplicity and transverse momentum distributions. • Initial tests of QCD; input to fragmentation functions. • Are parton distributions sufficiently well understood? • Correction for trigger biases • Important for all papers reporting cross-sections (All). • Longer term proton-proton physics – Pb-Pb physics • Resonances – sensitive to hadronic phase (Villalobos-Baillie). • Charmonium (J/) production – Debye screening (Lazzeroni). • High-pT and jet physics – energy loss (Barnby, Bombara, Evans, Lietava). • Anomalous high multiplicity pp events? – (Jones).

20. Outlook and Summary Unclear whether there will be a Pb-run in 2009. From 2010, expect 1 month of Pb per year. First few years, Pb-Pb collisions @ 5.5 TeV per nucleon. Option of changing beam species/energy in subsequent years. • e.g. p-Pb, symmetric light ions, lower energy(ies). LHC will achieve first collisions in March 2009. ALICE has a full physics programme. UK is helping to shape that programme. First physics  proton-proton collisions  Pb-Pb collisions.

21. 250 200 150 100 50 0 The nuclear phase diagram early universe Location of critical point uncertain: F. Karsch, BNL Workshop, 9-10 March 2006. Z. Fodor, S. Katz, JHEP 0203 (2002) 014, 0404 (2004) 050 C. R. Alton et al., Phys. Rev. D71 (2005) 054508 R. V. Gavai, S. Gupta, Phys. Rev. D71 (2005) 114014 T0 ≈ 4-5 Tc (LHC) T0 ≤ 2Tc (RHIC) Chemical Temperature Tch [MeV] quark-gluon plasma critical point? SPS AGS Lattice QCD deconfinement chiral restoration SIS hadron gas chemical freeze-out curve neutron stars atomic nuclei 0 200 400 600 800 1000 1200 Baryonic Potential B [MeV]

22. RHIC LHC ? Expectations from lattice QCD F Karsch: Quark Gluon Plasma 3 (World Scientific) Central Au+Au √sNN = 200 GeV Energy density at RHIC RHIC: T0/Tc = 1.5–2.0 LHC: T0/Tc = 3.0–4.0 J D Bjorken: Phys. Rev. D 27 (1983) 40 RHIC and LHC permit a detailed study of the high T phase of QCD

23. Surface bias • RAA sets a lower bound on the density Wicks, Horowitz, Djordjevic and Gyulassy, nucl-th/0512076 Origin of surviving jets (pT = 15 GeV/c) More penetrating probes needed to explore the medium.

24. Models of energy loss Initial ideas based on collisional energy loss. J D Bjorken, FERMILAB-Pub-82/59-THY Radiative energy loss was found to be dominant for light quarks. Soft gluon emission suppressed (Landau, Pomeranchuk, Midgal effect). Energy loss is independent of parton energy, E, and becomes a function of the path length L in the medium. Two example approaches (others exist) Few hard(er) interactions Multiple soft interactions GLV formalism BDMPS formalism Opacity (twist) expansion Static medium Transport coefficient For 1-d longitudinal expansion: Guylassy, Levai, Vitev, Wang, Wang, … Baier, Dokshitzer, Mueller, Peigne, Schiff, Salgado, Wiedemann, …

25. Eskola, Honkanen, Salgado, Wiedemann (2004) Use RAA to determine the medium density • Nuclear modification factor, RAA, for pions The medium is dense (30-50 x normal matter), but RAA provides limited sensitivity.

26. ALICE – Observables • ALICE is a general purpose detector • Access to a wide range of observables in one experiment!