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

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 GSI-SIS chemical freeze-out curve BNL-AGS CERN-SPS T0 ≤ 2Tc (RHIC) T0 ≈ 4-5 Tc (LHC) 200 250 150 100 50 Chemical Temperature Tch [MeV] critical point? quark-gluon plasma Lattice QCD deconfinement chiral restoration hadron gas neutron stars atomic nuclei 200 400 600 800 1000 1200 Baryonic Potential B [MeV]

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 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) 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 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 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) J.M. Nelson P.G. Jones L. Barnby M. Lamont (2002) J. Adams (2005) L. Gaillard (2008) A. Timmins (2008) T. Burton E. Elhahuli 1987 1994 1999 2008

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

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

Soft versus Hard QCD The advantage of high energy colliders , K, N, … , K, N, …  f Hadron gas s = 1? (H) QGP Light-cone trajectory (Q) s = 0.4 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”.

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

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

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

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

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

2-d ( correlations In vacuo (pp) fragmentation static medium broadening flowing medium anisotropic shape (Armesto et al, PRL 93, (2004); Eur. Phys. J. C 38 461) d+Au Au+Au Dh Dh Df Away-side 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?

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

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

UK–ALICE Birmingham’s role in ALICE ALICE trigger 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

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– c c g* p g b b K p p

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)

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? p + p  0 + X STAR

UK–ALICE Physics First physics Correction for trigger biases 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).

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.

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) 200 250 150 100 50 T0 ≤ 2Tc (RHIC) Chemical Temperature Tch [MeV] critical point? quark-gluon plasma SPS Lattice QCD AGS deconfinement chiral restoration hadron gas SIS chemical freeze-out curve neutron stars atomic nuclei 200 400 600 800 1000 1200 Baryonic Potential B [MeV]

Expectations from lattice QCD F Karsch: Quark Gluon Plasma 3 (World Scientific) Central Au+Au √sNN = 200 GeV RHIC LHC ? 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

Origin of surviving jets 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.

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, …

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

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