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Intermediate and High-pT Physics from STAR: What Did We Find at RHIC So Far?

This paper discusses the findings of the STAR Collaboration at RHIC regarding central Au+Au collisions, including the behavior of dense and rapidly thermalizing matter, the relevance of partonic degrees of freedom, and the opacity to jets. It also examines particle production and nuclear modification factors, as well as the probing of QCD matter with high-pT particles and jets.

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Intermediate and High-pT Physics from STAR: What Did We Find at RHIC So Far?

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  1. Intermediate and high-pT physics from STAR Jana Bielcikova Nuclear Physics Institute ASCR and Center for physics of ultra-relativistic heavy-ion collisions Prague, Czech Republic for the STAR Collaboration

  2. What did we find at RHIC so far? central Au+Au collisions produce denseandrapidly thermalizing matter: • it behaves almost like a‘perfect liquid’ (elliptic flow well described by ideal hydrodynamics) • the relevant degrees of freedom seem to be partonic (constituent quark scaling) • it is opaque to jets It is not the asymptotically free plasma of quarks and gluons. strongly coupled plasma (sQGP) ? STAR Collaboration, NPA 757,102 (2005) See e.g. STAR White Paper : Nucl.Phys. A757, 102 (2005) Bormio 2008

  3. Outline: • STAR experiment at RHIC • Particle production at intermediate and high-pT - nuclear modification factors, baryon/meson ratios: recombination vs pQCD, energy loss - two- and three-particle correlations: Mach cone, ridge • Summary Bormio 2008

  4. SolenoidalTrackerAtRHIC • Detectors: • Time Projection Chamber • (TPC): particle tracking, PID • Electromagnetic Calorimeter • (EMCal): trigger, g, p0,e PID • Time of Flight (TOF): PID • Silicon Vertex Tracker (SVT) • Forward Time Projection • Chamber (FTPC):2.7<|h|<3.9 • PID = particle identification • h= pseudorapidity Magnetic field:solenoidal, B=0.5 T Acceptance: full azimuth, |h|<1 Bormio 2008

  5. Au+Au collision as seen by STAR TPC + EMC ~ 1500 charged hadrons and leptons in central Au+Au collisions Bormio 2008

  6. Particle identification • Identification of strange particles • from V0-decay vertices: • Λ p + p- BR=64% • K0S  p+ + p- BR=69% TPC • cuts on dE/dx of daughters • topological cuts L K0S TOF mass (GeV/c2) mass (GeV/c2) Bormio 2008

  7. Probing QCD matter with high-pT particles and jets Au+Au p+p What happens to high-pT particles/jets which pass through the medium? Are they - similar to p+p ? - modified by the medium? Bormio 2008

  8. High-pT particle production c hadrons b a d hadrons • p+p: • parton scattering  fragmentation  jet • can be calculated in perturbative QCD • collinear factorization • A+A: • partons traversing medium lose energy • gluon radiation, elastic collisions • energy loss different for g, light/heavy quarks • (color factor, dead cone effect) leading particle Parton distribution function Matrix element Fragmentation function e+e- final state (energy loss?) measured in DIS initial state (saturation?) pQCD X.-N. Wang, M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480 Goal: Use in-medium energy loss to measure medium properties Bormio 2008

  9. Radiative energy loss in QCD Medium-induced radiation spectrum GLV BDMPS transport coefficient ‘average kT-kick per mean-free-path’ • found to be dominant for light quarks • independent of parton energy but • depends on the path length L in the medium • two example approaches: • medium properties can be characterized by a single constant: • static medium: DEL2due to interference effects, expanding medium: DE L Thin medium: few hard interactions Thick medium: multiple soft interactions GLV formalism BDMPS formalism Guylassy, Levai, Vitev, Wang, Wang, … Baier, Dokshitzer, Mueller, Peigne, Schiff, Armesto, Salgado, Wiedemann, … Salgado, Wiedemann PRD68 (2003) 014008 Bormio 2008

  10. AKK : S. Albino, B. A. Kniehl, and G. Kramer, Nucl. Phys. B 725 (2005) 181 KKP: B. A. Kniehl, G. Kramer and B. Potter, Nucl. Phys. B 597 (2001) 337 How well does pQCD describe p+p? STAR: Phys Lett B, 637 (2006) 161 • Do we understand our p+p baseline? p + p 0 + X NLO pQCD calculations by W. Vogelsang Depends on particle species and choice of fragmentation function. Bormio 2008

  11. Strange particle production in p+p STAR, PRC 75 (2007) • STAR measurement of strange particles in p+p constrained AKK FF AKK fragmentation functions agree well with both mesons and baryons at mid-rapidity. KKP (Kniehl-Kramer-Potter): NPB 582 (200) AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006) DSV (DeFlorian-Stratmann-Vogelsang): PRD57, 58111 (1998) Bormio 2008

  12. Nuclear modificiation factor in Au+Au • Direct photons: • - not suppressed • do not interact strongly • - binary scaling works • Light hadrons: • - strongly suppressed • (factor 5x) in central • Au+Au collisions T. Isobe (PHENIX), QM’06 Large energy loss of light partons in the formed nuclear matter at RHIC binary collision scaling p+p reference Bormio 2008

  13. Eskola et al, NPA 747 (2005) 511 RAA at 10 GeV/c Central RAA Data What do we learn from RAA ? Use RAA to extract medium density e: transport coefficient: ^ • large values of q = 5-15 GeV2/fm compatible with the data • suppression only supplies a lower bound on medium density  Bormio 2008

  14. Limited sensitivity of RAA thin plasma (‘GLV’) ~15 GeV thick plasma (‘BDMPS’) Wicks,Horowitz,Djordjevic,Gyulassy NPA 784, 426 (2007) Renk, Eskola, PRC 75, 054910 (2007) Energy loss distributions very different for BDMPS and GLV formalisms BUT! RAA is similar Reason: leading hadrons come preferentially from the surface  RAA has limited sensitivity to the region of the highest energy density More differential probes needed: JETS Bormio 2008

  15. STAR, PRL 97, 152301 (2006) Do baryons and mesons behave similarly? Intermediate-pT (pT =2-5 GeV/c): baryon/meson splitting RCP(meson)<RCP(baryon) High-pT (pT > 5 GeV/c): RCP()~RCP(p) RCP(K)~RCP() Does it mean similar energy loss of quarks and gluons ?

  16. D µ a 2 ˆ E C q L s R Gluon jet contribution factor increases from , K, p towards: e.g. pT = 8 GeV/c: 50% for p 90% for p If and At high pT for same beam energy, system and centrality: RCP() > RCP(p) RCP(K) > RCP() Gluon vs quark energy loss AKK = particle + anti-particle AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006)

  17. Baryon/meson ratios at RHIC STAR, PRL 97 (152301) 2006 • Au+Au: p/ ~ 1 Λ/K0S ~ 1.8 • p+p: p/ ~ 0.3 Λ/K0S ~ 0.6 • e++e-: p/ ~ 0.1-0.2 • large enhancement of B/M ratio in Au+Au relative to p+p collisions • - reaches maximum at pT~3 GeV/c • jet fragmentation is not a dominant source of particle production Bormio 2008

  18. cartoon Parton recombination at intermediate pT • in vacuo fragmentation of a high momentum quark to produce hadrons competes with in medium recombination of lower momentum quarks to produce hadrons • 6 GeV/c particle via : • fragmentation from high pT • meson • - 2 quarks at pT~3 GeV/c • baryon • - 3 quarks at pT~2 GeV/c baryon meson Recombination produces more baryons than mesons at intermediate pT R.J. Fries et al., PRL 90 (202303) 2003 V. Greco et al., PRL 90 (202302) 2003 Bormio 2008

  19. Towards more differential probes … Bormio 2008

  20. Trigger Associated A+A  flow p+p Associated “Jet-like” correlations Full jet reconstruction in A+A collisions at RHIC difficult due to underlying background: use azimuthal correlations of high-pT particles Correlated yield is related to ratio of di-hadron to single hadron fragmentation functions: near-side away-side RAA zT=pTassoc/pTtrig Bormio 2008

  21. 4 <pT(trig)<6 GeV/c, 2 GeV/c <pT(assoc)<pT(trig) increasing pT(assoc) 8 < pT(trig)< 15 GeV/c Jet-like correlations at RHIC STAR, Phys. Rev. Lett. 97 (2006) 162301 STAR, Phys Rev Lett 91, 072304 • central Au+Au collisions @ 200 GeV: • disappearance of away-side correlations observed at intermediate pT • d+Au and p+p similar -> jet suppression is a final state effect • Is there a punch through? YES Bormio 2008

  22. Fragmentation: near vs away side STAR, Phys. Rev. Lett. 97 (2006) 162301 8 < pT(trig)< 15 GeV/c Away-side yield strongly suppressed to level of RAA No dependence on zT in measured range Away side: - strongly suppressed to level of RAA - no dependence on zT Near side: no suppression Widths unchanged with centrality: seeing those partons that fragment in vacuum? Bormio 2008

  23. Away-side di-hadron suppression at high pT Zhang,Owens,Wang,Wang, PRL 98 212301 (2007) NLO pQCD + KKP FF + expanding medium • di-hadrons have a smaller surface bias •  a “better” differential probe • 2-minimum narrower for di-hadrons • stronger constraint on density • extracted medium properties: • e0 = 1.68 GeV/fm • q = 2.8±0.3 GeV2/fm ^ c2 (IAA) c2(RAA) Bormio 2008

  24. System size dependence of the away-side suppression Data: O. Catu (STAR), DNP 2007 • IAA ratio calculated relative to d-Au • model comparisons • modified fragmentation: • - Npart scaling, but misses shape • Parton Quenching Model (PQM): • - wrong relationship between • Cu+Cu and Au+Au • (wrong q evolution or geometry?) suppression scales with Npart H.Zhang, J.F. Owens, E. Wang, X.N. Wang, based on PRL 98 (2007), private communication 4< pT trig < 6 GeV STAR preliminary C. Loizides, Eur. Phys. J. C 49, 339-345 (2007) ^ 6< pT trig < 10 GeV agreement with models better at higher pTtrig STAR preliminary Bormio 2008

  25. Away-side di-hadron fragmentation functions 4< pT trig < 6 GeV 6< pT trig < 10 GeV O. Catu (STAR), DNP 2007 zT=pTassoc /pTtrig Theory curves: nucl-th/0701045 - H.Zhang, J.F. Owens, E. Wang, X.N. Wang Model: X.N.Wang private communication see also H.Zhang, J.F. Owens, E. Wang, X.N. Wang, PRL 98 (2007) • fragmentation functions more suppressed for higher Npart • theory curves using parameters from the fit to previous • Au+Au data at higher-pT match the Cu+Cu data Bormio 2008

  26. 4 < pT(trig) < 6 GeV/c, 0.15 < pT(assoc) < 4 GeV/c 4 <pT(trig)< 6 GeV/c, 2 GeV/c <pT(assoc)<pT(trig) Where did the energy go? STAR, Phys Rev Lett 91, 072304 STAR, PRL 95 (2005) 152301 lowering associated pT : - resurrects correlated yield at away side - near and away-side yields are enhanced with respect to d+Au - shape of the away-side peak is not Gaussian Bormio 2008

  27. Azimuhal correlations: pT dependence increasing pT(trig) increasing pT(assoc) M. Horner (STAR), J.Phys.G34, S995,2007 • increased statistics (year4):  full exploration of trigger and associated pT range Medium response: at lower pT,assoc away side enhanced and shape modified: broad peak with angular substructure STAR Preliminary Bormio 2008

  28. 40-60% Preliminary 0-12% Away-side peak: Mach cone? 2.5 < pTtrig< 4 GeV/c and 1< pTassoc < 2.5 GeV/c Stoecker, Casalderry-Solana et al, Muller et al.; Ruppert et al., … M. Horner (STAR), J.Phys.G34, S995,2007 vs~0.33 c 3-particle correlation studies needed! Bormio 2008

  29. near near near Medium Medium Medium away away π away di-jets 0 π 0 deflected jets mach cone Conical flow vs deflected jets J. Ulery (STAR), arXiv:0704.0224 1 1 13 2 12 3 3 2 3 < pT,trig < 4 GeV/c 1 < pT,assoc < 2 GeV/c 13 STAR uses two methods: 1. Cumulant unambiguous evidence for 3-particle correlations, although not definitive about conical emission. 2. Jet-flow background (shown) Model dependent analysis. Evidence for conical emission. Note: Large and complicated backgrounds. C. Pruneau (STAR), J.Phys.G34,S667,2007 J. Ulery (STAR), arXiv:0704.0224 [nucl-ex] 12 near-side away-side Bormio 2008

  30. pTtrig=3-6 GeV/c, 2 GeV/c <pTassoc< pTtrig d+Au, 40-100% ridge jet D. Magestro, Hard Probes 2004 P. Jacobs, EPJ C43 (2005) 467 3 < pT(trig) < 6 GeV/c2 < pT(assoc) < pT(trig) Au+Au, 0-5% Dh Df A closer look at the near-side peak … Additional near-side correlation in pseudo-rapidity () observed The near-side jet interacts with the medium! Bormio 2008

  31. after v2 subtraction jet+ridge 3 GeV/c < pTtrigger< 4 GeV/c and pTassoc> 2 GeV/c STAR preliminary  Jet+Ridge () Jet () Jet) yield,)   jet ridge  ridge Npart Centrality dependence of near-side yield (I) J. Putschke (STAR), J.Phys.G34:S679 (2007) • jet yield independent of Npart and consistent with d+Au • ridge yield increases ~ linearly with Npart Bormio 2008

  32. Jet Ridge Centrality dependence of near-side yield (II) -> “ridge” yield increases with centrality ridge for K0S trigger < ridge for Λtrigger • steep increase of near-side yield with centrality in Au+Au • ratio of yields in central Au+Au/d+Au ~ 4-5 -> “jet” yield is independent of centrality and agrees with d+Au J.B. (STAR), QM’2006 J.Phys.G34:S929 (2007) Bormio 2008

  33. “Jet” and ridge: pT dependence pT assoc> 2 GeV/c Ridge: solid symbols Jet: open symbols J. Putschke (STAR), J.Phys.G34:S679 (2007) J. Putschke (STAR), J.Phys.G34:S679 (2007) Jet: T(jet) > T(bulk) T(jet) increases with pTtrig Ridge: T(ridge) ~ T(bulk) T(ridge) ~ independent of pTtrig ridge persists up to pTtrig~ 10 GeV/c h-h correlations pTassociated>2GeV/c “jet” slope ridge slope inclusive slope Bormio 2008

  34. Particle composition in “jet” and ridge J.B. (STAR), arXiv:0707.3100 [nucl-ex] A hint? Ridge: B/M ratio closer to bulk Jet: B/M ratio ~ p+p More data needed ! Bormio 2008

  35. Is there ridge at forward rapidity? L. Molnar (STAR), J.Phys.G34, S593 (2007) pTassoc = 0.2-2.0 GeV/c: no near-side peak within systematic errors pTassoc > 1 GeV/c: : data suggest a non-zero correlation at near side (?) Bormio 2008

  36. Armesto et al, PRL 93 (2004) What is the origin of the ridge? (I) 1) Medium heating and parton recombination Chiu & Hwa Phys. Rev. C72:034903,2005 • hard parton enhances thermal parton distribution • (ΔT=15 MeV) •  recombination of thermal partons forms • a pedestal (ridge) • enhanced baryon/meson ratio 2) Parton radiation and its coupling to the longitudinal flow • gluon bremsstrahlung of hard-scattered parton • radiated gluon contributes to broadening Bormio 2008

  37. What is the origin of the ridge? (II) 3) Longitudinal broadening of quenched jets in turbulent color fields • A. Majumder, B. Mueller, S.A.Bass, hep-ph/0611135 • plasma instabilities in expanding medium •  non-thermal color fields • broadening of jet cone • wide ridge in rapidity at low pTassoc Bormio 2008

  38. What is the origin of the ridge? (III) 4) Correlations between jet and radial flow S. Voloshin, nucl-th/0312065, Nucl.Phys. A749, 287 (2005) E. Shuryak, arXiv: 0706.3531 [nucl-th] • radial expansion + jet quenching  correlation • ridge is independent of jet • particle spectra in ridge: • points of origin are biased towards surface •  ‘a bit’ stiffer slope than that of bulk • 5) Momentum kick • C.-Y. Wong , arXiv: 0707.2385, arXiv:0712.3282 • medium partons acquire ‘kick’ from propagating jet • T=470 MeV, q1 (mom. kick) and σy (rapidity distribution) • narrow peak in Dfdepends mainly on momentum kick • ridge in Dh depends on initial parton y distribution More quantitative theoretical predictions are needed! Bormio 2008

  39. Summary • Modification of jets: • inclusive measurements: large suppression of yields, • but RAA has only limited sensitivity to the medium density  • di-hadron measurements: • away-side associated jet-yield suppressed to the level of RAA • but shape of fragmentation function unmodified (pT, assoc > 3 GeV/c) • Medium response: • away-side: broadening, angular substructure • possible evidence of conical emission • near-side “ridge”: bulk-like properties • coupling of jet to the medium • Stay tuned for the upcoming Quark Matter Conference in India which is just around the corner … THANK YOU! Bormio 2008

  40. BACKUP Bormio 2008

  41. (J) ||<0.7 (J) ||<0.7 2 1 const bkg. subtracted 2 const bkg. subtracted (J+R) - (R)  v2 subtracted (J+R) ||<1.7 (J+R) ||<1.7 (J) no v2 subtraction needed Extracting near-side “jet” and “ridge” yields pTtrig=3-4 GeV/c, pTassoc>2 GeV/c J = “jet”, R= “ridge” J. Putschke (STAR),QM’2006 Bormio 2008

  42. Testing recombination with W and f The production of f and Ω is almost exclusively from thermal s-quarks even out to pT = 6-8 GeV/c (shower = “jet” contributions are strongly suppressed) S = shower (“jet”) T = thermal Prediction: 1. Ω/f ratio should rise linearly with pT 2. There should be no Ω or f di-hadron correlations at near side! R. Hwa, C.B. Yang , nucl-th/0602024 Bormio 2008

  43. Test 2: correlations with multi-strange particles B. Abelev (PhD 2007, Yale), J.B. (STAR) QM2006 There is a near-side peak for L, X and W-triggered correlations and its magnitude is independent of strangeness content ! This is in disagreement with the recombination picture. Bormio 2008

  44. Baryon/meson enhancement: model comparisons SOFT+QUENCH: Gyullassy, Levai, Vitev, PRL 85 (2000), Nucl.Phys. A 715, 779 (2003) TEXAS: Greco, Ko, Levai, PRL 90 (2003), DUKE: Fries, Mueller, Nonaka, Bass, PRC 68 (2003) OREGON: Hwa, Yang, PRC 67, 034902 (2003) Both, soft+quench and recombination type of models, predict a peak in Λ/K ratio, however they can’t reproduce the peak location in pT, its magnitude and shape in detail. Bormio 2008

  45. baryons mesons • scaling v2 and mT-m0 by quark content nq (baryon: nq= 3, meson: nq=2) • resolves baryon-meson separation Elliptic flow: constituent quark scaling Bormio 2008

  46. Di-hadron correlations: near-side peak Au+Au 0-10% STAR preliminary 3 GeV/c < pT trigger< 4 GeV/c, pT,assoc. > 2 GeV/c Near-side jet peak Near-side ‘ridge’ Modified away-side (+ v2) The near-side jet interacts with the medium! • What is the ridge? • 1) Medium heating and parton recombination • Chiu & Hwa PRC 72, (034903) 2005 • 2) Radial flow + high-pT trigger particle • Voloshin, nucl-th/0312065 NPA 749, 287 (2005) • 3) Parton radiation and its coupling • to the longitudinal flow • Armesto et al, PRL 93 (2004) J. Putschke (STAR), HP2006, QM2006 4) Momentum broadening in an anisotropic QGP Romatschke, PRC 75, 014901 (2007) 5) Longitudinal broadening of quenched jets in turbulent color fields Majumder, Mueller, Bass, hep-ph/0611135 Bormio 2008

  47. System size dependence of ridge yield relative ridge yield = ridge yield / jet() J. Putschke (STAR), QM2006 pTassoc. > 2 GeV/c STAR preliminary Au+Au @ 200 GeV Cu+Cu @ 200 GeV Au+Au @ 200 GeV (30-40 %) Cu+Cu @ 200 GeV (0-10 %) relative ridge yield relative ridge yield 3 GeV/c <pTtrigger<4 GeV/c STAR preliminary Relative ridge yield comparable in Au+Au and Cu+Cu at same Npart Bormio 2008

  48. pTtrig=3-6 GeV/c, 2 GeV/c <pTassoc< pTtrig after v2 subtraction jet+ridge ridge jet    jet ridge Dh Df  ridge Correlations at near-side v2 + away-side peak Near-side yield : Δɸ (J+R) = Δɸ (J) + Δɸ (R) Jet yield : Δɸ (J) = Δɸ(|Δη| < 0.7) - Δɸ(|Δη| > 0.7) : Δη (J) Ridge yield : Δɸ (R) = Δɸ (J+R) - Δɸ(J) Bormio 2008

  49. -/+: independent of pT p/p: model calculations with/without Eloss do not describe data Baryon junction and coalescence models describe data at intermediate pT X.-N. Wang et al., PRC 70 (2004) - Eloss I. Vitev et al.. NPA 715 (2003) - baryon junction V. Greco et al., PRC 71 (2005) - coalescence Energy dependence: anti-particle/particle ratios STAR, nucl-ex/0703040 STAR, PRL 97, 152301 (2006) STAR, nucl-ex/0703040 Bormio 2008

  50. RelativisticHeavyIonCollider 1 km PHOBOS BRAHMS RHIC PHENIX STAR v = 0.99995c BOOSTER AGS TANDEMS • 3.8 km circumference • 2 concentric rings of 1740 superconducting magnets • counter-rotating beams of ions from p to Au • √sNN = 20, 62, 130, 200 GeV Long Island, NY Bormio 2008

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