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STAR Physics at RHIC II

STAR Physics at RHIC II. James C. Dunlop Brookhaven National Laboratory. STAR Physics in the RHIC II Era. To test and extend QCD theory and its predictions STAR will: use hard (short wavelength) probes such as Inclusive jets and direct photons

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STAR Physics at RHIC II

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  1. STAR Physics at RHIC II James C. Dunlop Brookhaven National Laboratory RHIC II Science Workshop, Nov. 2004

  2. STAR Physics in the RHIC II Era To test and extend QCD theory and its predictions STAR will: • use hard (short wavelength) probes such as • Inclusive jets and direct photons • back to back jets (correlation of leading particles) • direct gamma + leading hadron from jet • flavor tagged jets to measure the differential energy loss for gluon, light quark and heavy quark probes which couple differently to the medium • spectra and yields of the full range of quarkonia, through the Upsilon family, to measure the thermodynamics of deconfinement • measure very large samples of “soft physics” events to study • mechanism of equilibration through heavy quark yields and flow • the velocity structure of the medium in complete detail • the medium’s temperature using leptons, photons • spectrum of extended hadronic matter (resonances) • broken / restored symmetries (e.g., CP violation, chiral restoration) Sensitivity to rare probes, plasma radiation, and characterization of bulk matter RHIC II Science Workshop, Nov. 2004

  3. Jet Quenching Pedestal&flow subtracted Experimentally well-established phenomenon at RHIC The promise: Use as a quantitative tool to characterize the medium’s properties J. Adams et al., Phys. Rev. Lett. 91, 072304 (2003). RHIC II Science Workshop, Nov. 2004

  4. Towards real tomography ? STAR Preliminary, nucl-ex/0407007 • Au+Au: Away-side suppression is larger in the out-of-plane direction compared to in-plane • Geometry of dense medium imprints itself on correlations RHIC II Science Workshop, Nov. 2004

  5. Geometry, Correlations, and “Fragile” RAA K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, hep-ph/0406319 • Quenching so strong that RAA loses sensitivity to the density of the medium: dominated by unquenched “halo” • Increased sensitivity only through detailed angular correlations and/or decreasing coupling strength A. Dainese, C. Loizides, G. Paic, hep-ph/0406201 RHIC II Science Workshop, Nov. 2004

  6. g + Leading Particle Quantitative measurement of partonic energy loss Measurement of the gluon density via direct  + jet and flavor-tagged jets to study the quark mass dependence of energy loss AuAu (b = 0), s1/2 = 200 GeV • Leading hadrons are very rare: only • ~0.1% of jets fragment hard enough • that hadrons are above incoherent • background • cross section for  + jet coincidences • (central Au+Au): • Eg=10 GeV: 6 nb/GeV • Eg=15 GeV: 0.6 nb/GeV • 50 weeks of Au+Au @ RHIC I design: • 10 nb-1 !!  luminosity upgrade • needed to access this physics! • Remaining question: fragmentation g dN/dyd2pT (y=0) (GeV-2c-3) PT (GeV/ c) RHIC II Science Workshop, Nov. 2004

  7. Heavy Quark Energy Loss • total energy loss comparable but smaller than in the massless case Armesto, Salgado, Wiedemann, PRD69 (2004) 114003 B.W. Zhang, E. Wang, X.N. Wang, PRL93 (2004) 072301 Djordjevic, Gyulassy, NPA733 (2004) 265 Flavor dependence of coupling: Less radiation, and so less suppression, for massive objects • vacuum radiation suppressed in the dead-cone q < m/E Dokshitzer, Kharzeev, PLB 519 (2001) 199 • medium-induced radiation fills the dead-cone Armesto, Salgado, Wiedemann, PRD69 (2004) 114003 massive dI/dk2 massless dead cone k2 RHIC II Science Workshop, Nov. 2004

  8. Open Charm nucl-ex/0407006, nucl-ex/0404029 Abundant production, but high backgrounds: upgrades and large data samples needed for precision measurements Limited window for testing E-loss flavor dependence: m/E large, but E perturbative RHIC II Science Workshop, Nov. 2004

  9. The next generation: b quark • Not just c quark: b quark also accessible at RHIC II • Heavier mass -> m/E appreciable deep into perturbative regime pT ~ 15 GeV/c: s (Au+Au) ~ 20mb/Gev 10 nb-1yields 200K b-bar pairs Non-photonic electrons in d+Au Tagging in Au+Au (sim) RHIC II Science Workshop, Nov. 2004

  10. What are the relevant degrees of freedom? PRL 92 (2004) 052302, nucl-ex/0403032 Growing evidence for recombination/coalescence picture: Window into relevant degrees of freedom at hadronization? Need to precisely characterize behavior of identified hadrons over fullest possible range of species and measures RHIC II Science Workshop, Nov. 2004

  11. Asymmetry in near-side correlations D. Magestro, Hard Probes 2004 At intermediate pT, significant increase in width in h of near-side cone Is this due to energy loss in a flowing medium? Or recombination effects (PID)? Or? STAR Preliminary What beyond density can be learned from strong coupling to the medium? Armesto, Salgado, Wiedemann, hep-ph/0405301, Phys. Rev. Lett. in press RHIC II Science Workshop, Nov. 2004

  12. Minijet angular correlation: elongated on h, narrowed on f Soft string fragments disappear Observed h width increase is 10 Armesto, et al. pQCD prediction – hep-ph/0405301 Minijet Deformation on (h,f) string fragments disappear nucl-ex/0411003 0.15 < pt < 0.5 GeV/c 0.15 < pt < 2 GeV/c Hijing default central Au-Au central p-p 200 GeV Au-Au 130 GeV STAR preliminary widths peripheral pt > 0.5 GeV/c h minijet angular deformation subtract cos(fD) and cos(2fD) path length RHIC II Science Workshop, Nov. 2004

  13. Minijets as velocity/temperature correlation structures on (h,f) Need PID to distinguish b from T Strong elongation on h and new negative same-side structure  pt Fluctuations →pt Correlations 70-80% f h 20-30% centrality 0-5% Parton interaction with longitudinally expanding color fluid drags pre-hadronic matter along pseudorapidity full STAR acceptance nucl-ex/0308033, hep-ph/0410180,hep-ph/0410182 h autocorrelation variance excess  pt fluctuations STAR preliminary fluctuation inversion subtract multipoles fluctuation scaling RHIC II Science Workshop, Nov. 2004

  14. Kinetic Thermalization: Open Charm Flow F. Laue et al, nucl-ex/0411007 Test mechanism for thermalization: charm heavy, so needs many collisions to reach kinetic equilibrium. Current measurements: indirect from electrons, and so suffer from large statistical and systematic errors Need direct open charm reconstruction (at low pT)! RHIC II Science Workshop, Nov. 2004

  15. Chemical Equilibration: Open Charm Yields • No thermal creation of c or b quarks; m(c) = 1.1GeV >> T • c and b quarks interact with lighter quarks  thermal recombination ? • Ds+/D0 very sensitive • J/y: suppression vs recombination ? • Precision necessary for J/y, dilepton baseline RHIC II Science Workshop, Nov. 2004

  16. Temperature and density: Onium RHIC • Yields and Spectra of the onium states (J/, Upsilon, and excited states) to measure the • thermodynamics of deconfinement through varying dissociation temperatures • To deeply probe the plasma through studies of (Debye) screening length l ~ 1 /gT and map in-medium QCD potential • Study vs. Pt • Study vs. centrality • Study in lighter systems • Study vs. a control ( the Upsilon) • Upsilon rate ~ 10-3 J/Y Yield in 10 weeks of AuAu running at design luminosity This physics requires luminosity upgrade 10 RHIC II Science Workshop, Nov. 2004

  17. Direct photons: temperature WA98: Phys. Rev. Lett. 93, 022301(2004) • As yet, no measurement at RHIC directly sensitive to temperature at early times • Intriguing possibility: direct photons, especially with low pT interferometric methods • Potentially sensitive to thermal black-body radiation from the plasma, and so T • However, significant theoretical uncertainties remain: what is needed to bring these under control? RHIC II Science Workshop, Nov. 2004

  18. Dileptons: chiral symmetry and temperature R. Rapp, PRC 63, 054907 (2001) Vector Meson Properties • Cleanest (only?) experimental signature of chiral symmetry restoration • Experimentally, need excellent e/h rejection at low p, precision vertexing to control Dalitz, conversion backgrounds • Large contribution from hadronic stage: needs precision, systematics, theoretical work Thermal Dileptons • Direct sensitivity to temperature as a complement to photons • Somewhat separate uncertainties may help to disentangle T RHIC II Science Workshop, Nov. 2004

  19. Enhancing the signal: “Hard” tagging of “soft” physics near away away Collimatedregion away side: whole: |Df-p|<2.0 collimated: |Df-p|<0.35 The data suggest the away side products approach equilibrium with the bulk medium traversed Suggests a means to study particles (e.g. leptonic decays of vector mesons such as the Φ) that have an increased probability of having been produced in a bulk medium which may be deconfined, and/or in which chiral symmetry is restored. Differences between yields and spectra, branching ratios, flavor composition… for products 180° versus 90° from the tagged high pt particle may provide access e.g. to the study of chiral symmetry φ e+ e- RHIC II Science Workshop, Nov. 2004

  20. Fundamental QCD: Strong CP Violation EC• HC  0  EC• HC  0 QCD “should ” include CP violation, but experimentally,  = 0 Under certain conditions around a de-confining phase transition, regions of space may be formed which behave as if   0 – spontaneous CP violation. (Kharzeev et al) No Helicity Correlations  Helicity Correlations  Simple momentum space asymmetry probably not good enough  look at e-by-e helicity balance of fermions () and search for fluctuation (too many positive helicity ) Estimated need: several hundred million events! (efficiency dependent) -0.004 0.004 N /(N +N) RHIC II Science Workshop, Nov. 2004

  21. Gluon distributions in Cold Nuclei BRAHMS, nucl-ex/0403005 STAR: nucl-ex/0408016, PRC in press • Ultra-Peripheral Collisions: large flux of g* • Use hard probes (such as J/y,U) to probe gluon distributions • d+Au: h dependence of spectra • Disagrees with expectations based on incoherent multiple scattering in the initial state (i.e. standard factorized pQCD explanation for Cronin enhancement) ds/dy HERA param. No shadowing Shadowed Leading Twist Calculation Frankfurt, Strikman & Zhalov, 2001 Y = 1/2 ln(2k/MV) RHIC II Science Workshop, Nov. 2004

  22. Can the boundaries on this diagram be mapped out experimentally? p0 Forward dAu: probing low x “Mono-jet” PT is balanced by many gluons Dilute parton system (deuteron) p0 Beam View Top View • Ep > 25 GeV •   4 Dense gluon field (Au) Large Dhp0+h± correlations f • Suppressed at small <xF> , <pT,p> • Consistent with CGC picture • Consistent in d+Au and p+p at larger <xF> and <pT,p> • as expected by HIJING 25<Ep<35GeV STAR Preliminary Fixed h, as E & pT grows Fixed h, as E & pT grows 35<Ep<45GeV Statistical errors only Shown at DIS’04 RHIC II Science Workshop, Nov. 2004

  23. Spin in the RHIC II Era • Comprehensive study of the spin structure and dynamics of the proton, in particular the nature of the proton sea, using polarized protons: “RHIC SPIN Baseline program” (DOE review, June 2004) • Gluon contributionto the proton spin using various probes involving: • Final-state jets such as inclusive jet production and di-jet production (Short-term) • Inclusive 0 production (Short-term) • Prompt photon production (Long-term) • Heavy-Flavor production (Long-term) • Flavor decomposition of quark and anti-quark polarization in W production (Long-term) • STAR RHIC-SPIN program M. Karliner and R.W. Robinett, Phys. Lett. B324 (1994) 209. • Heavy flavor production • Unique test of partonic aLL • Sensitive to gluon helicity with low background from quark helicities • NLO formalism available (Bojak and Stratmann) Low energy Typical STAR range Massless-Q limit RHIC II Science Workshop, Nov. 2004

  24. Spin in the RHIC II era: W production • Semi-inclusive DIS - sensitivity reduced by fragmentation functions and e2q weighting • Parity violating single-spin asymmetries at RHIC provide access to the quark flavor structure of the proton spin: • Flavor decomposition of quark and anti-quark polarization B. Dressler, Eur. Phys. J. C14 (2000) 147. Q2=2.5GeV2 Q2=2.5GeV2 • W± production in pp collisions forms the best means to probe the flavor structure of the proton sea J. Kiryluk RHIC II Science Workshop, Nov. 2004

  25. Spin: Beyond the Baseline With adequate integrated luminosity and longitudinal spin orientations, RHIC will answer two of the important open questions in nucleon spin structure: gluon helicity helicities of up and down quarks and anti-quarks e.g. ALL in prompt photon-jet production e.g. parity violating single spin asymmetries However, the RHIC baseline program will not provide much insight into strangequark and anti-quark helicities and , which are particularly sensitive to flavor symmetry scenario’s. RHIC-II’s luminosity upgrade may! It should provide access to rareprocesses, e.g. charm associated W production as well as, transversity, Drell-Yan, ... (c.f. Werner Vogelsang - this workshop) RHIC II Science Workshop, Nov. 2004

  26. Conclusion Sensitivity to rare probes, plasma radiation, and characterization of bulk matter STAR proposes a future program of QCD studies of unprecedented breadth and depth to study • the quark mass dependence of partonic energy loss • collective behavior in partonic systems • the nature of chiral symmetry breaking and its relation to the masses of the hadrons • the nature of a possible saturated gluon state in cold nuclei at low Bjorken x • the helicity preference of gluons inside a proton; the origin of the proton sea; the transversity distribution of quarks in a proton • And, of course, surprises: the program has just begun There are still significant questions about many of these observables: hopefully, this set of workshops can serve to at least clarify the questions RHIC II Science Workshop, Nov. 2004

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