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Direct observation of dijets in central Au+Au collisions at RHIC

Direct observation of dijets in central Au+Au collisions at RHIC. Dan Magestro, The Ohio State University for the STAR Collaboration. Jets and azimuthal correlations Raising the p T scale Dijet tomography of QCD matter Modified picture of jet suppression. ??. Quark Matter 2005.

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Direct observation of dijets in central Au+Au collisions at RHIC

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  1. Direct observation of dijets in central Au+Au collisions at RHIC Dan Magestro, The Ohio State University for the STAR Collaboration • Jets and azimuthal correlations • Raising the pT scale • Dijet tomography of QCD matter • Modified picture of jet suppression ?? Quark Matter 2005 August 5, 2005

  2. The key theoretical question What is the nature of high-pT suppression? Drees et al, PRC 71 (2005) 034909 • How is energy loss reflected in away-side suppression? • What is the relationship between jet “associated” yields and inclusive spectra? • Is there really a surface bias? Is there near-side energy loss? • Can data distinguish between different energy loss scenarios? (e.g. collisional vs. radiative) • What is the path length dependence of energy loss? • ..... Dan Magestro, STAR

  3. Evolution of  correlations at RHIC Trigger  Higher pT→ Away-side suppression Lower pT→ Away-side enhancement pT(assoc) > 2 GeV/c pT(assoc) > 0.15 GeV/c Pedestal&flow subtracted 4 < pT(trig) < 6 GeV/c STAR, PRL 91 (2003) 072304 STAR, nucl-ex/0501016  correlations Associated • “Trigger-associated” technique valuable for tagging jets in high-multiplicity environment (vs. jet-cone algorithms) • Probes the jet’s interaction with the QCD medium • Provides stringent test of energy-loss models Associated Dan Magestro, STAR

  4. Probing fragmentation region w/ pT • Successful RHIC Run 4 (2004) → ~10x more central, minbias Au+Au • STAR event reconstruction ongoing • Raising pT thresholds allows to: • Extend into fragmentation region • Significantly reduce uncorrelated background, relative v2 contribution • Look for leading away-side hadrons • How does  correlation evolve with increasing pT? • Does away-side jet “punch through”? • Is the fragmentation modified? If so, how? • Does away-side suppression disappear at very high pT? ?? ~pQCD Dan Magestro, STAR

  5. Emergence of dijets with increasing pT(trig) preliminary pT(trig) pT(assoc) > 2 GeV/c •  correlations (not background subtracted) Au+Au, 0-5% • Hint of narrow back-to-back peak for higher pT(trig) • Higher pT(trig) reflects higher-Q2 hard scattering Dan Magestro, STAR

  6. Emergence of dijets w/ increasing pT(assoc) 8 < pT(trig) < 15 GeV/c •  correlations (not background subtracted) pT(assoc) > 2 GeV/c pT(assoc) > 3 GeV/c pT(assoc) > 4 GeV/c pT(assoc) > 5 GeV/c pT(assoc) > 6 GeV/c pT(assoc) > 7 GeV/c pT(assoc) > 8 GeV/c • Narrow peak emerges cleanly above vanishing background Dan Magestro, STAR

  7. Centrality dependence of near, away-side yield • Near-side yields consistent within errors • Away-side yields decrease monotically with increasing NPart • Suppression pattern similar for two pT(assoc) ranges! Fit scaledby x2 8 < pT(trig) < 15 GeV/c Dan Magestro, STAR

  8. Initial physics thought on dijets • Are we seeing “tangential” dijets? A. Dainese et al, EJP 38 (2005) 461 C. Loizides: Poster #78 (Thur-Fri) • Hard scattering takes place at surface, with neither parton traversing matter • Dainese et al: use inclusive suppression to estimate probability for parton(s) to escape a L1 L2 Red lines determined from constraining ratio of b+c to total to measured RAA L1 c L2 b L1 L2 • From fit to RAA, probability is vanishingly small for both partons to escape Dan Magestro, STAR

  9. Hadron-triggered fragmentation functions pT(trig)  • Xin-Nian Wang: Modified di-hadron fragmentation function • Quantify energy loss in terms of away-side particle production per trigger particle zT = pT(assoc)/pT(trig) • Without knowledge of parton energy, Dhh(zT) is next best thing pT(assoc) Dan Magestro, STAR

  10. Hadron-triggered fragmentation functions • Away-side D(zT) suppressed, but shape unchanged Scaling factors ~0.54 ~0.25 8 < pT(trig) < 15 GeV/c Dan Magestro, STAR

  11. Dijet assoc. yields (IAA) vs. RAA 8 < pT(trig) < 15 GeV/c  = Near-side IAA  = Away-side IAA IAA = Yield(0-5% Au+Au) Yield(d+Au) Note: Very different quantities are being compared STAR, Phys. Rev. Lett. 91 (2003) 072304 • Near-side yields consistent with unity • Away-side associated yields similar to RAA values Dan Magestro, STAR

  12. Confronting predictions - 1  = STAR preliminary  = STAR preliminary X-N Wang, PLB 595 (2004) 165 Majumder, Wang, Wang, nucl-th/0412061 • Majumder et al:Near-side energy loss affects trigger bias • Trigger carries smaller <z> of higher-E parton → more energy for associated production, IAA >> 1 as pT(assoc) increases • X-N Wang: Away-side fragmentation altered relative to p+p • Drops steeply below zT~0.4, roughly constant above zT~0.5 • Majumder et al: Near-side energy loss affects trigger bias • Trigger carries smaller <z> of higher-E parton → more energy for associated production, IAA >> 1 as pT(assoc) increases • X-N Wang: Away-side fragmentation altered relative to p+p • Drops steeply below zT~0.4, roughly constant above zT~0.5 No evidence for near-side E-loss; away-side data more suppressed Dan Magestro, STAR

  13. Similarity of away-side widths • Widths similar • No evidence for away-side  broadening that is observed at lower pT 8 < pT(trig) < 15 GeV/c 4 < pT(trig) < 6 GeV/c pT(assoc) > 1.5 GeV/c J. Ulery, STAR (Parallel 3c) Dan Magestro, STAR

  14. Confronting predictions - 2 Calculation with realistic path-length distribution needed • However, data show no significant broadening (unlike lower-pT) Vitev, hep-ph/0501225 • Vitev: Away-side broadening due to gluon radiation, to high pT • Broadening due (entirely?!) to radiated gluons A. Dainese et al, EJP 38 (2005) 461 Dan Magestro, STAR

  15. Combining the observations   Dijets more suppressed from d+Au to central collisions Away-side fragmentation pattern unchanged  Away-side widths similar for central, noncentral Dan Magestro, STAR

  16. Theoretical basis for noninteracting dijets • First observation of unmodified fragmentation? • E-loss models contain noninteracting term • Wang, Wang: Modified fragmentation functions PRC 71 (2005) 014903 parent parton fragmentation radiated gluon(s) fragmentation vacuum fragmentation • Salgado, Wiedemann: Discrete term in quenching probability PRL 89 (2002) 092303 no medium-induced energy loss medium-induced energy loss Dan Magestro, STAR

  17. The beginnings of dijet tomography • These are the first precision measurements of dijets in the fragmentation regime in Au+Au collisions at RHIC • Dijet suppression driven by geometry (system, system size) • Near-side jet fragmentation in Au+Au consistent with p+p • Away-side fragmentation suppressed; shape, peak widths unchanged • Away-side IAA quantitatively consistent with RAA preliminary preliminary 8 < pT(trig) < 15 GeV/c 6 < pT(trig) < 8 GeV/c • What will they teach us about energy loss? • What is the nature of jet quenching? • What are the properties of induced radiation? • Does collisional energy loss need to be reconsidered? • What will they teach us about the medium? • Mechanisms leading to partonic equilibration? • Determine significant upper bound on the gluon density? • Idea: Constrain degrees of freedom? Müller, Rajagopal, hep-ph/0502174 Dan Magestro, STAR

  18. Potential back-up slides

  19. An upper bound on energy density? • Müller and Rajagopal: From entropy and jet quenching to deconfinement hep-ph/0502174 • The number of degrees of freedom can be determined independently of direct measurements of temperature Energy density: Entropy: Degrees of freedom: • A lower bound on (T) is constrained by an upper bound on energy density Dan Magestro, STAR

  20. Reminder: charge ordering in b2b paper • Charge ordering showed jetlike nature of near-side correlation • Comparison to Pythia showed agreement STAR, PRL 90 (2003) 082302 Dan Magestro, STAR

  21. Dijets: Like vs. unlike sign (first look) Like sign Unlike sign 3 < pT(assoc) < 4 4 < pT(assoc) < 5 • Strong near-side charge ordering; away-sides consistent • Comparison to Pythia → generic information about parton? 8 < pT(trig) < 12 GeV/c 5 < pT(assoc) < 6 pT(assoc) > 6 preliminary Dan Magestro, STAR

  22. Hadron-triggered F.F. vs. Pythia • Pythia agrees in magnitude with d+Au on away-side preliminary Scaling factors 0.54 0.25 Dan Magestro, STAR

  23.  correlations at 62 GeV • High-pT rate reduced → far fewer pairs in dataset preliminary • Away-side peak emerges in central events, not present for similar trig/assoc thresholds at 200 GeV ! • Weak near-side peak due to harder fragmentation Dan Magestro, STAR

  24.  correlations: a solid foundation • Amount of suppression sensitive to geometry • Path length dependence of energy loss →jet quenching • Suppression is not an initial-state effect • d+Au: Away-side jet alive and well, similar yield as p+p • Fits into jet-quenching picture • Suppression of back-to-back correlations in AA • Away-side correlation weakens with increasing centrality Pedestal&flow subtracted STAR, PRL 93 (2004) 252301 STAR, PRL 91 (2003) 072304 PRL 90 (2003) 082302 STAR, PRL 90 (2003) 082302 • Away-side spectra softer in central Au+Au • Larger away-side pT, Nch • Softer particle spectra on away-side → equilibration with medium STAR, nucl-ex/0501016, sub. to PRL Dan Magestro, STAR

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