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The transition observed in two-dimensional correlation data from STAR

The transition observed in two-dimensional correlation data from STAR. Lanny Ray University of Texas at Austin STAR Collaboration. Outline: p-p phenomenology Centrality evolution Same-side structures Away-side ridge Implications. Tamura Symposium – Nov. 20-22, 2008.

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The transition observed in two-dimensional correlation data from STAR

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  1. The transition observed in two-dimensional correlation data from STAR Lanny Ray University of Texas at Austin STAR Collaboration • Outline: • p-p phenomenology • Centrality evolution • Same-side structures • Away-side ridge • Implications Tamura Symposium – Nov. 20-22, 2008

  2. Begin with proton-proton spectra Two-component soft + (semi)hard model: PRD 74, 032006 (nucl-ex/0606028) + pQCD hard… “semi-hard” “soft” 200 GeV Spp replot on “transverse rapidity” Data – Spp semi-hard component: gaussian on yt pt spectra for increasing Nch

  3. Correlation measure ρ(p1,p2)= 2 particle density ρsibling(p1,p2) Event 1 ρreference(p1,p2) Event 2 Fill 2D histograms (a,b), e.g. (f1,f2), (h1,h2), (f1-f2,h1-h2), (pt1,pt2), etc. measures number of correlated pairs per final state particle square-root of rref(a,b); (for h,f space) normalized ratio of 2D binned histograms; acceptance cancellation; two-track ineff. corrections Motivated by p-p superposition null hypothesis

  4. Proton-Proton: from spectra to correlations Peak yt=2.66 yt=2.66 yt2 pt ~ 2.0 pt ~ 1.0 pt ~ 0.5 yt1 STAR Preliminary SOFT component – Levy Distribution HARD component – Gaussian on yt(!) PRD 74, 032006

  5. Proton-Proton components yt2 yt1 p-p transverse correlations p-p axial correlations STAR Preliminary φΔ ηΔ We hypothesize that this structure is caused by semi-hard partonic scattering & fragmentation - minijets soft component semi-hard component φΔ φΔ ηΔ ηΔ Longitudinal Fragmentation: 1D Gaussian onηΔ HBT peak at origin, LS pairs only Minijets: 2D Gaussian at origin plus broad away-side peak: -cos(φΔ)

  6. Why minijets? Jets observed by UA1 down to 5 GeV/c in p-pbar collisions at sqrt{s} = 200 – 900 GeV; accounted for with pQCD; angular correlations and pt structures are “jet-like.” pQCD calc. C. Albajar (UA1) Nucl. Phys. B 309, 405 (1988)

  7. Why minijets? STAR observes correlated charged hadron pairs with pt ~ 1 – 1.5 GeV/c corresponding to typical parton pt of order >(3/2)(2)(1 – 1.5) = 3-4.5 GeV/c, well within reach of the UA1 data and pQCD descriptions. In Au-Au we cannot hope to identify these low pt jets directly. There is also no identifiable trigger particle at lower pt; thus we use both angular and (pt1,pt2) correlations as suggested by Xin-Nian Wang a long time ago (Phys. Rev. D 46, R1900 (1992)). How would these low pt, minimum-bias jets, or minijets, appear in our 2D (h,f) angular correlations?

  8. Jet-A proton p-p 200 GeV parton-parton cm Jet-B NN cm A-A B-B h A-B B-A A-A B-B A-B B-A STAR preliminary proton Number of pairs Number of pairs h1-h2 0 Φ1 - Φ2 large azimuth peak 0 p fD=f1-f2 small angle peak η1 - η2 Example: di-jets sum over many events

  9. Minijets in HIJING & PYTHIA X.-N.Wang and M. Gyulassy implemented the UA1 observations into their Monte Carlo code HIJING (w. PYTHIA) (Phys. Rev. D 44, 3501 (1991)) using standard PDFs, pQCD parton-parton cross sections, and fragmentation functions. p + p at 200 GeV Hijing Jets on fD hD soft pt pairs removed additional sharp peak: HBT, conversion electrons fD Hijing Jets off hD STAR Preliminary fD hD In this analysis “minijets” are simply jets with no low pt cut-off.

  10. Centrality evolution of the 2D correlations (Expectation is that minijets will be thermalized.)

  11. proton-proton 200 GeV Au-Au data Analyzed 1.2M minbias 200 GeV Au+Au events; included all tracks with pt > 0.15 GeV/c,|η| <1, fullφ note: 38-46% not shown 84-93% 74-84% 64-74% 55-64% 46-55% φΔ ηΔ 18-28% 9-18% 28-38% 5-9% 0-5% φΔ ηΔ STAR Preliminary We observe the evolution of several correlation structures including the same-side low pt ridge From M. Daugherity’s Ph.D Thesis (2008)

  12. Fit function Same-side “Minijet” Peak, 2D gaussian Away-side -cos(φ) Proton-Proton fit function STAR Preliminary “soft” “hard” = + φΔ φΔ φΔ ηΔ ηΔ ηΔ dipole longitudinal fragmentation 1D gaussian HBT, e+e- 2D exponential cos(2φΔ) • Au-Au fit function • Use proton-proton fit function plus • cos(2φΔ) quadrupole term (~ elliptic flow). quadrupole Note: from this point on we’ll include entire momentum range instead of using soft/hard cuts φΔ ηΔ

  13. Centrality and energy trends Au-Au at 62 and 200 GeV 200 GeV 62 GeV Transition STAR Preliminary 200 GeV 62 GeV From M. Daugherity’s Ph.D Thesis (2008)

  14. fD < p/2 Same-side correlation structure The “soft” ridge

  15. Same-side 2D Gaussian & binary scaling - AuAu Peak Amplitude Peak η Width Peak φ Width STAR Preliminary STAR Preliminary STAR Preliminary Statistical and fitting errors as shown Systematic error is 9% of correlation amplitude 200 GeV 62 GeV constant widths peripheral central HIJING 1.382 default parameters, 200 GeV, quench off Binary scaling: Kharzeev and Nardi model Deviations from binary scaling represent new physics unique to heavy ion collisions

  16. Does the transition point scale? Transverse Particle Density Peak amplitude h width h/f aspect ratio volume S = overlap area (Monte Carlo Glauber) STAR Preliminary Peak Amplitude Peak Amplitude Peak η Width Peak η Width Bjorken Energy Density Npart STAR Preliminary STAR Preliminary STAR Preliminary STAR Preliminary 200 GeV 62 GeV 200 GeV 62 GeV εBJ εBJ Npart Npart Peripheral bins are compressed. Depends on formation time (assumed 1 fm/c), difficult to compare energies. Same-side 2D Gaussian amplitude, h-width, volume scale with particle density

  17. 2D angular correlations for pt pT minijet peak 0-30% centrality = inclusive mean pt Number pt 200 GeV Au+Au Same-side amplitude and widths pt correlations follow binary scaling well past the transition J Phys G 32 L37 This leads to the hypothesis that semi-hard partons continue to underlie the same-side gaussian number correlations above the transition.

  18. (yt1,yt2) correlations for same-side pairs Au-Au 200 GeV Like Sign peripheral central pp HBT peripheral Unlike Sign central pp minijets persist; pt dissipation STAR Preliminary From M. Daugherity’s Ph.D Thesis (2008)

  19. fD > p/2 Away-side correlation structure The away-side ridge

  20. Q ~ 2 GeV minijets, nucleon KT , acoplanarity Low-x parton KT ~ 1 GeV/c KT broadening pz Low-x parton events 1,2,3… p 0 sum events φΔ p 0 away-side φΔ -3p -pp 3p 0 Away-side ridge (dipole) – local pt conservation 200 GeV 62 GeV The dipole matches the centrality dependence of the same-side gaussian and shows the same transition point. It’s origin is pt conservation: global + jets STAR Preliminary calculated global pt conservation Hijing – jets on (no soft pt)

  21. (yt1,yt2) correlations for away-side pairs Au-Au 200 GeV Like Sign peripheral central pp minijets peripheral Unlike Sign central pp minijets persist; pt dissipation STAR Preliminary From M. Daugherity’s Ph.D Thesis (2008)

  22. Elliptic flow measurement using 2D (h,f) correlations

  23. Quadrupole component (~ elliptic flow) Data cos(2φΔ) component Amplitudes • 62 and 200 have the same shape • Substantial amp. changewith energy • no transition 200 GeV 62 GeV φΔ φΔ ηΔ ηΔ The η-dependence separates quadrupole (v2) from non-flow STAR Preliminary Au-Au quadrupole determined by initial conditions; medium (EoS, viscosity) not relevant. Initial state QCD source for v2: Boreskov et al., arXiv:0809.0625 [hep-ph] Kopeliovich et al., arXiv:0809.4327 [hep-ph] D. Kettler, T. Trainor arXiv:0704.1674

  24. Implications for theory and phenomenology

  25. Implications: superposition model Expected behavior: • Minijet shape unchanged, amplitude follows binary scaling. • Minijet peak on (yt,yt) unchanged except for amplitude. Comparison with data: Superposition model approximates data to the transition point but radically fails at higher density, more central collisions. STAR Preliminary

  26. Minijets & Quadrupole STAR Preliminary STAR Preliminary Below the transition the Au-Au system appears transparent, i.e. no collisional pressure build-up, no flow, at least up to the transition point. expected v2? What mechanism(s) produces a smooth v2?

  27. 1 2 3 pT minijet peak 0-30% central Implications: opaque, thermalized medium Expected behavior of minijets: • Semi-hard partons thermalize, sound waves(?) – butnumber correlations disperse • pt correlations on h,f disperse • Minijet peak on (yt,yt) dissipated Comparisons with data: Semi-hard partons persist; number correlations do not vanish, but increase dramatically. Minijet correlation region in (yt,yt) remains strong Peak Volume STAR Preliminary 200 GeV 62 GeV Narrowing azimuth width Any new mechanism above the transition must seamlessly match minijets. 8x increase Observations contradict expectations for a rapidly thermalized system.

  28. Implications: opaque core + hadronic corona Expected behavior: • pt correlations remain • (yt,yt) dissipates but amplitude remains at minijet yt • same-side 2D Gaussian remains • However, same-side yield decreases unless enough hadrons from surface are correlated with minijet. • Some jets will lose away-side partner, reducing –cos(fD) away-side pt escapes, but is dispersed among many more pairs Comparison with data: Ratio of away-side ridge to same-side Gaussian is ~constant from peripheral to most-central; data are inconsistent with this scenario

  29. Minijets & Quadrupole time hadrons novel QCD environment z beam beam if more central is hydro But the “soft-ridge” implies more: increasing hD correlation width For more central collisions the 2D Gaussian correlation must have an earlier source when the system is supposedly opaque (Perfect Liquid). Rather than diminish, the same-side correlations increase! means earlier source

  30. Theoretical models – same-side ridge • Chiu and Hwa, Phys. Rev. C72, 034903 (2005) – Jet driven; recombination; hot spots pushed out by radial flow. • Voloshin, Nucl. Phys. A749, 287c (2005); • Shuryak, Phys. Rev. C76, 047901 (2007) – • beam-jet fragments pushed out by radial flow. • Dumitru, Gelis, McLerran, Venugopalan, arXiv:0804.3858[hep-ph] - • glasma flux tubes pushed out by radial flow. • S. Gavin, Phys. Rev. Lett. 97, 162302 (2006) – initial state energy fluctuations spread along h by shear viscosity; pushed out by radial flow. • Majumder et al., Phys. Rev. Lett. 99, 042301 (2007); Dumitru et al., arXiv:0710.1223 [hep-ph] - Jet driven with plasma instability • C.-Y. Wong, Phys. Rev. C76, 054908 (2007) – Jet driven “momentum kick” model. • smooth extrapolation to p-p limit • away-side ridge • transition point • narrowing width on azimuth None explain all the observations:

  31. pz Implications for phenomenology Novel QCD field: forward scattered debris (diffractive), glasma flux tubes,… • Below the transition the system is transparent to • semi-hard scattered partons. • Above the transition pt transport begins suddenly. • Correlations on z map to width increase on h. • Azimuth width is constant if only pt is transported. • pt correlations are preserved. Results suggest pt transport which suddenly turns on at a critical density.

  32. Summary and Conclusions • Same-side 2D Gaussian (minijets) follows binary scaling, then a remarkable transition occurs which scales in Au-Au with transverse particle density. • The away-side ridge is crucial to understanding the RHIC data; it shows a transition, follows the minijet amplitude, and may be explained in terms of recoiling minijets. • Momentum dissipation is evident yet minijet correlation structures persist to most-central. • Preponderance of correlation data contradict expectations based on strong, random scattering such as in rapid thermalization scenarios. • The data suggest novel QCD processes in which pt transport/emission suddenly increases and v2 is generated in the initial stage.

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