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Low p t Ridge: Observation of a Dramatic Transition

Low p t Ridge: Observation of a Dramatic Transition. Lanny Ray For the STAR Collaboration University of Texas at Austin May 27, 2008. RHIC & AGS Annual User’s Meeting – May 2008. Introduction and Overview. We study the RHIC collision environment using low p t hadrons in

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Low p t Ridge: Observation of a Dramatic Transition

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  1. Low pt Ridge: Observation of a Dramatic Transition Lanny Ray For the STAR Collaboration University of Texas at Austin May 27, 2008 RHIC & AGS Annual User’s Meeting – May 2008

  2. Introduction and Overview • We study the RHIC collision environment using low pt hadrons in • analogy with Brownian motion: • The probe (correlation structures) is big enough to observe • yet small enough (low pt) to be strongly affected by the medium. • Our probes are the copious minijets observed in proton-proton • collisions which continue in Au-Au. • Anomalous centrality dependence was observed in the amplitude • and longitudinal width (Daugherity, QM 2008). • Theoretical explanations for the high pt ridge are considered. • Implications of these correlation data for the collision environment • at RHIC are discussed.

  3. 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

  4. Proton-Proton: 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. Correlation Measure ρ(p1,p2)= 2 particle density in momentum space ρsibling(p1,p2) Event 1 ρreference(p1,p2) Event 2 Start with a standard definition in statistics: Δρas a histogram on bin (a,b): ε = bin width, converts density to bin counts measures number of correlated pairs per final state particle Normalize

  6. 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)

  7. 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

  8. Fit Function (5 easy pieces) 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

  9. Same-side 2D gaussian & binary scaling 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 small increase before transition Binary scaling: Kharzeev and Nardi model STAR Preliminary Note the absence of a transition point in the quadrupole: v2 & elliptic flow Deviations from binary scaling represent new physics unique to heavy ion collisions

  10. Does the transition point scale? 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. Peak Amplitude Peak η Width Transverse Particle Density STAR Preliminary STAR Preliminary 200 GeV 62 GeV S = overlap area (Monte Carlo Glauber) Same-side gaussian amplitude and h-width scale with particle density

  11. (yt,yt) correlations, 200 GeV Au+Au proton-proton How the correlations evolve in transverse momentum (peripheral) (central) STAR Preliminary Sudden onset at lower yt corresponding to transition point for same-side gaussian. Correlations remain at original yt – surface jets? increase at higher yt. (protons: see arXiv:0710.4504)

  12. 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.

  13. Q ~ 2 GeV/c 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) – 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 Global pt conservation

  14. Implications: Superposition model Expected behavior: • Minijets unchanged, except amplitude increases with binary scaling; widths remain constant. • 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

  15. 1 2 3 pT minijet peak 0-30% central Implications: parton/hadron scattering model Expected behavior: • Widths of both number and pt angular correlations increase • Amplitude of pt correlation falls below binary scaling • Minijet peak on (yt,yt) dissipates to lower momentum Comparison with data: pt correlation amplitude follows binary scaling beyond transition; doesn’t decrease until here hwidths increase butfwidths decrease Minijet peak dissipates, strength remains at original yt, increases at higher yt

  16. 1 2 pT minijet peak 0-30% central Implications: opaque, thermalized medium Expected behavior: • Semi-hard partons stopped; produce local hot spots; isotropic thermal motion - number angular correlations vanish, radially flowing hot spots could produce correlations [e.g. +cos(fD)]. • momentum conserved - pt correlations on h,f may persist • Minijet peak on (yt,yt) completely dissipated; saddle shape appears at lower pt (J.Phys.G 34, 799) Comparison with data: Semi-hard partons persist; number correlations do not vanish, but increase dramatically. Peak Volume STAR Preliminary 200 GeV 62 GeV STAR Preliminary Narrow azimuth width from p-p to central Au-Au, no transition point. • width initially due to minijets. If other mechanisms contribute above the transition they must seamlessly match minijets. 8x increase

  17. Implications: opaque, thermalized medium b 3 4 Comparison with data (cont.): Boosted hot spots produce +cos(fD) correlations; opposite sign to data 200 GeV Au+Au peripheral The minijet correlation region in (yt,yt) does not vanish, but increases and extends to higher yt; a saddle shape develops (see J.Phys.G 34, 799) STAR Preliminary central The observed correlations contradict expectations for a rapidly thermalized system.

  18. Implications: opaque core + 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 relative to same-side Comparison with data: Ratio of away-side ridge (dipole) to same-side Gaussian is ~constant from peripheral to most-central.

  19. Theoretical Models: partonic collisions (intended for the higher pt ridge) • Armestoet al. Phys. Rev. Lett. 93, 242301 (2004) – Jet driven; radiated gluon interactions (order as) with longitudinally flowing medium, jet broadening on handf, modest width increases. Trigger pt = 4 – 6 GeV/c Width changes small and do not agree with low-pt ridge data • Romatschke, Phys. Rev. C75, 014901 (2007) – Jet driven pQCD LL; collisional broadening of heavy flavor jets in longitudinally expanding medium; obtain large h/f width ratios ~ 3. Relevance to low-pt ridge data unclear

  20. Theoretical Models: reco &radial boost • Chiu and Hwa, Phys. Rev. C72, 034903 (2005) – Jet driven; shower-thermal • recombination model in longitudinally expanding medium; local hot spots collimated • on azimuth via shower parton momentum. • Voloshin, Nucl. Phys. A749, 287c (2005); • Shuryak, Phys. Rev. C76, 047901 (2007) – • beam jet fragments pushed out by strong radial flow; • ridge shaped by flow and path length attenuation. Can these models distinguish number and pt same-side correlations? f narrowing? Transition point?

  21. Theoretical Models: plasma instabilities • Majumder et al., Phys. Rev. Lett. 99, 042301 (2007) Jet and plasma instability drivencolor turbulence; generates transverse color magnetic fields which deflect gluons from the jet parton toward beam direction; h broadening, azimuth width remains narrow. • Dumitru et al., arXiv:0710.1223 [hep-ph] – Jet and plasma non-Abelian • color field instability driven; transverse color magnetic fields and longitudinal • color electric fields, preferential deflection toward beam direction. • Strongest affect on lower pt partons, may agree with • different h-widths for number vs pt correlations. • Transverse momentum dissipation. • Jet recoil included. • Can the large increase in number correlations be explained?

  22. Theoretical Models: shearviscosity • C.-Y. Wong, Phys. Rev. C76, 054908 (2007) – Jet driven conjecture that the • fast parton’s momentum is transported to bulk via interactions (“momentum kick”) • which maintain same direction; h-width broadening generated by longitudinal • velocity gradients; radial expansion not required; azimuth width constant; • momentum dissipates; recoil included. Not clear what the model predicts for the h-ridge width. • S. Gavin, Phys. Rev. Lett. 97, 162302 (2006) – initial state fluctuations with shear viscosity; driven by hydrodynamic radial expansion Widths of same-side pt correlation reproduced Looks promising, but what about number correlations, p-p limit, recoil? – (see Sean Gavin’s talk later today)

  23. Implications for phenomenology (personal speculation) Novel, 1D Hubble expanding gluon field (in co-moving frame of parton) pz • transverse momentum loss; no change in direction • pt transfered to nearby gluons, increasing number • of correlated pairs • correlation along z maps to width increase on h • azimuth width stays constant • pt correlations preserved Same idea as C.-Y. Wong’s “momentum kick” PRC76, 054908 (2007) Results suggest pt transport which suddenly turns on at a critical density.

  24. Summary and Conclusions • Angular correlations on (h,f) were shown for p-p and Au+Au collisions at 62 and 200 GeV. • Same-side 2D Gaussian follows binary scaling (minijets) until an abrupt transition; • number of correlated pairs and h-width increase dramatically; f-width decreases. • This transition point appears to scale with transverse particle density. • Results indicate strong modification of parton scattering and fragmentation. • Theoretical models for the higher pt ridge were discussed in relation to the low pt ridge, some show promise. • The number (h,f) and (yt,yt) correlations contradict expectations based on rapid thermalization. • The data suggest a sudden increase in pt transport above the transition point: non-pQCD, plasma instabilities, shear viscosity…?

  25. Extra Slides

  26. 62 GeV Au-Au Data Analyzed 13M 62 GeV Au+Au minbias events; included all tracks with pT > 0.15 GeV/c, |η| < 1, full φ note: 37-46% not shown 84-95% 75-84% 65-75% 56-65% 46-56% 18-28% 28-37% 9-18% 5-9% 0-5% STAR Preliminary A similar evolution appears but is delayed on centrality relative to the 200 GeV data.

  27. 200 GeV Model Fit model STAR Preliminary 84-93% 75-84% 65-75% 55-65% 46-55% φΔ ηΔ 19-28% 28-38% 9-19% 5-9% 0-5% φΔ ηΔ 27

  28. 200 GeV Residual Fit residual = data - model STAR Preliminary 84-93% 75-84% 65-75% 55-65% 46-55% φΔ ηΔ 19-28% 28-38% 9-19% 5-9% 0-5% φΔ ηΔ We have a good fit with the simplest possiblefit function. Other than adding the cos(2φΔ) quadrupole term, no other modification was necessary. 28

  29. Centrality and Energy Trends

  30. Transition – close-up Does the transition from narrow to broad ηΔ occur quickly or slowly? data - fit (except same-side peak) STAR Preliminary 83-94% 55-65% 46-55% 0-5% ηΔ width Large change within ~10% centrality Smaller change from transition to most central low-pt manifestation of the “ridge” Shape changes little from peripheral to the transition The transition in same-side ridge occurs quickly

  31. Consistency Check Does interaction between same-side peak and cos(φΔ) terms cause the transition? Result 200 GeV: standard, two-stage fit Two-stage fit: cos(φΔ) cos(2φΔ) fix cos(φΔ) and cos(2φΔ) on away-side then fit remaining terms ν ν The results are consistent Cancellation in fit terms does not cause the amplitude increases. minijet peak minijet η width ν ν 31

  32. HIJING minijet predictions Peak Amplitude Peak η Width Peak φ Width STAR Preliminary STAR Preliminary STAR Preliminary 200 GeV 62 GeV HIJING 1.382 default parameters, 200 GeV, quench off Quench on causes slight amplitude decrease The observed minijet correlation is much larger than HIJING (factor of 4) mid (40-50%) HIJING 1.382 very little centrality dependence φΔ ηΔ

  33. Multiplicity fractions – same-side gaussian 1) Probability that minbias p-p collision produces semi-hard parton: 2) Number of semi-hard partons in Au-Au assuming binary scaling (pt correlations) 3) Total number of same-side 2D gaussian correlated pairs per event: 4) Number of final state particles associated with each semi-hard parton: 5) Fraction of total multiplicity associated with same-side gaussian correlation: Peak Volume STAR Preliminary 200 GeV 62 GeV For central Au+Au we estimate about 30%; a significant fraction of the bulk particles. 8x increase See also T. Trainor, arXiv:0710.4504, accepted to J Mod Phys E

  34. Implications: measures and media Suite of correlation and differential spectra measures: • Number of pair correlations on relative angles: (hD,fD) • pt correlations on (hD,fD) • 2D transverse momentum: (yt,yt) • Charge independent (CI) and dependent (CD) • PID dependent (not yet explored, need TOF) • Differential pt spectra (as in p-p analysis) Three example scenarios for RHIC collision environments: • Superposition of p-p collisions • Parton/hadron scattering, moderate cross sections • Opaque medium, zero mean-free path Focus attention on the 2D same-side gaussian

  35. What causes the reduction in azimuth width? Perhaps there is a competition between collisional broadening and an unknown narrowing mechanism which affects low-pt and depends on the first few N-N collisions. Interpretation: below the transition point STAR Preliminary approximate binary scaling time (lab) minijet fragmentation with moderate hD width increase hadrons moderate scattering and dissipation pre-hadrons z scattered parton beam beam

  36. STAR Preliminary Why does thefwidth remain narrow? Somehow the scattered parton’s azimuth direction of motion is transferred to the bulk hadrons which are associated/correlated with it. Interpretation: above the transition point (personal speculation) STAR Preliminary larger hD width parton fragments plus correlated hadrons spread over much larger hD range time hadrons earlier, stronger momentum dissipation novel QCD environment z scattered parton beam beam

  37. Implications for phenomenology (personal speculation) Semi-hard parton traversing thermal medium: • momentum loss • increased number of correlated pairs • Brownian motion induces h and f • width broadening – the latter is not seen 1D Hubble expanding gluon field (in co-moving frame of parton) • transverse momentum loss; no change in direction • pt transfered to gluons along z-coordinate, not f • correlation along z maps to width increase on h • azimuth width constant • increased number of correlated pairs • pt correlations preserved • But what causes the gluon field • to suddenly change ?

  38. Quadrupole Component Instead of removing a background, we can make a measurement Data cos(2φΔ) component Amplitudes 200 GeV 62 GeV • 62 and 200 have the same shape • Substantial amp. changewith energy φΔ φΔ ηΔ ηΔ STAR Preliminary STAR Preliminary v2{2} v2{2D} v2{4} D. Kettler, T. Trainor arXiv:0704.1674 accepted to J Mod Phys E flow data from PRC 72 014904 The η-dependence of correlations separates quadrupole from other components 38

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