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Correlations with a high-p T trigger over a broad η range

Correlations with a high-p T trigger over a broad η range. G á bor Veres Eötvös Loránd University, Budapest for the collaboration. based on a PHOBOS talk at QM’2008 by E. Wenger -. Támogatók : OTKA F 49823, NKTH-OTKA H 07 -C 74248

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Correlations with a high-p T trigger over a broad η range

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  1. Correlations with a high-pT triggerover a broad η range • Gábor Veres • Eötvös Loránd University, Budapest • for the collaboration • based on a PHOBOS talk at QM’2008 by E. Wenger- Támogatók: OTKA F 49823, NKTH-OTKA H07-C 74248 Magyary Zoltán Felsőoktatási Közalapítvány

  2. The Collaboration Burak Alver, Birger Back,Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Joshua Hamblen, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Bolek Wysłouch ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

  3. high-pT azimuthal correlations STAR, PRL 91, 072304 (2003) Motivation One of the most fundamental discoveries at RHIC is that partons strongly interact as they traverse the produced medium • single-particle spectra, RAA PHOBOS, PRL 96, 212301 (2006)

  4. Beep! Beep! Associated high-pT particles Where does the energy go? Look at associated particles at lower momentum!

  5. Enhanced correlation (“ridge”) at =0 and large  • broadening in  of away-side compared to p+p Au+Au STAR, arXiv:nucl-ex/0701074v2 d+Au • There is no “ridge”without medium:  PHENIX, arXiv:0705.3060v2 Associated low-pT particles Existing triggered correlation measurements show novel features in heavy-ion collisions

  6. Interpretations of the “ridge” Different proposed mechanisms qualitatively describe “ridge” at |Δη|<2: • Coupling of induced radiation to longitudinal flow • Recombination of shower + thermal partons • Anisotropic plasma • Turbulent color fields • Bremsstrahlung + transverse flow + jet-quenching • Splashback from away-side shock • Momentum kick imparted on medium partons Armesto et al., Phys. Rev. Letters 93, 242301 Hwa, Nucl. Phys. A783, 57 (2007) Romatschke, Phys. Rev. C 75, 014901 Majumder, Muller, Bass, Phys. Rev. Lett. 99, 042301 (2007) Shuryak, Phys. Rev. C76, 047901 (2007) Pantuev, arXiv:0710.1882v1 [hep-ph] Wong, Phys. Rev. C 76, 054908 (2007)

  7. The advantage of • Can use the uniquely broad acceptance of the PHOBOS multiplicity detectors to measure the ridge at largeΔ… • …to constrain the possible explanations for correlated particle production far in  from a high-pT trigger ? ?

  8. Experimental layout Associated hits Full  coverage Broad η coverage (-3 < η < 3) Single layer of silicon No pTinformation pT> 35 MeV/c (at η=0) High pTtrigger track pT > 2.5 GeV/c 0 < ηtrig < 1.5 Small acceptance in  Octagonholes are filled using hits from the first layers of theSpectrometer andVertexdetectors TRIGGER PARTICLE 

  9. = { - a [ ] } V = <v2trig><v2assoc> “Background”: Acceptance-corrected mixed-event pairs (per trigger particle) Signal/Background. Detector acceptance cancels in the ratio Modulation from elliptic flow How can these three terms be measured? Constructing the correlation = - · a [ ]

  10. 1. Normalized background is the mixed-event pair distribution corrected for the pair acceptance, per trigger particle In other words, it is the single-particle distribution (dN/dη) convoluted with the ηtrigger distribution Au+Au200 GeV η ηtrigger Δη PHOBOS: PRL 91, 052303 (2003)

  11. 2. Signal and mixed events 15-20% central 3mm < vz < 4mm Au+Au  averaged over the vertex position: -15cm < vz < 10cm 

  12. 3. Estimating the flow contribution Parametrize the published PHOBOS v2 results in a factorized form: v2(Npart,pT,η) = A(Npart) B(pT) C(η) A(Npart) B(pT) C(η) PHOBOS: PRC 72,051901 PHOBOS: PRC 72,051901 • Correct v2(Npart,<pTtrig>,ηtrig) for occupancy and v2(Npart,<pTassoc>,ηassoc)for secondaries • V2 is available at any Npart, pT, η V = <v2trig><v2assoc>

  13. Subtracting elliptic flow -a[ ] PHOBOS preliminary 10-30% central -0.5 < Δη < 0.0 a  The scale factor, a, is calculated such that the yield after subtraction is zero at its minimum (ZYAM) Ajitanand et al. PRC 72, 011902(R) (2005)

  14. Systematic uncertainties PHOBOS preliminary 10-30% central -0.5 < Δη < 0.0 • Dominant systematic error: • the uncertainty of v2trig v2assoc • ~14% error on v2trig v2assoc (η=0) • ~20% error on v2trig v2assoc (η=3) • In the most central collisions - where flow is small compared to the correlation - the relative error on v2trig v2assoc can exceed 50%.  PHOBOS preliminary 0-6% central -0.5 < Δη < 0.0 

  15. PYTHIA: p+p reference • p+p data from PHOBOS is limited by statistics • We will compare our Au+Au results to PYTHIA, which reproduces STAR p+p data reasonably well: |η| < 1 4 < pTtrig < 6 GeV/c 0.15 < pTassoc < 4 GeV/c STAR, PRL95,152301 (2005)

  16. Results pTtrig > 2.5 GeV/c pTassoc20 MeV/c p+p PYTHIA v6.325 Au+Au 0-30% central PHOBOS preliminary

  17. A closer look… Correlated yield, short-range (|Δη| < 1): Au+Au Significant broadening on the away side

  18. The ridge extends to high  Correlated yield, long-range (-4 < Δη < -2): Au+Au Ridge Broadening, away side

  19. Short-range; centrality dependence CENTRAL |Δη| < 1 Au+Au The broadeningis growing withcentrality PERIPHERAL

  20. Long-range; centrality dependence CENTRAL -4 < Δη < -2 Au+Au PERIPHERAL Both featuresare growing with centrality

  21. || < 1 rad Integrated yield of the “ridge” -4 <  < -2 Integrate ridge over || < 1

  22. The extent of the ridge in  Correlated yield on near-side (|| < 1): Au+Au Ridge persiststo high ||!

  23. Trigger q Momentum kick σy = rapidity width of partons Comparison to predictions ||<1 C.Y. Wong, private communication C.Y. Wong, PRC 76, 054908 (2007)

  24. Broadening of the away-side correlation in (relative to p+p) persists over the complete range covered • The near-side correlation at   0 and large  (“ridge”) extends to  4 • The yield of the “ridge” at large  gradually disappears going from central to peripheral Au+Au collisions Conclusions Correlations with a high-pT trigger were measured in an extended pseudorapidity region. Looking forward to LHC, where similar studies willbe feasible with a comparableregion.

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