Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 1 Correlations with a high-p T trigger over 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 Magyary Zoltán Felsőoktatási Közalapítvány

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 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 LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 3 Motivation PHOBOS, PRL 96, (2006) One of the most fundamental discoveries at RHIC is that partons strongly interact as they traverse the produced medium single-particle spectra, R AA STAR, PRL 91, (2003) high-p T azimuthal correlations

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 4 Beep! Associated high-p T particles Where does the energy go? Look at associated particles at lower momentum!

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 5 Associated low-p T particles Existing triggered correlation measurements show novel features in heavy-ion collisions STAR, arXiv:nucl-ex/ v2 Enhanced correlation (“ridge”) at  =0 and large  Au+Au d+Au There is no “ridge” without medium: PHENIX, arXiv: v2  broadening in  of away-side compared to p+p

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 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, Hwa, Nucl. Phys. A783, 57 (2007) Shuryak, Phys. Rev. C76, (2007) Romatschke, Phys. Rev. C 75, Majumder, Muller, Bass, Phys. Rev. Lett. 99, (2007) Pantuev, arXiv: v1 [hep-ph] Wong, Phys. Rev. C 76, (2007)

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 7 The advantage of Can use the uniquely broad acceptance of the PHOBOS multiplicity detectors to measure the ridge at large Δ  … …to constrain the possible explanations for correlated particle production far in  from a high-p T trigger ? ?

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 8 Experimental layout High p T trigger track p T > 2.5 GeV/c 0 < η trig < 1.5 Small acceptance in  Octagon holes are filled using hits from the first layers of the Spectrometer and Vertex detectors Associated hits Full  coverage Broad η coverage (-3 < η < 3) Single layer of silicon No p T information p T > 35 MeV/c (at η=0)  TRIGGER PARTICLE

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 9 Constructing the correlation = { - a [ ] } V = “Background”: Acceptance-corrected mixed-event pairs (per trigger particle) Signal/Background. Detector acceptance cancels in the ratio = - · a [ ] Modulation from elliptic flow How can these three terms be measured?

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’ 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 η trigger distribution PHOBOS: PRL 91, (2003) Δηη trigger η Au+Au 200 GeV

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’ Signal and mixed events 15-20% central 3 mm < v z < 4 mm averaged over the vertex position: -15cm < v z < 10cm Au+Au  

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’ Estimating the flow contribution Parametrize the published PHOBOS v 2 results in a factorized form: v 2 (N part,p T,η) = A(N part ) B(p T ) C(η) PHOBOS: PRC 72, Correct v 2 (N part,,η trig ) for occupancy and v 2 (N part,,η assoc ) for secondaries V = A(N part )B(p T )C(η) V 2 is available at any N part, p T, η

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 13 Subtracting elliptic flow The scale factor, a, is calculated such that the yield after subtraction is zero at its minimum (ZYAM) - a [ ] Ajitanand et al. PRC 72, (R) (2005) a PHOBOS preliminary 10-30% central -0.5 < Δη < 0.0 

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 14 Systematic uncertainties PHOBOS preliminary 10-30% central -0.5 < Δη < 0.0 PHOBOS preliminary 0-6% central -0.5 < Δη < 0.0  Dominant systematic error: the uncertainty of v 2 trig v 2 assoc ~14% error on v 2 trig v 2 assoc (η=0) ~20% error on v 2 trig v 2 assoc (η=3) In the most central collisions - where flow is small compared to the correlation - the relative error on v 2 trig v 2 assoc can exceed 50%.

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 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: STAR, PRL 95, (2005) |η| < 1 4 < p T trig < 6 GeV/c 0.15 < p T assoc < 4 GeV/c

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 16 Results PHOBOS preliminary p+p PYTHIA v6.325Au+Au 0-30% central p T trig > 2.5 GeV/c p T assoc  20 MeV/c

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 17 A closer look… Correlated yield, short-range (|Δη| < 1): Significant broadening on the away side Au+Au

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 18 The ridge extends to high  Correlated yield, long-range (-4 < Δη < -2): Ridge Broadening, away side Au+Au

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 19 Short-range; centrality dependence |Δη| < 1 The broadening is growing with centrality CENTRAL PERIPHERAL Au+Au

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 20 Long-range; centrality dependence -4 < Δη < -2 Both features are growing with centrality CENTRAL PERIPHERAL Au+Au

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 21 Integrated yield of the “ridge” -4 <  < -2 Integrate ridge over |  | < 1 |  | < 1 rad

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 22 The extent of the ridge in  Correlated yield on near-side (|  | < 1): Au+Au Ridge persists to high |  |!

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 23 Comparison to predictions C.Y. Wong, PRC 76, (2007) Momentum kick Trigger σ y = rapidity width of partons q |  |<1 C.Y. Wong, private communication

Tokaj, Hungary, March 17, 2008Gábor VeresHigh-p T physics at the LHC, Tokaj ’08 24 Conclusions Correlations with a high-p T trigger were measured in an extended pseudorapidity region. Looking forward to LHC, where similar studies will be feasible with a comparable  region. 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