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Yoshitaka Hatta (U. Tsukuba) Eccentricity and v2 in proton-proton collisions at the LHC in collaboration with E. Avsar, C. Flensburg, J.-Y. Ollitrault, T. Ueda arXiv:1009.5643v2 [hep-ph]

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Contents High-multiplicity events at the LHC Flow in pp? Dipole model & DIPSY Eccentricity and v2 in pp Nonflow correlations Signatures of flow

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The first data from the LHC

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High-multiplicity pp events

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Like an AA collision ! p T >0.1GeV/c high multiplicity pp 7TeV comparable to ~18 nucleon pairs, each colliding at 62.4GeV in CuCu CMSPHOBOS -2-2 +2+2 Phenomena usually discussed in the context of AA collisions may be observed in pp collisions.

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Ridge in pp CMS Collaboration, arXiv:1009.4122 First unexpected result from the LHC ! near-side, long-range rapidity correlation

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Elliptic flow at RHIC Hallmark of collective expansion, widely regarded as a signal of the QGP.

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Elliptic flow in pp? Luzum,Romatschke 0901.4588 Prasad, Roy, Chattopadhyay 0910.4844 d’Enterria, et al. 0910.3029 Bozek 0911.2392 Bautista, Cunqueiro, de Deus, Pajares 0905.3058 Pierog, Porteboeuf, Karpenko, Werner, 1005.4526 Casalderrey-Solana, Wiedemann 0911.4400 Toy models : Woods-Saxon, Glauber, Hard Sphere…. “Hot spot” model Partons + flux tube model

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Participant eccentricity Event plane may be different from the reaction plane. Can be nonzero even at vanishing impact parameter due to fluctuations. AA fluctuation of nucleons pp fluctuation of small-x gluons

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QCD dipole model Mueller (1994~) Coordinate space formulation of the BFKL evolution. Strong fluctuation in the gluon multiplicity Strong correlation in impact parameter space These correlations are important to study correlations observables.

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Multiplicity fluctuation Probability distribution of the number of dipoles (gluons)Salam (1995)

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Correlation in impact parameter space For an initially dilute system (like a proton), the double dipole scattering amplitude does not factorize at least at the onset of unitarity corrections. YH and Mueller (2007) Avsar and YH (2008)

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DIPSY Full-fledged Monte Carlo event generator for pp based on the dipole model. BFKL Running coupling (NLO) Energy conservation ( ) Saturation effects Confinement effects Multiple scattering, Underlying events, Parton shower (ARIADNE) Hadronization (Pythia) Flensburg, Gustafson, Lonnblad, COMING SOON extension of Avsar, Gustafson, Lonnblad (2005~) Featuring: “Lund gang”

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Some sample results from DIPSY Flensburg, 1009.5323 Adjust (a few) parameters to fit the total cross section. No ad hoc re-tuning of the parameters at different energies.

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Multiple parton-parton collisions In high-multiplicity pp events, there are >10 subcollisions ! (high-multiplicity events) = (upward fluctuation in the gluon number) +(multiple gluon collisions)

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Eccentricity in pp at 7 TeV ~40% eccentricity, comparable to AA collisions at RHIC Shape of the area occupied by the “liberated” gluons.

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Nature of eccentricity in pp The conventional definition of the eccentricity at fixed b is negative, in stark contrast to AA ! x y “Overlapping area of the two protons”…largely irrelevant.

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Elliptic flow at 7 TeV 6-7% Comparable to Au-Au at RHIC Use the empirical formula Drescher et al. (2007)

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“Nonflow” effects “flow” “nonflow” Suppressed like if subcollisions are uncorrelated, which is the case in AA. Nonflow correlations from 2 2 hard collisions, resonance decays. In pp, nonflow effects are very large due to jets, etc. BUT, they have never been estimated before. We’ll do it !

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Nonflow correlations in pp Do NOT decrease as Nch increases ! It’s even increasing a bit. All pairs within One particle from the other from

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Signature of flow in pp Nonflow correlations get replaced by flow correlations. Approach the flow limit either from above, or from below. 100% nonflow 100% flow

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Four-particle correlations corresponding to We find Negative !

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Signature of flow in pp (cont’d) Low-NchHigh-Nch real and positive

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Conclusions High-multiplicity events : important to understand the low-x structure of the proton. Well simulated by DIPSY. Fluctuations in the transverse plane lead to a large (40%) eccentricity. Challenging to distinguish from non-flow correlations. First (serious) quantitative estimates and proposal of signatures.

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