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Eccentricity and v2 in proton-proton collisions at the LHC Yoshitaka Hatta (U. Tsukuba) in collaboration with E. Avsar, C. Flensburg, J.-Y. Ollitrault, T. Ueda arXiv: [hep-ph] TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAA A A

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Contents High-multiplicity events at the LHC Flow in pp? DIPSY Eccentricity and v2 in pp

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High-multiplicity pp events at the LHC

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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 Phenomena usually discussed in the context of nucleus collisions may be observed in proton collisions !

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

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Elliptic flow in pp Luzum,Romatschke Prasad, Roy, Chattopadhyay d’Enterria, et al Bozek Ortona, Denicol, Mota, Kodama, Bautista, Cunqueiro, de Deus, Pajares Pierog, Porteboeuf, Karpenko, Werner, Casalderrey-Solana, Wiedemann Toy models : Woods-Saxon, Glauber, Hard Sphere, Gaussian…. Hot spot model Parton + flux tube model Proton’s transverse structure is either predetermined or random.

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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. Important because high-multiplicity events mostly come from central collisions. AA fluctuation of nucleons pp fluctuation of small-x gluons

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QCD dipole model Mueller (1994~) Very strong fluctuation in the gluon multiplicity Salam (1995) Very strong correlation in impact parameter space YH, Mueller (2007) Avsar, YH (2008) These correlations are important to study correlations observables Coordinate space formulation of the BFKL equation

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Gluon number fluctuation Probability distribution of the number of gluons in the dipole model Salam (1995)

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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, arXiv: extension of Avsar, Gustafson, Lonnblad (2005~) Featuring: Energy dependence is a prediction. No ad hoc retuning of parameters at different energies.

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Some sample results from

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Deconstructing high-multiplicity events High-multiplicity pp events = Upward fluctuation in the gluon number in proton’s w.f. + Multiple (more than 10) parton-parton interactions

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Eccentricity in pp at 7 TeV Shape of the area occupied by the “liberated” gluons.

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Comparison with Pb-Pb Figure by Luzum Comparable to a 30% central Pb-Pb collision

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Nature of eccentricity in pp Conventional definition of the eccentricity at fixed b is negative ! x y Nominal “overlapping region” is a poor guide in pp.

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Empirical relation between the eccentricity and v2. Drescher et al. (2007) In order to obtain large v2, a single event has to be both elliptic in shape and have high density. taking care of incomplete equilibration …and a similar relation between and

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Elliptic flow at 7 TeV ~ 6% comparable to AA at LHC, RHIC Aamodt et al, PRL105,252302

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ALICE data for ALICE preliminary Bilandzic, talk at QCHS IX dominate over the flow contribution…. “flow” “nonflow” Large nonflow contribution due mostly to jets. in AA

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ALICE preliminary ALICE data for Bilandzic, talk at QCHS IX positive from DIPSY negative in the data and in MCs Sign change at large Nch Possible signature of flow.

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Conclusions DIPSY can simulate high-multiplicity pp events including proper QCD dynamics in the transverse plane. Eccentricity is 30%, comparable to semi-central Pb collisions. Challenging to distinguish from non-flow correlations. Sign change of (v2{4})^4 can be a signature.

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