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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.: AAAAA

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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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**comparable to ~18 nucleon pairs, each colliding at 62.4GeV in CuCu**

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

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**Ridge in pp First unexpected result from the LHC !**

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

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Elliptic flow in pp Toy models : Woods-Saxon, Glauber, Hard Sphere, Gaussian…. 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 Parton + flux tube model Hot spot 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~)**

Coordinate space formulation of the BFKL equation 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

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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 Featuring: BFKL Running coupling (NLO) Energy conservation ( )**

Full-fledged Monte Carlo event generator for pp based on the dipole model. Flensburg, Gustafson, Lonnblad, arXiv: extension of Avsar, Gustafson, Lonnblad (2005~) Featuring: BFKL Running coupling (NLO) Energy conservation ( ) Saturation effects Confinement effects Multiple scattering, Underlying events, Parton shower (ARIADNE) Hadronization (Pythia) Energy dependence is a prediction. No ad hoc retuning of parameters at different energies.

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**Some sample results from DIPSY @7TeV**

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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 Comparable to a 30% central Pb-Pb collision**

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 ! y x Nominal “overlapping region” is a poor guide in pp.

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**Empirical relation between the eccentricity and v2.**

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

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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 Large nonflow contribution due mostly to jets.**

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

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**ALICE data for negative in the data and in MCs positive from DIPSY**

Bilandzic, talk at QCHS IX ALICE preliminary negative in the data and in MCs positive from DIPSY 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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