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: v2 [hep-ph]

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

The first data from the LHC

High-multiplicity pp events

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  Phenomena usually discussed in the context of AA collisions may be observed in pp collisions.

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

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

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

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

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.

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

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)

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”

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

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)

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

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.

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

“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 !

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

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

Four-particle correlations corresponding to We find Negative !

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

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.