eccentricity and v2 in proton-proton collisions at the lhc

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]

Contents

High-multiplicity events at the LHCFlow in pp?Dipole model & DIPSYEccentricity and v2 in ppNonflow correlationsSignatures of flow

The first data from the LHC

High-multiplicity pp events

Like an AA collision !

pT>0.1GeV/c

high multiplicity pp 7TeVcomparable to ~18 nucleon

pairs, each colliding at 62.4GeV in CuCu

CMS PHOBOS

-2

+2

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

Ridge in pp CMS Collaboration, arXiv:1009.4122

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 0901.4588Prasad, Roy, Chattopadhyay 0910.4844d’Enterria, et al. 0910.3029Bozek 0911.2392

Bautista, Cunqueiro, de Deus, Pajares 0905.3058Pierog, Porteboeuf, Karpenko, Werner, 1005.4526

Casalderrey-Solana, Wiedemann 0911.4400

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 modelMueller (1994~)

Coordinate space formulation of the BFKL evolution.

Strong fluctuation in the gluon multiplicity Strong correlation in impact parameter space

zdYdyzzx

yxdP s

222

2

)()(

)(

These correlations are important to study correlations observables.

Multiplicity fluctuationProbability 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 factorizeat least at the onset of unitarity corrections.

YH and Mueller (2007)

Avsar and YH (2008)

DIPSYFull-fledged Monte Carlo event generator for pp based on the dipole model.

BFKLRunning coupling (NLO)Energy conservation ( )Saturation effectsConfinement effectsMultiple scattering,Underlying events,Parton shower (ARIADNE)Hadronization (Pythia)

Flensburg, Gustafson, Lonnblad, COMING SOONextension of Avsar, Gustafson, Lonnblad (2005~)

Featuring: “Lund gang”

Some sample results from DIPSY

Flensburg, 1009.5323

Adjust (a few) parameters to fit the total cross section. No ad hoc re-tuning of theparameters 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 22 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% nonflow

100% flow

Four-particle correlations

corresponding to

We find

Negative !

Signature of flow in pp (cont’d)

Low-Nch High-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.