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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:1009.5643 v2 [hep-ph]. Contents. High-multiplicity events at the LHC Flow in pp? Dipole model & DIPSY - PowerPoint PPT Presentation
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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]
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.