1. 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]
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2. Contents High-multiplicity events at the LHC Flow in pp? DIPSY
Eccentricity and v2 in pp

3. High-multiplicity pp events at the LHC

4. 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 !

5. 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?

6. 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.

7. 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

8. 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

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

10. 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.

11. Some sample results from DIPSY @7TeV

12. 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

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

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

15. 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.

16. 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.

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

18. 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….

19. 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.

20. 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.