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Eccentricity and v2 in pp collisions from parton evolution

Eccentricity and v2 in pp collisions from parton evolution. Hatta Yoshitaka. 八田　佳孝 (U. Tsukuba). in collaboration with. E. Avsar, C. Flensburg, J.-Y. Ollitrault, T. Ueda. arXiv:1009.5643 version2 [hep-ph]. Contents. Motivation: High-multiplicity pp events Flow in pp? DIPSY

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Eccentricity and v2 in pp collisions from parton evolution

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  1. Eccentricity and v2 in pp collisions from parton evolution HattaYoshitaka 八田　佳孝 (U. Tsukuba) in collaboration with E. Avsar, C. Flensburg, J.-Y. Ollitrault, T. Ueda arXiv:1009.5643version2 [hep-ph]

  2. Contents • Motivation: High-multiplicity pp events • Flow in pp? • DIPSY • Eccentricity and v2 in pp • Nonflow correlations • Signatures

  3. First data from the LHC !!!

  4. High-multiplicity pp events

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

  6. Ridge in pp CMS Collaboration, arXiv:1009.4122 Near—side correlation at large rapidity separation. First unexpected result from LHC !

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

  8. Elliptic flow in pp? Luzum,Romatschke 0901.4588 Prasad, Roy, Chattopadhyay 0910.4844 d’Enterria, et al. 0910.3029 Bozek 0911.2392 Bautista, Cunqueiro, de Deus, Pajares 0905.3058 Pierog, Porteboeuf, Karpenko, Werner, 1005.4526 Casalderrey-Solana, Wiedemann 0911.4400 Toy models (Woods-Saxon, Glauber, Hard Sphere…) Partons + flux tube model “Hot spot” model

  9. Participant eccentricity Event plane may be different from the reaction plane. Can be nonzero even at due to fluctuations. Important in pp because high-multiplicity events mostly come from AA  fluctuation of nucleons pp  fluctuation of small-x gluons

  10. QCD dipole model Mueller (1994~) Coordinate space formulation of the BFKL evolution (hard Pomeron). Very strong fluctuation in the gluon multiplicity Salam, 1995 Very strong correlation in impact parameter space YH, Mueller, 2007 These correlations are important to study correlation observables !

  11. DIPSY Full-fledged Monte Carlo event generator for pp based on the dipole model. Flensburg, Gustafson, Lonnblad, COMING SOON extension of Avsar, Gustafson, Lonnblad (2005~) “Lund gang” Featuring: BFKL (Hard Pomeron) Running coupling (NLO) Energy conservation ( ) Saturation effects Confinement effects Multiple scattering, Underlying events, Parton shower (ARIADNE) Hadronization (Pythia)

  12. Some sample results from DIPSY Flensburg, 1009.5323 Adjust parameters to fit the total cross section. No ad hoc re-tuning of parameters at different energies.

  13. Multiple parton-parton collisions In high—multiplicity events, there are >10 subcollisions !

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

  15. Elliptic flow at 7 TeV 6-7% Comparable to Au-Au at RHIC No significant difference at 14 TeV

  16. “Nonflow” effects “flow” “nonflow” Nonflow correlations from 22 hard collisions, resonance decays. Suppressed like if subcollisions are uncorrelated, which is the case in AA. In pp, nonflow effects are very large due to jets, BFKL, etc. BUT, they have never been estimated before. Let’s do it !

  17. Nonflow effects in pp All pairs within One particle from the other from Do NOT decrease as Nch increases ! It’s even increasing a bit.

  18. Signature of flow in pp no flow scenario 100% flow scenario no flow scenario Nonflow correlations get replaced by flow correlation. Approach the flow limit either from above, or from below.

  19. DIPSY predicts Negative ! Four-particle correlations Less effected by nonflow effects…in AA If there is flow, it is proportional to

  20. Another signature real and positive Low-Nch High-Nch

  21. Conclusions • High-multiplicity events: low-x structure of the proton crucial. DIPSY is the ideal MC for this purpose. • Large fluctuation from pQCD evolution gives a large eccentricity. • Challenging to distinguish from non-flow correlations. First (serious) quantitative estimates and proposal of signatures.

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