Ridge Formation and Long Range Correlations in pp Collisions at CMS

1. Ridge Formation and Long Range Correlations in pp Collisions at CMS C.B. Yang Institute of Particle Physics Central China Normal University Wuhan 430079, China Based on PRC83, 024911 (2011) by R.C. Hwa and C.B. Yang

2. Outline • Ridge from experiments • Ridge in AuAu collisons from RHIC • Ridge and long range correlations in pp collisions at CMS • Existing theoretical model explanations • Our model for ridge and long-range correlations • Discussion

3. Ridge from RHIC • STAR: auto-correlation, w/wo trigger • PHOBOS: triggered analysis

4. STAR Auto-correlation

6. J. Putschke, QM 2006

8. PHOBOS: High pt Triggered Ridge from Edward Wenger, RHIC & AGS User’s Meeting, ‘08

9. long range correlations PHOBOS: High pt Triggered Ridge from Edward Wenger, RHIC & AGS User’s Meeting, ‘08

10. Ridge in pp collisions from CMS

11. Long-range near-side angular correlations Signal = same event pairs • pT-inclusive two-particle angular correlations in minimum bias collisions Ratio Signal/Background Background = mixed-event pairs JHEP 09 (2010) 091

12. G. Roland’s talk Long-range near-side angular correlations 7 TeV Peaks aretruncated !

13. G. Roland’s talk Long-range near-side angular correlations 7 TeV Peaks aretruncated !

14. G. Roland’s talk Long-range near-side angular correlations 7 TeV Peaks aretruncated !

15. Long-range near-side angular correlations 7 TeV Peaks aretruncated ! new ridge-likestructure at Df ~ 0

16. G. Roland’s talk Long-range near-side angular correlations • No such structure is seen in Monte Carlo simulations :PYTHIA8, PHYTHIA6, Herwig++, MadGraph • The effect is small, but clearly seen for large Dh and multiplicities > 90 • It is most pronounced at intermediate pT (1–3 GeV/c) • It is the first observation of such an effect in pp (or p-pbar) collisions • Further studies ongoing for a better understanding of the effect • The heavy ion run will provide additional input

17. Experimental summary • A peak at relative angle • Widedistribution in pseudo-rapidity • Most obvious for particles with 1<pT<3 GeV/c • Spectrum of secondaries harder than a bulk one • A composition very different from jets

18. Theoretical explanations for ridgein AuAu collisions hard ridge explanations -- jet interactions with matter • N. Armesto, C.A. Salgado, U.A. Wiedemann, Phys. Rev. Lett. 93, 242301 (2004) • P. Romatschke, Phys. Rev. C 75, 014901 (2007) • A. Majumder, B. Muller, S. A. Bass, Phys. Rev. Lett. 99, 042301 (2007) • C. B. Chiu, R. C. Hwa, Phys. Rev. C 72, 034903 (2005) • C. Y. Wong, arXiv:0712.3282 [hep-ph] • R. C. Hwa, C. B. Yang, arXiv:0801.2183 [nucl-th] • T. A. Trainor, arXiv:0708.0792 [hep-ph] • A. Dumitru, Y. Nara, B. Schenke, M. Strickland, arXiv:0710.1223 [hep-ph] • E. V. Shuryak, Phys. Rev. C 76, 047901 (2007) • C. Pruneau, S. Gavin, S. Voloshin, Nucl.Phys.A802:107-121,2008 soft ridge -- similar but no jet -- collective behavior • S. Gavin and M. Abdel-Aziz, Phys. Rev. Lett. 97, 162302 (2006) • S. A. Voloshin, Phys. Lett. B 632, 490 (2006) • S. Gavin and G. Moschelli, arXiv:0806.4366 [nucl-th] • A. Dumitru, F. Gelis, L. McLerran and R. Venugopalan, arXiv:0804.3858 [hep-ph] • S. Gavin, L. McLerran, G. Moschelli, arXiv:0806.4718 [nucl-th] • F. Gelis, T. Lappi, R. Venugopalan, arXiv:0807.1306 [hep-ph]

19. Ridge in pp collisionsby Edward Shuryak • Independent string breaking -> small , arbitrary • If the string moves as a whole->a ridge can be seen in all events. But the data show ridge is there only for events with high multiplicity • Explosion for Nch>100 in CMS? Can hydrodynamics be applied to pp collisions?

20. Our consideration on ridgefor AuAu collisions • Without using hydrodynamics explicitly • Semihard scattering near the surface is the driving force of the azimuthal anisotropy • The lost energy from (mini)jets heats the medium system • This heating effect depends on the position of the semihard scattering point • The enhanced soft medium->ridge

21. Φdetermined by overlap geometry

23. From the azimuthal dependence of R, v2 can be calculated

26. Ridge in pp collisions shows • Soft partons of high density created (?) • Those soft partons affect passage of jets • Origin of ridge in pp collisions at CMS may be the same as in AuAu collisions at RHIC

27. Long-range correlations • CMS data

30. Initial fluctuations • Transverse momentum conservation • Correlation induced by hadro-fluctuations

31. Initial fluctuations Cross sectional slices are the same • Correlated Particles come from the same tube • originate at the earliest stages of the collision • information on particle production mechanism What’s the pT dependence of correlations? Why long range correlations can be seen only in high multiplicity events?

32. Transverse momentum conservation • With suitably parameters one can fit the data • Momentum is conserved in all elementary processes in MC codes. Why PYTHIA cannot explain experimental data? • The conservation effect is stronger for events with low multiplicity. Contrary to experimental discovery

33. T T T’ T’ ΔT=T’-T depends on multiplicity, because the initial fluctuations at some point need to bemediated into phase space well separated. Heating simultaneously two points at large Δηresults in long range correlations

34. Lessons from STARPRD74, 032006 (06) • Hard component increases with multiplicity • ΔT larger for higher multiplicity

36. If dense medium is produced in pp collisions at CMS • anisotropy in the spectrum->v2 • harder spectrum in ridge • high p/πratio, about 0.5 at 3-5 GeV/c

37. Summary • Experimental data on ridge are revisited • Model explanations are introduced briefly • Long range correlations in pp collisions at CMS may be induced by ridge • Possible phenomena are predicted • More work needed!!

38. Thank you !

39. Discussion • Hydrodynamic influence to ridge formation at CMS • Dependence of ridge yield and correlations on direction of triggered particle • Particle spectra in ridge • pT dependence of particle ratio in ridge