1. Yen-Jie Lee (CERN) 1 MBUE working group 2012 Yen-Jie Lee (CERN) for the CMS Collaboration MBUE working group CERN 3 rd Dec, 2012 Two-particle correlations in pPb collisions from the CMS Collaboration

2. Yen-Jie Lee (CERN) 2 MBUE working group 2012 pp Introduction Two-particle correlation: study of particle production mechanism

3. Yen-Jie Lee (CERN) 3 MBUE working group 2012 Two-particle correlation: study of particle production mechanism and possible collective effects in high particle density environment created at the LHC energies pp Pb Introduction

4. Yen-Jie Lee (CERN) 4 MBUE working group 2012 pp Pb ? p Pb Introduction Two-particle correlation: study of particle production mechanism and possible collective effects in high particle density environment created at the LHC energies

5. Yen-Jie Lee (CERN) 5 MBUE working group 2012 Very large coverage for full tracking (|  | up to 5.0)! Compact Muon Solenoid Inner tracker: charged particles primary vertex solenoid EM and Hadron calorimeters photons, jet Muon HCAL ECAL Tracker |η|< 2.5 |η|< 3.0 |η|< 5.2 |η|< 2.4 proton Pb

6. Yen-Jie Lee (CERN) 6 MBUE working group 2012 High multiplicity events recorded by CMS pp PbPb pPb

7. Yen-Jie Lee (CERN) 7 MBUE working group 2012 Defining two-particle correlation Event 1 Event 2 same event pairs mixed event pairs Signal pair distribution:Background pair distribution: Δη = η 1 -η 2 Δφ = φ 1 -φ 2

8. Yen-Jie Lee (CERN) 8 MBUE working group 2012 Defining two-particle correlation Event 1 Event 2 same event pairs mixed event pairs Signal pair distribution:Background pair distribution: JHEP 09 (2010) 091 Associated hadron yield per trigger: Δη = η 1 -η 2 Δφ = φ 1 -φ 2

9. Yen-Jie Lee (CERN) 9 MBUE working group 2012 Understanding the correlation function “Near-side” ( ,  ~ 0) correlations from single jets JHEP 09 (2010) 091 pp

10. Yen-Jie Lee (CERN) 10 MBUE working group 2012 Understanding the correlation function “Near-side” ( ,  ~ 0) correlations from single jets “Away-side” (  ~  ) back-to-back jet correlations JHEP 09 (2010) 091 pp Z axis adjusted to reveal the detail of the correlation function (peak truncated)

11. Yen-Jie Lee (CERN) 11 MBUE working group 2012 Understanding the correlation function Striking “ridge-like” structure extending over  at  ≈0 In high multiplicity events, N≥110 where: N ≡ number of offline tracks with p T >0.4 GeV/c JHEP 09 (2010) 091 pp

12. Yen-Jie Lee (CERN) 12 MBUE working group 2012 Understanding the correlation function p T trig : 4–6 GeV/c p T assoc : 2–4 GeV/c CMS PbPb 2.76 TeV 35-40% PbPb 35-40% centrality arXiv:1201.3158 CMS PbPb 2.76 TeV EPJC 72 (2012) 2012 Pb

13. Yen-Jie Lee (CERN) 13 MBUE working group 2012 p T trig : 4–6 GeV/c p T assoc : 2–4 GeV/c CMS PbPb 2.76 TeV 35-40% PbPb 35-40% centrality arXiv:1201.3158 CMS PbPb 2.76 TeV EPJC 72 (2012) 2012 Understanding the correlation function Pb Particle azimuthal distributions: dN/d  Σ  v n cos(n(   ))

14. Yen-Jie Lee (CERN) 14 MBUE working group 2012 Any guesses for pPb correlations? What do we expect to see in (high-multiplicity) pPb? p Pb N=235 event

15. Yen-Jie Lee (CERN) 15 MBUE working group 2012 A ridge! p Pb N ≡ number of offline tracks with p T >0.4 GeV/c What do we expect to see in (high-multiplicity) pPb? arXiv 1210.5482 Accepted by PLB

16. Yen-Jie Lee (CERN) 16 MBUE working group 2012 A (relatively big) ridge! p Pb JHEP 09 (2010) 091 pp 7 TeV Much bigger than in pp! Physical origin still unclear arXiv 1210.5482 Accepted by PLB N ≡ number of offline tracks with p T >0.4 GeV/c

17. Yen-Jie Lee (CERN) 17 MBUE working group 2012 A (relatively big) ridge! p Pb Initial-state geometry + collective expansion arXiv 1210.5482 Accepted by PLB N ≡ number of offline tracks with p T >0.4 GeV/c

18. Yen-Jie Lee (CERN) 18 MBUE working group 2012 Multiplicity Evolution Divide into 4 multiplicity bins: p Pb Low multiplicityLow transverse density arXiv 1210.5482 Accepted by PLB N ≡ number of offline tracks with p T >0.4 GeV/c

19. Yen-Jie Lee (CERN) 19 MBUE working group 2012 Multiplicity Evolution Divide into 4 multiplicity bins: p Pb Increasing multiplicityIncreasing transverse density arXiv 1210.5482 Accepted by PLB N ≡ number of offline tracks with p T >0.4 GeV/c

20. Yen-Jie Lee (CERN) 20 MBUE working group 2012 Multiplicity Evolution Divide into 4 multiplicity bins: p Pb Increasing multiplicityHigh transverse density arXiv 1210.5482 Accepted by PLB N ≡ number of offline tracks with p T >0.4 GeV/c

21. Yen-Jie Lee (CERN) 21 MBUE working group 2012 Multiplicity Evolution Divide into 4 multiplicity bins: p Pb Increasing multiplicityHighest transverse density arXiv 1210.5482 Accepted by PLB N ≡ number of offline tracks with p T >0.4 GeV/c

22. Yen-Jie Lee (CERN) 22 MBUE working group 2012 Quantitative evolution of ridge effect Want to use the same approach as in pp ridge paper for “apples-apples” comparison

23. Yen-Jie Lee (CERN) 23 MBUE working group 2012 Quantitative evolution of ridge effect Average over ridge region (2<|Δη|<4) Want to use the same approach as in pp ridge paper for “apples-apples” comparison

24. Yen-Jie Lee (CERN) 24 MBUE working group 2012 Quantitative evolution of ridge effect Average over ridge region (2<|Δη|<4) Want to use the same approach as in pp ridge paper for “apples-apples” comparison

25. Yen-Jie Lee (CERN) 25 MBUE working group 2012 Multiplicity and p T dependence Multiplicity N<35 35 ≦ N<90 90 ≦ N<110 N ≧ 110 0 – 1 1 – 2 2 – 3 3 – 4 p T (GeV/c) N ≡ number of offline tracks with pT>0.4 GeV/c

26. Yen-Jie Lee (CERN) 26 MBUE working group 2012 Multiplicity and p T dependence 0 – 1 1 – 2 2 – 3 3 – 4 p T (GeV/c) N ≡ number of offline tracks with pT>0.4 GeV/c Multiplicity N<35 35 ≦ N<90 90 ≦ N<110 N ≧ 110

27. Yen-Jie Lee (CERN) 27 MBUE working group 2012 Multiplicity and p T dependence 0 – 1 1 – 2 2 – 3 3 – 4 p T (GeV/c) N ≡ number of offline tracks with pT>0.4 GeV/c Multiplicity N<35 35 ≦ N<90 90 ≦ N<110 N ≧ 110

28. Yen-Jie Lee (CERN) 28 MBUE working group 2012 No ridge in pPb MC Compare to AMPT and HIJING pPb AMPT pPb, N>=100 HIJING pPb, N>=120 Generator-level No ridge in these pPb MCs!

29. Yen-Jie Lee (CERN) 29 MBUE working group 2012 Ridge Associated Yield ZYAM example N ≡ number of offline tracks with pT>0.4 GeV/c arXiv 1210.5482 Accepted by PLB

30. Yen-Jie Lee (CERN) 30 MBUE working group 2012 Ridge Associated Yield In the signal (N>110) region, the strength of the effect rises and falls with p T ZYAM example N ≡ number of offline tracks with pT>0.4 GeV/c arXiv 1210.5482 Accepted by PLB

31. Yen-Jie Lee (CERN) 31 MBUE working group 2012 Ridge Associated Yield In the p T range where the yield is the strongest, the ridge turns on at N≈50 In the signal (N>110) region, the strength of the effect rises and falls with p T ZYAM example N ≡ number of offline tracks with pT>0.4 GeV/c arXiv 1210.5482 Accepted by PLB

32. Yen-Jie Lee (CERN) 32 MBUE working group 2012 Correlations from planar particle production Correlations from back to back jets Correlations from planar particle production Summary and Conclusions A significant ridge is observed in high multiplicity (central) pPb collisions at 5 TeV – strong mechanism to produce particles in a plane – much larger than in pp

33. Yen-Jie Lee (CERN) 33 MBUE working group 2012 Summary and Conclusions A significant ridge is observed in high multiplicity (central) pPb collisions at 5 TeV – strong mechanism to produce particles in a plane – much larger than in pp Effect turns on slightly above average minimum bias multiplicity Effect rises and falls with p T – similar trend as observed in both PbPb and pp ridge before

34. Yen-Jie Lee (CERN) 34 MBUE working group 2012 Outlook All this came from a few hours of LHC pPb test running, only one fill! Thank you LHC! Several questions to be asked: – What is the physics origin of the ridge? Collective effect? Modification of jet structure? – Have we created a medium in pPb collision? Elliptic flow measurement (v n ) ? Jet quenching ? Hope for more surprises from the full pPb run coming up in January!

35. Yen-Jie Lee (CERN) 35 MBUE working group 2012 Backup