Centrality Categorization and its Application to Physics Effects in High-Energy d+A Collisions

1. Centrality Categorization and its Application to Physics Effects in High-Energy d+A Collisions Javier Orjuela-Koop University of Colorado Boulder For the PHENIX Collaboration gaveston_entertainment by optictopic CC/BY 30th Winter Workshop on Nuclear Dynamics Galveston, Texas April 6-12, 2014

2. Motivation 30th WWND J. Orjuela-Koop Numerous physics effects can be studied in d+Aucollisions.

3. Motivation 30th WWND J. Orjuela-Koop Numerous physics effects can be studied in d+Aucollisions. • Modifications of parton distribution functions in nuclei • Gluon saturation • Color neutralization • Collective flow in small systems (!)

4. Motivation 30th WWND J. Orjuela-Koop Numerous physics effects can be studied in d+Aucollisions. • Modifications of parton distribution functions in nuclei • Gluon saturation • Color neutralization • Collective flow in small systems (!) We need to characterize the event geometry!

5. Motivation 30th WWND J. Orjuela-Koop Numerous physics effects can be studied in d+Aucollisions. • Modifications of parton distribution functions in nuclei • Gluon saturation • Color neutralization • Collective flow in small systems (!) We need to characterize the event geometry! Au • How to select centrality classes? • How to validate the selection? • Are there any bias effects? • How do they affect ‘the physics’ at the GeVand TeV scales? b d

6. Experiment 30th WWND J. Orjuela-Koop PHENIX determines centrality by cutting on backward (Au-going) rapidity multiplicity. Au d Au d SIDE VIEW TOP VIEW

7. Experiment 30th WWND J. Orjuela-Koop PHENIX determines centrality by cutting on backward (Au-going) rapidity multiplicity. Au d Au d SIDE VIEW TOP VIEW • BBCs • Vertex pos. • Centrality • Event timing BBC Range:

8. Experiment 30th WWND J. Orjuela-Koop PHENIX determines centrality by cutting on backward (Au-going) rapidity multiplicity. Au d Au d SIDE VIEW TOP VIEW • ZDC • Spectator • neutrons • BBCs • Vertex pos. • Centrality • Event timing BBC Range: ZDC Range:

9. Centrality Categorization 30th WWND J. Orjuela-Koop Monte Carlo Glauber Event Geometry Ncoll, Npart, etc. Physical Observable Charge in BBC (Au-going)

10. Centrality Categorization 30th WWND J. Orjuela-Koop Monte Carlo Glauber Event Geometry Ncoll, Npart, etc. Physical Observable Charge in BBC (Au-going) http://arxiv.org/abs/1310.4793 Hulthén Wavefunction (deuteron)

11. Centrality Categorization 30th WWND J. Orjuela-Koop Monte Carlo Glauber Event Geometry Ncoll, Npart, etc. Physical Observable Charge in BBC (Au-going) http://arxiv.org/abs/1310.4793 Hulthén Wavefunction (deuteron) Woods-Saxon Distribution (Au)

12. Centrality Categorization 30th WWND J. Orjuela-Koop Monte Carlo Glauber Event Geometry Ncoll, Npart, etc. Physical Observable Charge in BBC (Au-going) http://arxiv.org/abs/1310.4793 http://arxiv.org/abs/1310.4793 Hulthén Wavefunction (deuteron) We use Monte Carlo methods to simulate d+Au collisions by sampling the position of nucleons froma Hulthénwavefunction (d) and a Woods- Saxon distribution (Au). Woods-Saxon Distribution (Au)

13. Centrality Categorization 30th WWND J. Orjuela-Koop Ok, so we know Ncoll and Npart . How to map them to experimental observables? BBC Charge (Au-going) Ncoll 30-40% 20-30% 10-20% 5-10% 0-5% http://arxiv.org/abs/1310.4793 MinBias trigger requirement: At least one hit in both (Au-going and d-going) BBCs.

14. Centrality Categorization 30th WWND J. Orjuela-Koop Ok, so we know Ncoll and Npart . How to map them to experimental observables? BBC Charge (Au-going) Ncoll Fluctuations from Negative Binomial Distribution 30-40% 20-30% 10-20% 5-10% 0-5% http://arxiv.org/abs/1310.4793 MinBias trigger requirement: At least one hit in both (Au-going and d-going) BBCs.

15. Centrality Categorization 30th WWND J. Orjuela-Koop Ok, so we know Ncoll and Npart . How to map them to experimental observables? BBC Charge (Au-going) Ncoll Fluctuations from Negative Binomial Distribution 30-40% 20-30% 10-20% 5-10% 0-5% determined from fitting experimental data. http://arxiv.org/abs/1310.4793 MinBias trigger requirement: At least one hit in both (Au-going and d-going) BBCs.

16. Centrality Categorization 30th WWND J. Orjuela-Koop Ok, so we know Ncoll and Npart . How to map them to experimental observables? BBC Charge (Au-going) Ncoll Fluctuations from Negative Binomial Distribution 30-40% 20-30% 10-20% 5-10% 0-5% determined from fitting experimental data. Fold in the GlauberNcoll distribution Gl(n) http://arxiv.org/abs/1310.4793 MinBias trigger requirement: At least one hit in both (Au-going and d-going) BBCs.

17. Centrality Categorization 30th WWND J. Orjuela-Koop Ok, so we know Ncoll and Npart . How to map them to experimental observables? BBC Charge (Au-going) Ncoll Fluctuations from Negative Binomial Distribution 30-40% 20-30% 10-20% 5-10% 0-5% determined from fitting experimental data. Trigger inefficiency for low charge! Fold in the GlauberNcoll distribution Gl(n) http://arxiv.org/abs/1310.4793 MinBias trigger requirement: At least one hit in both (Au-going and d-going) BBCs.

18. Centrality Categorization 30th WWND J. Orjuela-Koop Using Monte Carlo Glauber to map BBC multiplicity to geometry, we get numbers like… http://arxiv.org/abs/1310.4793

19. Centrality Categorization 30th WWND J. Orjuela-Koop Using Monte Carlo Glauber to map BBC multiplicity to geometry, we get numbers like… http://arxiv.org/abs/1310.4793 What about the ZDCs? What information can they provide?

20. Checking Centrality Selection 30th WWND J. Orjuela-Koop Au What if only one nucleon in the deuteron collides with the Au? D=4 fm Measure spectator neutron energy with d-going ZDC. D=14 fm d

21. Checking Centrality Selection 30th WWND J. Orjuela-Koop Au What if only one nucleon in the deuteron collides with the Au? D=4 fm Measure spectator neutron energy with d-going ZDC. D=14 fm d Monte Carlo Glauber Single Neutron Peak Data http://arxiv.org/abs/1310.4793 http://arxiv.org/abs/1310.4793 Double Interaction Contribution Exp. Backg. Good agreement between data and Monte Carlo Glauber values!

22. Systematic Uncertainties 30th WWND J. Orjuela-Koop http://arxiv.org/abs/1310.4793 • Nucleon-nucleon inelastic cross section • Woods-Saxon Au nucleus parameters • Include hard-core repulsive potential • Change z-vertex range • BBC charge proportional to Npart(not shown in plot)

23. Bias Factor Corrections 30th WWND J. Orjuela-Koop Event Geometry Invariant Yields BBC Multiplicity How is this mapping affected for events with charged particles at midrapidity? Consider p+p@ 200 GeV BBC MB trigger fires 52% of the time BBC MB trigger fires 75% of the time for events with charged particle at midrapidity

24. Bias Factor Corrections 30th WWND J. Orjuela-Koop Event Geometry Invariant Yields BBC Multiplicity How is this mapping affected for events with charged particles at midrapidity? Consider p+p@ 200 GeV BBC MB trigger fires 52% of the time BBC MB trigger fires 75% of the time for events with charged particle at midrapidity Small chance of particle production at midrapidity

25. Bias Factor Corrections 30th WWND J. Orjuela-Koop Event Geometry Invariant Yields BBC Multiplicity How is this mapping affected for events with charged particles at midrapidity? Consider p+p@ 200 GeV BBC MB trigger fires 52% of the time BBC MB trigger fires 75% of the time for events with charged particle at midrapidity Small chance of particle production at midrapidity MB Trigger biased towards non-diffractive events with greater particle production at midrapidity!

26. Bias Factor Corrections 30th WWND J. Orjuela-Koop What are the implications for d+Au? N collisions 1 Biased N-1 Unbiased http://arxiv.org/abs/1310.4793

27. Bias Factor Corrections 30th WWND J. Orjuela-Koop What are the implications for d+Au? N collisions 1 Biased N-1 Unbiased http://arxiv.org/abs/1310.4793 BBC charge deposition is scaled up!

28. Bias Factor Corrections 30th WWND J. Orjuela-Koop What are the implications for d+Au? N collisions 1 Biased N-1 Unbiased http://arxiv.org/abs/1310.4793 BBC charge deposition is scaled up!

29. Bias Factor Corrections 30th WWND J. Orjuela-Koop http://arxiv.org/abs/1310.4793 The bias effect seems to be pT dependent. Conservation of energy Change in the rapidity distribution

30. HIJING Study 30th WWND J. Orjuela-Koop We use the HIJING MC Generator to simulate p+p and d+Au events @ 200 GeV HIJING results are model-dependent, but they capture the desired bias! 12

31. HIJING Study 30th WWND J. Orjuela-Koop We use the HIJING MC Generator to simulate p+p and d+Au events @ 200 GeV HIJING results are model-dependent, but they capture the desired bias! Data Simulation http://arxiv.org/abs/1310.4793 http://arxiv.org/abs/1310.4793 12

32. HIJING Study at RHIC Energy 30th WWND J. Orjuela-Koop d+Au @ 200 GeV

33. HIJING Study at RHIC Energy 30th WWND J. Orjuela-Koop d+Au @ 200 GeV

34. HIJING Study at RHIC Energy 30th WWND J. Orjuela-Koop d+Au @ 200 GeV http://arxiv.org/abs/1310.4793 We observe a slight pT dependence of the bias factors. Less than 5% variation. Good agreement with Glauber calculation!

35. HIJING Study at LHC Energy 30th WWND J. Orjuela-Koop p+Pb @ 5.02 TeV HIJING Large auto-correlation in p+p. Large and strongly pT-dependent bias correction factors. http://arxiv.org/abs/1310.4793

36. HIJING Study at LHC Energy 30th WWND J. Orjuela-Koop p+Pb @ 5.02 TeV HIJING Large auto-correlation in p+p. Large and strongly pT-dependent bias correction factors. http://arxiv.org/abs/1310.4793 Recall QpPb~ 1/Bias Factor

37. Charged Particle and ET Production 30th WWND J. Orjuela-Koop dET/dh dNch/dh Npart Npart Linear scaling with Npart. Central d+Au matches peripheral Au+Au.

38. Bjorken Energy Density 30th WWND J. Orjuela-Koop Npart

39. Bjorken Energy Density 30th WWND J. Orjuela-Koop Npart The overlap area is centrality-dependent

40. Collective Flow in d+Au 30th WWND J. Orjuela-Koop There is recent compelling evidence for collective flow in p+Pb and d+Au. This is an example of a physics observable where comparison with theory requires an understanding of the geometry.

41. Collective Flow in 3He+Au 30th WWND J. Orjuela-Koop 3He+Au collisions have been proposed as a test system whose intrinsic triangularity can be used to discriminate initial state effects and those from viscous damping.

42. Collective Flow in 3He+Au 30th WWND J. Orjuela-Koop 3He+Au collisions have been proposed as a test system whose intrinsic triangularity can be used to discriminate initial state effects and those from viscous damping. http://arxiv.org/abs/1312.4565

43. Summary 30th WWND J. Orjuela-Koop • We described the PHENIX methodology for selecting centrality classes. • The centrality determination method is subject to inherent bias effects thatcan be accounted for. • Bias correction factors are small for d+Au @ 200 GeV, but large and pT-dependent for p+Pb @ 5.02 TeV. • There is evidence for collective flow in small systems. • This is one of many physics analyses that depend on a careful characterizationof the event geometry.

44. 30th WWND J. Orjuela-Koop Thank you!

45. 30th WWND J. Orjuela-Koop Backup Slides

46. Double-Interaction Study 30th WWND J. Orjuela-Koop Double inelastic interactions result in greater BBC charge deposition. We need to account for this extra charge when categorizing centrality.