Event structure analysis in pp minimum bias events in alice

1. Event structure analysis in pp minimum bias events in alice Guy Paić for the ALICE collab. Instituto de CienciasNucleares, UNAM<Mexico

2. outline • Motivations • The event structure analysis - sphericity • Results for sphericity • Results for mean sphericity and mean pt • Aplications • Conclusions

3. The minimum bias collisions are an interesting subject of investigations for different reasons Work done in collaboration with Antonio Ortiz

4. Motivation I Motivation I – from earlier experiments CDF ChrissQuigg: arXiv:1004.0975v1 [hep-ph] Albajar, C., et al. Analysis of the Highest Transverse Energy Events Seen in the UA1 Detector at the SppS Collider. Z. Phys. C36 (1987), UA1

5. Motivation II • The generators are not able to fit (today) simultaneously the pt and the multiplicity spectra • Introducing more variables on the market might shed light • Understand the details of the interaction in proton-proton beyond simple averaging of quantities since it is the bench mark for HI • Understand the event by event fluctuations • as a function of multiplicity • Reach the highest possible multiplicities where the energy density might approach the one of collisions of light ions

6. . Status of the fits: at 0.9 TeV Physics Letters B 693 (2010) 53-68 Eur. Phys. J. C (2010) 68: 89-108  

7. Motivation IIIthe usual representation of pp collisions

8. The Inelastic Non-Diffractive Cross-Section Occasionally one of the parton-parton collisions is hard (pT > ≈2 GeV/c) Majority of “min-bias” events! “Semi-hard” parton-parton collision (pT < ≈2 GeV/c) + + + + … Multiple-parton interactions (MPI)!

10. Event samples Three energies 0.9 TeV 3.5 million MB events 2.76TeV 40 million MB events TeV 40 million MB events Usual Physics selection cuts

11. The transverse sphericity observablefrom a pencil to a hedgehog

12. The sphericity spectrum in bins of multiplicity The general trend with multiplicity is trivial: more multiplicity – > sphericity rises Detail: do the generators reproduce the trend? Actually not! The generator sphericity spectra CROSS the experimental spectrum at ST of ≈ 0.5 at low multiplicity and 0.7 at the highest multiplicities

13. HIGHER MULTIPLICITIES Conclusion :the mean sphericitydifferences between the generators and data should be smaller than the difference in parts of the spectra – let’s check whether this is true?

14. Choice of events by hardness To maximise the effect of the analysis it was made for two types of events selected by hardness (and for their sum). Hard events Soft events Firstconclusion: Soft events represent the majority of pp events even at 7TeV, and some generators do have problems reproducing the data

15. The <ST> at 0.9 TeV soft hard bulk The ratios get worse for hard events!

16. The <ST> at 7 TeV Conclusion: the <ST> differences between generators and data are rather small ≈20% at higher multiplicities and is caused by the hard events while in the earlier sphericity spectra we saw larger disagreements The hard part is more difficult to fit then the soft part The trends at high multiplicities is different

17. <PT>vs multiplicity Again the same scenario: large discrepancies found at large multiplicities for hard events

18. An interplay of the sphericity and pt dependence ? The difference in <pT> is the result of the different weights of the individual sphericity bins This is in our opinion the explanation of the observation

19. How to reconcile pT and sphericity? 7 TeV data

20. Evolution of the sphericity spectra in function of hardness and multiplicity

21. bulk soft hard

22. bulk soft hard

23. bulk soft hard

24. bulk bulk soft hard

27. Interesting observation the jet heart is very concentrated the topology is squashed in phi and extended in eta! The tail is generally of very lo pt particles! Perhaps the jetty events of low sphericity are more interesting than the hedgehog ones!

28. Dependence of of <ST> on incident energy in function of multiplicity

29. Variation for soft events

30. Variation of the sphericity with energy for hard events Variation observed: rather strong disagreement with some generators

31. Possible application: Initial state radiation contribution to various sphericity events and understanding of the width of the peaks

32. Different azimuthal correlations belong to well defined sphericity intervals of the events Pythia6, ATLAS-CSC, pp @ 7 TeV ST and correlations were computed using primary charged particles in |η|<0.8

33. Pythia6, ATLAS-CSC, pp @ 7 TeV

34. ST and Correlations in |η|<0.8, pT>0.5 GeV/c Pythia6, ATLAS-CSC, pp @TeV ISR enhances the production of isotropic events

35. Conclusions • The sphericity analysis allows to identify event topologies pertaining most probably to very different types of interactions • The sphericity variable constitutes a rich mine to study the details of the interaction. • A collision of multiplicity N can contain both very “jetty” events and events of the kind mentioned at the beginning. • The Hedgehog events are neither extraordinary nor outliers they are predicted by the generators but with a lesser yield! • Many possibilities of further applications

36. corrections

37. True multiplicity

38. Systematic uncertainties