Event Shape Analysis in minimum bias pp collisions in ALICE.

1. Event Shape Analysis in minimum bias pp collisions in ALICE. Antonio Ortiz Velasquez for the ALICE Collaboration 5th Workshop on High Pt Physics at LHC, 27.September.2010

2. ESA Outline 1. Introduction. 2. Event Shapes at Hadron Colliders. 3. Event Shape Analysis (ESA) in MB interactions. 4. Strategy to measure the event shapes in ALICE. 5. First ALICE results. <St> vs multiplicity: “bulk,” “soft” and “hard” events. Comparison with different models. Characteristics of high multiplicity events. 6. Conclusions.

3. What are the event shapes? • They measure the geometrical properties of the energy flow in QCD events. In e+e- colliders and DIS, the event shapes were among the most studied QCD observables, both theoretically and experimentally: • Extraction of the strong coupling constant. • Tunning the parton showers and non - perturbative components of MC. • For example: sphericity was used to show the evidence for the existence of jets. The variable oblateness was used for describing processes where three prolong jets were produced. Discovery of three-jet events and a test of QCD at PETRA. (PRL, vol 43 No. 12, 1979). Historical picture SLAC (PRL vol 35 No. 24, 1975).

4. T=2/ Event Shapes at Hadron Colliders • Thrust is defined as: , and we can catch different configurations like: • Thrust minor is a measurement of the momentum perpendicular to the plane defined by the thrust and beam axes. • Transverse Sphericity is defined in terms of the eigen-values of the transverse momentum tensor: =0, “pencil-like” events. =1, isotropic events.

5. Event Shapes at Hadron Colliders At hadron colliders there are not enough studies of the ES. D0 • Few measurements at Tevatron. CDF Studies using high Et jet data to test QCD

6. Event Shapes at Hadron Colliders • Simulation study by CMS Sensitivity of the ES distributions to different models of multi-jet production. • Interesting phenomenology studies of the ES. But also concentrated in hard events (A. Banfi et. al., arXiv:1001.4082v1). • The transverse sphericity has been mentioned in the ATLAS and CMS discussions of perspective analyses to be used in the searches of supersymmetric particle decays and black-hole decay. (arXiv:0901.0512, J. Phys. G 34 (2007) 995)

7. Event Shape Analysis in MB interaction. • The previous studies were dedicated to measure perturbative effects. • However the ES are very sensitive to the so-called Underlying Event (In Pythia: MPI, ISR, BR). Jet 1 Jet 1 proton proton proton proton Jet 2 Jet 2 pQCD Soft partonic interactions also occur in the remains of the hard parton scattering and are not associated with the hard process, their understanding important for many physics analyses!

8. Event Shape Analysis in MB interaction. Using primary charged particles, |eta|<0.8, pt>0.5 GeV/c, Nch>2. Pure Generator!!! The non-perturbative effect decrease with the hardness. Different sensitivities.

9. We can discriminate events according with their topologies. The low sphericity events look more like events without UE. Event Shape Analysis in MB interaction. Average ES versus multiplicity. • The ES is a property of the event. • The highest NP effects are manifested at High Multiplicity!

10. ESA Event Shape Analysis in MB interaction. Why are we so excited for applying ESA to MB data? • Fast identification of events with unusual topology. • Interesting tool to study the high multiplicity events. • Event-by-event classification of events with hard and soft topology. • Sensitivity to multi parton interactions (MPI)‏. Constraining MPI parameters. • Monte Carlo tuning.

11. Performance 11.08.2010 Strategy to measure the ES in ALICE. MC studies to measure the transverse sphericity. Events are selected if they were triggered as MB interactions. Events with reconstructed primary vertex. In real data an offline selection is applied in order to reduce the background event contribution. Events with more than two primary tracks. Only primary tracks reconstructed with TPC and ITS are considered in the computations (|eta|<0.8, pt>500 MeV/c). Differences < 5 % Multiplicity-Response-Matrix From unfolding procedure.

12. First ALICE results. • Primary charged particles. • Hardness of events: we chose to separate the sample in soft and hard using as the limit events that have pt>2GeV/c, this is not trivial! Pure MC • We see that the soft events are still prevailing so that the overall fits with generators are somehow less sensitive to the hard part.

13. Performance 11.08.2010 ESA First ALICE results. Possibly more active MPI. Statistical errors are shown. At first approximation the systematic error due to: procedure + trigger + vertex + pile-up is < 6 % at high multiplicity.

14. Performance 11.08.2010 Performance 11.08.2010 First ALICE results. Leading particle with pt<2 GeV/c. MC produces soft high multiplicity events. More isotropic! St > 0.6

15. Performance 11.08.2010 Performance 11.08.2010 First ALICE results. With this selection, the events look more jetty!!!

16. Performance 11.08.2010 First ALICE results. • MC and ALICE show that “soft” and “hard” events have <St> vs multiplicity independent of the energy. • On the other hand, if we compare the hardness of the events produced at 7 TeV with respect to the 0.9 ones. We observe that Perugia-0 is harder than ALICE data. This explains the differences in the “bulk” between <St> at both energies for Perugia-0.

17. Performance 11.08.2010 Performance 11.08.2010 First ALICE results. Sphericity spectra in different multiplicity bins.

18. Performance 11.08.2010 First ALICE results. Sphericity spectra for the largest multiplicities: Nch > 30. MC overestimates the jetty-events production. The measurements of ALICE in the high multiplicity show more events with high St with respect to the models.

19. ESA Conclusions. • The average sphericity (<St>) in function of multiplicity was measured in ALICE using minimum bias (MB) events of pp collisions at 0.9 and 7 TeV. The results were compared with 3 different MB models: PYTHIA (tunes: PERUGIA-0 and ATLAS-CSC) and PHOJET. • We observe that the soft events at 7TeV are still dominating in a ratio ~2:1 the hard events. We have treated the two classes separately. • At 0.9 TeV the “bulk” measurements are in agreement with the models for multiplicities up to 13. However for the “soft” and “hard” events, slight differences are observed. • At 7 TeV the high multiplicity is formed mostly by the “hard” processes. The models fail in describing such events. • The “soft” events are well described by the PYTHIA models, however they fail in the description of the “bulk” where the ALICE measurements indicate a more spherically symetrical distribution of the transverse momentum.

20. ESA Conclusions. • The “soft” events are in general more isotropic than the “hard” ones. For example in real data at 7 TeV: while the “bulk” events have a mean sphericity between ~0.4 and 0.6; the “soft” ones have <St> between ~0.48 and 0.7. In contrast the “hard” events have <St> between 0.2 up to 0.6. • The high sphericity events in the limit St → 1 we observe more events in the data than in the in the simulation.