Heavy Ion Physics at CMS

1. 1 Heavy Ion Physics at CMS Prashant Shukla Nuclear Physics Division BARC, Mumbai India 28th February 2011, BARC

2. 2 Physics motivations of LHC Most publicized motivations of LHC experiments: • pp collisions: Searching the particle (Higgs) responsible for masses of the fundamental particles. Plus new physics beyond standard model. • PbPb collisions: The matter is made of quarks and gluons always confined inside nucleons. PbPb collisions aim to produce a soup of quarks and gluons. This state is known as Quark Gluon Plasma existing at early universe.

3. 3 Heavy Ion Collisions at LHC • At LHC hottest matter ever created in the laboratory • New probes open up • High tech detectors for precise measurement • Hard probes (approved CMS Results) • Dijets and jet quenching (HIN-10-04) • Z0 results (HIN-10-03) • Quarkonia (under progress) • Soft probes (Under progress) • Multplicity • Elliptic Flow • Charged particle spectra • Correlations

4. 4 LHC at CERN Experiments for Heavy Ions CMS Geneva Lake ALICE ATLAS CERN CERN

5. 5 Heavy Ion plans for the LHC • Physics proton-proton run at the LHC Started in November 2009 √s = 0.9, 2.36, 7 TeV. • The heavy-ion run started 8th November 2010 Pb+Pb collisions at √s = 2.76 TeV per nucleon pair • CMS LHC run stopped on 5th December --> Luminosity next slide • The second heavy-ion run is expected in the November-December 2011 at the same or little more energy with but increase in luminosity. • After the LHC upgrade 2012 we hope to have in 2013 Pb+Pb collisions at √s = 5.5 TeV per nucleon pair 11/23/10 5

6. 6 PbPb data taking with CMS PbPb run finished with integrated Luminosity 9.59 b-1 delivered, 8.72 b-1 recorded corresponding pp equivalent ~ 0.3pb-1 (pp delivered so far 40 pb-1) 11/23/10 6

7. 7 The CMS Experiment Compact Muon Solenoid 7 7

8. 8 The CMS as a Heavy Ion Experiment CMS is a superb and versatile detector for heavy ion physics Excellent performance in high pT (ET) region and for muon pairs Quarkonia, Jet physics, Z0, . . . . Silicon Tracker • Good efficiency and purity for pT > 1 GeV/c • p/p 1–2 % for pT < 100 GeV/c Calorimeters: high resolution and segmentation Good performance for jet studies Muon Tracking: from Z0, J/,  • Wide rapidity coverage: || < 2.4 • σm 70 MeV/c2at the mass in || < 1

9. 9 A typical PbPb central event 9

10. 10 A typical PbPb peripheral event 10

11. 11 The Muon reconstruction 11

12. Trigger condition Minimum bias trigger: Using information from the two Beam Scintillator Counters (BSC) and Forward Hadronic Calorimeters (HF). Statistics: Number of events = 54,965,553 12 12

13. Centrality Determination 13 13

14. 14 Heavy Ion Observables

15. 15 Soft Probes

16. 16 Soft probes in progress • Multiplicity • Elliptic Flow • Charged Particle Spectra • Two Particle Correlations

17. py px Z y X Anisotropic Flow 17 The Elliptic Flow v2 Peripheral Collisions  phi = atan (py/px)‏ V2= < cos 2 phi > V2 gives pressure transfer from y to x direction measures collectivity

18. 18 The Elliptic Flow v2 - v2 measures collectivity - v2 (pT) for |η|<0.8 , 0.8 <|η|<1.6, 1.6 <|η|<2, 2<|η|<2.4 – Integrated v2 vs η : for several centralities – v2 (integrated) mid-rapidity scaled by Npart as a function of centrality – v2 results with events selected on the basis of presence of identified jets

19. 19 New observation by CMS in pp collisions “Ridge”-effect in high-multiplicity events at 7 TeV Long-Range Near-Side Angular Correlations in pp Shown alongwith RHIC results

20. 20 Hard Probes

21. 21 Jet Quenching • At RHIC Strong quenching effects were observed in single particle spectra and particle correlations. • Direct jet reconstruction possible but very difficult with RHIC detectors. Jet suppression is indicated by leading particle.

22. 22 Di Jet events • First hours of LHC running We have seen di-jet events We have seen di-jets with unbalanced energy

23. 23 Study of Di Jet events • Leading jet is required to have at least 120 GeV Above trigger threshold • Sub-leading jet is required to have at least 50 GeV Above background fluctuations • Select back-to-back jets  phi > 2.5 • To study jet quenching effects use jet energy asymmetry AJ = (PT,1 – PT,2) / (PT,1 + PT,2) arXiv:1102.1957 [nucl-ex]

24. 24 Di Jet energy imbalance arXiv:1102.1957 [nucl-ex] A significant dijet imbalance, well beyond that expected from unquenched MC embedded in real data, appears with increasing collision centrality

25. 25 Quantifying the imbalance Fraction of unbalanced dijets • Fraction of jets with imbalance smaller than 0.15 • Plot as a function of number of participating nucleons (volume) averaged over centrality bin arXiv:1102.1957 [nucl-ex]

26. 26 Jet quenching What we conclude: • A significant dijet energy imbalance. • The imbalance is well beyond that expected from unquenched MC embedded in real data. • The imbalance increases with collision centrality The robustness checks: Imbalance MC with and without embedding in data. By smearing the jet resolution by 10 to 50 % in simulation. Leading jet cut off ( 120, 130, 140). Sub leading jet cut off ( 35, 50 , 55). ...................

27. 27 Z0 bosons QGP probes are modified in the medium: a baseline needed. Z0 does not interact with medium (like photons) - Probe of initial state effects: shadowing (10-20 %), multi-parton scattering (2 %), Isospin (3 %) First Z0 Candidate in HI collisions

28. 28 Z0 analysis framework MuSkim: • Event passed DiMuon Trigger • HLT_HIL2DoubleMu3_Core • DiMuonSkim: • Three exclusive dimuon categories. • i) DiMuonsGlobal • ii) DiMuonsGlobalSTA • iii) DiMuonsSTA • DiMuon2DPlots • 2D histograms (M vs pT), (M vs Y), (M vs Cent) for each dimuon category. • Z0MassFit • V. Kumar + P Shukla

29. 29 Z0+- signal in PbPb • All heavy ion statistics between [30,120] GeV/c2, • with some loose quality criteria • Resolution comparable to p+p 2.9 pb-1 [60,120] GeV/c2

30. 30 RAA for Z0+- in PbPb

31. 31 RAA for Z0+- in PbPb

32. 32 Z-> ee Candidate

33. 33 Future Study : gamma+jet

34. 34 Future Study : Z0+jet

35. 35 Quarkonia as probes of QGP • Large Masses produced early in the collisions via gluon fusion • Strongly bound and weakly coupled to light mesons Quarkonia should melt in QGP: • SPS: J/ψ suppression seen. But there are alternative explainations. • RHIC: Suppresion vs rapidity not completely understood. • LHC: Regeneration of J/ψ from the (large) number of uncorrelated ccbar pairs. Upsilons open up.

36. 36 High pT J/+- • Subset of the statistics in HI, dimuon pT in [6.5, 30] GeV/c2 • Very good resolution also in HI collisions ! • Background in HI already low with basic quality criteria in this pT window

37. 37 High pT Y +- • Subset of the statistics in HI, single muon pT in [4, 30] GeV/c2 • Good resolution also in HI collisions • Background is more than J/psi

38. 38 Quarkonia The goal of the first analysis: absolute cross sections/ RCP All Physics Data Set Crucial to separate prompt and non-prompt J/ψ Need to tune muon identification cuts  Abdulla Abdulsalam

39. 39 Summary and Outlook • CMS has collected a good quality data with heavy ion collisions. The detector has shown excellent performance in all major sectors. • Observation of new phenomena in heavy ion collisions Large number of dijets with unbalanced energies indicative of jet quenching • Z0 measurement: Within uncertainty no modification was observed. At higher luminosity it can be used to study the modifications in PDFs in the initial state. • Reconstruction of J/psi and Upsilon with similar mass resolutions as in pp. Rigorously pursued now. • Soft Probes: Interesting results are expected soon for: Multplicity Elliptic Flow Charged particle spectra Correlations

40. 40 CMS Heavy Ion Crew

41. 41 Back Up

42. 42 Reconstruction of Jets