Measurement of Quenched Energy Flow for Dijets in PbPb collisions with CMS

Transcription

1 Measurement of Quenched Energy Flow for Dijets in PbPb collisions with CMS For the CMS Collaboration NPA Seminar Yale, USA 15 October, 2015

2 Relativistic Heavy Ion Collisions Trying to answer two important questions in the high density QCD: Where is the critical point of the QCD phase diagram? What is the property of Quark Gluon Plasma? 2

3 Ideal World Quark Gluon Plasma Brick High Energy Quark Medium 3

4 Parton Cascade in Vacuum Parton shower Hadronization High Energy Jet Large Virtuality Q Hadrons 4

5 Parton Cascade in Medium Parton shower Hadronization High Energy Jet Large Virtuality Q Medium Medium Changes vs. Time Space-time information is also important in heavy ion environment Hadrons 5

6 Parton Energy Loss Models Two theoretical approaches: Perturbative QCD Weak coupling limit Holographic calculation Strong coupling limit Collisional energy loss Radiative energy loss AdS/CFT drag force 6

7 The High Energy Frontier Large Hadron Collider 27 km circumference Pb+Pb collisions 2.76 TeV ( ) 14x jump with respect to RHIC! 2015: 5.1 TeV Lake Geneva CMS France LHCb RHIC ALICE ATLAS Switzerland LHC 7 7

8 CMS Detector 8

9 CMS Detector EM and Hadron calorimeters photons, isolation, jet reconstruction Pb Inner tracker: charged particles vertex, isolation solenoid Pb Muon HCAL ECAL η < 2.4 η < 5.2 η < 3.0 Tracker η < 2.5 9

10 Particle Reconstruction with CMS γ Neutral Hadron e ± μ ± Charged Hadron 10

11 Heavy Ion Collision Recorded by the CMS Detector 11

12 Underlying Event Background Jet Goal: To study high p T jets event-by-event How do we subtract the background from soft scatterings? Take advantage of the large acceptance CMS detector: Predict with the measured energy in the forward calorimeter -5<η<-3 η <2.5 3<η<5 12

13 Underlying Event in Heavy Ion Collisions Training done with minimum-bias events (optimized by SVD method) CMS DP 2013/018 13

14 Jet Reconstruction and Composition UE subtracted particles Jet Clustering Anti-k T algorithm is used in the most CMS publications On average, charged hadrons carry ~65% of the jet momentum Measure the known part Correct the rest by MC simulation Optimize the use of calorimeter and tracker Example: Particle Flow in CMS A typical high p T jet 14

15 CMS Jet Reconstruction Strategy correction Raw jet energy Background subtraction Jet energy correction Jet energy Remove underlying events contribution MC Simulation PYTHIA Validated with dijet, photon(z)-jet data 15

16 Nuclear Modification Factor Jet Transverse Momentum Spectra Nuclear Modification Factor (R AA ): Ratio of the jet cross-section in PbPb and pp scaled by the number of nucleon-nucleon collisions ppb Collisions No significant modification Head-on PbPb Collisions Large suppression of high p T jet Large final state effect observed in PbPb collisions Is the jet structure modified in PbPb collisions? 16

17 Inclusive Jet Shape and Fragmentation Function Jet shape PbPb / pp Charged particle in cone PbPb - pp Jet shape: the jet energy distribution as a function of R Jet Fragmentation function: how transverse momentum is distributed inside the jet cone Energy Jet R 17

18 Inclusive Jet Shape and Fragmentation Function Jet shape PbPb / pp Charged particle in cone PbPb - pp 0-10% PbPb 120 < Jet p T < 300 GeV/c One more low p T particle in the jet cone R=0.3 Energy R Jet R Observation of energy redistribution inside the jet cone (Some modification at large R; ~ one more low track p T ) The bulk of the Jet structure is actually pretty similar to that in pp 18

19 Probe the QGP with High Energy Quarks and Gluons medium PP PbPb Increased rate of asymmetric dijets in central PbPb collisions 19

20 Probe the QGP with High Energy Quarks and Gluons Small A J Large A J (A J ~0.5) medium PP PbPb PRC 84 (2011) PLB 712 (2012) 176 Increased rate of asymmetric dijets in central PbPb collisions Where does the energy go? 20

21 Significant Energy Flow Out of the Jet Cone Tracks in the jet cone ΔR<0.8 Tracks out of the jet cone ΔR>0.8 CMS PRC 84 (2011)

22 Significant Energy Flow Out of the Jet Cone Tracks in the jet cone ΔR<0.8 Tracks out of the jet cone ΔR>0.8 Jet collimation CMS PRC 84 (2011) Partons undergo Brownian motion Medium trims away soft component to large angle Casalderrey-Solana, Milhano, Wiedemann J.Phys. G38 (2011)

23 Significant Energy Flow Out of the Jet Cone Tracks in the jet cone ΔR<0.8 CMS Tracks out of the jet cone ΔR>0.8 Jet collimation Decoherence PRC 84 (2011) Color coherence (angular ordered radiation) is destroyed in the strongly interacting medium Open up the phase space for wide angle radiation Ex: Mehtar-Tani, Ex: Mehtar-Tani, Salgado Mehtar-Tani, Salgado PLB Salgado PLB 707 PLB 707 (2012) 707 (2012) (2012)

24 Significant Energy Flow Out of the Jet Cone Tracks in the jet cone ΔR<0.8 CMS Tracks out of the jet cone ΔR>0.8 Jet collimation Decoherence Turbulance cascade PRC 84 (2011) Blaizot, Iancu, Mehtar-Tani PRL (2013) Effect of multiple branchings in medium: Quasidemocratic branching Effective mechanism which brings soft partons to large angle 24

25 Significant Energy Flow Out of the Jet Cone Tracks in the jet cone ΔR<0.8 CMS Tracks out of the jet cone ΔR>0.8 Jet collimation Decoherence Turbulance cascade PRC 84 (2011) Third jet quenching Vacuum-like parton shower also extend to large angle as large as R~1.0 The third jet may be quenched and produce soft particles out of the small jet cone R=0.3 25

26 Significant Energy Flow Out of the Jet Cone Tracks in the jet cone ΔR<0.8 CMS Tracks out of the jet cone ΔR>0.8 Jet collimation Decoherence Turbulence cascade PRC 84 (2011) The medium takes energy away from the high energy parton and turns that energy into heat / collective motion of the medium. Produce extra soft particles in the final state Casalderrey-Solana, Gulhan, Mihano, Pablos, Rajagopal, Third jet quenching Strongly coupling approach, hydro 26

27 Significant Energy Flow Out of the Jet Cone Jet collimation Decoherence Turbulance cascade Third jet quenching (1) How many particles are carrying the missing energy? (2) What is the angular distribution of the quenched energy flow with respect to the dijet axis? Strongly coupling approach, hydro 27

28 Measurement of the Quenched Energy Flow Subleading jet Leading jet Idea: Use all charged particles (p T >0.5 GeV/c) Study the transverse momentum balance Difficulty: Large PbPb underlying event (UE) 28

29 Datasets and Event Selection = 2.76 TeV 5.3 pb -1 pp Jet trigger with p T > 80 GeV/c Track reconstruction: p T > 0.2 GeV/c Anti-k T Calorimeter jet With R= μb -1 PbPb Jet trigger with p T > 80 GeV/c Track reconstruction: p T > 0.4 GeV/c Anti-k T Calorimeter jet with R=0.3 HF/Voronoi UE subtraction Dijet selection: p T,1 > 120 GeV/c p T,2 > 50 GeV/c η 1, η 2 < 1.6 (0.5) Δφ > 5π/6 Charged particles: p T > 0.5 GeV/c η <

30 Multiplicity Difference φ 2 What is the multiplicity of the particles that balance the extra lost p T? φ 1 30

31 Multiplicity Difference φ 2 What is the multiplicity of the particles that balance the extra lost p T? Compare the multiplicities in the leading and subleading jet hemispheres φ dijet φ 1 Direction of the dijet is defined as: φ dijet =½(φ 1 + (π-φ 2 )) (In contrast to PRC 84 (2011) , where the leading jet direction was used) Provide UE cancellation differential in ΔR 31

32 Multiplicity Difference (subleading leading jet) φ 2 What is the multiplicity of the particles that balance the extra lost p T? Compare the multiplicities in the leading and subleading jet hemispheres Direction of the dijet is defined as: φ dijet =½(φ 1 + (π-φ 2 )) φ dijet φ 1 Δ mult = (In contrast to PRC 84 (2011) , where the leading jet direction was used) Provide UE cancellation differential in ΔR N ch in subleading jet hemisphere - N ch in leading jet hemisphere 32

33 Multiplicity Difference (subleading leading jet) A J = (p T,1 -p T,2 )/(p T,1 +p T,2 ) arxiv φ 2 Three / multi-jet events φ dijet φ 1 Symmetric Dijet Events Multiplicity difference between the subleading and leading hemisphere is increasing vs. dijet asymmetry in pp and peripheral PbPb. There are more charged particles in the subleading hemisphere 33

34 Multiplicity Difference (subleading leading jet) arxiv φ 2 PbPb-PP φ dijet φ 1 A J = (p T,1 -p T,2 )/(p T,1 +p T,2 ) This increase is larger in central PbPb The enhancement in PbPb compared to pp increases with centrality Large A J, 0-10%: ~16 extra particles (p T >0.5 GeV) in the subleading jet hemisphere 34

35 Missing p T φ i, p T i φ 2 What is the multiplicity and p T spectra of the particles that balance the lost p T??? Charged particle azimuthal angle φ dijet ½(φ 1 + (π-φ 2 )) φ 1 Projection to dijet axis Dijet axis 35

36 Missing p T vs. A J More energy flow in the subleading jet direction PP arxiv % PbPb More energy flow in the leading jet direction The momentum imbalance inside the jet cone is restored if we consider all particles in the event (in both pp and PbPb collisions) 36

37 Missing p T vs. A J More energy flow in the subleading jet direction PP arxiv % PbPb More energy flow in the leading jet direction Missing p T from high p T particles increases as a function of A J In pp Balanced by 2-8 GeV/c particles In 0-10% PbPb Balanced by particles with p T < 4 GeV/c 37

38 Missing p T vs. Δ φ 2 What is the angular distribution of these particles with respect to the dijet system? Calculate the missing? p? T for charged particles that fall in slices of Δ φ dijet φ 1 38

39 Missing p T vs. Δ φ 2 What is the angular distribution of these particles with respect to the dijet system? Calculate the missing? p? T for charged particles that fall in slices of Δ φ dijet φ 1 39

40 Missing p T vs. Δ φ 2 What is the angular distribution of these particles with respect to the dijet system? Calculate the missing? p? T for charged particles that fall in slices of Δ φ dijet φ 1 40

41 Missing p T vs. Δ in pp Subleading jet direction Contribution from third jet GeV/c Integrated curve from 0-ΔR Leading jet direction Asymmetry inside the jet cone arxiv

42 Missing p T vs. Δ in pp Subleading jet direction Contribution from third jet Integrated curve from 0 to Δ Leading jet direction Asymmetry inside the jet cone arxiv

43 Missing p T vs. Δ Subleading jet direction Leading jet direction Inclusive A J arxiv

44 Missing p T vs. Δ Subleading jet direction Slight modification of the cumulative energy flow Leading jet direction Open circles: Integrated over particle p T Inclusive A J arxiv

45 Missing p T vs. Δ Subleading jet direction Slight modification of the cumulative energy flow High p T imbalance at small Δ Leading jet direction Inclusive A J arxiv

46 Missing p T vs. Δ Subleading jet direction Slight modification of the cumulative energy flow High p T imbalance at small Δ Leading jet direction Inclusive A J Balanced by low p T particles in subleading jet direction Extends upto large Δ arxiv

47 Anti-k T Jets with Different R Parameters Jet shape from PYTHIA Jets reconstructed with R=0.5 gives wider jet Jets reconstructed with R=0.2 gives narrower jet Jets are only meaningful if the algorithm is defined Different parameter R select different sets of dijet events!! Jet width dependent studies 47

48 Shooting Jets with Different Width through the Medium pp R= 0.2 R= 0.3 R= 0.4 R= 0.5 PbPb PbPb - pp 48

49 Shooting Jets with Different Width through the Medium Narrower jets Wider jets Quenched energy distribution depends on the R parameter used in the Anti-k T algorithm 49

50 Shooting Jets with Different Width through the Medium Narrower jets Wider jets Quenched energy distribution depends on the R parameter used in the Anti-k T algorithm Hint of narrower leading jet (or wider subleading jet) in PbPb collisions? 50

51 Shooting Jets with Different Width through the Medium Narrower jets Wider jets Quenched energy distribution depends on the R parameter used in the Anti-k T algorithm Hint of narrower leading jet (or wider subleading jet) in PbPb collisions? 51

52 Outlook Comparison to Theory and Outlook LHC Run II (2015) 52

53 Significant Energy Flow Out of the Jet Cone Jet collimation Decoherence Turbulence cascade Third jet quenching Strongly coupling approach, hydro 0-10% PbPb collision ~16 extra particles (p T >0.5 GeV) in the subleading jet hemisphere 53

54 Significant Energy Flow Out of the Jet Cone Jet collimation Decoherence Turbulence cascade Third jet quenching Strongly coupling approach, hydro 54

55 Jet Data vs. JEWEL event generator Hadron R AA JHEP03(2013)080 Jet R AA A Jet Quenching Monte Carlo based on weak coupling approach: Jet Fragmentation Function Reasonable description of jet FF, jet R AA and charged hadron R AA 55

56 Hybrid Model with Strong Coupling Approach JHEP 1410 (2014) 19 PYTHIA+ Ads Drag Dijet Asymmetry Jet R AA Jet FF Quark/Gluon Ratio Motivate high p T flavor tagged jet analysis: (heavy) quark vs. gluon jets 56

57 Heavy Flavor Meson PRL 108, (2012) PLB 666, 320 (2008) Very high p T heavy flavor meson (from quark jets): complementary to the high p T heavy flavor jets 57

58 Particle Identification with CMS Excellent capability of (decay) particle identification with CMS Υ(1S) D 0 Υ(2S) 58

59 Flavor Dependence of Jet Quenching Fully reconstructed D 0 K - π + over a wide kinematic range CMS-PAS-HIN b J/ψ D 0 Charged Hadrons 59

60 Outlook:Identified Heavy Flavor Jet and Mesons TeV PbPb data (0.15/nb) b-jet (Artist s impression) D (*) meson TeV ppb data (35/nb) B meson b-jet c-jet TeV PbPb data (~1.5/nb) D (*) meson B meson c-jet b-jet HL-LHC (10/nb): (b)-jet quenching at O(TeV) 60

61 Outlook An iterative feedback cycle between theory and experiment has started! Quenched jet event generator A systematical comparison between models and data Used to derive correction or to compare with data Generator Experiment Feedback and improve the generator / model LHC Run II: High statistics Photon-Jet and the first Z-Jet measurements High statistics (di-)jet measurement up to p T ~ 1 TeV Multi-jet correlation Flavor dependence of the parton energy loss from low to high p T with jets and mesons Stay tuned!!! 61

62 Backup slides BACK-UP 62