Lecture 3: jets in heavy ion collisions

Transcription

1 Lecture 3: jets in heavy ion collisions Marco van Leeuwen, Nikhef and Utrecht University JET School, UC Davis, 9-2 June 204

2 Jets and parton energy loss Motivation: understand parton energy loss by tracking the lost energy Qualitatively two scenarios: ) In-cone radiation: R AA =, change of fragmentation 2) Out-of-cone radiation: R AA < 2

3 Jets at LHC ALICE ϕ Transverse energy map of event η Clear peaks: jets of fragments from high-energy quarks and gluons And a lot of uncorrelated soft background 3

4 Jet reconstruction algorithms Two categories of jet algorithms: Sequential recombination k T, anti-k T, Durham Define distance measure, e.g. d ij = min(p Ti,p Tj )*R ij Cluster closest Cone Draw Cone radius R around starting point Iterate until stable η,ϕ jet = <η,ϕ> particles Sum particles inside jet Different prescriptions exist, most natural: E-scheme, sum 4-vectors Jet is an object defined by jet algorithm If parameters are right, may approximate parton For a complete discussion, see: 4

5 Collinear and infrared safety Illustration by G. Salam Jets should not be sensitive to soft effects (hadronisation and E-loss) - Collinear safe - Infrared safe 5

6 Collinear safety Illustration by G. Salam Note also: detector effects, such as splitting clusters in calorimeter (π 0 decay) 6

7 Infrared safety Illustration by G. Salam Infrared safety also implies robustness against soft background in heavy ion collisions 7

8 Jet algorithm examples simulated p+p event Cacciari, Salam, Soyez, arxiv:

9 Di-jet imbalance measurements

10 Di-jet asymmetry Observation: some events have two jets with different energy! (However: one swallow does not make spring) 0

11 Jet energy asymmetry Centrality ATLAS, arxiv:0.682 (PRL) Jet-energy asymmetry E A 2 = J E + 2 E E Large asymmetry seen for central events Suggests large energy loss: many GeV ~ compatible with expectations from RHIC+theory However: Only measures reconstructed di-jets (don t see lost jets) Not corrected for fluctuations from detector+background Both jets are interacting Not a simple observable

12 Energy dependence of asymmetry CMS, arxiv: (Relative) asymmetry decreases with energy However: difference pp vs PbPb remains energy loss finite at large E 2

13 Energy dependence of AJ Peripheral Central 0.8 CMS p /p /p T, T, T, PbPb pp s NN = 2.76 TeV, s = 2.76 TeV, PYTHIA+HYDJET Ldt = 50 µb Ldt = 23 nb % p T,2 > 30 GeV/c φ > 2 π 2 3 p T, % 0-20% PbPb - MC p T, (GeV/c) PbPb - MC Asymmetry decreases for larger jet energy! Similar effect in pp (Pythia): difference stays ~constant 3

14 γ-jet imbalance Centrality CMS, arxiv: γ-jet asymmetry x = Jγ p p Advantage: γ is a parton: know parton kinematics Disadvantage: low rate (+background π 0 γγ) jet T γ T Translates into: low pt,γ cut > 60 GeV Dominant contibution: qg qγ γ Potentially important observable more stats in next run(s) 4

15 Corrections Always ask: for which effects is the measurement corrected?! Important for any measurement, but in particular for jets! A J is basically uncorrected! Background subtracted, detector effects corrected on average! Hard to correct for detector effects+fluctuations! Spectra and recoil measurements are corrected for! Detector effects! Background:! Overall background, measured outside jet cone, details differ between experiments! Background fluctuations! Motivation: compare to theory without detector effects and without background (may be ill-defined) 5

16 Measuring the jet spectrum 6

17 Charged and full jets Full jets: charged + neutral particles (except neutrinos)! Hadronic + Electromagnetic Calorimetry (ATLAS)! + tracking (particle flow; CMS)! Tracking + EMCal (ALICE)! Charged jets: only charged particles! Used by ALICE because of limited acceptance of EMCal Counts / 2 GeV Reconstructed energy 00 GeV jets (particle level) Charged + Charged Leading charged particle [GeV] cone E T Charge to neutral fluctuations! Full jets strongly preferred for original goal: recover jet energy 7

18 Detector corrections Definitions:! Particle level: as generated by event generator, e.g. Pythia! Detector level: as reconstructed (Pythia+detector simulation)! (Parton level: parton energy; ill-defined?) Standard practice:! Charged jets are corrected to charged jets at the particle level! main effect: tracking efficiency! Full jets are corrected to full jets at the particle level! Calorimetric jets: HCal response! Tracking+EMCal: Unmeasured hadrons (neutrons, K 0 L, tracking efficiency) Probability/Bin(0.04) ALI PUB ALICE simulation p part T,ch jet (GeV/c) Anti-k T R = 0.3 track p > 0.5 GeV/c T (p det T,ch jet - p part T,ch jet )/p part T,ch jet 8

19 PbPb jet background Jet finding illustration Background density vs multiplicity Cacciari et al η-ϕ space filled with jets Many background jets Background contributes up to ~80 GeV per unit area Subtract background: = pt, ρ A Statistical fluctuations remain after subtraction p sub T, jet raw jet 9

20 PbPb jet background Toy Model Main challenge: large fluctuations of uncorrelated background energy Size of fluctuations depends on p T cut, cone radius 20

21 Background jets Raw jet spectrum Event-by-event background subtracted Low p T : combinatorial jets - Can be suppressed by requiring leading track - However: no strict distinction at low p T possible Next step: Correct for background fluctuations and detector effects by unfolding/deconvolution 2

22 Removing the combinatorial jets Raw jet spectrum Fully corrected jet spectrum ALICE arxiv: Correct spectrum and remove combinatorial jets by unfolding Results agree with biased jets: reliably recovers all jets and removed bkg 22

23 Charged jets, R=0.3 PbPb jet spectra R CP, charged jets, R=0.3 ALICE arxiv: Jet reconstruction does not recover much of the radiated energy Jet spectrum in Pb+Pb: charged particle jets Two cone radii, 4 centralities 23

24 Pb+Pb jet R AA Jet R AA measured by ATLAS, ALICE, CMS Good agreement between experiments Despite different methods: ATLAS+CMS: hadron+em jets ALICE: charged track jets R AA < : not all produced jets are seen; out-of-cone radiation and/or absorption For jet energies up to ~250 GeV; energy loss is a very large effect 24

25 Comparing hadrons and jets R CP Ch. particles Pb-Pb 0 ALICE (0-0%)/(50-80%) CMS (0-5%)/(50-90%) s NN =2.76 TeV Jets p track, p T ALICE Ch. Jets R=0.3 (0-0%)/(50-80%) ATLAS Calo Jets R=0.3 (0-0%)/(60-80%) jet T 2 0 (GeV/c) Suppression of hadron (leading fragment) and jet yield similar Is this natural? No (visible) effect of in-cone radiation? 25

26 Comparison to JEWEL energy loss MC R AA R=0.2 JEWEL+PYTHIA ALICE data 0.4 R AA CMS data, 0-5% centrality ALICE data, 0-5% centrality JEWEL+PYTHIA charged hadrons Ratio p [GeV] R AA R= ALICE data JEWEL+PYTHIA p t [GeV] JEWEL shows the same feature: jet RAA ~ hadron RAA Ratio p [GeV] 26

27 Path length dependence: v2 of Jets ATLAS, arxiv: v 2 = 0.06 ± meas % jet jet v 2 = ± meas 0-20 % ATLAS jet v % anti- R = % ATLAS k t ] - N jet 2 d [GeV d φ meas T dp N jet L dt = 0.4 nb Pb+Pb = 2.76 TeV jet s NN v 2 = ± meas % v 2 = ± meas % jet φ anti-k t R = < p < 80 GeV T jet v 2 meas = 0.04 ± % jet v 2 = ± meas % φ jet v 2 jet v % Pb+Pb - L dt = 0.4 nb % s NN = 2.76 TeV % % Significant azimuthal modulation of jet yield! jet v2 ~ 0.03 at high pt p T [GeV] p T [GeV] 27

28 Comparing to JEWEL energy loss MC 5-0% centrality 40-50% centrality (5-0)%, 45 GeV < p < 60 GeV (40-50)%, 45 GeV < p < 60 GeV /Njet d 2 Njet/dp d φ corr [GeV ] MC/data ATLAS data JEWEL+PYTHIA φ /Njet d 2 Njet/dp d φ corr [GeV ] MC/data ATLAS data JEWEL+PYTHIA φ K. Zapp, arxiv: Good agreement between JEWEL and jet v2 results Geometry: Glauber overlap with Bjorken expansion 28

29 Generic expectations from energy loss E jet k T ~µ λ fragmentation after energy loss? Longitudinal modification: out-of-cone energy lost, suppression of yield, di-jet energy imbalance in-cone softening of fragmentation Transverse modification out-of-cone increase acoplanarity k T in-cone broadening of jet-profile Out-of-cone effects are large, so expect combination of all of the above 29

30 Looking outside the jet cone // p miss = p cos( ϕ ϕ ) T, T jet tracks In Cone R<0.8 PYTHIA+HYDJET Out of Cone R>0.8 Momentum imbalance restored by hadrons at large angle R>0.8 and small p T < 2 GeV/c CMS measured CMS, arxiv:

31 Jet broadening: R dependence of RAA R CP ATLAS 0-0 % Centrality 58 < p T 89 < 50 < 38 < p T p T p T < 82 GeV < 03 GeV < 58 GeV < 44 GeV Pb+Pb L dt s NN - = 7 µb = 2.76 TeV R = 0.2 R = 0.3 R = 0.4 R = /R CP R R CP ATLAS, arxiv: Ratio of spectra with different R Pb+Pb s NN = 2.76 TeV L dt - = 7 µb ATLAS p T 0-0 % R = 0.3 R = 0.4 R = 0.5 [GeV] Larger jet cone: catch more radiation! Jet broadening However, R = 0.5 still has R AA < Hard to see/measure the radiated energy 3

32 Changes in fragmentation Transverse fragment distributions CMS, arxiv: Longitudinal fragment distributions CMS preliminary 0-0% PAS CMS-HIN-2-03! Enhancement at large R, low pt No modification at small R, large pt: physics or auto-correlation? 32

33 Again: background fluctuations Toy model spectrum Fragment distributions (simulation) /N jet dn h /dz Pythia Pythia+Hydjet z-subtracted /N jet dn h /d PbPb s=2.76 TeV 0-0% (Hydjet) anti-k t, R=0.4, y < Cacciari et al, arxiv: z Background fluctuations migrate yield to higher pt! At fixed pt: pick up above-average background contributions! ξ 4 pt 2 GeV Current measurements mostly pt > 2 GeV 33

34 Jet Quenching Summary I So, jet RAA is not close to! Large out-of-cone radiation, low pt, large angles! NB: even the fragmentation measurements do not capture the initial energy What is the (dominant) mechanism?! Several lines of investigation!! - No angular ordering the the medium; large angle radiation allowed (Mehtar- Tani, Salgado, Tywoniuk)! - Interplay of scales: medium density/mean free path vs opening angle of radiation! - Multiple interactions thermalise the radiation (Renk, Wiedemann, Caselderrey-Solana)! - Large angle democratic gluon splitting allowed in the medium (Blaizot, Iancu, Mehtar-Tani)! - Kinematics, (trigger-)biases also play a role! - Thorsten Renk: effect of Angular Ordering is small in Pythia 34

35 R CP Comparing to energy loss models Jet observables: need explicit modelling of multi-particle final states JEWEL: RCP vs R ATLAS data JEWEL+PYTHIA R=0.2 R=0.3 ( 0 ) R=0.4 ( 0 2 ) K. Zapp et al, arxiv: Mehtar-Tani, Tywoniuk, arxiv: R=0.5 ( 0 3 ) p [GeV] JEWEL gets the right suppression for R=0.2,! but not the increase with R! (Treatment of recoil partons?) Fragment distributions sensitive to coherence effects (NB: no geometry model yet) 35

36 Hadron trigger vs jet trigger Are jets an unnecessary complication? If hadron and jet RAA are similar, why not use hadron observables? Hadron trigger: strong surface bias maximizes recoil path length Hadron trigger Full jet trigger: no geom. bias partially cancelled by bkg fluctuations Jet trigger T.Renk, PRC Biases are different! Can be exploited to constrain models 36

37 Hadron-recoil jet measurements 37

38 Hadron-triggered recoil jet distributions G. de Barros et al., arxiv: p T,jet < 20 GeV/c: No change with trigger p T Combinatorial background p T,jet > 20 GeV/c: Evolves with trigger p T Recoil jet spectrum 38

39 Background subtraction: Δ recoil Remove background by subtracting spectrum with lower p T trig: Δ recoil =[(20-50)-(5-20)] Reference spectrum (5-20) scaled by ~0.96 to account for conservation of jet density Δ recoil measures the change of the recoil spectrum with p T trig Unfolding correction for background fluctuations and detector response 39

40 Ratio of Recoil Jet Yield ΔI AA PYTHIA pp reference: PYTHIA (Perugia 200) R=0.4! Constituents: p T const > 0.5 GeV/c! no additional cuts (fragmentation bias) on recoil jets Recoil jet yield ΔI AA PYTHIA 0.75, approx. constant with jet p T 40

41 Recoil Jet ΔI AA PYTHIA : R dependence R=0.4 R=0.2 Similar ΔI AA PYTHIA for R=0.2 and R=0.4 No visible broadening within R=0.4 (within exp uncertainties) 4

42 Hadrons vs jets II: recoil Hadrons Jets PRL Hadron I AA = In approx. agreement with models; elastic E-loss would give larger I AA Jet I AA = Jet I AA > hadron I AA Not unreasonable NB/caveat: very different momentum scales! 42

43 Model comparison I AA JEWEL: Zapp et al., EPJ C69, 67 Pythia / recoil Pb-Pb = recoil Pythia I AA const anti-k T, R=0.4, p > 0.5 GeV/c T Pb-Pb 0-20% PYTHIA ALICE I AA (20-50)-(5-20) PYTHIA YAJEM I AA (20-50) 0-0% PYTHIA JEWEL I AA (20-50)-(5-20) s NN = 2.76 TeV ch p (GeV/c) T,jet ALI DER JEWEL correctly describes inclusive jet R AA JEWEL ΔI AA ~0.4, below measured YAJEM agrees with measurement Difference in energy loss or geometry? 43

44 Summary Jets: a new tool for parton energy loss measurements Large out-of-cone radiation (R = ) Energy asymmetry R AA <, similar to hadrons I AA < Radial shapes Remaining jet has small modifications: Longitudinal and transverse structure similar at small r, large z Deviations at large r, low z Most of the radiation is at low p T Scale set by medium temperature? Democratic branchings? Interplays of many effects: impossible to read simple conclusions off the plots! Need (detailed) calculations to draw conclusions e.g. JEWEL and YaJEM energy loss MCs agree with many of the observed effects! Does this constrain the energy loss mechanism(s)?! Ongoing work 44

45 Extra slides

46 46 A consistent view of jet quenching 200 data: arxiv: arxiv: G. Roland@QM202 Change from ξ to p T PbPb pp (/GeV) No change at small r, high p T Consistent with 200 result! Recall (200 vs 20): Track p T > 4 GeV vs p T > GeV Leading vs inclusive jet 0-30% vs 0-0% and 0-30% Radius r Narrowing/depletion at intermediate r, p T Broadening/excess at large r, low p T! (~2% of jet energy)

47 47 A consistent view of jet quenching G. Roland@QM202 Charged particles from p T =50-00 GeV: z = p T (track)/p T (jet) = ξ < Looking at the same parton p T range PbPb fragmentation function = pp for ξ < Consistent message from charged hadron R AA, inclusive jet R AA and fragmentation functions!

48 Jet fragment distributions PbPb measurement Ratio to pp ATLAS M. Low p T enhancement: soft radiation Intermediate z: depletion: E-loss NB: z is wrt observed E jet initial E parton 48

49 Jet fragment distributions CMS, Frank Low p T enhancement: soft radiation Intermediate z, p T : depletion: E-loss 49

50 Jet broadening: transverse fragment distributions ρ (r) pp /ρ(r) PbPb ρ(r) CMS, s NN 70-00% = 2.76 TeV anti-k T jets: R = 0.3 jet p > 00 GeV/c T jet 0.3 < η < 2 p > GeV/c track T r - pp, L dt = 5.3 pb PbPb pp reference 50-70% r - PbPb, L dt = 50 µb 30-50% ρ (r) pp /ρ(r) PbPb ρ(r) r ρ (r) % r pp /ρ(r) PbPb ρ(r) r 0-0% r r CMS, arxiv: CMS PAS HIN-2-03 PbPb Jet broadening: radiation at large angles PbPb 50

51 Comparing to JEWEL energy loss MC R CP ATLAS data JEWEL+PYTHIA R=0.2 R=0.3 ( 0 ) R=0.4 ( 0 2 ) MC/data MC/data MC/data R=0.2 R=0.3 R=0.4 K. Zapp et al, arxiv: R=0.5 ( 0 3 ) MC/data R= p [GeV] p [GeV] JEWEL gets the right suppression for R=0.2,! but not the increase with R! May be treatment of recoil patrons 5

52 Full jet comparison R AA ALICE Preliminary Pb-Pb anti-k T R = 0.2 ALICE 0-0% ALICE 0-30% CMS 0-5% CMS 0-30% s NN = 2.76 TeV 0.5 CMS: Read from HIN PAS CMS: Syst. Unc. R = ALI-PREL p T,jet (GeV/c) Good agreement between experiments; hint of pt dependence 52