Hard Probes of the Quark Gluon Plasma Lecture III: jets

1. Hard Probesof the Quark Gluon PlasmaLecture III: jets Marco van Leeuwen, Nikhef and Utrecht University Lectures at: Quark Gluon Plasma and Heavy Ion Collisions Siena, 8-13 July 2013

2. Jets and parton energy loss Motivation: understand parton energy loss by tracking the gluon radiation • Qualitatively two scenarios: • In-cone radiation: RAA = 1, change of fragmentation • Out-of-cone radiation: RAA < 1

3. Jets at LHC ALICE Transverse energy map of 1 event j h Clear peaks: jets of fragments from high-energy quarks and gluons And a lot of uncorrelated ‘soft’ background

4. Jet reconstruction algorithms Two categories of jet algorithms: • Sequential recombination kT, anti-kT, Durham • Define distance measure, e.g. dij = min(pTi,pTj)*Rij • Cluster closest • Cone • Draw Cone radius R around starting point • Iterate until stable h,jjet = <h,j>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: http://www.lpthe.jussieu.fr/~salam/teaching/PhD-courses.html

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

6. Collinear safety Illustration by G. Salam Note also: detector effects, such as splitting clusters in calorimeter (p0 decay)

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

8. PbPb jet background Subtract background: Background density vs multiplicity Jet finding illustration Cacciari et al h-j space filled with jets Many ‘background jets’ Background contributes up to ~180 GeV per unit area Statistical fluctuations remain after subtraction

9. Jet energy asymmetry Centrality ATLAS, arXiv:1011.6182 (PRL) Large asymmetry seen for central events Jet-energy asymmetry 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 intereracting – No simple observable

10. Studying the imbalance In Cone R<0.8 Out of Cone R>0.8 PYTHIA+HYDJET Momentum imbalance restored by hadrons at large angle R>0.8 and small pT < 2 GeV/c CMS, arXiv:1102.1957 CMS measured

11. Energy dependence of asymmetry CMS, arXiv:1202.5022 (Relative) asymmetry decreases with energy However: difference pp vs PbPb remains – energy loss finite at large E

12. g-jet imbalance  Centrality CMS, arXiv:1502.0206 g-jet asymmetry Advantage: g is a parton: know parton kinematics Disadvantage: low rate (+background p0→gg) Dominant contibution: qg → qg

13. Hadron-triggered recoil jet distributions G. de Barros et al., arXiv:1208.1518 pT,jet< 20 GeV/c: No change with trigger pT Combinatorial background pT,jet> 20 GeV/c: Evolves with trigger pT Recoil jet spectrum

14. Background subtraction: Δrecoil Remove background by subtracting spectrum with lower pTtrig: Δrecoil=[(20-50)-(15-20)] Reference spectrum (15-20) scaled by ~0.96 to account for conservation of jet density Δrecoil measures the change of the recoil spectrum with pTtrig Unfolding correction for background fluctuations and detector response

15. Ratio of Recoil Jet Yield ΔIAAPYTHIA • pp reference: PYTHIA (Perugia 2010) R=0.4 Constituents: pTconst > 0.15 GeV/c no additional cuts (fragmentation bias) on recoil jets Recoil jet yield ΔIAAPYTHIA ≈0.75, approx. constant with jet pT

16. Recoil Jet ΔIAAPYTHIA: R dependence R=0.4 R=0.2 Similar ΔIAAPYTHIA for R=0.2 and R=0.4 No visible broadening within R=0.4 (within exp uncertainties)

17. Hadrons vs jets II: recoil Hadrons Jets PRL108 092301 Hadron IAA = 0.5-0.6 Jet IAA = 0.7-0.8 In approx. agreement with models; elastic E-loss would give larger IAA Jet IAA > hadron IAA Not unreasonable NB/caveat: very different momentum scales !

18. Measuring the jet spectrum

19. PbPb jet background Toy Model Main challenge: large fluctuations of uncorrelated background energy Size of fluctuations depends on pT cut, cone radius

20. Background jets • Low pT: ‘combinatorial jets’ • Can be suppressed by requiring leading track • However: no strict distinction at low pT possible Raw jet spectrum Event-by-event background subtracted Next step: Correct for background fluctuations and detector effects by unfolding/deconvolution

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

22. PbPb jet spectra M. Verweij@HP, QM Charged jets, R=0.3 RCP, charged jets, R=0.3 Jet reconstruction does not‘recover’ much of the radiated energy Jet spectrum in Pb+Pb: charged particle jets Two cone radii, 4 centralities

23. Pb+Pb jet RAA Jet RAA measured byATLAS, ALICE, CMS Good agreementbetween experiments Despite different methods: ATLAS+CMS: hadron+EM jets ALICE: charged track jets RAA < 1: 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. g, hadrons, jets compared Jets g, hadrons Suppression of hadron (leading fragment) and jet yield similar Is this ‘natural’? No effect of in-cone radiation?

25. Model comparison Jet RAA Hadron RAA U. Wiedemann@QM2012 JEWEL: K. Zapp et al, Eur Phys J C69, 617 Schukraft et al, arXiv:1202.3233 M. Verweij@HP, QM2012 At least one model calculation reproduces the observed suppression  Understand mechanism for out-of-cone radiation?

26. Model comparison IAA JEWEL: Zapp et al., EPJ C69, 617 JEWEL preliminary JEWEL correctly describes inclusive jet RAA Predicts ΔIAA~0.4, below measured

27. Hadron trigger vs jet trigger • Hadron trigger: strong “surface bias” • maximizes recoil path length • (T.Renk, private com.) Hadron trigger Jet trigger • Full jet trigger: no geom. bias • partially cancelled by bkg fluctuations T.Renk, PRC85 064908 • Centrality and reaction plane biases: • finite, but only weak trigger pT dependence for high pTtrig

28. Jet broadening: R dependence Ratio of spectra with different R Larger jet cone: ‘catch’ more radiation  Jet broadening ATLAS, A. Angerami, QM2012 However, R = 0.5 still has RAA < 1 – Hard to see/measure the radiated energy

29. Jet broadening: transverse fragment distributions Pb Pb Pb Pb CMS, P. Kurt@QM12 CMS PAS HIN-12-013 Jet broadening: Soft radiation at large angles

30. Jet fragment distributions PbPb measurement Ratio to pp ATLAS M.Rybar@QM12 M. Rybar@QM2012 Low pT enhancement: soft radiation Intermediate z: depletion: E-loss NB: z is wrt observedEjet≠ initial Eparton

31. Jet fragment distributions CMS, Frank Ma@QM12 Low pT enhancement: soft radiation Intermediate z, pT: depletion: E-loss

32. A consistent view of jet quenching G. Roland@QM2012 arXiv:1205.5872 2010 data: arXiv:1205.5872 PbPb – pp (1/GeV) Change from “ξ” to “pT” No change at small r, high pT Narrowing/depletion at intermediate r, pT • Consistent with 2010 result • Recall (2010 vs 2011): • Track pT > 4 GeVvspT> 1 GeV • Leading vs inclusive jet • 0-30% vs 0-10% and 10-30% Broadening/excess at large r, low pT (~2% of jet energy) Radius r

33. A consistent view of jet quenching G. Roland@QM2012 Looking at the same parton pT range Charged particles from pT =50-100 GeV: z = pT(track)/pT(jet) = 0.4-0.6 x < 1 PbPb fragmentation function = pp for ξ <1 Consistent message from charged hadron RAA, inclusive jet RAA and fragmentation functions!

34. Direct photons

35. Early times: direct photons • Thermal photons • Low-Q2 scatterings in HG, QGP • Thermal glow of hot matter • Photons from initial scattering • Dominant at pT > few GeV • Small reinteraction prob

36. Direct photons at RHIC Idea: hot quark-gluon matter radiates photons which escape PHENIX, PRL 104, 132301 • Difficult measurement: • Large background p0→gg • Thermal photons at low pT Excess of photons seen at RHIC

37. Direct (thermal) photons at LHC First measurement of low-pT (thermal) direct photons at LHC M. Wilde, ALICE, QM12 • Method: • Measure g/p0 • Low pT: use conversions • Divide g/p0 by theory expectation pp: g/p0 agrees with NLO PbPb: excess over NLO at low pT Multiply by p0 to get g spectrum Also at LHC: low pT thermal photons T ≈ 300 MeV

38. Summary • Jets: a new tool for parton energy loss measurements • Large out-of-cone radiation (R = 0.2-0.4) • Energy asymmetry • RAA < 1 • IAA < 1 • Radial shapes • Remaining jet is pp-like: • Fragment distribution at large z same as pp • RAA similar for jets and hadrons • Most of the radition is at low pT • Scale set by medium temperature? • Direct photons • High pT: photon is a parton • Low pT: thermal (?) radiation in Pb+Pb Most conclusions here are qualitative/phenomenology What does this (quantitatively?) mean for the mechanism of energy loss?

39. Jets at LHC ALICE Large jet energies clearly visible above HI background

40. Jet imbalance calculations Coleman-Smith, Qin, Bass, Muller, arXiv:1108.5662 Qin, Muller, arXiv:1012.5280 Parton transport (brick) Radiation plus evolution Several calculations describemeasured imbalance Young, Schenke, Jeon, Gale, arXiv:1103.5769 Need to keep track of fragments; Leading particle approximations do not work Cannot yet check consistency with leading hadrons… Most natural approach: parton showers (MARTINI, qPYTHIA, qHERWIG, JEWEL) MARTINI: AMY+MC