Hard Probes of the Quark Gluon Plasma Lecture III: jets Lectures at: Quark Gluon Plasma and Heavy Ion Collisions Siena, 8-13 July 2013 Marco van Leeuwen, Nikhef and Utrecht University

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

3 Jets at LHC ALICE   Transverse energy map of 1 event 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 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 ,  jet = particles For a complete discussion, see: 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

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

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

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

9 Jet energy asymmetry Centrality ATLAS, arXiv: (PRL) Jet-energy asymmetry Large asymmetry seen for central events 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 Suggests large energy loss: many GeV ~ compatible with expectations from RHIC+theory

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

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

12  -jet imbalance CMS, arXiv: Centrality  -jet asymmetry Advantage:  is a parton: know parton kinematics Disadvantage: low rate (+background  0 →  )  Dominant contibution: qg → q 

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

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

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

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

17 Hadrons vs jets II: recoil PRL HadronsJets 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 !

18 Measuring the jet spectrum

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

20 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

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

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

23 Pb+Pb jet R AA Jet R AA measured by ATLAS, ALICE, CMS R AA < 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 ATLAS+CMS: hadron+EM jets ALICE: charged track jets Good agreement between experiments Despite different methods:

24 , hadrons, jets compared , hadrons Jets Suppression of hadron (leading fragment) and jet yield similar Is this ‘natural’? No effect of in-cone radiation?

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

26 Predicts ΔI AA ~0.4, below measured JEWEL preliminary Model comparison I AA JEWEL: Zapp et al., EPJ C69, 617 JEWEL correctly describes inclusive jet R AA

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

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 R AA < 1 – Hard to see/measure the radiated energy

29 Jet broadening: transverse fragment distributions PbPb PbPb CMS PAS HIN CMS, P. Jet broadening: Soft radiation at large angles

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

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

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

33 A consistent view of jet quenching Charged particles from p T = GeV: z = p T (track)/p T (jet) =  < 1 Looking at the same parton p T range Consistent message from charged hadron R AA, inclusive jet R AA and fragmentation functions! PbPb fragmentation function = pp for ξ <1 G.

34 Direct photons

35 Early times: direct photons Photons from initial scattering Dominant at p T > few GeV Small reinteraction prob Thermal photons Low-Q 2 scatterings in HG, QGP Thermal glow of hot matter

36 Direct photons at RHIC PHENIX, PRL 104, Idea: hot quark-gluon matter radiates photons which escape Difficult measurement: Large background  0 →  Thermal photons at low p T Excess of photons seen at RHIC

37 Direct (thermal) photons at LHC Method: Measure  /  0 Low p T : use conversions Divide  /  0 by theory expectation pp:  /  0 agrees with NLO PbPb: excess over NLO at low p T Multiply by  0 to get  spectrum M. Wilde, ALICE, QM12 Also at LHC: low p T thermal photons T ≈ 300 MeV First measurement of low-p T (thermal) direct photons at LHC

38 Summary Jets: a new tool for parton energy loss measurements –Large out-of-cone radiation (R = ) Energy asymmetry R AA < 1 I AA < 1 Radial shapes –Remaining jet is pp-like: Fragment distribution at large z same as pp R AA similar for jets and hadrons –Most of the radition is at low p T Scale set by medium temperature? Direct photons –High p T : photon is a parton –Low p T : 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 Qin, Muller, arXiv: Radiation plus evolution Parton transport (brick) Coleman-Smith, Qin, Bass, Muller, arXiv: MARTINI: AMY+MC Young, Schenke, Jeon, Gale, arXiv: Several calculations describe measured imbalance Most natural approach: parton showers (MARTINI, qPYTHIA, qHERWIG, JEWEL) Need to keep track of fragments; Leading particle approximations do not work Cannot yet check consistency with leading hadrons…