Jet reconstruction in heavy ion collisions: status and issues Peter Jacobs Lawrence Berkeley National Laboratory 11/17/20081 Jets in Heavy Ion Collisions

Jet quenching: iconic plots 11/17/2008Jets in Heavy Ion Collisions2 p T trigger >8 GeV/c Yield per trigger STAR, PRL 97, (2006) Strong, experimentally robust signals in multiple measurement channels Systematic uncertainties are well-controlled and small → Jet quenching is fully established → Awaits similarly accurate theoretical interpretation

So what is missing? 11/17/2008 Jets in Heavy Ion Collisions trigger recoil How to do better? Full jet reconstruction High p T (leading) hadrons bias towards jets that have not interacted indirect measurement of jet quenching limited sensitivity to dynamics and modification of jet structure Jet probes of the QGP3 Recover full energy/momentum flow → unbiased view of quenching New class of observables much greater kinematic reach than other channels (esp.  +jet) But how to beat the background problem…?

Broad jet kinematic reach: new probes? 4 Casalderrey-Solana and Wang arXiv: e.g. XN Wang and J Casalderay-Solano: think about jet quenching like Deep Inelastic Scattering jet QGP small x large x x = partonic momentum fraction  jet energy variation probes QGP structure at varying length scales… Urgently needs additional theory input…

5 One way measure jets in HIC: LHC Pb+Pb at 5.5 TeV P. Jacobs and M. van Leeuwen Nucl. Phys A774, 237 (2006) 11/17/2008Jets in Heavy Ion Collisions

6 Another way: full jet reconstruction at RHIC STAR Phys. Rev. Lett. 97 (2006) STAR 200 GeV p+p (‘03 and ‘04 runs) : int lumi = 0.2 nb -1 measured inclusive jet spectrum: statistical reach >40 GeV STAR 200 GeV Au+Au (’07 run) “p+p equivalent” int lumi ~5 pb -1 → jets should be there!! Normalization uncertainty ~50% 11/17/2008Jets in Heavy Ion Collisions

Jets in STAR Au+Au 200 GeV data 11/17/2008Jets in Heavy Ion Collisions7 STAR Preliminary Au+Au Central   Au+Au central; E T ~47 GeV Au+Au central; E T ~21GeV → jets should be there…and indeed they are

8 Jet reconstruction algorithms used in the STAR analysis Cone algorithms – Leading Order High Seed Cone (LOHSC)‏ – Mid Point Cone (Used in p+p only) Merging & Splitting Sequential recombination algorithms – K T – Cambridge/ Aachen Jet Fragmentation process Hard scatter Cone jet K T jet STAR p+p Phys. Rev. Lett. 97 (2006) M. Cacciari, G. Salam, G. Soyez JHEP 0804:005 (2008) 11/17/2008Jets in Heavy Ion Collisions

k t algorithm → soft radiation (k t ~0) always clusters first… Sequential recombination algorithms Cambridge/Aachen algorithm → no dependence on k t so clustering sequence depends on relative differences 11/17/20089Jets in Heavy Ion Collisions Each algorithm has a jet “resolution parameter” R ~ cone radius

The FastJet Algorithms 11/17/2008Jets in Heavy Ion Collisions10 Cacciari, Salam, Soyez ‘08 Suite of modern Colinear-safe and InfRed-safe jet algorithms seq recomb: k T, Cambridge/Aachen, anti-k T cone: SISCone (Seedless InfRed-safe Cone) Two important algorithmic advances: 1.Numerical tricks → large improvements to processing time vs. event multiplicity → k T was previously unusable at hadron colliders 2.Completely general definition of jet area enables much more precise subtraction of diffuse event background

A = jet area ρ = area density of noise  = noise fluctuations (all three are measurable quantities!) Modified jet clustering response due to bkgd (NLO accuracy) Jet Area and Background Corrections 11/17/2008Jets in Heavy Ion Collisions11 M. Cacciari and G. Salam, Phys. Lett. B659, 119 (2008) Jets are irregular objects Jet area is non-trivial (≠ πR 2 ) for any algorithm beyond simple LO cone

FastJet measurement of Jet Area 11/17/2008Jets in Heavy Ion Collisions12 M. Cacciari, G. Salam, G. Soyez JHEP 0804:005 (2008) For any infra-red safe jet algorithm: The addition of an arbitrary number of low energy “ghost” particles (E<< QCD ) will not alter the structure of a high energy jet 1.Add randomly distributed ghost particles of known density d to the event 2.Run the jet algorithm 3.Count the number n of ghost particles assigned to the jet 4.Jet area A= n/d Accounts for event-wise fluctuations in shape and area of jet → much improved diffuse background correction

LHC example: Z’→dijets in 14 TeV p+p collisions at high luminosity 11/17/2008Jets in Heavy Ion Collisions13 L=10 34 /cm 2 /s ~20 minbias p+p collisions per LHC bunch crossing Calorimetric jet measurements cannot disentangle them →pileup Compare blue and black: FastJet pileup correction is essential to recover signal

14 Unbiased jet reconstruction in heavy ions? Check the cross-section Phys. Rev. Lett. 97 (2006) Jet energy and momentum flow are conserved, regardless of the change (softening) in jet structure due to quenching Unbiased reconstruction: jet energy flow is recovered accurately for all jets Analogy to (non- interacting) photons: Au+Au yield/evt equals N binary -scaled p+p cross section 11/17/2008Jets in Heavy Ion Collisions

15 Check the cross-section: p+p reference spectrum Unbiased jet reconstruction requires: calculable (±5%) Measure in STAR Au+Au data STAR Phys. Rev. Lett. 97 (2006) This is our tool of choice to measure jet reconstruction biases… 11/17/2008Jets in Heavy Ion Collisions

16 Cross section compared to binary-scaled p+p: Seq Recomb algorithms, negligible p T cut E T [GeV] dN Jet /dE T (per event)‏ E T [GeV] dN Jet /dE T (per event)‏ Au+Au 0-10% STAR Preliminary R=0.4 p T cut =0.1 GeV KT Statistical Errors Only Au+Au 0-10% STAR Preliminary R=0.4 p T cut =0.1 GeV CAMB Statistical Errors Only N bin scaled p+p  MB-Trig O HT-Trig  MB-Trig O HT-Trig S. Salur, Hard Probes 2008 Agreement (~factor 2) with binary-scaled p+p seedless algorithms minimal p T cut bias (p T >100 MeV) spectrum correction factors /17/2008Jets in Heavy Ion Collisions

Suppression of backgrounds in heavy ions…? 11/17/2008Jets in Heavy Ion Collisions17 STAR Preliminary Au+Au Central   J R p T cut Limit jet resolution parameter R and track/calorimeter p T that contributes to jet signal But will bias jet measurement- what happens in practice?

Bias due to p T cut 11/17/2008Jets in Heavy Ion Collisions18 S.Salur, HP08 p T cut >0.1 GeV p T cut >1.0 GeVp T cut >2.0 GeV E T (GeV) STAR Preliminary Au+Au Central   J p T cut

Fake jets? 11/17/2008Jets in Heavy Ion Collisions19 trigger recoil ? Acoplanarity Energy balance LHC simulations suggest large “fake jet” rates due to incoherent HI bkgd fluctuatons STAR: Dijet correlations indicate that this is small to negligible at RHIC

Some shortcuts in current STAR analysis and how to fix them 11/17/2008Jets in Heavy Ion Collisions20 Main issues are background corrections and energy resolution Current spectrum corrections based on “embedded PYTHIA” wrong but done deliberately for a first shot may not be so wrong in practice if radiation is ~colinear Fixes in progress: New PYTHIA-based MCs incorporating quenching (JEWEL, qPYTHIA) exploit coincidences for “data-driven” corrections: compare and contrast p+p, d+Au, Au+Au di+jets, hadron+jet: acoplanarity, energy balance, recoil spectrum conditional yields gamma+jet not as helpful: insufficient kinematic reach ;-(

11/17/2008Jets in Heavy Ion Collisions21 STAR Preliminary Au+Au Central   J Backreaction from HI background  = Diffuse noise,  = noise fluctuations

Back-reaction and vs jet algorithm 11/17/2008Jets in Heavy Ion Collisions22 Cacciari, Salam, Soyez: Matrix element for emission of soft perturbative gluon p T2 at angle  12 Mean shift in jet energy due to back-reaction from background density ρ= : an opportunity?

Back-reaction and vs jet algorithm (cont’d) 11/17/2008Jets in Heavy Ion Collisions23 Can we put this differential back-reaction sensitivity to use? → use different algorithms to assess systematics of background correction How well controlled theoretically? (above is simplified NLO estimate) Experimentally: compare E T (antikT) vs. E T (kT) vs. E T (SISCone)… R, p T dependence… → how constraining in practice? Experimental studies in progress; need additional theory guidance

So where is the jet quenching? 11/17/2008Jets in Heavy Ion Collisions24  =ln( E Jet / p hadron ) p T hadron ~2 GeV Jet quenching Borghini and Wiedemann ‘06: look at modified Fragmentation Function Modified Leading Log Approx (MLLA): good description of semi-incl hadron distribution in jets → medium-induced modifications?

Compare p+p and Au+Au FragFns 11/17/2008Jets in Heavy Ion Collisions25 No evidence of quenching: but HT-trig dataset requires high p T   → biased jet sample fully consistent with current understanding of quenching redo analysis with MinBias triggered dataset → in progress J. Putschke, HP08

Fragmentation Function: how useful in practice? 11/17/2008Jets in Heavy Ion Collisions26 JEWEL (K. Zapp et al.) arXiv: String fragmentation: significant hadronization corrections MLLA: precise representation of fragmentation to low momentum Is the Fragmentation Function a robust observable for quenching?

More robust observable? subjet distributions 11/17/2008Jets in Heavy Ion Collisions27 Cone jet K T jet K. Zapp et al., arXiv: R large R small 3 jets 2 jets 1 jet R dial jet resolution parameter R to small values count number of “subjets” – tracks hard radiation in fragmentation LEP data well understood in pQCD → jet quenching effects?

Conclusions 11/17/2008Jets in Heavy Ion Collisions28

11/17/2008Jets in Heavy Ion Collisions29 Extra slides

What really happens in hadronic collisions… 30 Beam Remnants Beam Remnants p = (uud) p = ,K,p,n,…} Jet Initial State Radiation (ISR) Hadronization Final State Radiation (FSR) Detector 11/17/2008Jets in Heavy Ion Collisions

11/17/2008Jets in Heavy Ion Collisions31

11/17/2008Jets in Heavy Ion Collisions32

33 Cross section compared to binary-scaled p+p: seeded cone algorithm, moderate p T cut E T [GeV] dN Jet /dE T (per event)‏ N bin scaled p+p Au+Au 0-10%  MB-Trig O HT-Trig R=0.4 p T cut =1 GeV Seed=4.6 GeV LOHSC Statistical Errors Only S. Salur, Hard Probes 2008 Star Preliminary Inclusive spectrum correction based on PYTHIA fragmentation MB-Trig: agreement within errors with binary-scaled p+p → unbiased jet reconstruction? HT-Trig: hardware cluster trigger (requirement of ~7.5 GeV) in EMC → large trigger bias persists beyond 30 GeV MB-Trig is essential for unbiased measurement factor ~20 more on tape than shown here 11/17/2008Jets in Heavy Ion Collisions

First look at FragFn in STAR Au+Au data 11/17/2008Jets in Heavy Ion Collisions34 J. Putshcke, HP08 Uses HT-trig dataset (which we know is biased…)