High-pT probes of QCD matter Marco van Leeuwen, Utrecht University

Plan of Lecture IV Yesterday’s summary: Today’s lecture: Best achievable goal: determine P(DE) experimentally (Or at least some features of it) Difficult in practice: RAA (at RHIC) not sensitive IAA limited sensitivity (fragmentation bias) Today’s lecture: New approaches to reduce fragmentation bias Multi-hadron correlations g-jet measurements at RHIC and LHC Jet measurements at LHC Mix of existing results and ideas for future

pQCD illustrated fragmentation jet spectrum ~ parton spectrum CDF, PRD75, 092006

Naive picture for di-hadron measurements Fragment distribution (fragmentation fuction) Out-of-cone radiation: PT,jet2 < PT,jet1 In-cone radiation: PT,jet2 = pT,jet1 Softer fragmentation Ref: no Eloss PT,jet,1 PT,jet,2 Naive assumption for di-hadrons: pT,trig measures PT,jet So, zT=pT,assoc/pT,trig measures z

Fragmentation bias in di-hadrons Test case: Calculate away-side spectra Use two different frag functions LEP: Quarks: D(z) ~ exp(-8.2 z) Gluons: D(z) ~ exp(-11.4 z) pTa pTt Away-side spectra not sensitive to slope of fragmentation function More detailed analysis shows: mainly depends on power n of partons spectrum

Di-jet triggered analysis pTa1 pTt1 pTt2 T1T2 correlation Raw, uncorrected signal Df -1 -2 1 2 3 4 5 a.u. 0.6 0.4 0.2 O. Barannikova, F. Wang, QM08 Idea: use back-to-back hadron pair to trigger on di-jet and study assoc yield T1: pT>5 GeV/c T2: pT>4 GeV/c p ± 0.2 Tune/control fragmentation bias and possibly geometry/energy loss bias di-hadron trigger pairs contain combinatorial background

3-hadron analysis backgrounds T1: pT>5GeV/c, T2: pT>4GeV/c, A: pT>1.5GeV/c Signal + Background Background Signal a.u. T1T2 2 4 Df (T1T2) ZDC central Au+Au 12% O. Barannikova, F. Wang, QM08 Df (T2A1) -1 -2 1 2 3 4 5 1 _dN_ Ntrig d(Df ) STAR Preliminary T2A1_T1 flow subt. T2A1 T1A1 pTa1 pTt1 pTt2 Subtract two combinatorial terms: random T1, random T2

Di-jet triggered in d+Au T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c O. Barannikova, F. Wang, QM08 1 _dN_ Ntrig d(Df ) T2A1_T1 T2A1 d+Au 200 GeV Di-jet trigger pTa1 pTt1 pTt2 2 1 STAR Preliminary -1 -2 1 2 3 4 5 Df (T2A1) Requiring away-side trigger increases yield Fragmentation bias changes: higher Q2 Single hadron trigger pTa1 pTt1

Di-jet triggers in Au+Au T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c Di-jet trigger 1 _dN_ Ntrig d(Df ) 4 pTa1 pTt1 pTt2 central 0-12% T2A1_T1 T2A1 2 Single hadron trigger pTa1 pTt1 STAR Preliminary -2 Df -1 -2 1 2 3 4 5 Single trigger: broad away-side Di-jet trigger: jet peaks on both near and away side Di-jet trigger selects different events, has different bias

Au+Au vs d+Au comparison T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c T1A1_T2 T2A1_T1 Df -1 -2 1 2 3 4 5 1 _dN_ Ntrig d(Df ) STAR Preliminary 200 GeV Au+Au, 12% central Au+Au d+Au Df -1 -2 1 2 3 4 5 1 _dN_ Ntrig d(Df ) STAR Preliminary 200 GeV Au+Au & d+Au Distributions wrt to both triggers similar Au+Au similar to d+Au Di-jet trigger selects jet pairs with little or no energy loss

Di-jet trigger model calculation Renk, Phys. Rev. C 75, 054910 (2007) <DE> for back-to-back jets T1 Thorsten Renk, private comm. 2 density models T2 The energy deposition on the near side is smaller than 0.6 GeV in all cases (this has to do with the fact that there's a bias for a quark leading to the most energetic hadron due to the harder fragmentation function, which maps into another bias of having preferentially a gluon on the away side from pQCD, which maps into yet another bis towards smaller energy loss on the near side due to the different color charge of the quark). If one compares the numbers to a situation where only the near side hadron is triggered (say in the 6-8 GeV momentum range), the expected energy deposition on the away side is about 10 GeV, i.e. any signal of a cone structure should be stronger by a factor 5 or so. Model agrees: ptT1 ~ ptT2 reduces energy loss Next step: increase ptT1 - ptT2

Multi-hadron cluster triggers Idea: Reduce fragmentation bias by clustering hadrons ‘proto-jet’ Away-side spectrum 0-12% Au+Au Add 12-15 GeV trigger B. Haag, QM08 Multi-hadron trigger STAR Preliminary R Seed Secondary Seeds Associated track Use cluster energy for trigger: - R = 0.3 - pT,seed > 5 GeV - pT,sec seed > 3 GeV Single-hadron and multi-hadron triggers give similar result Fragmentation bias does not change? - Needs further study

Probing the photon energy with g-jet events T. Renk, PRC74, 034906 Nuclear modification factor Away-side spectra in g-jet Eg = 15 GeV RAA insensitive to P(DE)  Away-side spectra for g-jet are sensitive to P(DE) g-jet: know jet energy  sensitive to P(DE)

g-jet in Au+Au Use shower shape in EMCal to form p0 sample and g-rich sample Combinatorial subtraction to obtain direct-g sample

Away-side suppression with direct-g triggers A. Hamed et al QM08 First g-jet results from heavy ion collisions are becoming available* Measured suppression agrees with theory expectations Next step: measure pTassoc dependence to probe DE distribution * both PHENIX and STAR

Large Hadron Collider at CERN CMS 2008: p+p collisions @ 14 TeV 2009: Pb+Pb collisions @ 5.5 TeV ALICE ATLAS 3 Large general purpose detectors ALICE dedicated to Heavy Ion Physics, PID p,K, out to pT > 10 GeV ATLAS, CMS: large acceptance, EM+hadronic calorimetry

From RHIC to LHC Most abundant probe: jets, light hadrons RHIC: s=200 GeV Au+Au LHC: s=5.5 TeV Pb+Pb Pion spectra Hard process yields much larger at LHC 10k/year (orders of magnitude at high-pT) Most abundant probe: jets, light hadrons Robust yields to pT>200 GeV for jets Larger initial density  Validate understanding of RHIC data

RAA at LHC GLV BDMPS T. Renk, QM2006 RHIC RHIC S. Wicks, W. Horowitz, QM2006 LHC: typical parton energy > typical E Expected rise of RAA with pT depends on energy loss formalism Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)

Medium modification of fragmentation MLLA calculation: good approximation for soft fragmentation extended with ad-hoc implementation medium modifications Borghini and Wiedemann, hep-ph/0506218 pThadron ~2 GeV for Ejet=100 GeV =ln(EJet/phadron) z 0.37 0.14 0.05 0.02 0.007 Trends intuitive: suppression at high z, enhancement at low z Recent progress: showering with medium-modified Sudakov factors, see Carlos’s talk and arXiv:0710.3073

Jet modifications at LHC Jet reconstruction Expectations from QCD+jet quenching PQM with fragmentation of radiated gluons (A. Morsch) Fragmentation function Radial profile Ejet = 125 GeV Energy loss depletes high-z and populates low-z Low-z fragments from gluon radiation at large R z 0.37 0.14 0.05 0.02 0.007 =ln(EJet/phadron)‏ In-medium energy loss redistributes momenta in jets Model has the main phenomenology included; use as benchmark

Need for a calorimeter Jet energy response ALICE PPR, part II 100 GeV Jets R=0.4 Note: tail due to jet-splitting Charged particle tracking only sees ~50 % of jet energy TPC+EMCal recovers large fraction of jet energy Moreover: EMCal provides important trigger capability

Improves jet energy resolution ALICE EMCal US-France-Italy project ALICE-EMCal project: Approved in 2007 Full detector by 2011 EMCal module Testbeam: Support frame installed Lead-scintillator sampling calorimeter ||<0.7, =110o ~13k towers (x~0.014x0.014)‏ Improves jet energy resolution Provides jet triggers

Jets in heavy ion events Simulation: 100 GeV jet in Hijing Finding jets is relatively easy Energy (GeV) Challenges: Measuring the energy in presence of cuts to reduce background Reconstructing lower energy jets, jets with softer fragmentation

Reducing the background Radial distribution of jet energy Charged background energy fluctuations PYTHIA HERWIG pTcharged>5 GeV pTcharged>30 GeV 80% no pt -cut pt > 2 GeV/c saturation model scaling (Eskola et al, hep-ph/0506049) CDF, PRD65, 092002 (2002) CDF: ~80% of jet energy contained in R < 0.2 Use smaller cone size, e.g. R=0.4 to reduce bkg Apply pT-cut to reduce background Disadvantage: Not infrared safe Background from 5.5 TeV Pb+Pb: dET/dh ~ 3700 GeV, ET(R<0.2) ~ 75 GeV

Small cones: split jets Reconstructed energy Split jet fraction # Jets all particles, R=0.3, pT > 2GeV R=0.3, pt>2GeV all particles charged+pi0 charged input - Njets,rec=1 - Njets,rec>=1 highest jet Njets,rec>=1 mid-cone - Njets,rec>=1 sum Fraction of events Njets,rec.>1 Jet Energy (GeV) Jet Energy (GeV) Jet-splitting affects energy recnstruction Jet shape fluctuate; need stable algorithm (infrared safe?) Possible solutions: SISCone, anti-kT Still lots of room to improve jet-finding in heavy ions (if you’re interested, let me know)

Full jet reconstruction performance Simulation input Simulated result reference Medium modified (APQ)‏ Full jet reco in ALICE is sensitive to modification of fragmentation function E > E, explore dynamics of energy loss process

Under Study: Di-jet Correlations Acoplanarity pp PbPb: PYQUEN T = 1 GeV charged jets J. Casalderrey-Solana and XNW, arXiv:0705.1352 [hep-ph]. BDMPS Effect seems to be measurable, but large effect from initial state radiation

g-jet rates at RHIC and LHC g, p0 rates Jacak, Vogelsang, QM06 gdirect/gdecay small at LHC: ~10% at 100 GeV Challenging measurement: g/p < 0.1 in accessible kinematic region

Identifying prompt g in ALICE Backgrounds are large, need isolation cut pp R = 0.3, SpT < 2 GeV/c Efficiency: 69% Background rejection: 1/170 PbPb R = 0.2, pTthresh = 2 GeV/c Efficiency: 50% Background rejection: 1/14  5 signal

Make sure you don’t miss out! Summary/conclusions Goal of high-pT measurements (my opinion) Measure/constrain P(DE) Detemine medium properties Dominant limitations of current measurements: Fragmentation bias: single, di-hadron not very sensitive to P(DE) At RHIC: DE ~ Ejet Promising new measurements: g-jet at RHIC and LHC jet reconstruction at LHC (maybe also at RHIC) Experimental methods and theory understanding are under continuous development Make sure you don’t miss out!

Heavy-ion backgrounds Background energy fluctuations Jet cone energy pT > 2 GeV ALICE PPR Vol II 100 GeV 50 GeV Dominant source: Impact parameter fluctuations already taken out for these plots 50 GeV jets: need to restrict cone size >100 GeV jets: can use larger size (R=0.5-1) Note: also here large uncertainty: multiplicity and mean-pT can only be guessed

Getting at the jet energy Simulations: Pythia 6.319 (CDF tune A) ‘Ideal’ jets reconstructed using all final state particles Cone algorithm R=1 Ejet = SpT Jet energy contributions: 65% in charged particles 20% EM (mostly p0) Small contributions from K0L , neutrons, etc (approx -1 leading particle) + Long tails Leading particle ~15 %

Jets on a steep spectrum Leading particle: mostly lower limit on jet energy (+finite ‘efficiency’ at larger pT) Charged jets: ‘sharp’ turn-on. Still mainly lower cut at RHIC Charged+EM selects narrow range in E-jet at RHIC & LHC Caveat: energy loss may transport energy outside cuts (cone, pT)