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High-p T probes of QCD matter. Marco van Leeuwen, Utrecht University. Plan of Lecture IV. Yesterday’s summary:. Best achievable goal: determine P( D E) experimentally. (Or at least some features of it). Difficult in practice: R AA (at RHIC) not sensitive
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High-pT probes of QCD matter Marco van Leeuwen, Utrecht University
Plan of Lecture IV Yesterday’s summary: 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 Out-of-cone radiation: PT,jet2 < PT,jet1 In-cone radiation: PT,jet2 = pT,jet1 Softer fragmentation Fragment distribution (fragmentation fuction) 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 pTt1 pTa1 a.u. Raw, uncorrected signal 0.6 0.4 pTt2 0.2 -2 -1 2 0 1 3 4 5 Df T1T2 correlation 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 biasand possibly geometry/energy loss bias di-hadron trigger pairs contain combinatorial background
3-hadron analysis backgrounds ZDC central Au+Au 12% a.u. T1T2 Signal + Background Background Signal 0 4 2 Df (T1T2) 2 1 _dN_ Ntrigd(Df ) -2 -1 2 0 1 3 4 5 T2A1_T1 flow subt. pTt1 T1A1 T2A1 1 pTa1 0 pTt2 STAR Preliminary Df (T2A1) T1: pT>5GeV/c, T2: pT>4GeV/c, A: pT>1.5GeV/c O. Barannikova, F. Wang, QM08 Subtract two combinatorial terms: random T1, random T2
Di-jet triggered in d+Au d+Au 200 GeV -2 -1 2 0 1 3 4 5 pTt1 pTa1 pTt1 pTa1 pTt2 T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c O. Barannikova, F. Wang, QM08 1 _dN_ Ntrigd(Df ) T2A1_T1 T2A1 Di-jet trigger 2 1 0 STAR Preliminary Df (T2A1) Requiring away-side trigger increases yield Fragmentation bias changes: higher Q2 Single hadron trigger
Di-jet triggers in Au+Au Single trigger: broad away-side Di-jet trigger: jet peaks on both near and away side -2 -1 2 0 1 3 4 5 pTt1 pTa1 pTt1 pTa1 pTt2 Df T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c Di-jet trigger 1 _dN_ Ntrigd(Df ) 4 central 0-12% T2A1_T1 T2A1 2 0 Single hadron trigger STAR Preliminary -2 Di-jet trigger selects different events, has different bias
Au+Au vs d+Au comparison 200 GeV Au+Au, 12% central 1 _dN_ Ntrigd(Df ) STAR Preliminary 200 GeV Au+Au & d+Au T1A1_T2 T2A1_T1 1 _dN_ Ntrigd(Df ) STAR Preliminary Au+Au d+Au 2 3 2 0 1 0 -2 -2 -1 -1 2 2 0 0 1 1 3 3 4 4 5 5 Df Df T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c 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 Model agrees: ptT1 ~ ptT2 reduces energy loss Next step: increaseptT1 - ptT2
Multi-hadron cluster triggers R Seed Associated track Secondary Seeds 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 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 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-zand populates low-z Low-z fragments from gluon radiation at large R =ln(EJet/phadron) z 0.37 0.14 0.05 0.02 0.007 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
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 80% PYTHIA pTcharged>30 GeV HERWIG pTcharged>5 GeV 80% Radial distribution of jet energy Charged background energy fluctuations 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 Apply pT-cut to reduce background Use smaller cone size, e.g. R=0.4 to reduce bkg 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 all particles, R=0.3, pT > 2GeV # Jets 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 reference Simulated result 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 signal 5 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
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 reconstructedusing 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)