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Jet quenching at RHIC and the LHC

Peter Jacobs, LBNL. Jet quenching at RHIC and the LHC. Radiative energy loss. BDMPS transport coefficient:. Energy loss:. D E~ L 2 D E linearly dependent on color charge C R D E ~independent of partonic energy E. At most: logarithmic dependence of D E on E

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Jet quenching at RHIC and the LHC

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  1. Peter Jacobs, LBNL Jet quenching at RHIC and the LHC

  2. Radiative energy loss BDMPS transport coefficient: Energy loss: • DE~L2 • DE linearly dependent on color charge CR • DE ~independent of partonic energy E • At most: logarithmic dependence of DE on E •  need logarithmically large variation of parton (jet) energy to see its evolution

  3. D. d’Enterria STAR, Phys Rev Lett 91, 072304 Jet quenching at RHIC… Medium-modified fragmentation?

  4. pTassoc > 0.15 GeV cos(Df) STAR, Phys Rev Lett 95, 152301 Response of medium to lost energy? 4< pTtrig < 6 GeV pTassoc > 2 GeV Near-side ridge correlated with jets? STAR, Phys Rev Lett 91, 072304 • High momentum recoil suppressed  low momentumenhanced • Recoil distribution soft and broad ~ thermalized? angular substructure?? • Qualitative picture consistent with jet quenching •  quantitative study of dynamics at low pT?

  5. STAR preliminary 8 < pT(trig) < 15 GeV/c Di-hadrons at yet higher pT • Away-side yield is suppressed but finite and measurable  set upper bound on energy loss? • Suppression without angular broadening or modification of high z fragmentation: why?

  6. A. Dainese et al, hep-ph/0511045 ? Inclusive hadrons: surface bias distance to origin Dihadrons: tangential dominates Dihadrons: ~volume emission? T. Renk, hep-ph/0602045 angle wrt ray to origin trigger direction High pT di-hadrons and geometric bias Where are the surviving pairs generated? SW quenching weights+geometry+dynamics

  7. Jet quenching at RHIC: summary • Jets are quenched in very dense matter: unique probes of the medium • But current picture is largelyqualitative: • leading hadrons:fragmentation and geometric biases • pT ~2-5 GeV/c: baryon/meson anomaly not fully understood • no direct evidence yet for radiative energy loss • where is the radiation? is it also quenched in the medium? • color charge, quark mass, length dependence? • role of collisional energy loss? • response of medium to lost energy? • Future RHIC measurements: new instrumentation and larger datasets • Jet studies at the LHC complement and greatly extend the RHIC measurements

  8. Large Hadron Collider at CERN mid-late 2007: commission 14 TeV p+p end 2008: first long 5.5 TeV Pb+Pb run heavy ion running: 4 physics weeks/year

  9. (h++h-)/2 p0 √s = 5500 GeV 200 GeV LO p+p y=0 17 GeV LHC RHIC SPS From RHIC to the LHC… • Heavy ions at LHC: • hard scattering at low x dominates particle production • low x: calculable (CGC) initial conditions? • fireball hotter and denser, lifetime longer than at RHIC • dynamics dominated by partonic degrees of freedom • huge increase in yield of hard probes

  10. I. Vitev and M. Gyulassy, PRL 89, 252301(2002) A. Dianese et al., Eur.Phys.J. C38, 461(2005) RHIC vs LHC First jet quenching measurement at the LHC: inclusive hadron suppression Initial gluon density atLHC~ 5-10 x RHIC: • But no dramatic effects: RAA (LHC) ~ 0.1-0.2 ~ RAA(RHIC): • inclusive hadrons have limited sensitivity to initial density •  measure jet structure

  11. The jet landscape for 5.5 TeV Pb+Pb collisions Inclusive jet rates very high g+jet, Z+jet: precision measurements, but cover only limited dynamic range  study of the evolution of jet quenching must utilize inclusive jet and multi-jet measurements

  12. Jet measurements for LHC heavy ion collisions • High energy jets: fully reconstructable without fragmentation bias(?) unbiased jet population  comprehensive study of energy loss (contrast leading particle biases) Large kinematic reach  evolution of energy loss • New channels: heavy quark jets at high ET, multi-jet events, Z+jet, very hard di-hadrons,… Color charge, quark mass dependence over broad range  basic tests of energy loss mechanisms Comparison of similar measurements at RHIC + LHC will provide deep insight

  13. I. Vitev, hep-ph/0603010 What is necessary dynamic range? Rough argument: • small modification to fragmentation for Ejet>~200 GeV • GLV Calculation (I.Vitev):Medium-induced gluon multiplicity saturates at • Ejet> ~100 GeV  need to measure to ETjet~200 GeV Ejet (GeV)

  14. Borghini and Wiedemann, hep-ph/0506218 pThadron~2 GeV for Ejet=100 GeV =ln(EJet/phadron) Medium modification of fragmentation • MLLA: parton splitting+coherence angle-ordered parton cascade • good description of vacuum fragmentation (PYTHIA) • introduce medium effects at parton splitting Fragmentation strongly modified at pThadron~1-5 GeV even for the highest energy jets

  15. A. Morsch, ALICE EJet=100 GeV: 2.0 0.7 GeV Sensitivity of fragmentation to medium properties • largest medium effects for pT~1-5 GeV • background limits to >~5 (??)

  16. jet Salgado and Wiedemann kT Jet broadening kT (tranverse to jet) in jet cone R=C Medium-induced broadening at kT~2 GeV/c  longitudinal momentum ~few GeV/c

  17. ALICE Size: 16 x 26 meters Weight: 10,000 tons TOF TRD HMPID ITS PMD Muon Arm PHOS TPC

  18. ALICE Tracking Silicon Vertex Detector (ITS): 4 cm < r < 44 cm, 6 layers, >6 m2 Time Projection Chamber (TPC): 85 cm < r < 245 cm, L=1.6m, 159 pad rows Transition Radiation Detector (TRD) 290 cm < 370 cm, 6 layers of 3 cm tracklets modest solenoidal field (0.5 T)  good pattern recognition long lever arm  good momentum resolution small material budget: vertexTPC outer field cage < 0.1 X0  robust, redundant tracking: 100 MeV to 100 GeV Momentum resolution TPC dE/dx s~5.5-6.5% ~ 5% @ 100 GeV 5 par. fit 107 central Pb

  19. ALICE Electromagnetic Calorimeter • upgrade to ALICE • ~17 US and European institutions • Current expectations: • 2009 run: partial installation • 2010 run: fully installed and commissioned Lead-scintillator sampling calorimeter Shashlik fiber geometry Avalanche photodiode readout Coverage: |h|<0.7, Df=110o ~13K towers (DhxDf~0.014x0.014) depth~21 X0 Design resolution: sE/E~1% + 8%/E

  20. EMCal support rails average Frenchman EMCal: 120 tons, 50 m2 ~same area and weight as STAR barrel calorimeter

  21. Kinematic reach of ALICE+EMCal • 104/year for minbias Pb+Pb: • inclusive jets: ET>200 GeV • dijets: ET>170 GeV • p0: pT~75 GeV • inclusive g: pT~45 GeV • inclusive e: pT~25 GeV

  22. What does the EMCal bring to ALICE? • fast trigger (level 0/1): enhancement of high pTg, p0, electron and jet statistics by factors 10-60 • significant improvement in jet reconstruction performance • extension of direct photon measurements at high pT • electron-tagged heavy quark jets at high ET

  23. ALICE+EMcal in the larger LHC context • We can agree that large statistics and broad kinematic reach are good! • But rate and kinematic reach are not the only issues: • main fragmentation modifications are at pT<~5 GeV even for the highest energy jets • interaction with medium is per definition soft physics • hadronization effects may be a central issue  particle ID • how critical are 300 GeV jets? • ALICE+EMCal effectively trade acceptance/rate in favor of robust tracking and PID over a broad kinematic range • There are significant measurements that ALICE+EMcal cannot do: • 3-jet events, forward rapidity (not yet), Z+jet,… •  heavy ion jet measurements must be done by both ALICE and CMS/ATLAS

  24. Jets reconstruction in heavy ion events • Goal: reconstruct jet independent of details of fragmentation • unbiased measurement of energy loss 50 GeV jet (Pythia) + central Pb+Pb background (Hijing) • jet structure clearly visible even for modest energy jets • but large uncertainties in background fluctuations and energy loss effects  current studies are only a rough sketch

  25. Jet energy fraction outside cone R=0.3 CDF preliminary Energy in cone R: background and jets Central Pb+Pb R Jet reconstruction and heavy ion background • Large jet cone integrates large background •  bkgd fluctuations overwhelm jet measurement • Unmodified (p+p) jets: over 80% of energy within R~0.3 • Baseline algorithm to suppress heavy ion background: • small jet cones R~0.3, track pT>2 GeV/c

  26. R=0.3, pt>2GeV, Njets,rec. =2 • input • - highest jet • - second jet • mid-cone • - sum # Jets R=0.3, pt>2GeV all particles charged+em charged fraction of evenst with Njets,rec.>1 Jet Energy [GeV] Jet Energy [GeV] Jet splitting for small cones(hard radiation) • Suggests modified kT-type algorithm: • best resolution from summation of small clusters (hot spots) •  study has only just begun…

  27. Armesto, Dainese, Salgado and Wiedemann, PhysRev D71, 054027 (2005) RD/h RB/h High pT heavy quarks: color charge dependence • Light hadrons dominantly from gluon jets • B-mesons less suppressed even at high pT (quark jets) •  quark vs gluon color charge

  28. High pT electrons • Significant electron yield to pT~25 GeV/c with e/p~0.01 • EMCal provides electron trigger •  reconstruct heavy quark jet (ETjet~50+ GeV)

  29. 103 e 1/pion efficiency h 20 GeV E/p electron efficiency EMCal: e/h discrimination at high pT • Geant, all material • E/p from EMCal/tracking; shower-shape • First look: good hadron rejection at 20 GeV • Not yet addressed: electron backgrounds

  30. Summary • Jet quenching as an experimental observation is well established • But key issues remain open: • radiative vs collisional? • quark mass, color charge dependence? • response of lost energy to medium? • Jet studies in LHC heavy ion collisions provide: • similar observables for a (presumably) very different physical system • huge kinematic and statistical reach, new observables to elucidate the energy loss mechanisms in detail • ALICE+EMcal are crucial for full exploitation of jets as a probe of dense matter • The future is upon us!

  31. Extra slides

  32. g/p0 Pb+Pb p+p CERN Yellow Report Direct photons • Not an easy measurement: • g/p0 < 0.1 for p+p • (better in central Pb+Pb due to hadron suppression) • QCD bremsstrahlung photons significant for pT<50 GeV/c  isolation cuts • tricky issue in heavy ion collisions

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