Jet quenching at RHIC and the LHC Peter

Jet quenching at RHIC and the LHC Peter Jacobs, LBNL

Radiative energy loss BDMPS transport coefficient: Energy loss: • DE~L 2 • 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) Winter Workshop, energy to see its evolution March 12, 2006 Jet Quenching at RHIC and LHC 2

Jet quenching at RHIC… D. d’Enterria STAR, Phys Rev Lett 91, 072304 Medium-modified fragmentation? Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 3

Response of medium to lost energy? p. Tassoc > 2 Ge. V 4< p. Ttrig < 6 Ge. V p. Tassoc > 0. 15 Ge. V cos(Df) Near-side ridge correlated with jets? STAR, Phys Rev Lett 91, 072304 STAR, Phys Rev Lett 95, 152301 High momentum recoil suppressed low momentum enhanced Recoil distribution soft and broad ~ thermalized? angular substructure? ? Qualitative picture consistent with jet quenching quantitative study of dynamics at low p. T? Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 4

Di-hadrons at yet higher p. T STAR preliminary 8 < p. T(trig) < 15 Ge. V/c • Away-side yield is suppressed but finite and measurable set upper bound on energy loss? • Suppression without angular broadening or modification of Winter Workshop, high z fragmentation: why? March 12, 2006 Jet Quenching at RHIC and LHC 5

High p. T di-hadrons and geometric bias Where are the surviving pairs generated? ? SW quenching weights+geometry+dynamics Inclusive hadrons: surface bias Dihadrons: tangential dominates distance to origin A. Dainese et al, hep-ph/0511045 angle wrt ray to origin Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC Dihadrons: ~volume emission? T. Renk, hep-ph/0602045 trigger direction 6

Jet quenching at RHIC: summary Jets are quenched in very dense matter: unique probes of the medium But current picture is largely qualitative: • leading hadrons: fragmentation and geometric biases • p. T ~2 -5 Ge. V/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 Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 7

Large Hadron Collider at CERN mid-late 2007: commission 14 Te. V p+p end 2008: first long 5. 5 Te. V Pb+Pb run heavy ion running: 4 physics weeks/year Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 8

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 Winter Workshop, March 12, 2006 LO p+p y=0 (h++h-)/2 p 0 √s = 5500 Ge. V 200 Ge. V 17 Ge. V LHC RHIC SPS Jet Quenching at RHIC and LHC 9

First jet quenching measurement at the LHC: inclusive hadron suppression I. Vitev and M. Gyulassy, PRL 89, 252301(2002) A. Dianese et al. , Eur. Phys. J. C 38, 461(2005) Initial gluon density at LHC ~ 5 -10 x RHIC: RHIC vs LHC But no dramatic effects: RAA (LHC) ~ 0. 1 -0. 2 ~ RAA(RHIC): inclusive hadrons have limited sensitivity to initial density measure jet structure Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 10

The jet landscape for 5. 5 Te. V 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 Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 11

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 Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 12

What is necessary dynamic range? Rough argument: small modification to fragmentation for Ejet>~200 Ge. V GLV Calculation (I. Vitev): Medium-induced gluon multiplicity saturates at Ejet> ~100 Ge. V I. Vitev, hep-ph/0603010 need to measure to ETjet~200 Ge. V Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC Ejet (Ge. V) 13

Medium modification of fragmentation • MLLA: parton splitting+coherence angle-ordered parton cascade • good description of vacuum fragmentation (PYTHIA) • introduce medium effects at parton splitting Borghini and Wiedemann, hep-ph/0506218 p. Thadron~2 Ge. V for Ejet=100 Ge. V =ln(EJet/phadron) Winter Workshop, March 12, 2006 Fragmentation strongly modified at p. Thadron~1 -5 Ge. V even for the highest energy jets Jet Quenching at RHIC and LHC 14

Sensitivity of fragmentation to medium properties A. Morsch, ALICE EJet=100 Ge. V: 2. 0 0. 7 Ge. V • largest medium effects for p. T~1 -5 Ge. V • background limits to >~5 (? ? ) Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 15

Jet broadening Salgado and Wiedemann k. T (tranverse to jet) in jet cone R= C jet k. T Medium-induced broadening at k. T~2 Ge. V/c longitudinal momentum ~few Ge. V/c Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 16

Size: 16 x 26 meters Weight: 10, 000 tons TOF TRD HMPID ITS PMD Muon Arm PHOS Winter Workshop, March 12, 2006 TPC Jet Quenching at RHIC and LHC ALICE 17

ALICE Tracking Silicon Vertex Detector (ITS): 4 cm < r < 44 cm, 6 layers, >6 m 2 Time Projection Chamber (TPC): 85 cm < r < 245 cm, L=1. 6 m, 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 X 0 robust, redundant tracking: 100 Me. V to 100 Ge. V Momentum resolution TPC d. E/dx s~5. 5 -6. 5% ~ 5% @ 100 Ge. V 5 par. fit 107 central Pb Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 18

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=110 o ~13 K towers (Dhx. Df~0. 014 x 0. 014) depth~21 X 0 Design resolution: s. E/E~1% + 8%/ E Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 19

EMCal support rails average Frenchman Winter Workshop, March 12, 2006 EMCal: 120 tons, 50 m 2 ~same area and weight as STAR barrel calorimeter Jet Quenching at RHIC and LHC 20

Kinematic reach of ALICE+EMCal 104/year for minbias Pb+Pb: • inclusive jets: ET>200 Ge. V • dijets: ET>170 Ge. V • p 0: p. T~75 Ge. V • inclusive g: p. T~45 Ge. V • inclusive e: p. T~25 Ge. V Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 21

What does the EMCal bring to ALICE? • fast trigger (level 0/1): enhancement of high p. T g, p 0, electron and jet statistics by factors 10 -60 • significant improvement in jet reconstruction performance • extension of direct photon measurements at high p. T • electron-tagged heavy quark jets at high ET Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 22

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 p. T<~5 Ge. V 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 Ge. V 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 Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 23

Jets reconstruction in heavy ion events Goal: reconstruct jet independent of details of fragmentation unbiased measurement of energy loss 50 Ge. V 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 Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 24

Jet reconstruction and heavy ion background Large jet cone integrates large background bkgd fluctuations overwhelm jet measurement Jet energy fraction outside cone R=0. 3 CDF preliminary Energy in cone R: background and jets Central Pb+Pb R • Unmodified (p+p) jets: over 80% of energy within R~0. 3 • Baseline algorithm to suppress heavy ion background: Winter Workshop, small jet cones. Jet. R~0. 3, March 12, 2006 Quenchingtrack at RHICp and LHCGe. V/c T>2 25

R=0. 3, pt>2 Ge. V R=0. 3, p >2 Ge. V, N # Jets fraction of evenst with Njets, rec. >1 Jet splitting for small cones (hard radiation) t jets, rec. all particles charged+em charged =2 - input - highest jet - second jet - mid-cone - sum Jet Energy [Ge. V] Suggests modified k. T-type algorithm: best resolution from summation of small clusters (hot spots) study has only just begun… Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 26

High p. T heavy quarks: color charge dependence Armesto, Dainese, Salgado and Wiedemann, Phys. Rev D 71, 054027 (2005) RD/h RB/h Light hadrons dominantly from gluon jets B-mesons less suppressed even at high p. T (quark jets) quark vs gluon color charge Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 27

High p. T electrons Significant electron yield to p. T~25 Ge. V/c with e/p~0. 01 EMCal provides electron trigger reconstruct heavy quark jet (ETjet~50+ Ge. V) Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 28

EMCal: e/h discrimination at high p. T • Geant, all material • E/p from EMCal/tracking; shower-shape 1/pion efficiency 103 e h 20 Ge. V E/p electron efficiency • First look: good hadron rejection at 20 Ge. V • Not yet addressed: electron backgrounds Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 29

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! Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 30

Extra slides Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 31

Direct photons Not an easy measurement: • g/p 0 < 0. 1 for p+p (better in central Pb+Pb due to hadron suppression) g/p 0 • QCD bremsstrahlung photons significant for p. T<50 Ge. V/c isolation cuts • tricky issue in heavy ion collisions Winter Workshop, March 12, 2006 Pb+Pb p+p CERN Yellow Report Jet Quenching at RHIC and LHC 32