Jet quenching at RHIC and the LHC Peter Jacobs, LBNL

Jet Quenching at RHIC and LHC Winter Workshop, March 12, Radiative energy loss At most: logarithmic dependence of  E on E  need logarithmically large variation of parton (jet) energy to see its evolution BDMPS transport coefficient: Energy loss:  E~L 2  E linearly dependent on color charge C R  E ~independent of partonic energy E

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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, cos(  ) p T assoc > 0.15 GeV STAR, Phys Rev Lett 95, Response of medium to lost energy? 4< p T trig < 6 GeV 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 ? STAR, Phys Rev Lett 91, p T assoc > 2 GeV Near-side ridge correlated with jets?

Jet Quenching at RHIC and LHC Winter Workshop, March 12, Di-hadrons at yet higher p T 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? 8 < p T (trig) < 15 GeV/c STAR preliminary

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

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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, 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 Large Hadron Collider at CERN

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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, First jet quenching measurement at the LHC: inclusive hadron suppression Initial gluon density at LHC ~ 5-10 x RHIC: But no dramatic effects: R AA (LHC) ~ ~ R AA (RHIC): inclusive hadrons have limited sensitivity to initial density  measure jet structure I. Vitev and M. Gyulassy, PRL 89, (2002) A. Dianese et al., Eur.Phys.J. C38, 461(2005) RHIC vs LHC

Jet Quenching at RHIC and LHC Winter Workshop, March 12, The jet landscape for 5.5 TeV Pb+Pb collisions Inclusive jet rates very high  +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

Jet Quenching at RHIC and LHC Winter Workshop, March 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 E T, 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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, What is necessary dynamic range? Rough argument:  small modification to fragmentation for E jet >~200 GeV I. Vitev, hep-ph/ E jet (GeV) GLV Calculation (I.Vitev): Medium-induced gluon multiplicity saturates at E jet > ~100 GeV  need to measure to E T jet ~200 GeV

Jet Quenching at RHIC and LHC Winter Workshop, March 12, 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 p T hadron ~1-5 GeV even for the highest energy jets  =ln( E Jet / p hadron ) p T hadron ~2 GeV for E jet =100 GeV Borghini and Wiedemann, hep-ph/

Jet Quenching at RHIC and LHC Winter Workshop, March 12, Sensitivity of fragmentation to medium properties largest medium effects for p T ~1-5 GeV background limits to  >~5 (??) 2.0 A. Morsch, ALICE 0.7 GeVE Jet =100 GeV:

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

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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, ALICE Tracking Silicon Vertex Detector (ITS): 4 cm 6 m 2 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 X 0  robust, redundant tracking: 100 MeV to 100 GeV ~ 100 GeV Momentum resolution 5 par. fit 10 7 central Pb TPC dE/dx  ~ %

Jet Quenching at RHIC and LHC Winter Workshop, March 12, ALICE Electromagnetic Calorimeter Lead-scintillator sampling calorimeter Shashlik fiber geometry Avalanche photodiode readout Coverage: |  |<0.7,  =110 o ~13K towers (  x  ~0.014x0.014) depth~21 X 0 Design resolution:  E /E~1% + 8%/  E upgrade to ALICE ~17 US and European institutions Current expectations: 2009 run: partial installation 2010 run: fully installed and commissioned

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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, Kinematic reach of ALICE+EMCal 10 4 /year for minbias Pb+Pb: inclusive jets: E T >200 GeV dijets: E T >170 GeV   : p T ~75 GeV inclusive  : p T ~45 GeV inclusive e: p T ~25 GeV

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

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

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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, 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 R Energy in cone R: background and jets Central Pb+Pb 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 p T >2 GeV/c

Jet Quenching at RHIC and LHC Winter Workshop, March 12, Jet splitting for small cones (hard radiation) Suggests modified k T -type algorithm: best resolution from summation of small clusters (hot spots)  study has only just begun… Jet Energy [GeV] fraction of evenst with N jets,rec. >1 all particles charged+em charged R=0.3, p t >2GeV Jet Energy [GeV] # Jets R=0.3, p t >2GeV, N jets,rec. =2 - input - highest jet - second jet - mid-cone - sum

Jet Quenching at RHIC and LHC Winter Workshop, March 12, High p T heavy quarks: color charge dependence Armesto, Dainese, Salga do and Wiedemann, PhysRev D71, (2005) R D/h R B/h Light hadrons dominantly from gluon jets B-mesons less suppressed even at high p T (quark jets)  quark vs gluon color charge

Jet Quenching at RHIC and LHC Winter Workshop, March 12, High p T electrons Significant electron yield to p T ~25 GeV/c with e/  ~0.01 EMCal provides electron trigger  reconstruct heavy quark jet (E T jet ~50+ GeV)

Jet Quenching at RHIC and LHC Winter Workshop, March 12, EMCal: e/h discrimination at high p T Geant, all material E/p from EMCal/tracking; shower-shape e h E/p 1/pion efficiency 10 3 electron efficiency 20 GeV First look: good hadron rejection at 20 GeV Not yet addressed: electron backgrounds

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

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

Jet Quenching at RHIC and LHC Winter Workshop, March 12, Direct photons Not an easy measurement:   < 0.1 for p+p (better in central Pb+Pb due to hadron suppression) QCD bremsstrahlung photons significant for p T <50 GeV/c  isolation cuts tricky issue in heavy ion collisions p+p Pb+Pb // CERN Yellow Report