Jet quenching at RHIC and the LHC

Peter Jacobs, LBNL

Radiative energy loss

BDMPS transport coefficient: Energy loss: D

E med

~ 

S q

ˆ

C R

  2

q

 ˆ

L

2 • • • 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  need logarithmically large variation of parton (jet) Winter Workshop, March 12, 2006 energy to see its evolution Jet Quenching at RHIC and LHC 2

Jet quenching at RHIC…

D. d’Enterria Winter Workshop, March 12, 2006 Medium-modified fragmentation?

Jet Quenching at RHIC and LHC STAR, Phys Rev Lett 91, 072304 3

Response of medium to lost energy?

4< p T trig < 6 GeV p T assoc > 2 GeV p T assoc > 0.15 GeV 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

STAR preliminary

Di-hadrons at yet higher p

T 8 < p T (trig) < 15 GeV/c • Away-side yield is suppressed but finite and measurable  set

upper

bound on energy loss?

• Suppression without angular broadening or modification of March 12, 2006 high z fragmentation: why?

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

?

Dihadrons: tangential dominates A. Dainese et al, hep-ph/0511045 Inclusive hadrons: surface bias Dihadrons: ~volume emission?

T. Renk, hep-ph/0602045 Winter Workshop, angle wrt ray to origin March 12, 2006 Jet Quenching at RHIC and LHC 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 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 Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 7

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 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 LO p+p y=0 (h + +h )/2 p 0 √s = 5500 GeV 200 GeV 17 GeV LHC • huge increase in yield of hard probes SPS RHIC Winter Workshop, March 12, 2006 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. C38, 461(2005) Initial gluon density at LHC ~ 5-10 x RHIC:

RHIC

~ 10 GeV 2 /fm

LHC

~ 70 GeV 2 /fm RHIC vs LHC But no dramatic effects: R inclusive hadrons have limited sensitivity to initial density  AA measure jet structure (LHC) ~ 0.1-0.2 ~ R AA (RHIC): Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 10

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

What is necessary dynamic range?

Rough argument:

dN g d

   5  10  

dN g d

 

q

ˆ

LHC

 7 

q

ˆ

RHIC LHC RHIC

D

E

~ few GeV  D

E

~ 30 GeV

RHIC LHC

 small modification to fragmentation for E jet >~200 GeV GLV Calculation (I.Vitev): Medium-induced gluon multiplicity saturates at E jet > ~100 GeV  need to measure to E T jet ~200 GeV Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC I. Vitev, hep-ph/0603010 E jet (GeV) 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 T hadron ~2 GeV for E jet =100 GeV Winter Workshop, March 12, 2006  =ln( E Jet / p hadron ) Fragmentation strongly modified at p T hadron ~1-5 GeV even for the highest energy jets Jet Quenching at RHIC and LHC 14

Sensitivity of fragmentation to medium properties

A. Morsch, ALICE Winter Workshop, March 12, 2006 E Jet =100 GeV: 2.0 0.7 GeV • largest medium effects for p T ~1-5 GeV • background limits to  >~5 (??) 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 Winter Workshop, March 12, 2006 Medium-induced broadening at k T ~2 GeV/c  longitudinal momentum ~few GeV/c Jet Quenching at RHIC and LHC 16

Size

: 16 x 26 meters

Weight

: 10,000 tons

HMPID TOF TRD PMD ITS PHOS

Winter Workshop, March 12, 2006

Muon Arm 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=1.6m, 159 pad rows Transition Radiation Detector (TRD) 290 cm < 370 cm, 6 layers of 3 cm tracklets modest solenoidal field (0.5 T)  long lever arm  good pattern recognition good momentum resolution small material budget: vertex  TPC outer field cage < 0.1 X 0  robust, redundant tracking: 100 MeV to 100 GeV Momentum resolution TPC dE/dx s ~5.5-6.5% Winter Workshop, March 12, 2006 ~ 5% @ 100 GeV Jet Quenching at RHIC and LHC 5 par. fit 10 7 central Pb 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: |  |<0.7, Df =110 o ~13K towers ( D x Df ~0.014x0.014) 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

10 4 /year for minbias Pb+Pb: • inclusive jets: E T >200 GeV • dijets: E T >170 GeV • p 0 : p T ~75 GeV • inclusive g : p T ~45 GeV • inclusive e: p T ~25 GeV 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 electron and jet statistics by factors 10-60 g , p 0 , • significant improvement in jet reconstruction performance • extension of direct photon measurements at high p T • electron-tagged heavy quark jets at high E T 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 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 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 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 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 cone R

 D  2  D f 2 • Unmodified (p+p) jets: over 80% of energy within R~0.3

• Baseline algorithm to suppress heavy ion background: Winter Workshop, March 12, 2006 small jet cones R~0.3, track p T >2 GeV/c R 25

Jet splitting for small cones (hard radiation)

all particles charged+em charged

R=0.3, p t >2GeV t >2GeV, N jets,rec.

=2 - input - highest jet - second jet - mid-cone - sum Jet Energy [GeV] Jet Energy [GeV] 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, Salga do and Wiedemann, PhysRev D71, 054027 (2005) R D/h R B/h Light hadrons dominantly from gluon jets B-mesons less suppressed even at high p T (quark jets) Winter Workshop,  quark vs gluon color charge March 12, 2006 Jet Quenching at RHIC and LHC 27

High p

T

electrons

Significant electron yield to p T ~25 GeV/c with e/ p ~0.01

EMCal provides electron trigger  reconstruct heavy quark jet (E T jet ~50+ GeV) 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 10 3 e h 20 GeV Winter Workshop, March 12, 2006 E/p electron efficiency • First look: good hadron rejection at 20 GeV • Not yet addressed: electron backgrounds 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!

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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) • QCD bremsstrahlung photons significant for p T <50 GeV/c  isolation cuts • tricky issue in heavy ion collisions g / p 0 Pb+Pb p+p CERN Yellow Report Winter Workshop, March 12, 2006 Jet Quenching at RHIC and LHC 32