Jet quenching at RHIC and the LHC

Jet quenching at RHIC and the LHCPeter Jacobs, LBNL

2Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006

Radiative energy loss

At most: logarithmic dependence of E on E need logarithmically large variation of parton (jet) energy to see its evolution

2ˆ~ LqCE RSmed

BDMPS transport coefficient: 2

ˆ q

Energy loss:

• E~L2

• E linearly dependent on color charge CR

• E ~independent of partonic energy E


Jet quenching at RHIC…

STAR, Phys Rev Lett 91, 072304

D. d’Enterria

Medium-modified fragmentation?


cos()

pTassoc > 0.15 GeV


Response of medium to lost energy? 4< pT

trig < 6 GeV

High momentum recoil suppressed low momentum enhancedRecoil distribution soft and broad ~ thermalized? angular substructure??Qualitative picture consistent with jet quenching

quantitative study of dynamics at low pT?


pTassoc > 2 GeV

Near-side ridge correlated with jets?


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?

8 < pT(trig) < 15 GeV/c

STAR preliminary


High pT di-hadrons and geometric biasWhere are the surviving pairs generated?

SW quenching weights+geometry+dynamics

dist

ance

to o

rigin

angle wrt ray to origin

A. Dainese et al, hep-ph/0511045

Dihadrons: tangential dominates

trigger direction

Inclusive hadrons: surface bias

Dihadrons: ~volume emission?T. Renk, hep-ph/0602045

?


Jet quenching at RHIC: summaryJets are quenched in very dense matter: unique probes of the mediumBut current picture is largely qualitative:

• 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


mid-late 2007: commission 14 TeV p+pend 2008: first long 5.5 TeV Pb+Pb runheavy ion running: 4 physics weeks/year

Large Hadron Collider at CERN


From RHIC to the LHC…

LHC

RHICSPS

(h+

+h-)/20

17 GeV

200 GeV

5500 GeV=√s

LO p+p y=0Heavy 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


First jet quenching measurement at the LHC: inclusive hadron suppression

Initial gluon density at LHC ~ 5-10 x RHIC:

/fmGeV10~ˆ 2RHICq

But no dramatic effects: RAA (LHC) ~ 0.1-0.2 ~ RAA(RHIC): inclusive hadrons have limited sensitivity to initial density measure jet structure

I. Vitev and M. Gyulassy, PRL 89, 252301(2002)A. Dianese et al., Eur.Phys.J. C38, 461(2005)

RHIC vs LHC/fmGeV70~ˆ 2LHCq


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 measurements for LHC heavy ion collisionsHigh 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


What is necessary dynamic range?Rough argument:

RHICLHCRHIC

g

LHC

g qqd

dNd

dNˆ7ˆ105

GeV30~GeVfew~ LHCRHIC EE small modification to fragmentation for Ejet>~200 GeV

I. Vitev, hep-ph/0603010

Ejet (GeV)

GLV Calculation (I.Vitev):Medium-induced gluon multiplicity saturates at

Ejet> ~100 GeV

need to measure to ET

jet~200 GeV


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

=ln(EJet/phadron)

pThadron~2 GeV for

Ejet=100 GeV

Borghini and Wiedemann, hep-ph/0506218


Sensitivity of fragmentation to medium properties

• largest medium effects for pT~1-5 GeV • background limits to >~5 (??)

2.0

A. Morsch, ALICE

0.7 GeVEJet=100 GeV:


Jet broadeningSalgado and Wiedemann

jet

kT

kT (tranverse to jet) in jet cone R=C

Medium-induced broadening at kT~2 GeV/c longitudinal momentum ~few GeV/c


ALICE

HMPID

Muon Arm

TRD

PHOS

PMDITS

TOF

TPC

Size: 16 x 26 metersWeight: 10,000 tons


ALICE TrackingSilicon 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 rowsTransition Radiation Detector (TRD) 290 cm < 370 cm, 6 layers of 3 cm tracklets

modest solenoidal field (0.5 T) good pattern recognitionlong lever arm good momentum resolutionsmall material budget: vertexTPC outer field cage < 0.1 X0

robust, redundant tracking: 100 MeV to 100 GeV

~ 5% @ 100 GeV

Momentum resolution

5 par. fit107 central Pb

TPC dE/dx

~5.5-6.5%


ALICE Electromagnetic Calorimeter

Lead-scintillator sampling calorimeterShashlik fiber geometry Avalanche photodiode readout

Coverage: ||<0.7, =110o

~13K towers (x~0.014x0.014)depth~21 X0

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


EMCal support rails

average Frenchman

EMCal: 120 tons, 50 m2

~same area and weight as STAR barrel calorimeter


Kinematic reach of ALICE+EMCal

104/year for minbias Pb+Pb:

• inclusive jets: ET>200 GeV

• dijets: ET>170 GeV

• : pT~75 GeV

• inclusive : pT~45 GeV

• inclusive e: pT~25 GeV


What does the EMCal bring to ALICE?

• fast trigger (level 0/1): enhancement of high pT , , 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


ALICE+EMcal in the larger LHC contextWe 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


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 reconstruction and heavy ion backgroundLarge jet cone integrates large background

bkgd fluctuations overwhelm jet measurement

Jet energy fraction outside cone R=0.3

CDF preliminary

22 R

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 pT>2 GeV/c

coneR


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…

Jet Energy [GeV]

frac

tion

of e

vens

t with

Nje

ts,re

c.>1

all particlescharged+emcharged

R=0.3, pt>2GeV

Jet Energy [GeV]

# Je

ts R=0.3, pt>2GeV, Njets,rec. =2 - input - highest jet- second jet- mid-cone- sum


High pT heavy quarks: color charge dependence

Armesto, Dainese, Salgado and Wiedemann, PhysRev D71, 054027 (2005)

RD/h

RB/h

Light hadrons dominantly from gluon jetsB-mesons less suppressed even at high pT (quark jets)

quark vs gluon color charge


High pT electrons

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

EMCal provides electron trigger

reconstruct heavy quark jet (ET

jet~50+ GeV)


EMCal: e/h discrimination at high pT• Geant, all material• E/p from EMCal/tracking; shower-shape

e

h

E/p1/

pion

eff

icie

ncy103

electron efficiency

20 GeV

• First look: good hadron rejection at 20 GeV• Not yet addressed: electron backgrounds


SummaryJet 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!


Extra slides


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 pT<50 GeV/c isolation cuts

• tricky issue in heavy ion collisions

p+p

Pb+Pb/

CERN Yellow Report

Jet quenching at RHIC and the LHC

Documents

Transcript of Jet quenching at RHIC and the LHC