Jet quenching at RHIC and the LHC
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Transcript of 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
3Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
Jet quenching at RHIC…
STAR, Phys Rev Lett 91, 072304
D. d’Enterria
Medium-modified fragmentation?
4Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
cos()
pTassoc > 0.15 GeV
STAR, Phys Rev Lett 95, 152301
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?
STAR, Phys Rev Lett 91, 072304
pTassoc > 2 GeV
Near-side ridge correlated with jets?
5Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
6Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
?
7Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
8Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
9Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
10Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
11Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
12Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
13Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
14Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
15Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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:
16Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
17Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
ALICE
HMPID
Muon Arm
TRD
PHOS
PMDITS
TOF
TPC
Size: 16 x 26 metersWeight: 10,000 tons
18Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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%
19Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
20Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
EMCal support rails
average Frenchman
EMCal: 120 tons, 50 m2
~same area and weight as STAR barrel calorimeter
21Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
22Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
23Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
24Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
25Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
26Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
27Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
28Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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)
29Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
30Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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!
31Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
Extra slides
32Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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