jet quenching at rhic and the lhc peter jacobs, lbnl
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![Page 1: Jet quenching at RHIC and the LHC Peter Jacobs, LBNL](https://reader036.vdocuments.site/reader036/viewer/2022062320/56649d7e5503460f94a61912/html5/thumbnails/1.jpg)
Jet quenching at RHIC and the LHCPeter Jacobs, LBNL
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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
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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?
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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?
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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
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6Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
High pT di-hadrons and geometric bias
Where are the surviving pairs generated?
SW quenching weights+geometry+dynamics
dist
ance
to o
rigi
n
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
?
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7Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
Jet quenching at RHIC: summary
Jets 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
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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
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9Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
From RHIC to the LHC…
LHC
RHIC
SPS
(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
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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
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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
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12Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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
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13Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
What is necessary dynamic range?Rough argument:
RHICLHC
RHIC
g
LHC
g qqd
dN
d
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
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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
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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:
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16Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
Jet broadening
Salgado 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
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17Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
ALICE
HMPID
Muon Arm
TRD
PHOS
PMD
ITS
TOF
TPC
Size: 16 x 26 meters
Weight: 10,000 tons
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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. fit
107 central Pb
TPC dE/dx
~5.5-6.5%
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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
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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
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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
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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
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23Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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 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
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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
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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
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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
even
st w
ith
Nje
ts,r
ec.>
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
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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
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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)
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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
ncy
103
electron efficiency
20 GeV
• First look: good hadron rejection at 20 GeV• Not yet addressed: electron backgrounds
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30Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
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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31Jet Quenching at RHIC and LHCWinter Workshop,March 12, 2006
Extra slides
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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