Hard Probes
of the Quark Gluon Plasma
Lecture III: jets
Marco van Leeuwen,
Nikhef and Utrecht University
Lectures at:
Quark Gluon Plasma and Heavy Ion Collisions
Siena, 8-13 July 2013
Jets and parton energy loss
Motivation: understand parton energy loss by tracking the gluon radiation
Qualitatively two scenarios:
1) In-cone radiation: RAA = 1, change of fragmentation
2) Out-of-cone radiation: RAA < 1
2
Jets at LHC
ALICE
Transverse energy map of 1 event
j
h
Clear peaks: jets of fragments
from high-energy quarks and gluons
And a lot of uncorrelated ‘soft’ background
3
Jet reconstruction algorithms
Two categories of jet algorithms:
• Sequential recombination kT, anti-kT, Durham
– Define distance measure, e.g. dij = min(pTi,pTj)*Rij
– Cluster closest
• Cone
– Draw Cone radius R around starting point
– Iterate until stable h,jjet = <h,j>particles
Sum particles inside jet
Different prescriptions exist, most natural: E-scheme, sum 4-vectors
Jet is an object defined by jet algorithm
If parameters are right, may approximate parton
For a complete discussion, see: http://www.lpthe.jussieu.fr/~salam/teaching/PhD-courses.html
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Collinear and infrared safety
Illustration by G. Salam
Jets should not be sensitive to soft effects
(hadronisation and E-loss)
- Collinear safe
- Infrared safe
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Collinear safety
Illustration by G. Salam
Note also: detector effects,
such as splitting clusters in calorimeter (p0 decay)
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Infrared safety
Illustration by G. Salam
Infrared safety also implies robustness
against soft background in heavy ion collisions
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PbPb jet background
Jet finding illustration
Background density vs multiplicity
Cacciari et al
h-j space filled with jets
Many ‘background jets’
Background contributes up to ~180 GeV per unit area
Subtract background:
pTsub, jet  pTraw
, jet   A
Statistical fluctuations remain after subtraction
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Jet energy asymmetry
Centrality
ATLAS, arXiv:1011.6182 (PRL)
Jet-energy asymmetry AJ 
E2  E1
E2  E1
Large asymmetry seen
for central events
Suggests large energy loss: many GeV
~ compatible with expectations from RHIC+theory
However:
• Only measures reconstructed di-jets (don’t see lost jets)
• Not corrected for fluctuations from detector+background
• Both jets are intereracting – No simple observable
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Studying the imbalance
p T ,miss 
//
p
T
cos(j  j jet )
tracks
Out of Cone R>0.8
PYTHIA+HYDJET
CMS measured
CMS, arXiv:1102.1957
Momentum imbalance
restored by hadrons at
large angle R>0.8 and
small pT < 2 GeV/c
In Cone R<0.8
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Energy dependence of asymmetry
CMS, arXiv:1202.5022
(Relative) asymmetry decreases with energy
However: difference pp vs PbPb remains – energy loss finite at large E
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g-jet imbalance
Centrality
CMS, arXiv:1502.0206
pTjet
g-jet asymmetry xJg  g
pT
g
Advantage: g is a parton: know parton kinematics
Disadvantage: low rate (+background p0→ gg)
Dominant contibution: qg → qg
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Hadron-triggered recoil jet distributions
G. de Barros et al., arXiv:1208.1518
pT,jet< 20 GeV/c:
No change with trigger pT
Combinatorial background
pT,jet> 20 GeV/c:
Evolves with trigger pT
Recoil jet spectrum
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Background subtraction: Δrecoil
Remove background by
subtracting spectrum with
lower pTtrig:
Δrecoil =[(20-50)-(15-20)]
Reference spectrum (15-20)
scaled by ~0.96 to account for
conservation of jet density
Δrecoil measures the change of the recoil spectrum with pTtrig
Unfolding correction for background fluctuations and detector response
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Ratio of Recoil Jet Yield ΔIAAPYTHIA
pp reference: PYTHIA
(Perugia 2010)
R=0.4
Constituents:
pTconst > 0.15 GeV/c
no additional cuts
(fragmentation bias) on
recoil jets
Recoil jet yield ΔIAAPYTHIA ≈0.75, approx. constant with jet pT
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Recoil Jet ΔIAAPYTHIA: R dependence
R=0.4
R=0.2
Similar ΔIAAPYTHIA for R=0.2 and R=0.4
No visible broadening within R=0.4
(within exp uncertainties)
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Hadrons vs jets II: recoil
Hadrons
Jets
PRL108 092301
Hadron IAA = 0.5-0.6
Jet IAA = 0.7-0.8
In approx. agreement with models;
elastic E-loss would give larger IAA
Jet IAA > hadron IAA
Not unreasonable
NB/caveat: very different momentum scales !
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Measuring the jet spectrum
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PbPb jet background
Toy Model
Main challenge: large fluctuations of uncorrelated background energy
Size of fluctuations depends on pT cut, cone radius
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Background jets
Raw jet spectrum
Event-by-event background subtracted
Low pT: ‘combinatorial jets’
- Can be suppressed by requiring
leading track
- However: no strict distinction at
low pT possible
Next step: Correct for background
fluctuations and detector effects by
unfolding/deconvolution
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Removing the combinatorial jets
Raw jet spectrum
Fully corrected jet spectrum
Correct spectrum and remove combinatorial jets by unfolding
Results agree with biased jets: reliably recovers all jets and removed bkg
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PbPb jet spectra
Charged jets, R=0.3
M. Verweij@HP, QM
RCP, charged jets, R=0.3
Jet reconstruction does not
‘recover’ much of the radiated energy
Jet spectrum in Pb+Pb: charged particle jets
Two cone radii, 4 centralities
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Pb+Pb jet RAA
Jet RAA measured by
ATLAS, ALICE, CMS
Good agreement
between experiments
Despite different methods:
ATLAS+CMS: hadron+EM jets
ALICE: charged track jets
RAA < 1: not all produced jets are seen;
out-of-cone radiation and/or ‘absorption’
For jet energies up to ~250 GeV; energy loss is a very large effect
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g, hadrons, jets compared
g, hadrons
Jets
Suppression of hadron (leading fragment) and jet yield similar
Is this ‘natural’? No effect of in-cone radiation?
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Model comparison
Jet RAA
Hadron RAA
JEWEL: K. Zapp et al, Eur Phys J C69, 617
Schukraft et al, arXiv:1202.3233
U. Wiedemann@QM2012
M. Verweij@HP, QM2012
At least one model calculation reproduces the observed suppression
 Understand mechanism for out-of-cone radiation?
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Model comparison IAA
JEWEL: Zapp et al., EPJ C69, 617
JEWEL preliminary
JEWEL correctly describes
inclusive jet RAA
Predicts ΔIAA~0.4, below measured
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Hadron trigger vs jet trigger
Hadron trigger: strong “surface bias”
maximizes recoil path length
Hadron trigger
(T.Renk, private com.)
Full jet trigger: no geom. bias
partially cancelled by bkg fluctuations
T.Renk, PRC85 064908
Jet trigger
Centrality and reaction plane biases:
• finite, but only weak trigger pT dependence for high pTtrig
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Jet broadening: R dependence
Ratio of spectra with different R
Larger jet cone:
‘catch’ more radiation
 Jet broadening
ATLAS, A. Angerami, QM2012
However, R = 0.5 still has RAA < 1
– Hard to see/measure the radiated energy
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Jet broadening: transverse fragment distributions
Pb
Pb
CMS, P. Kurt@QM12
PbPb
CMS PAS HIN-12-013
Jet broadening: Soft radiation at large angles
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Jet fragment distributions
PbPb measurement
Ratio to pp
ATLAS M.Rybar@QM12
M. Rybar@QM2012
Low pT enhancement:
soft radiation
Intermediate z:
depletion: E-loss
NB: z is wrt observed Ejet ≠ initial Eparton
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Jet fragment distributions
CMS, Frank Ma@QM12
Low pT enhancement:
soft radiation
Intermediate z, pT:
depletion: E-loss
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A consistent view of jet quenching
G. Roland@QM2012
PbPb – pp (1/GeV)
arXiv:1205.5872
2010 data: arXiv:1205.5872
Change
from
“ξ” to “pT”
No change at
small r, high pT
Narrowing/depletion
at intermediate r, pT
Consistent with 2010 result
Recall (2010 vs 2011):
•
Track pT > 4 GeV vs pT > 1 GeV
•
Leading vs inclusive jet
•
0-30% vs 0-10% and 10-30%
Broadening/excess
at large r, low pT
Radius r
(~2% of jet energy)
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A consistent view of jet quenching
G. Roland@QM2012
Looking at the same
pT range
Charged particles from
parton
pT =50-100 GeV:
z = pT(track)/pT(jet) = 0.4-0.6
x<1
PbPb fragmentation function = pp for ξ <1
Consistent message from charged hadron RAA,
inclusive jet RAA and fragmentation functions!
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Direct photons
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Early times: direct photons
Thermal photons
• Low-Q2 scatterings in HG, QGP
• Thermal glow of hot matter
Photons from initial scattering
• Dominant at pT > few GeV
• Small reinteraction prob
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Direct photons at RHIC
PHENIX, PRL 104, 132301
Idea: hot quark-gluon matter
radiates photons which escape
Difficult measurement:
• Large background p0 → gg
• Thermal photons at low pT
Excess of photons seen at RHIC
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Direct (thermal) photons at LHC
First measurement of
low-pT (thermal) direct photons
at LHC
M. Wilde, ALICE, QM12
Method:
• Measure g/p0
• Low pT: use conversions
• Divide g/p0 by theory
expectation
pp: g/p0 agrees with NLO
PbPb: excess over NLO
at low pT
Multiply by p0 to
get g spectrum
Also at LHC: low pT thermal photons
T ≈ 300 MeV
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Summary
• Jets: a new tool for parton energy loss
measurements
– Large out-of-cone radiation (R = 0.2-0.4)
•
•
•
•
Energy asymmetry
RAA < 1
IAA < 1
Radial shapes
– Remaining jet is pp-like:
• Fragment distribution at large z same as pp
• RAA similar for jets and hadrons
– Most of the radition is at low pT
• Scale set by medium temperature?
• Direct photons
– High pT: photon is a parton
– Low pT: thermal (?) radiation in Pb+Pb
Most conclusions here are qualitative/phenomenology
What does this (quantitatively?) mean for the mechanism of energy loss?
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Jets at LHC
ALICE
Large jet energies clearly
visible above HI background
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Jet imbalance calculations
Qin, Muller, arXiv:1012.5280
Coleman-Smith, Qin, Bass, Muller, arXiv:1108.5662
Parton transport (brick)
Young, Schenke, Jeon, Gale, arXiv:1103.5769
Radiation plus evolution
Several calculations describe
measured imbalance
Need to keep track of fragments;
Leading particle approximations do not work
Cannot yet check consistency
with leading hadrons…
Most natural approach: parton showers
(MARTINI, qPYTHIA, qHERWIG, JEWEL)
MARTINI: AMY+MC
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