Transcript Jet Analysis in Heavy-Ion Collisions Elena Bruna INFN Torino & Yale University 5th International School on QGP.

Jet Analysis in Heavy-Ion
Collisions
Elena Bruna
INFN Torino & Yale University
5th International School on QGP. Torino, March 2011
“The same thrill, the same awe and mystery, come again and
again when we look at any problem deeply enough. With more
knowledge comes deeper, more wonderful mystery, luring one
on to penetrate deeper still. With pleasure and confidence we
turn over each stone to find unimagined strangeness.”
R. Feynman
These (experimental) lectures won’t probably tell you
everything you would ever wanted to know about jets…but I
hope some of the young minds will be inspired and
start/continue working on hard probes to turn over more
stones…
5th International School on QGP. Torino, March 2011
Past, Present, Future…
Pb+Pb @ √sNN=2.76 TeV
LHC
RHIC
LEP
e+e-
• Full jet reconstruction  energy of the hard scattering but
challenging in A+A
• New jet-finding tools
• Do physics with jets !
Elena Bruna (Yale&INFN Torino)
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Outline
Jet I:
Intro & Motivations
Jet II:
Full Jet Reconstruction
Jet III:
Results
Jet IV:
The Present: from RHIC to LHC
Elena Bruna (Yale&INFN Torino)
Jet I:
Intro & Motivations
Jets in high-energy collisions
Collins, Soper, Sterman
Nucl. Phys. B263 (1986) 37
pQCD Factorization:
PDF
p
a, xa
σab
Partonic x-section
b, xb
Fragmentation function
Factorization: assumed between the
perturbative hard part and the
universal non-perturbative
fragmentation (FF) and parton
distribution functions (PDF)
p
Universality: fragmentation functions
and parton distribution functions are
universal (i.e. FF from ee, PDF from
ep, use for pp)
Elena Bruna (Yale&INFN Torino)
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Jets in high-energy collisions
Collins, Soper, Sterman
Nucl. Phys. B263 (1986) 37
pQCD Factorization:
PDF
Partonic x-section
Fragmentation function
QCD factorization works!
p + p  p0
p+p √s=200 GeV
p + p  p
p + p  p
p+p √s=200 GeV
Elena Bruna (Yale&INFN Torino)
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Jets in high-energy collisions
Collins, Soper, Sterman
Nucl. Phys. B263 (1986) 37
pQCD Factorization:
PDF
Partonic x-section
Fragmentation function
PDFs:
Probability for a parton a(b) to carry a
fraction xa(xb) of the hadron momentum
p
a, xa
σab
b, xb
p
Universal  can be measured with fit to
experimantal data for one or more
processes that can be calculated with
perturbative QCD, i.e. deep inelastic
scattering DIS (like e-p), Drell-Yan
processes (qq  l+l-) and others
Many PDFs on the market (CTEQ, GRV,
MRST,…)
Elena Bruna (Yale&INFN Torino)
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Jets in high-energy collisions
Collins, Soper, Sterman
Nucl. Phys. B263 (1986) 37
pQCD Factorization:
PDF
Partonic x-section
Fragmentation function
Hard scattering:
dσ/dt = parton cross section calculable in powers of αS
LO
NLO
Elena Bruna (Yale&INFN Torino)
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Jets in high-energy collisions
Collins, Soper, Sterman
Nucl. Phys. B263 (1986) 37
pQCD Factorization:
PDF
Partonic x-section
Fragmentation function
Fragmentation Functions:
probability to find, at scale Q, a hadron h with a fraction z of the parton c momentum
universal and measured with fits to experimental data
Many D on the market (KKP, AKK, …)
z=
p(hadron)
p (parton)
z
Elena Bruna (Yale&INFN Torino)
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Jets in Nucleus-Nucleus
collisions
Detector
Jet Tomography!
Hard processes make
perturbative QCD applicable
 high momentum transfer Q2
Self-generated
“hard” probes
Hard processes scale as Nbin
Calibrated
LASER/x-ray
Elena Bruna (Yale&INFN Torino)
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Jets in Nucleus-Nucleus
collisions
jet energy loss
in the medium
Questions:
1) How does the parton lose energy?
2) What happens to the radiated energy?
3) Collisional energy loss?
4) Does the energy loss depend on the parton type?
Interpretation: Gluon radiation
DEloss ~ ρgluon (gluon density)
DEloss ~ ΔL2 (medium length) [~ ΔL with expansion]
DEgluon > DEquark, m=0 > DEquark, m>0
 Important to measure DE of gluons  light  heavy quarks…
Transport coefficient: q^ = m2 / L is the <pT2> transferred from the parton to a
gluon per unit path length
Elena Bruna (Yale&INFN Torino)
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Jets in Nucleus-Nucleus
collisions
Eskola, Honkanen, Salgado, Wiedemann
Nucl Phys A747 (2005) 511
q^ = 5 – 15 GeV2 / fm
Elena Bruna (Yale&INFN Torino)
from RHIC RAA Data
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Some Predictions: FF
nchin jet : increases
zin jet : decreases
Gyulassy et al., nucl-th/0302077
Renk, Phys. Rev. C79:054906,2009
Borghini and Wiedemann, hep-ph/0506218
z=ph/pjet
ph
pjet
ξ stretches
the low z part
Elena Bruna (Yale&INFN Torino)
Energy loss in the medium
 softer fragmentation
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Some Predictions: Jet shapes
If energy loss by gluon radiation  broadening of the jet energy profile
R
= jet radius (on η-ϕ plane) =√(Δϕ2+Δη2)
ωmin (pTmin) = minimum pT on particles in the jet
Energy loss ratio goes down with larger b.
Energy loss ratio becomes smaller with
smaller R and larger ωmin.
Limit of large R and ωmin=0  no out-ofcone energy  ΔEin~E
I Vitev, S Wicks, B-W Zhang,
JHEP 0811,093 (2008); EPJC 62, 139 (2009).
Elena Bruna (Yale&INFN Torino)
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Some Predictions: Jet shapes
I Vitev, S Wicks, B-W Zhang,
JHEP 0811,093 (2008); EPJC 62, 139 (2009).
Vitev, Zhang, PRL 104 (2010) 132001, arXiv: 0910.1090
Limits:
• small Rmax and large ωmin  single particle
suppression.
• large Rmax and small ωmin  all jet energy
recovered  RAAjet=1 ! (jet production is hard process,
scales as Nbin)
Elena Bruna (Yale&INFN Torino)
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Some Predictions: Jet shapes
I Vitev, S Wicks, B-W Zhang,
JHEP 0811,093 (2008); EPJC 62, 139 (2009).
Vitev, Zhang, PRL 104 (2010) 132001, arXiv: 0910.1090
Limits:
• small Rmax and large ωmin  single particle
suppression.
• large Rmax and small ωmin  all jet energy
recovered  RAAjet=1 ! (jet production is hard process,
scales as Nbin)
Elena Bruna (Yale&INFN Torino)
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Jet quenching from single high-pT
hadrons
Observations at RHIC:
1.Large suppression of high-pT
hadrons: factor ~ 5
2.Photons are not suppressed
• They don’t interact with the
medium (good!)
• Nbin scaling works
Elena Bruna (Yale&INFN Torino)
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Jet quenching from single high-pT
hadrons
Observations at RHIC:
1.Large suppression of high-pT
hadrons: factor ~ 5
2.Photons are not suppressed
• They don’t interact with the
medium (good!)
• Nbin scaling works
3. Also Heavy Flavor is
suppressed at RHIC
•same as light quarks
•role of bottom?
•collisional energy loss/resonant
elastic scattering?
Elena Bruna (Yale&INFN Torino)
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Jet quenching from single high-pT
hadrons
ALICE, Phys. Lett. B 696 (2011) 30.
RHIC suppression < LHC
RHIC: high pT hadrons hadronize from quarks
LHC: from gluons (larger color charge!)
Prediction: Vitev(hep-ph/050322v1)
• GLV – pQCD factorization
• medium-induced gluon brems.
Elena Bruna (Yale&INFN Torino)
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Jet quenching from di-hadrons
Start from a high-pt “trigger” particle and
look on the away side (in f).
Azimuthal correlation function shows
~complete absence of “away-side” jet
Partner in hard scatter is absorbed in the
dense medium
not the case in d+Au  final state effect
Elena Bruna (Yale&INFN Torino)
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Jet quenching from di-hadrons
Start from a high-pt “trigger” particle make
azimuthal correlation
~complete absence of “away-side” jet
Partner in hard scatter is strongly interacting
with the dense medium
not the case in d+Au  final state effect !
Path-length dependence of di-jet topologies
Out-of-plane
y
in-plane
x
Back-to-back suppression out-of-plane stronger than in-plane
Elena Bruna (Yale&INFN Torino)
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Jet quenching from di-hadrons
increasing pTtrig 
increasing pTassoc 
STAR, Phys.Rev.C82 024912 (2010)
Elena Bruna (Yale&INFN Torino)
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Jet quenching from di-hadrons
increasing pTtrig 
At low trigger pT & low pTassoc:
• Mach Cone – conical emission?
• Cherenkov Radiation?
• pure 3D hydro?
[won’t discuss this]
At high trigger pT:
increasing pTassoc 
double bump:
Phys.Rev.C82 024912 (2010)
• re-emergenceSTAR,
of away-side
jet (punch thru)?
or
• tangential jets?
Elena Bruna (Yale&INFN Torino)
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Trigger and Surface Biases
Experiments online-trigger dependent:
• Large pT or energy deposition triggers bias towards hard
fragmentation!
• EM calorimetry bias towards large EM fraction
Trigger particles biased toward
the surface  Surface bias, as
seen in hydro models
Elena Bruna (Yale&INFN Torino)
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High pT: towards jets
ALICE, Phys. Lett. B 696 (2011) 30.
What we have so far:
• Suppression of high-pT hadrons in A+A (at RHIC and
LHC) w.r.t. p+p
• Evidence for parton energy loss in the medium
But:
• Geometrical bias: dominated by surface jets
• Jet energy not constrained
• Limited kinematic reach
Renk and Eskola, hepph/0610059
What we want:
• Precise measurement of the parton energy loss
• Measurement of the modified fragmentation function
How?
Elena Bruna (Yale&INFN Torino)
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High pT: towards jets
How?
1.
•
g-Jet
Jet energy well constrained
• limited kinematic reach (x-sec scales as αSαem)
• Difficult to have a clean measurement of photons
Di-Hadron
z = p(h)/pparton
E = pparton
p pparton
Courtesy Thomas Ullrich
Elena Bruna (Yale&INFN Torino)
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Direct g-hadron fragmentation
functions
STAR, Phys. Rev. C 82 (2010) 34909
1. Good agreement w/ theory models
2. more assoc h± for p0 than for g
 different parton energies for p0and g
(p0come from fragmentation of higher
energy parton)
Trig particle= g or p0
Assoc particle: h±
3. Au+Au: different path-length for the
recoil jet for p0and gand triggers
Elena Bruna (Yale&INFN Torino)
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Direct g-hadron fragmentation
functions
IAA= ratio of associated yield per trigger
in Au+Au to that in p+p
Trig particle= g or p0
Assoc particle: h±
8<Etrig<16 GeV/c
1. IAA < 1 for zT>0.3
2. data can distinguish between different
theoretical models
3. low zT: expected differences between
p0and gIAA due to path-length
dependence of the energy loss
Measurements do not indicate path-length or color-charge dependence !
Elena Bruna (Yale&INFN Torino)
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High pT: towards jets
How?
1.
•
g-Jet
Jet energy well constrained
• limited kinematic reach (x-sec scales as αSαem)
• Difficult to have a clean measurement of photons
2. Full Jet Reconstruction
•
Larger kinematic reach
• large background  complex and challenging !
z = p(h)/pparton
Ejet = pparton
E = pparton
Courtesy Thomas Ullrich
Elena Bruna (Yale&INFN Torino)
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Jet II:
Full Jet Reconstruction
Elena Bruna (Yale&INFN Torino)
Jets: Theory vs Experiment
Theory (pQCD): jet = High-pT parton produced in hard scatterings, or
the closest object to a parton
Experiment: jet = spray of collimated hadrons
GOAL: measure the parton energy in experiments  do jet physics!
Tool: Full jet reconstruction with jet-finding algorithms
•
for both Theory and Experiment !
Elena Bruna (Yale&INFN Torino)
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pT
Theoretical requirements
pT
cone iteration
y
Jet 1
y
Jet 1
Collinear safety
 replaces one parton by two at the same place
 the algorithm should be insensitive to any collinear
radiation.
Jet 2
Infrared safety
 a soft emissions that add very soft gluon
 the jet-finding algorithm should not be sensitive to soft radiation
Elena Bruna (Yale&INFN Torino)
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Experimental
requirements
• Detector independence: the
performance of the jet algorithm should not
be dependent on detector segmentation,
energy resolution, …
• Stability with luminosity: jet finding
should not be strongly affected by multiple
hard scatterings at high beam luminosities.
• Fast
• Efficient: the jet algorithm should find as
many physically interesting jets as
possible, with good energy resolution
Elena Bruna (Yale&INFN Torino)
CDF
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Jet Finding algorithms
Review of CDF Jet Algorithms, arXiv:hep-ex/0005012v2
FastJet JHEP 0804:005, arXiv:0802.1188
FastJet JHEP 0804, 063 (2008), arXiv:0802.1189v2
Particles are combined into jets
• the larger experimental coverage, the better
Which particles? The measured ones:
• charged tracks (TPC)
• neutral towers (EMC)
• charged energy (Hcal)
Different ways of combining particles  jet-finding algorithms
Sequential Recombination
Cone
kT
CDF JetClu, MidPoint
Anti-kT
D0 Cone
Cambridge - Aachen
CMS Iterative Cone
ATLAS Cone
PyCell
SISCone
Elena Bruna (Yale&INFN Torino)
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Sequential Recombination
dij  min(kTi ,kTj )(Dyij  Dfij )/R2
2p
2p
2
2
kTi,j= particle transverse momentum (pT)
kT: p>0 (soft particles merged first)
Anti-kT: p<0 (hard particles merged first)

R=resolution
parameter
Example: Anti-kT
Blue = highest pT particle
2π
If dij<kTi-2  merged
ϕ
0
-1
Elena Bruna (Yale&INFN Torino)
η
+1
If dij>kTi-2  not merged
 call it a jet
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kT vs anti-kT
FastJet M. Cacciari, G. Salam, G. Soyez 0802.1188
• ALL particles are clustered into “jets”
• kT  not bound to a circular structure
• Anti-kT  circular shape, “cone” radius ~R parameter
– Expected to be less sensitive to background/“back reaction” (it starts from
high-pT particles)  ideal choice in heavy-ion collision
• Recombination algorithms are collinear and infrared safe
Elena Bruna (Yale&INFN Torino)
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R matters!
Elena Bruna (Yale&INFN Torino)
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R matters!
Elena Bruna (Yale&INFN Torino)
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R matters!
Elena Bruna (Yale&INFN Torino)
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R matters!
Elena Bruna (Yale&INFN Torino)
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R matters!
The choice of R depends on
• The system we are looking at (e+e-, pp, AuAu, PbPb,…)
• Tradeoff: don’t want to loose too much out-of-cone
radiation (corrections for hadronization become difficult)
but want to have a small background in the jet area
In pp: ~80% of jet
energy within R=0.4
for 20 GeV jets
Elena Bruna (Yale&INFN Torino)
p+p 200 GeV
STAR Preliminary
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Jets in Heavy-Ion Collisions at
RHIC and LHC
Central Au+Au √sNN=200 GeV
STAR EMC + tracking data
ETjet ~ 21 GeV
Central Pb+Pb√sNN=2.76 TeV
ALICE tracking data
STAR preliminary
Why measure jets in heavy ion collisions? [inclusive, di-jets, jet-hadron, g-jet,..]
• Access kinematics of the binary hard-scattering
• Characterize the parton energy loss in the hot QCD medium
− modified fragmentation, energy flow within jets, quark vs gluon jet difference
− flavor and mass dependence
• Study medium response to parton energy loss – establish properties of the medium
Elena Bruna (Yale&INFN Torino)
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