Transcript From yesterday Jet II: Full Jet Reconstruction Goal: set the Jet Energy Scale • Different systematics to take into account (tracking,…) • Background fluctuations: the.

From yesterday
Jet II:
Full Jet Reconstruction
Goal: set the Jet Energy Scale
• Different systematics to take into account (tracking,…)
• Background fluctuations: the challenge
Jet III:
Results
p+p and d+Au: reference/control measurements
Broadening observed at RHIC
Elena Bruna (Yale&INFN Torino)
Today
Jet III:
Results
p+p and d+Au: reference/control measurements
Broadening observed at RHIC
More on PHENIX vs STAR
More quenching observables:
di-jets, jet-hadron
Jet IV:
The Present: from RHIC to LHC
Elena Bruna (Yale&INFN Torino)
Hard Probes at LHC vs RHIC
Results on quenching at LHC
Jet-finding in PHENIX
CAVEAT:
jet-finder based on unmodified jet-shapes
⇒ veto against modified/quenched jets
“Anti-quenching” biases!
Elena Bruna (Yale&INFN Torino)
3
Jet-finding in PHENIX
Step back: how does jet-finding work in PHENIX?
Jet-finding based on a Gaussian kernel (σ=0.3, 0.4)
Focuses on the core of the jet
Direct rejection of fake jets (i.e. jet by jet):
May select specific jet fragmentation
1) Sum pT2 inside a Gaussian kernel to obtain a discriminant:
2) Keep jets with g0.1 > threshold
Are those jets that keep a Gaussian shape only minimally interacting?
That would explain the suppressed RAA to the level of surface emission
Elena Bruna (Yale&INFN Torino)
4
Jets in A+A: possible biases
CAVEAT:
jet-finder based on unmodified jet-shapes
⇒ veto against modified/quenched jets
“Anti-quenching” biases!
pT cut to minimize background
⇒ bias towards less-interacting
jets
Can we exploit the biases?
Elena Bruna (Yale&INFN Torino)
5
Di-jet measurements
Trigger jets are biased towards the surface.
Recoil jets are exposed to a maximum pathlength in the medium.
Large energy loss expected.
Anti-kT, R=0.4
Trigger Jet: pT,cut=2 GeV/c, pT(trig)>20 GeV/c
Coincidence rate:
how often I measure a recoil
jet once the trigger jet is found
σ=6.5 GeV/c
Elena Bruna (Yale&INFN Torino)
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di-jet coincidence rate
Recoil Jet: R=0.4, pT,cut=0.2 GeV/c
Trigger Jet: pT,cut=2 GeV/c
pT,cut on trigger jet: allows
similar trigger jet population
in p+p and Au+Au
Recoil jets measured per
trigger jet  coincidence
rate
STAR preliminary
Significant suppression in di-jet coincidence measurements
 broadening and/or absorption?
Elena Bruna (Yale&INFN Torino)
7
Broadening or absorption?
If broadening:
Energy shift?
p+p
Absorption?
Au+Au
Jet energy spread outside R
pTJet(R)<pTJet(true)  shift of spectrum
towards lower pT
If absorption:
Jet is so quenched that the jet-finding
algorithms do not find it  Jet is lost
Or both?
Elena Bruna (Yale&INFN Torino)
8
Broadening scenario
possible interpretation of di-jet suppression
How much would the Recoil AuAu spectra need to be shifted in order to recover
unbiased pp?
(simple assumption = constant shift of the spectra, i.e. constant energy loss)
AuAu shifted/pp
Compatible with a jet pT shift Δ = 7-8 GeV/c
Elena Bruna (Yale&INFN Torino)
9
Broadening or absorption?
Energy shift?
p+p
Absorption?
Au+Au
If we were able to measure unambiguously
the jet energy (even in presence of
quenching) we could measure the
Fragmentation Functions (FF) and:
If absorption:
FF(A+A) = FF(p+p)
jets that come out are pp-like jets
If broadening:
FF(A+A) < FF(p+p)
 jets are modified !
Remark: measure of jet energy (background + possible medium effects) is
challenging!
Elena Bruna (Yale&INFN Torino)
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Jet-hadron correlations
Towards Fragmentation Functions
If tangential (halo) emission:
 Away side yield in Au+Au similar to p+p,
also for low pT,assoc
If energy loss:
Decrease of high-pT,assoc particles
Strong enhancement of low pT,assoc
Broadening
Elena Bruna (Yale&INFN Torino)
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Jet-hadron correlations
Trigger jet: Anti-kt R=0.4, pt,cut>2 GeV/c, ptjet>20 GeV/c
0.1<pt,assoc<1 GeV/c
STAR Preliminary
0-20% Au+Au
Open symbols p+p
1<pt,assoc<2.5 GeV/c
STAR Preliminary
0-20% Au+Au
pt,assoc>2.5 GeV/c
STAR Preliminary
0-20% Au+Au
Open symbols p+p
Open symbols p+p
Significant broadening on the recoil side
Observed modification of “Fragmentation Function”
Remark: flat bkg subtraction by ZYAM - jet v2 under investigation
Elena Bruna (Yale&INFN Torino)
12
Jet-hadron correlations
Broadening
π
Significant (Gaussian) broadening of the away side.
Broadening decreases with jet energy.
Out-of-cone (R>0.4) energy ~ 6-9 GeV.
 in agreement with broadening scenario in di-jet analysis!
Elena Bruna (Yale&INFN Torino)
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Jet-hadron correlations
Softening
IAA
Y away (AuAu)

Y away ( pp)

π
Softening of “jet fragmentation”
Significant enhancement at low pT (pT<2 GeV)
Suppression at low pT
Elena Bruna (Yale&INFN Torino)
14
Jets IV:
The Present:
From RHIC to LHC…
Elena Bruna (Yale&INFN Torino)
LHC: the hard probes factory
Elena Bruna (Yale&INFN Torino)
16
Remarks on Jet Kinematics (1)
jet1
Jet production:
qq(gq,gg)  qq(gq,gg)
Energy-momentum conserved.
-h
q2
q1
+h
 jets back-to-back in f
Not necessarily back-to-back in h!
Why?
jet2
Mqq  x1x2sNN
2
Example: q1 + q2  j1 + j2
q1=(x1,0,0,x1)
q2=(x2,0,0,-x2)
1 x1 
y12  ln 
2 x 2 
If x1=x2  y12=0  jets
back-to-back in h !

x1 
M qq
x2 
M qq
sNN
sNN
e y12
ey12
 this is more likely for high-pT jets, where the total energy goes into the transverse plane
If x1 ≠x2  y12≠0  jets not back-to-back in h !

Elena Bruna (Yale&INFN Torino)

17
Remarks on Jet Kinematics (1)
jet1
Simulation:
PYTHIA p+p
√sNN= 200 GeV
-h
q2
q1
+h
jet2
h range
Simulation:
PYTHIA p+p
√sNN= 200 GeV
The higher the jet pT, the more
peaked at mid-rapidity it is
Elena Bruna (Yale&INFN Torino)
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Remarks on Jet Kinematics:
RHIC vs LHC
1) For the same x1 at RHIC and LHC, the
higher the √sNN energy, the larger the
rapidity gap between the di-jets!
2) For fixed hadron pT,
different parton energies are
sampled at RHIC vs LHC !

x1 
M qq
x2 
M qq
sNN
sNN
e y12
ey12

Near side has higher pTparton than
away side
Fixed pTtrig & pTassoc  larger pTparton
at LHC
di-hadron
pTtrig>8 GeV
Keep in mind (1) and (2) when comparing di-hadron/di-jets at RHIC vs LHC
Elena Bruna (Yale&INFN Torino)
19
Hard processes: from RHIC to LHC
Large increase of jet x-section from RHIC to LHC!
Elena Bruna (Yale&INFN Torino)
20
LHC: the hard probes factory
LO p+p y=0
(h++h-)/2
0
From RHIC to LHC:
• fireball hotter, denser,
longer lifetime
√s
=
5500 GeV
200 GeV
17 GeV
LHC
• huge increase of hard
probes!
 need high-pT triggers !
RHIC
SPS
Elena Bruna (Yale&INFN Torino)
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LHC: the hard probes factory
jet cross section in p+p: RHIC vs LHC
Simulation:
PYTHIA
Anti-kT, R=0.4
huge increase of hard
probes!
 need high-pT triggers !
Cross-section falls with a smaller (power-law) exponent
 less sensitivity to the energy scale: important for background treatment
Elena Bruna (Yale&INFN Torino)
22
Hard processes: from RHIC to LHC
xT
N. Glover CTEQ,
Rhodes, (2006)
•Different xT range:
RHIC: 0.15 – 0.45
LHC: 0.02- 0.2
• RHIC is quark dominated.
LHC is gluon dominated
Elena Bruna (Yale&INFN Torino)
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LHC detectors for Jet analysis
tracking
ECAL
HCAL
muon
hadron PID
counters
lumi.
Complimentary measurements:
• large acceptance for charged hadrons, leptons and neutral energy (ATLAS, CMS)
• Hadron PID in ALICE (|h|<1)
• ALICE: full tracking to very low pT
• ATLAS,CMS also low pT with vertex detector (pp)
Elena Bruna (Yale&INFN Torino)
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Jet x-section measurement in
ALICE: p+p (PYTHIA)
ALICE EMCal PPR (2009)
EMCal acceptance: | |<0.7,  =110o
EMCal needed for triggering
and for neutral jet energy
component
TPC used for charged tracks
Large kinematical reach
in 1 year ALICE p+p running
Elena Bruna (Yale&INFN Torino)
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Jet x-section measurement in
ALICE: Pb+Pb (qPYTHIA)
ALICE EMCal PPR (2009)
EMCal acceptance: | |<0.7,  =110o
Large kinematical reach
in 1 year ALICE running
Precise measurement:
Effect of background fluctuations in jet
spectrum suppressed due to harder
underlying partonic spectrum!
Elena Bruna (Yale&INFN Torino)
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Underlying background at LHC
ALICE EMCal PPR (2009)
PYTHIA jet spectrum √s=5.5 TeV:
• embedded in HIJING 0-10% Pb+Pb
• unfolded assuming Gaussian
fluctuations with s=12 GeV/c
• unfolded spectrum within 20% of the
input spectrum!
 background fluctuations under control
because of the harder jet spectrum at LHC
wrt RHIC !
LHC: background less dangerous because of the harder parton spectrum
Elena Bruna (Yale&INFN Torino)
27
Underlying background at LHC
Jet p+p x-sec (PYTHIA)
Ratio: Jet p+p x-sec / Jet p+p ✕
Bkg fluctuations
Simulation:
PYTHIA
Anti-kT, R=0.4
LHC: background less dangerous because of the harder parton spectrum
Elena Bruna (Yale&INFN Torino)
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Let’s look at the data..
Elena Bruna (Yale&INFN Torino)
29
Jet quenching at the LHC
ALICE, Phys. Lett. B 696 (2011) 30.
Central Pb+Pb suppressed !
Peripheral suppressed less
Elena Bruna (Yale&INFN Torino)
30
Jet quenching at the LHC
ALICE, Phys. Lett. B 696 (2011) 30.
LHC RAA:
• sharp rise above 7 GeV
• minimum at ~ 0.5 RHIC RAA
Next:
• PID
• increase statistics
• take pp reference at 2.76 TeV
LHC RAA<RHIC RAA
RHIC: high-pT hadrons hadronize from quarks
LHC: high-pT hadrons hadronize from gluons (larger color charge)
Elena Bruna (Yale&INFN Torino)
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Jet quenching at the LHC
Di-Jet asymmetry:
pT ,1  pT ,2
Aj 
pT ,1  pT ,2
arXiv:1011.6182
Anti-kT R=0.4

Elena Bruna (Yale&INFN Torino)
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Jet quenching at the LHC
Di-Jet asymmetry:
CMS
pT ,1  pT ,2
Aj 
pT ,1  pT ,2
arXiv:1102.1957

Anti-kT
Iterative cone
R=0.5
Elena Bruna (Yale&INFN Torino)
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Quenching or fluctuations?
Cacciari, Salam, Soyez, arXiv:1101.2878
• Use HYDJET instead of HIJING
• Fluctuations might potentially have an impact on the dijet asymmetry
From the paper: “It is not our intention to claim that the striking di-jet
asymmetry results are an artifact of fluctuations. Nevertheless fluctuations
can significantly affect the main observable Aj”.
Elena Bruna (Yale&INFN Torino)
34
Quenching or fluctuations?
Cacciari, Salam, Soyez, arXiv:1101.2878
•Next
Use HYDJET
steps: instead of HIJING
•- Other
Fluctuations
might potentially
impact
the dijet asymmetry
observables:
jet energy have
profilean(jet
core),onR-dependence
of Aj, jet-hadron
From
the
paper:
“It
is
not
our
intention
to
claim
that
the
striking
di-jet
correlations
asymmetry
results
are an artifact
of fluctuations.
Nevertheless
fluctuations
-More exhaustive
investigation
of different
scenarios of
fluctuations and
quenching
can significantly affect the main observable Aj”.
Elena Bruna (Yale&INFN Torino)
35
Summary
Jet I:
Intro & Motivations
Jet II:
Full Jet Reconstruction
Jet III:
Results
Why jets in heavy ion collisions? Jet Tomography!
• Access kinematics of the binary hard-scattering
• Characterize the parton energy loss in the hot
QCD medium
• Study medium response to parton energy loss
Jet-finding connects Theory and Experiment
Goal: set the Jet Energy Scale
• Different systematics to take into account (double
counting,…)
• Background fluctuations: the challenge
p+p and d+Au: reference/control measurements
Broadening observed at RHIC
• From RHIC to LHC: huge increase of hard
probes!
Jet IV:
• LHC: Less sensitivity to the energy scale:
The Present: from RHIC to LHC important for background treatment
• First observation of quenching!
Elena Bruna (Yale&INFN Torino)
“Science is a way to teach how something gets to be known, what is not known,
to what extent things are known (for nothing is known absolutely), how to think
about things so that judgments can be made, how to distinguish truth from fraud,
and from show”.
R. Feynman
Thanks for this fruitful school !
Elena Bruna (Yale&INFN Torino)
BACKUP
Elena Bruna (Yale&INFN Torino)
Jet Energy resolution with di-jets
Particle-Detector jet Res:
pTJet(Part.Lev) – pTJet(Det.Lev)
~10-25 %
di-jet Res:
pTJet 1– pTJet 2
(PY Det. Lev.) ~
good!
(dijet data) :
But:
(dijet PY Det. Lev.) >
(Part-Det)
di-jet imbalance includes both energy
resolution and kT (initial state) effect!
[kT=pTjet sinfdijet]
kT: good agreement between data and
simulation
Use PYTHIA to determine the jet energy resolution
Elena Bruna (Yale&INFN Torino)
39
Jet-finding and systematics..
Tracking performance
Tracking is limited by misalignment, luminosity, resolution…
• Rare processes as high-pT jets are likely to come from high luminosity runs
Example of high-luminosity distortion? Space-charge effect  accumulation of
space charge in the TPC that causes an anomalous transport of drifting electrons in
the TPC, affecting the tracking performance by shifting the momentum up or down
(depending on the charge)
•Tracking resolution at high-pT is
expected to deteriorate  need to apply
an upper pT cut on tracks
PYTHIA simulation: p+p 200 GeV
effect of upper pT cut on jet energy scale
Elena Bruna (Yale&INFN Torino)
40
Jet-finding and systematics..
Unobserved neutral energy
Experiments like STAR and ALICE do not detect neutral, long-lived particles
(neutrons, K0L)
PYTHIA simulation:
p+p at 200 GeV
• mean missed E ~ 9%
• median missed E <0.3 %
• 50% of jets loose no energy
• model dependent
Elena Bruna (Yale&INFN Torino)
41
Fragmentation Functions
large uncertainties due to background
(further systematic evaluation needed)
AuAu (Jet+Bkg)
AuAu (Bkg)
STAR preliminary
Jet energy determined in R=0.4
pT Jet(trig)>20 GeV
pTcut=2 GeV
Charged particle FF: R(FF)=0.7
xrec=ln( pT,Jet rec / pT,hadr)
AuAu: FF(Jet)=FF(Jet+Bkg)-FF(bkg)
Bkg estimated from charged particle spectra out of jet cones
Bkg dominates at low pT
Elena Bruna (Yale&INFN Torino)
42
Fragmentation Functions
“trigger” jet
No apparent modification of FF of recoil jets
with pTrec>25 GeV would imply non-interacting
jets, but:
“recoil” jet
Jet broadeningEnergy shift harder FF
Need to better determine the jet energy
Elena Bruna (Yale&INFN Torino)
43
Fake jets in PHENIX
Pedestal comes from combinatorics of residual fake jets
When 17.8 (GeV/c)2 used as standard fake rejection cut level:
 < 10% contamination at 7.5 GeV/c
Elena Bruna (Yale&INFN Torino)
44
Jet Yields in ALICE
Elena Bruna (Yale&INFN Torino)
45
DCal for Di-Jet analysis @ ALICE
Elena Bruna (Yale&INFN Torino)
46