Post on 08-Jan-2016
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Elena Bruna (Yale&INFN Torino)
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 measurementsBroadening observed at RHIC
Elena Bruna (Yale&INFN Torino)
Today
Jet III: Results
p+p and d+Au: reference/control measurementsBroadening observed at RHIC
More on PHENIX vs STARMore quenching observables:di-jets, jet-hadron
Jet IV:The Present: from RHIC to LHC
Hard Probes at LHC vs RHIC
Results on quenching at LHC
Jet-finding in PHENIX
Elena Bruna (Yale&INFN Torino) 3
CAVEAT:jet-finder based on unmodified jet-shapes
⇒ veto against modified/quenched jets
“Anti-quenching” biases!
Jet-finding in PHENIX
Elena Bruna (Yale&INFN Torino) 4
Step back: how does jet-finding work in PHENIX?
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
Jet-finding based on a Gaussian kernel (σ=0.3, 0.4)Focuses on the core of the jet
Are those jets that keep a Gaussian shape only minimally interacting?That would explain the suppressed RAA to the level of surface emission
Jets in A+A: possible biases
Elena Bruna (Yale&INFN Torino) 5
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?
Di-jet measurements
Elena Bruna (Yale&INFN Torino) 6
EMC trigger
Trigger jet
Recoil jet
Trigger jets are biased towards the surface. Recoil jets are exposed to a maximum path-length in the medium. Large energy loss expected.
σ=6.5 GeV/c
Anti-kT, R=0.4Trigger 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
di-jet coincidence rate
7Elena Bruna (Yale&INFN Torino)
Significant suppression in di-jet coincidence measurements broadening and/or absorption?
STAR preliminary
Recoil Jet: R=0.4, pT,cut=0.2 GeV/cTrigger 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
Broadening or absorption?
Elena Bruna (Yale&INFN Torino) 8
p+p
Au+Au
Energy shift?
Absorption?
If broadening:
Jet energy spread outside RpT
Jet(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?
Broadening scenario
Compatible with a jet pT shift Δ = 7-8 GeV/c
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)
possible interpretation of di-jet suppression
9Elena Bruna (Yale&INFN Torino)
AuAu shifted/pp
Broadening or absorption?
Elena Bruna (Yale&INFN Torino) 10
p+p
Au+Au
Energy shift?
Absorption?
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!
Jet-hadron correlations
Elena Bruna (Yale&INFN Torino) 11
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
Jet-hadron correlations
Elena Bruna (Yale&INFN Torino) 12
0.1<pt,assoc<1 GeV/c 1<pt,assoc<2.5 GeV/c pt,assoc>2.5 GeV/cSTAR Preliminary0-20% Au+Au
STAR Preliminary0-20% Au+Au
STAR Preliminary0-20% Au+Au
Open symbols p+p Open symbols p+p
Open symbols p+p
Trigger jet: Anti-kt R=0.4, pt,cut>2 GeV/c, ptjet>20 GeV/c
Significant broadening on the recoil side
Observed modification of “Fragmentation Function”Remark: flat bkg subtraction by ZYAM - jet v2 under investigation
Jet-hadron correlations
Elena Bruna (Yale&INFN Torino) 13
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!
Broadening
Jet-hadron correlations
Elena Bruna (Yale&INFN Torino) 14
π
€
IAA =Y away(AuAu)
Y away (pp)
Softening
Softening of “jet fragmentation” Significant enhancement at low pT (pT<2 GeV)Suppression at low pT
Elena Bruna (Yale&INFN Torino)
Jets IV: The Present:
From RHIC to LHC…
LHC: the hard probes factory
Elena Bruna (Yale&INFN Torino) 16
Remarks on Jet Kinematics (1)
Elena Bruna (Yale&INFN Torino) 17
Jet production:qq(gq,gg) qq(gq,gg)
Energy-momentum conserved. jets back-to-back in
Not necessarily back-to-back in !Why?
Example: q1 + q2 j1 + j2q1=(x1,0,0,x1) q2=(x2,0,0,-x2)
€
Mqq2 = x1x2sNN
- +
jet1
jet2
q2 q1
€
Δy12 =1
2lnx1
x2
⎛
⎝ ⎜
⎞
⎠ ⎟
€
x1 =Mqq
sNNe+Δy12
€
x2 =Mqq
sNNe−Δy12
If x1=x2 Δy12=0 jets back-to-back in ! this is more likely for high-pT jets, where the total energy goes into the transverse plane
If x1 ≠x2 Δy12≠0 jets not back-to-back in !
Simulation:PYTHIA p+p √sNN= 200 GeV
Simulation:PYTHIA p+p √sNN= 200 GeV
range
Remarks on Jet Kinematics (1)
Elena Bruna (Yale&INFN Torino) 18
- +
jet1
jet2
q2 q1
The higher the jet pT, the more peaked at mid-rapidity it is
Remarks on Jet Kinematics: RHIC vs LHC
Elena Bruna (Yale&INFN Torino) 19
€
x1 =Mqq
sNNe+Δy12
€
x2 =Mqq
sNNe−Δy12
1) For the same x1 at RHIC and LHC, the higher the √sNN energy, the larger the rapidity gap between the di-jets!
di-hadronpTtrig>8 GeV
2) For fixed hadron pT, different parton energies are sampled at RHIC vs LHC !Near side has higher pT
parton than away side
Fixed pTtrig & pTassoc larger pT
parton at LHC
Keep in mind (1) and (2) when comparing di-hadron/di-jets at RHIC vs LHC
Hard processes: from RHIC to LHC
Elena Bruna (Yale&INFN Torino) 20
Large increase of jet x-section from RHIC to LHC!
LHC: the hard probes factory
Elena Bruna (Yale&INFN Torino) 21
LHC
RHIC
SPS
(h++h-)/2
π0
17 GeV
200 GeV
5500 GeV=√s
LO p+p y=0
From RHIC to LHC:• fireball hotter, denser, longer lifetime
• huge increase of hard probes! need high-pT triggers !
Elena Bruna (Yale&INFN Torino) 22
Simulation:PYTHIAAnti-kT, R=0.4
LHC: the hard probes factoryjet cross section in p+p: RHIC vs LHC
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
Hard processes: from RHIC to LHC
Elena Bruna (Yale&INFN Torino) 23
•Different xT range: RHIC: 0.15 – 0.45 LHC: 0.02- 0.2
• RHIC is quark dominated. LHC is gluon dominated
N. Glover CTEQ, Rhodes, (2006)
xT
LHC detectors for Jet analysis
Elena Bruna (Yale&INFN Torino) 24
trackingECALHCALmuonhadron PIDcounters lumi.
Complimentary measurements:• large acceptance for charged hadrons, leptons and neutral energy (ATLAS, CMS)• Hadron PID in ALICE (||<1)• ALICE: full tracking to very low pT
• ATLAS,CMS also low pT with vertex detector (pp)
ALICE EMCal PPR (2009)
Jet x-section measurement in ALICE: p+p (PYTHIA)
EMCal needed for triggeringand for neutral jet energy component TPC used for charged tracks
Large kinematical reachin 1 year ALICE p+p running
EMCal acceptance: ||<0.7, Δφ=110o
25Elena Bruna (Yale&INFN Torino)
Jet x-section measurement in ALICE: Pb+Pb (qPYTHIA)
Large kinematical reachin 1 year ALICE running
Precise measurement:
Effect of background fluctuations in jet spectrum suppressed due to harder underlying partonic spectrum!
ALICE EMCal PPR (2009)
EMCal acceptance: ||<0.7, Δφ=110o
26Elena Bruna (Yale&INFN Torino)
Underlying background at LHC
Elena Bruna (Yale&INFN Torino) 27
ALICE EMCal PPR (2009)
PYTHIA jet spectrum √s=5.5 TeV: • embedded in HIJING 0-10% Pb+Pb• unfolded assuming Gaussian fluctuations with =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
Underlying background at LHC
Elena Bruna (Yale&INFN Torino) 28
LHC: background less dangerous because of the harder parton spectrum
Simulation:PYTHIAAnti-kT, R=0.4
Jet p+p x-sec (PYTHIA)
Ratio: Jet p+p x-sec / Jet p+p ✕ Bkg fluctuations
Let’s look at the data..
Elena Bruna (Yale&INFN Torino) 29
Jet quenching at the LHC
Elena Bruna (Yale&INFN Torino) 30
ALICE, Phys. Lett. B 696 (2011) 30.
Central Pb+Pb suppressed !
Peripheral suppressed less
Jet quenching at the LHC
Elena Bruna (Yale&INFN Torino) 31
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 quarksLHC: high-pT hadrons hadronize from gluons (larger color charge)
Jet quenching at the LHC
Elena Bruna (Yale&INFN Torino) 32
€
A j =pT ,1 − pT ,2
pT ,1 + pT ,2
Di-Jet asymmetry:
Anti-kT R=0.4arXiv:1011.6182
Elena Bruna (Yale&INFN Torino) 33
CMS
€
A j =pT ,1 − pT ,2
pT ,1 + pT ,2
Di-Jet asymmetry:
Jet quenching at the LHC
arXiv:1102.1957
Anti-kT Iterative cone R=0.5
Cacciari, Salam, Soyez, arXiv:1101.2878
Quenching or fluctuations?
Elena Bruna (Yale&INFN Torino) 34
• Use HYDJET instead of HIJING• Fluctuations might potentially have an impact on the dijet asymmetryFrom 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”.
Cacciari, Salam, Soyez, arXiv:1101.2878
Quenching or fluctuations?
Elena Bruna (Yale&INFN Torino) 35
• Use HYDJET instead of HIJING• Fluctuations might potentially have an impact on the dijet asymmetryFrom 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”.
Next steps:- Other observables: jet energy profile (jet core), R-dependence of Aj, jet-hadron correlations-More exhaustive investigation of different scenarios of fluctuations and quenching
Elena Bruna (Yale&INFN Torino)
Summary
Jet I: Intro & Motivations
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 II: Full Jet Reconstruction
Jet-finding connects Theory and ExperimentGoal: set the Jet Energy Scale• Different systematics to take into account (double counting,…)• Background fluctuations: the challenge
Jet III: Results
p+p and d+Au: reference/control measurementsBroadening observed at RHIC
Jet IV:The Present: from RHIC to LHC
• From RHIC to LHC: huge increase of hard probes! • LHC: Less sensitivity to the energy scale: important for background treatment• First observation of quenching!
Elena Bruna (Yale&INFN Torino)
Thanks for this fruitful school !
“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
Elena Bruna (Yale&INFN Torino)
BACKUP
Jet Energy resolution with di-jets
Elena Bruna (Yale&INFN Torino) 39
Particle-Detector jet Res:pT
Jet(Part.Lev) – pTJet(Det.Lev)
~10-25 %
di-jet imbalance includes both energy resolution and kT (initial state) effect![kT=pT
jet sinΔdijet]
kT: good agreement between data and simulation
di-jet Res:pT
Jet 1– pTJet 2
(PY Det. Lev.) ~ (dijet data) : good!
But: (dijet PY Det. Lev.) > (Part-Det)
Use PYTHIA to determine the jet energy resolution
Jet-finding and systematics..
Elena Bruna (Yale&INFN Torino) 40
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 GeVeffect of upper pT cut on jet energy scale
Jet-finding and systematics..
Elena Bruna (Yale&INFN Torino) 41
Unobserved neutral energy
Experiments like STAR and ALICE do not detect neutral, long-lived particles (neutrons, K0
L)
PYTHIA simulation:p+p at 200 GeV
• mean missed E ~ 9%• median missed E <0.3 %• 50% of jets loose no energy• model dependent
ST
AR
pre
limin
ary
Fragmentation Functions
AuAu: FF(Jet)=FF(Jet+Bkg)-FF(bkg)
Bkg estimated from charged particle spectra out of jet cones
Bkg dominates at low pT
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AuAu (Jet+Bkg)
AuAu (Bkg)
pT Jet(trig)>20 GeV
pTcut=2 GeV
large uncertainties due to background (further systematic evaluation needed)
rec=ln( pT,Jet rec / pT,hadr)
low zhigh z
Charged particle FF: R(FF)=0.7
Jet energy determined in R=0.4
Elena Bruna (Yale&INFN Torino)
No apparent modification of FF of recoil jets with pTrec>25 GeV would imply non-interacting jets, but:
Jet broadeningEnergy shift harder FF
Need to better determine the jet energy43
“recoil” jet“recoil” jet
“trigger” jet“trigger” jet
EMC trigger
Elena Bruna (Yale&INFN Torino)
Fragmentation Functions
Fake jets in PHENIX
Elena Bruna (Yale&INFN Torino) 44
Pedestal comes from combinatorics of residual fake jetsWhen 17.8 (GeV/c)2 used as standard fake rejection cut level: < 10% contamination at 7.5 GeV/c
Jet Yields in ALICE
Elena Bruna (Yale&INFN Torino) 45
DCal for Di-Jet analysis @ ALICE
Elena Bruna (Yale&INFN Torino) 46