From yesterday

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


Today

Jet III: Results


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


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


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


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?


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


AuAu shifted/pp

Broadening or absorption?


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


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



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



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



π

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IAA =Y away(AuAu)

Y away (pp)

Softening

Softening of “jet fragmentation” Significant enhancement at low pT (pT<2 GeV)Suppression at low pT


Jets IV: The Present:

From RHIC to LHC…

LHC: the hard probes factory


Remarks on Jet Kinematics (1)


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)


- +

jet1

jet2

q2 q1

The higher the jet pT, the more peaked at mid-rapidity it is

Remarks on Jet Kinematics: RHIC vs LHC


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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


Large increase of jet x-section from RHIC to LHC!

LHC: the hard probes factory


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 !


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


•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


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


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!


EMCal acceptance: ||<0.7, Δφ=110o


Underlying background at LHC



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


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..


Jet quenching at the LHC


ALICE, Phys. Lett. B 696 (2011) 30.

Central Pb+Pb suppressed !

Peripheral suppressed less



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)



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A j =pT ,1 − pT ,2

pT ,1 + pT ,2

Di-Jet asymmetry:

Anti-kT R=0.4arXiv:1011.6182


CMS

€

A j =pT ,1 − pT ,2

pT ,1 + pT ,2

Di-Jet asymmetry:


arXiv:1102.1957

Anti-kT Iterative cone R=0.5

Cacciari, Salam, Soyez, arXiv:1101.2878

Quenching or fluctuations?


• 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?


• 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


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


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!


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


BACKUP

Jet Energy resolution with di-jets


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..


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..


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

42

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


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


Fragmentation Functions

Fake jets in PHENIX


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


DCal for Di-Jet analysis @ ALICE


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