Hard Probesof the Quark Gluon Plasma

Lecture III: jets

Lectures at: Quark Gluon Plasma and Heavy Ion Collisions

Siena, 8-13 July 2013

Marco van Leeuwen, Nikhef and Utrecht University

2

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 fragmentation2) Out-of-cone radiation: RAA < 1

3

Jets at LHCALICE

Transverse energy map of 1 event

Clear peaks: jets of fragments from high-energy quarks and gluons

And a lot of uncorrelated ‘soft’ background

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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 ,jet = <,>particles

For a complete discussion, see: http://www.lpthe.jussieu.fr/~salam/teaching/PhD-courses.html

Sum particles inside jet Different prescriptions exist, most natural: E-scheme, sum 4-vectors

Jet is an object defined by jet algorithmIf parameters are right, may approximate parton

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Collinear and infrared safetyIllustration 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

Note also: detector effects, such as splitting clusters in calorimeter (0 decay)

Illustration by G. S

alam

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Infrared safety

Infrared safety also implies robustness against soft background in heavy ion collisions

Illustration by G. S

alam

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PbPb jet background

Cacciari et al

Background density vs multiplicity

- space filled with jetsMany ‘background jets’

Background contributes up to ~180 GeV per unit area

Statistical fluctuations remain after subtraction

Subtract background: App rawjetT

subjetT ,,

Jet finding illustration

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Jet energy asymmetryCentrality

12

12

EEEE

AJ

AT

LA

S, a

rXiv:1

01

1.6

18

2 (P

RL

)

Jet-energy asymmetry Large asymmetry seen for central events

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

Suggests large energy loss: many GeV~ compatible with expectations from RHIC+theory

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Studying the imbalanceC

MS

, arXiv:1102.1957

In Cone R<0.8 Out of Cone R>0.8

PYTHIA+HYDJET

CMS measured

tracks

jetTppmissT

)cos(//

,

Momentum imbalance restored by hadrons at large angle R>0.8 and

small pT < 2 GeV/c

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Energy dependence of asymmetryC

MS

, arXiv:1202.5022

(Relative) asymmetry decreases with energyHowever: difference pp vs PbPb remains – energy loss finite at large E

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-jet imbalance

CMS, arXiv:1502.0206Centrality

T

jetT

J pp

x -jet asymmetry

Advantage: is a parton: know parton kinematicsDisadvantage: low rate (+background 0→ )

Dominant contibution: qg → q

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G. de Barros et al., arXiv:1208.1518

pT,jet< 20 GeV/c: No change with trigger pT

Combinatorial background

Hadron-triggered recoil jet distributions

pT,jet> 20 GeV/c: Evolves with trigger pT

Recoil jet spectrum

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Remove background by subtracting spectrum with lower pT

trig:

Δrecoil =[(20-50)-(15-20)]

Reference spectrum (15-20) scaled by ~0.96 to account for

conservation of jet density

Background subtraction: Δrecoil

Unfolding correction for background fluctuations and detector response

Δrecoil measures the change of the recoil spectrum with pTtrig

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Recoil jet yield ΔIAAPYTHIA ≈0.75, approx. constant with jet pT

R=0.4

Constituents: pT

const > 0.15 GeV/c

no additional cuts (fragmentation bias) on recoil jets

pp reference: PYTHIA (Perugia 2010)

Ratio of Recoil Jet Yield ΔIAAPYTHIA

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R=0.4

Recoil Jet ΔIAAPYTHIA: R dependence

Similar ΔIAAPYTHIA for R=0.2 and R=0.4

R=0.2

No visible broadening within R=0.4

(within exp uncertainties)

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Hadrons vs jets II: recoil

PR

L108 092301

Hadrons Jets

Hadron IAA = 0.5-0.6In approx. agreement with models; elastic E-loss would give larger IAA

Jet IAA = 0.7-0.8Jet 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 jetsRaw 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

Correct spectrum and remove combinatorial jets by unfolding

Results agree with biased jets: reliably recovers all jets and removed bkg

Raw jet spectrum Fully corrected jet spectrum

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PbPb jet spectraCharged jets, R=0.3

Jet spectrum in Pb+Pb: charged particle jetsTwo cone radii, 4 centralities

M. Verweij@HP, QM

RCP, charged jets, R=0.3

Jet reconstruction does not‘recover’ much of the radiated energy

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Pb+Pb jet RAA

Jet RAA measured byATLAS, ALICE, CMS

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

ATLAS+CMS: hadron+EM jets

ALICE: charged track jets

Good agreementbetween experiments

Despite different methods:

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, hadrons, jets compared, hadrons Jets

Suppression of hadron (leading fragment) and jet yield similarIs this ‘natural’? No effect of in-cone radiation?

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Model comparison

M. Verweij@HP, QM2012

JEW

EL: K

. Za

pp et al, E

ur Ph

ys J C6

9, 617

U. Wiedemann@QM2012

Hadron RAAJet RAA

Schukraft et al, arX

iv:1202.3233

At least one model calculation reproduces the observed suppression Understand mechanism for out-of-cone radiation?

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Predicts ΔIAA~0.4, below measured

JEWEL preliminary

Model comparison IAA

JEWEL: Zapp et al., EPJ C69, 617

JEWEL correctly describes inclusive jet RAA

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Centrality and reaction plane biases:• finite, but only weak trigger pT dependence for high pT

trig

Jet trigger

Hadron trigger

Hadron trigger vs jet trigger

Hadron trigger: strong “surface bias”

maximizes recoil path length (T.Renk, private com.)

Full jet trigger: no geom. bias

partially cancelled by bkg fluctuations

T.R

enk, PR

C85 06490

8

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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

PbPbPbPb PbPbPbPb

CM

S P

AS

HIN

-12-013C

MS

, P. K

urt@Q

M12

Jet broadening: Soft radiation at large angles

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Jet fragment distributionsPbPb measurement

Ratio to pp

Low pT enhancement:soft radiation

Intermediate z:depletion: E-lossNB: z is wrt observed Ejet ≠ initial Eparton

AT

LAS

M.R

ybar@Q

M12

M. Rybar@QM2012

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Jet fragment distributions

Low pT enhancement:soft radiation

Intermediate z, pT:depletion: E-loss

CMS, Frank Ma@QM12

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A consistent view of jet quenching

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%

2010 data: arXiv:1205.5872arXiv:1205.5872

Change from

“ξ” to “pT”

Pb

Pb

– p

p (

1/G

eV

)

Broadening/excess at large r, low pT

(~2% of jet energy)

Narrowing/depletion at intermediate r, pT

No change at small r, high pT

Radius r

G. Roland@QM2012

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A consistent view of jet quenching

Charged particles from pT =50-100 GeV:

z = pT(track)/pT(jet) = 0.4-0.6 < 1

Looking at the same parton pT range

Consistent message from charged hadron RAA, inclusive jet RAA and fragmentation functions!

PbPb fragmentation function = pp for ξ <1

G. Roland@QM2012

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Direct photons

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Early times: direct photons

Photons from initial scattering• Dominant at pT > few GeV• Small reinteraction prob

Thermal photons• Low-Q2 scatterings in HG, QGP• Thermal glow of hot matter

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Direct photons at RHICP

HE

NIX

, PR

L 1

04

, 13

23

01

Idea: hot quark-gluon matter radiates photons which escape

Difficult measurement:• Large background 0 → • Thermal photons at low pT

Excess of photons seen at RHIC

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Direct (thermal) photons at LHC

Method:• Measure /0

• Low pT: use conversions• Divide /0 by theory

expectation

pp: /0 agrees with NLO

PbPb: excess over NLO at low pT

Multiply by 0 to get spectrum

M. Wilde, ALICE, QM12

Also at LHC: low pT thermal photonsT ≈ 300 MeV

First measurement of low-pT (thermal) direct photons

at LHC

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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/phenomenologyWhat 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 calculationsQin, Muller, arXiv:1012.5280

Radiation plus evolution

Parton transport (brick)

Coleman-Smith, Qin, Bass, Muller, arXiv:1108.5662

MARTINI: AMY+MC

Young, S

chenke, Jeon, Gale, arX

iv:1103.5769

Several calculations describemeasured imbalance

Most natural approach: parton showers(MARTINI, qPYTHIA, qHERWIG, JEWEL)

Need to keep track of fragments;Leading particle approximations do not work

Cannot yet check consistency with leading hadrons…