Jet modifications at RHIC

Marco van Leeuwen, Utrecht University

2

QCD and quark parton model

S. Bethke, J Phys G 26, R27

Running coupling:s grows with decreasing Q2

Asy

mpt

otic

free

dom

At low energies, quarks are confined in hadrons

At high energies, quarks and gluons are manifest

gqqee

Running coupling: from confinement to asymptotic freedomQCD governs both extremes.

Can we study/conceptualise the evolution?

This is the basic theory, but what is the phenomenology?

QCD Lagrangian

3

Hard probes of QCD matter

Use the strength of pQCD to explore QCD matter

Use ‘quasi-free’ partons from hard scatterings

to probe ‘quasi-thermal’ QCD matterInteractions between parton and medium:-Radiative energy loss-Collisional energy loss-Hadronisation: fragmentation and coalescence

Sensitive to medium density, transport properties

Calculable with pQCD

Quasi-thermal matter: dominated by soft (few 100 MeV) partons

4

Radiative energy loss in QCD

2

ˆ q

2ˆ~ LqE Smed

Energy loss process characterized by a single constant

Transport coefficient

43~~ˆ glueq

Transport coefficient is a fundamental parameter of QCD matter

Energy loss

kT~

pQCD expectation

Transport coefficient sets medium properties

Non-perturbative: is a Wilson loopq̂

(Wiedemann)

369.26ˆ TNq cSYMSYM (Liu, Rajagopal, Wiedemann)e.g. N=4 SUSY: From AdS/CFT

(Baier et al)

5

STARSTAR

Relativistic Heavy Ion Collider

PHENIX STAR

Au+Au sNN= 200 GeV

RHIC: variety of beams: p+p, d+Au, Au+Au, Cu+CuTwo large experiments: STAR and PHENIX

Smaller experiments: PHOBOS, BRAHMS decomissionedDedicated to study QCD: proton spin and Quark Gluon Plasma

6

High-pT hadron suppression

Size of medium

ppTbin

AuAuTAA dpdNN

dpdNR

/

/

Compare Au+Au spectra to properly scaled p+p spectra:

‘nuclear modification factor’

: no interactions

Hadrons: energy loss

RAA = 1

RAA < 1

Direct photons confirm volume scaling

Hadrons suppressed: energy loss

7

Energy loss in QCD matter

ppTbin

AuAuTAA dpdNN

dpdNR

/

/

: RAA = 1

0, h±: RAA ≈ 0.2

Au+Au 200 GeV, 0-5% centralCompare Au+Au spectra to properly scaled p+p spectra:

‘nuclear modification factor’

D. d’Enterria

Hard partons lose energy in the hot matter

Hadron suppression ~ independent of pT for pT>~4 GeV

: no interactions

Hadrons: energy loss

RAA = 1

RAA < 1

8

Di hadron correlations

associated

trigger

8 < pTtrig < 15 GeV

pTassoc > 3 GeV

Use di-hadron correlations to probe the jet-structure in p+p, d+Au

Near side Away side

and Au+Au

Combinatorialbackground

pT assoc > 3 G

eVp

T assoc > 6 GeV

d+Au Au+Au 20-40% Au+Au 0-5%

Suppression of away-side yield in Au+Au collisionsMeasures energy loss in di-jet events

No detectable broadening or change of peak shape: fragmentation after energy loss

High-pT hadron production in Au+Au dominated by (di-)jet fragmentation

Highest pT: focus on fragmentation

10

Di hadron yield suppression

No suppression Suppression byfactor 4-5 in central Au+Au

Away-side: Suppressed by factor 4-5 large energy loss

Near side Away side

STAR PRL 95, 152301

8 < pT,trig < 15 GeV

Yield of additional particles in the jet

Yield in balancing jet, after energy loss

Near side: No modification Fragmentation outside medium?

11

Theory vs. data IPHENIX, arXiv:0801.1665,J. Nagle WWND08

PQM (Loizides, Dainese, Paic),Multiple soft-scatttering approx (Armesto, Salgado, Wiedemann)Realistic geometry

GLV (Gyulassy, Levai, Vitev), opacity expansion (L/), average path length

WHDG (Wicks, Horowitz, Djordjevic, Gyulassy)GLV + realistic geometry

ZOWW (Zhang, Owens, Wang, Wang) Medium-enhanced power corrections (higher twist) Hard sphere geometry

AMY (Arnold, Moore, Yaffe) Finite temperature effective field theory (Hard Thermal Loops)

For each model:

1. Vary parameter and predict RAA

2. Minimize 2 wrt data

Models have different but ~equivalent parameters:

• transport coeff. • gluon density dNg/dy• energy density 0

• coupling constant S

q̂

12

Medium density from RAA

PQM <q> = 13.2 GeV2/fm +2.1- 3.2

^

GLV dNg/dy = 1400 +270- 150

WHDG dNg/dy = 1400 +200- 375

ZOWW 0 = 1.9 GeV/fm +0.2- 0.5

AMY s = 0.280 +0.016- 0.012

Quantitative extraction gives medium density to 10-20% Method takes into account only exprimental uncertainties Theory uncertainties need to be further evaluted by comparing different formalisms and other model parameters

Different models approximately agree – except PQM, high density

Density 30-50x cold nuclear matter

13

d-Au

Au-Au

Medium density from di-hadron measurement

IAA constraintDAA constraintDAA + scale uncertainty

J. Nagle, WWND2008

associated

trigger

0=1.9 GeV/fm single hadrons

Medium density from away-side suppression and single hadron suppression agreeSome open questions: disagreement in d+Au?

14

Fundamental quantity P(E)

~15 GeV

Renk, Eskola, hep-ph/0610059

Salgado and Wiedemann, Phys. Rev. D68, 014008

Radiation spectrum Radiation in realistic medium

In realistic systems, energy loss is a broad distribution P(E)

Single-hadron and di-hadron observables fold production spectra with P(E)

Can we access P(E) experimentally?Need to fix parton energy: -jet events

Ejet = E

15

-jet in Au+Au

Use shower shape in EMCal to form 0 sample and -rich sample

Combinatorial subtraction to obtain direct- sample

A. Hamed, STAR, QM08

16

Away-side suppression with direct- triggers

A. Hamed et al QM08

First -jet results from heavy ion collisions

Measured suppression agrees with theory expectations

Next step: measure pTassoc dependence to probe E distribution

Model predictions tuned to hadronic

measurements

17

Lowering pT: gluon fragments/bulk response

3 < pt,trigger < 4 GeVpt,assoc. > 2 GeV

Au+Au 0-10%STAR preliminaryassociated

trigger

d+Au 40-100%Jet-like peak

`Ridge’: associated yield at large dN/d approx. independent of

Strong - asymmetry suggests coupling to longitudinal flowLong. flow Long. flow

J. Putschke, M. van Leeuwen, et al

18

More medium effects: away-side

3.0 < pTtrig < 4.0 GeV/c

1.3 < pTassoc < 1.8 GeV/c

A. Polosa, C. Salgado

Vitev, PLB630

Mach Cone/Shock wave

T. Renk, J. Ruppert

Stöcker, Casseldery-Solana et al

Gluon radiation+Sudakov

Au+Au 0-10%

d+Au

Near side:Enhanced yield in Au+Au consistent with ridge-effect Away-side:

Strong broadening in central Au+Au‘Dip’ at =

Medium response (shock wave)or gluon radiation with kinematic constraints?

(other proposals exist as well: kT-type effect or

Cherenkov radiation)

M. Horner, M. van Leeuwen, et al

Trigger particle

Note also: not shown is large background – some non-trivial may be hiding there?

19

pT evolution of correlations

Increase trigger pT

arXiv:0801.4545

A. Hanks et al, WWND08PHENIX arXiv 0801.4545

Low pT: recoil clear double-peak

Effect reduces with increasing pT?

20

Conclusion

• High-pT hadron production at RHIC probes jet structure and parton energy loss• Data/theory comparisons becoming quantitative

– Qhat ~ 10 GeV2/fm, dN/dy ~ 1500 medium density 30-50x nuclear matter

• ‘Golden probe’ -jet:– First results consistent with hadron measurements– Can we extract P(E)? Statistics needed …

• Intermediate pT 1 - 4 GeV, new phenomena:– Ridge: - assymetry, yield at large – Broad recoil distribution

Luminosity still increase… so is theoretical understandingMore to come!

21

0-12%

4.0 < pTtrig < 6.0 GeV/c 6.0 < pT

trig < 10.0 GeV/c3.0 < pTtrig < 4.0 GeV/c

Preliminary

Au+Au 0-12%1.3 < pT

assoc < 1.8 GeV/c

Low pTtrig: broad shape, two peaks High pT

trig: broad shape, single peak

Away-side shapes

Fragmentation becomes ‘cleaner’ as pTtrig goes up

Suggests kinematic effect?

M. Horner, M. van Leeuwen, et al

22

Background level for di-hadrons

Signal is few per centSo is v2-modulation

Δ12

2-Part Correlation

Flow background

“Jetty”signal

C. Pruneau, Q

M06

23

Energy loss in a QCD medium

+ alternative scenarios, e.g. shock wave

Energy loss and fragmentation

Unmodified fragmentation after energy loss

Fragmentation in the medium completely modified

A more complete picture

FragmentationIn-medium energy loss

Or in-medium fragmentation

Or

Time-scales matterh

Th m

p~Hadron formation time

Lower pTassoc: measure radiation fragments

Lower pTtrig: explore timescale

24

Baryon enhancement

Large baryon/meson ratio in Au+Au ‘intermediate pT‘

Hadronisation by coalescence?3-quark pT-sum wins over fragmentation

M. Konno, QM06

High pT: Au+Au similar to p+p Fragmentation dominates

p/ ~ 1, /K ~ 2

25

Hadronisation through coalescence

fragmenting parton:ph = z p, z<1

recombining partons:p1+p2=ph

Fries, Muller et alHwa, Yang et al

Baryon pT=3pT,parton

MesonpT=2pT,parton

‘Shower-thermal’ recombinationwill result in larger associated

B/M at intermediate pT

Recombination of thermal (‘bulk’) partons

produces baryons at larger pT No associated yield

(Hwa, Yang)

Recombination enhancesbaryon/meson ratios

Hard parton

Hot matter

26

Associated yields from coalescence

Baryon pT=3pT,parton

MesonpT=2pT,parton

Expect large baryon/meson ratio associated with high-pT

trigger

No associated yield with baryons from coalescence:

Expect reduced assoc yield with baryon triggers 3<pT<4 GeV

(Hwa, Yang)Hard parton

Hot matter

Baryon pT=3pT,parton

MesonpT=2pT,parton

Hard parton

Hot matter

Recombination of thermal (‘bulk’) partons

‘Shower-thermal’ recombination

27

Baryon/meson ratios in ‘jets’

• Shape similar for mesons and baryons– provides constraint on

models describing modification of away-side

• Baryon to Meson ratio similar to the bulk– inconsistent with vacuum

fragmentation– consistent with jet induced

medium excitation

PHENIX arXiv, A. Hanks WWND 08

28

STAR Preliminary

Associated baryon/meson ratios

STAR Preliminary

p/ ratio in jet-peak < inclusive p/ ratio in ridge > inclusive

Ridge and jet-peak have different hadro-chemistry, different production mechanism

Jet-peak Ridge regionpT

trig > 4.0 GeV/c

2.0 < pTAssoc

< pTtrig

29

Jet-like peak: (Λ+Λ)/2K0

S ≈0.5

STAR Preliminary

Associated baryon/meson ratios

STAR Preliminary

Ridge: (Λ+Λ)/2K0S ≈ 1

Note: systematic error due to v2 not shown

Similar to p+p inclusive ratio

Baryon/meson enhancement in the ridge?

L. Gaillard, J. Bielcikova, C. Nattras et al.

No shower-thermal contribution?

30

Separating jet and ridge: pT-spectraJet spectra

Yie

ld (p

t,ass

oc >

pt,a

ssoc

,cut

)

Ridge spectra

Yie

ld (p

t,ass

oc >

pt,a

ssoc

,cut

)

pt,assoc,cutpt,assoc,cut

Jet (peak) spectra harden with pT,trig

Peak dominated by jet fragmentation Radiated gluons ‘thermalise’ in the medium?

Jet and ridge separate dynamics

inclusive

Ridge yield and spectra independent of pT,trigSlope of spectra similar to inclusives

J. Putschke, M

. van Leeuwen, et al

inclusive

31

Properties of medium at RHIC

Broad agreement between different observables, and with theory

432ˆ qpQCD:

2.8 ± 0.3 GeV2/fmq̂

(Baier)

23 ± 4 GeV/fm3

T 400 MeV

Transport coefficient

Total ETViscosity

10.008.0ˆ

25.13

q

Ts

(model dependent)

= 0.3-1fm/c

~ 5 - 15 GeV/fm3 T ~ 250 - 350 MeV

(Bjorken)

sTS

34

From v2

dydE

RVE T

02

1

GeV580dy

dET1.0s

(Majumder, Muller, Wang)

Lattice QCD:/s < 0.1

A quantitative understanding of hot QCD matter is emerging

(Meyer)

32

Fixing the jet energy:-jet events

T. Renk, PRC74, 034906

-jet: monochromatic source sensitive to P(E)

Expectations for different P(E)

E = 15 GeV

-jet events are rare, need large luminosity

First results from 2007 RHIC run

p+p

33

RHIC summary

• Jets interact strongly with medium at RHIC

• High pT: yield suppression, but no change in shapes– Fragmentation after energy loss

• Lower pT: enhancement, strongly modified shapes– Gluon fragments, medium response, etc

• Large Baryon/Meson ratio suggests coalescence of ‘free’ quarks– Test shower-thermal contribution by di-hadron correlations

2.8 ± 0.3 GeV2/fmq̂Transport coefficient from high-pT results:

34

Extra slides

35

Extracting the transport coefficient

Zhang, H et al, nucl-th/0701045

Di-hadrons provide stronger constrain on density

Extracted transport coefficient from singles and di-hadrons consistent

2.8 ± 0.3 GeV2/fmqq ˆ~00

2-minimum narrower for di-hadrons

Di-hadron suppression

Inclusive hadron suppression

Di-hadronsInclusive hadrons

36

Theory vs. data II

HEDP/HEDLA meeting APS St. Louis Apr 08 Jet probes of the QGP 36

016.0012.0S

32.05.00

200375

g

270150

g

21.22.3

280.0AMY

GeV/fm9.1ZOWW

1400dy

dNWHDG

1400dy

dNGLV

fm/GeV2.13ˆPQM/ASW

q Model parameters are constrained within ~20%

Values are large: ~30-50 times cold nuclear matter density!

Additional assumptions → different models are broadly consistent (except PQM – much larger than others)

Strong conclusions: • initially generated medium is highly opaque to energetic partons • very dense, high temperature matter has been created

PHENIX ’08; J. Nagle WWND08

37

Two extreme scenarios

p+p

Au+Au

pT

1/N

bin d

2 N/d

2 pT

Scenario IP(E) = (E0)

‘typical energy loss’

Shifts spectrum to left

Scenario IIP(E) = a (0) + b (E)

‘partial transmission’

Downward shift

(or how P(E) says it all)

P(E) encodes the full energy loss process

RAA cannot distinguish those two extreme scenarios

… need more differential probes

38

Radiative energy loss: calculational frameworks

38

A. Majumder, nucl-th/0702066

GLV (Gyulassy, Levai, Vitev): systematic expansion in small number of scatterings (“opacity”) n=L/

ASW (Armesto, Salgado, Wiedemann): multiple soft interactions

ZOWW (Zhang, Owens, Wang Wang): medium-enhanced power corrections to vacuum fragmentation function (higher twist)

AMY (Arnold, Moore, Yaffe): finite temperature effective field theory (Hard Thermal Loops) at small coupling

39

The extremes of QCD

This is the basic theory, but what is the phenomenology?

Small coupling Quarks and gluons

are quasi-free

Calculable with pQCD

Two basic regimes in which QCD theory gives quantitative results:Hard scattering and bulk matter

QCD Lagrangian

Nuclear matter Quark Gluon Plasma

High densityQuarks and gluons

are quasi-free

Bulk QCD matter

Calculable with Lattice QCD

Hard scattering

40

Expectations for hot QCD matter

Bernard et al. hep-lat/0610017

Tc ~ 170 -190 MeV

Energy density from Lattice QCD

Deconfinement transition: sharp rise of energy density at Tc Measure energy density

does not reach Boltzmann limit

Signals remaining interactions, structure?

Measure transport properties-Viscosity -Transport coefficient q̂

c ~ 1 GeV/fm3

41

Bulk QCD matter in heavy ion collisions

Azimuthal anisotropy:

v 2

Elliptic flow

pT (GeV)

We create bulk QCD matter at RHIC

))(2cos21( 2 RvddN

Initial state pressure accelerates matter

v2 = 0 free streaming

Au+Au event

dNch/dy 600

For central Au+Auat √sNN = 200 GeV

Low pT: Qualitative agreement with hydrodynamics:

viscosity, mean free path small

42

Experimental probes of energy loss

• Particle spectra– High statistics– Integrates over all production mechanisms

• Di-hadron correlations– Probe jet-structure– Some control over parton kinematics

• Identified particles– Probe hadronisation mechanisms– Heavy flavours: systematics of energy loss

Focus of this talk

43

Model dependence of

Different calculational frameworks

C. Loizides hep-ph/0608133v2

/fmGeV 24ˆ6 2 q2.8 ± 0.3 GeV2/fmq̂

Di-hadronsInclusive hadrons

Zhang, H et al, nucl-th/0701045

q̂

Multiple soft scattering (BDMPS, Wiedemann, Salgado,…)

Twist expansion (Wang, Wang,…)

Different approximations to the theory give significantly different results

Main uncertainties:- Formalism for QCD radiation- Geometry (density profile)

44

ALICE

2008: p+p collisions @ 14 TeV2009: Pb+Pb collisions @ 5.5 TeV

3 Large general purpose detectors

ALICE dedicated to Heavy Ion Physics, PID p,K, out to pT > 10 GeV

Large Hadron Collider at CERN

ATLAS

CMS

45

From RHIC to LHC

RHICs=200 GeV Au+Au s=5.5 TeV Pb+Pb

LHC

- Larger pT-reach:typical parton energy > typical E

- New observablese.g. jet reconstruction fix parton energy

Larger initial density= 10-15 GeV/fm3 at RHIC ~ 100 GeV/fm3 at LHC

10k/year

Large cross sections for hard processes

Including heavy flavours

Validate understanding of RHIC data

Direct access to energy loss dynamics, P(E)

And others, e.g. gluon saturation

46

RAA at LHC

S. Wicks, W. Horowitz, QM2006

T. Renk, QM2006

Expected rise of RAA with pT depends on energy loss formalism

Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)

LHC: typical parton energy > typical E

GLV BDMPS

RHIC RHIC

47

Jet modifications at LHC

Radial profileFragmentation function

PQM with fragmentation of radiated gluons (A. Morsch)

Energy loss depletes high-zand populates low-z

Low-z fragments from gluon radiation at large R

In-medium energy loss redistributes momenta in jetsMeasure these modifications to extract P(E), medium properties

Expectations from QCD+jet quenchingJet reconstruction

Ejet = 125 GeV

=ln(EJet/phadron)z 0.37 0.14 0.05 0.02 0.007

48

ALICE EMCal

Lead-scintillator sampling calorimeter||<0.7, =110o

~13k towers (x~0.014x0.014)

ALICE-EMCal project: -Approved in 2007 -Full detector by 2011

US-France-Italy project

Testbeam:E

E %11%2)(

Support frame installed

EMCal module

Improves jet energy resolutionProvides jet triggers

49

Full jet reconstruction performanceSimulation input

referenceMedium modified (APQ)

Simulated result

Full jet reco in ALICE is sensitive to modification of fragmentation function

E > E, explore dynamics of energy loss process

50

Conclusion

• Large effects of medium on parton fragmentation– Lower pT: various effects

• Large baryon/meson ratio• Near-side ridge• Away-side broadening

– High pT: fragmentation after energy loss• Quantitative understanding

– Transport coefficient– High energy density ~ 10 - 30 GeV/fm3

– Low viscosity /s ~ 0.1

Clear picture of in-medium energy loss and medium properties at RHIC developing

Future at RHIC and LHC:Direct measurements of energy loss -jet- High-pT : E > E- Full jet reconstruction

Crucial tests of energy loss theory

2.8 ± 0.3 GeV2/fmq̂

51

Thank you for your attention

52

M. Lamont (STAR), J.Phys.G32:S105-S114,2006J. Bielcikova (STAR), v:0707.3100 [nucl-ex]

(Λ

+Λ

)/2K

0 S

Baryon/meson ratio in jets, ridge and inclusive

2 < pT,trig < 3 GeV

53

PreliminaryPreliminary

Near side yield||>0.9

Away side yield||<0.9

8 < pTtrig < 15 GeV 8 < pT < 15 GeV

zT=pTassoc/pT

trigzT=pTassoc/pT

trig

Energy loss in action

Both near- and away-side show yield enhancement at low pT

Possible interpretation:

di-jet → di-jet (lower Q2) + gluon fragments

‘primordial process’ High-pT fragmentsas in vacuum

Near side: ridgeAway-side: broadening

M. H

orner, M. van Leeuw

en, et al

Au+

Au

/ d+A

u

8 < pT < 15 GeV

Near side yield ratio

zT=pTassoc/pT

trig

0.2

1.0Lower pTtrig

Preliminary

Away side yield ratio

zT=pTassoc/pT

trig

Au+

Au

/ d+A

u

M. Horner, M. van Leeuwen, et al

Lower pTtrig

54

Results and interpretation

Extraction of direct away-side yields

R=Y-rich+h/Y0+hnear near

Y+h = (Y-rich+h - RY0+h )/1-Raway away

Assume no near-side yield for direct

 then the away-side yields per trigger obey

55

Results and interpretationDirect away-side yields

The away-side yield of the associated particle per trigger in -jet is suppressed.