What do we understand about parton energy loss at RHIC?

An experimentalists viewpoint

Marco van Leeuwen,Utrecht University

2

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

3

Energy loss in QCD matter

radiated gluon

propagating parton

kTQCD bremsstrahlung(+ LPM coherence effects)

Density of scattering centers:

Nature of scattering centers, e.g. mass: radiative vs elastic loss

Or no scattering centers, but fields synchrotron radiation?

1

2

ˆTk

q

2ˆ~ LqE Smed

Transport coefficient

Energy loss

Energy loss probes:

4

Questions about energy loss

• What is the dominant mechanism: radiative or elastic?– Heavy/light, quark/gluon difference, L2 vs L dependence

• How important is the LPM effect?– L2 vs L dependence

• Can we use this to learn about the medium? – Density of scattering centers?– Temperature?– Or ‘strongly coupled’, fields are dominant?

Phenomenological questions:Large vs small angle radiationMean E?How many radiations?Virtuality evolution/interplay with fragmentation?

5

0 RAA – high-pT suppression

Hard partons lose energy in the hot matter

: no interactions

Hadrons: energy loss

RAA = 1

RAA < 1

0: RAA ≈ 0.2

: RAA = 1

Factor 4-5 suppression – A large effect!

6

Parton energy loss and RAA modeling

Qualitatively:

)/()( , jethadrTjetshadrT

EpDEPdEdN

dpdN

`known’ from e+e-knownpQCDxPDF

extract

Parton spectrum Fragmentation (function)Energy loss distribution

Contains medium propertiese.g. density profile

‘Same’ in p+pand Au+Au

Vacuum fragmentation (?)

7

Four theory approaches

• Multiple-soft scattering (ASW-BDMPS)– Full interference (vacuum-medium + LPM)

– Approximate scattering potential

• Opacity expansion (GLV/WHDG)– Interference terms order-by-order (first order default)

– Dipole scattering potential 1/q4

• Higher Twist– Like GLV, but with fragmentation function evolution

• Hard Thermal Loop (AMY)– Most realistic medium

– LPM interference fully treated

– No interference between vacuum frag and medium

Allo

w d

efin

ition

of

‘que

nchi

ng w

eigh

ts’ P

(E

)

8

Extracting , TB

ass e

t al, P

RC

79

, 02

49

01

All can be fit to RAA – RAA is not decisive

Large differences between formalisms

q̂

Not all approaches can be ‘correct’

Can we decide which formalism(s) is/are correct?

9

Two extreme scenarios

p+p

Au+Au

pT

1/N

bin

d2 N/d

2 pT

Scenario IP(E) = (E0)

‘Energy loss’

Shifts spectrum to left

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

‘Absorption’

Downward shift

(or how P(E) says it all)

P(E) encodes the full energy loss process

RAA not sensitive to details of mechanism

Would need E/E ~ 0.2 to get RAA ~ 0.2

Would need a = 0.2, b = 0.8to get RAA ~ 0.2

10

Energy loss spectrum

BrickL = 2 fm, E/E = 0.2E = 10 GeV

Typical examples with fixed L

E/E> = 0.2 R8 ~ RAA = 0.2

Different theoretical approximation (ASW, WHDG) give different results – significant?

Significant probability to lose no energy (P(0))

Broad distribution, large E-loss (several GeV, up to E/E = 1)

Theory expectation: mix of partial transmission+continuous energy loss– Can we see this in experiment?

11

How geometry complicates mattersM

. V

erw

eij,

UU

L < R, small q L > R, smaller q

For ‘typical partons’ L and q are correlated

L ~ R, large q

Resulting P(E/E) peaked at ~ 0 and 1Black-white scenario

No sensitivity to continuous E-loss?

Energy loss distributionsummed over all partons

Static medium

12

Path length dependence I

Centrality

Au+Au

Cu+Cu

<L>, density increase with centralityVary L and density independently by changing Au+Au Cu+Cu

Turns out to be not very precise

In-plane

Out of plane

Change L in single system in-plane vs out of plane

Collision geometry

2ˆ~ LqE Smed

13

RAA vs reaction plane angle

Azimuthal modulation, path length dependence largest in ASW-BDMPS

Data prefer ASW-BDMPS

C. V

ale

, PH

EN

IX, Q

M0

9

But why? – No clear answer yet

14

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

15

Di hadron yield suppression

No suppressionSuppression 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?

Note: per-trigger yields can be same with energy-loss

Near sideassociated

trigger

Away side associated

trigger

16

RAA and IAA in a single model

Armesto, Cacciari, Salgado et al.

RAA and IAA give similar densityModel has L2 built in – But how sensitive is it?

Hydro + ASW-BDMPS

17

L scaling: elastic vs radiativeT. Renk, PRC76, 064905

RAA: input to fix density Radiative scenario fits data; elastic scenarios underestimate

suppression

Indirect measure of path-length dependence: single hadrons and di-hadrons probe different path length distributions

Confirms L2 dependence radiative loss dominates

18

L-dependence from surface bias

Near side trigger, biases to small E-loss

Away-side large L

Away-side suppression IAA samples different path-length distribution than inclusives RAA

19

Dead cone effectRadiated wave front cannot

out-run source quark

Heavy quark: < 1

Result: minimum angle for radiation

light

M.D

jordjevic PR

L 94

Wicks, H

orowitz et al, N

PA

784, 426

Expected energy loss

Most pronounced for bottom

Dead cone effect reduces E-loss

20

Heavy quark suppressionP

HE

NIX

nucl-ex/0611018, ST

AR

nucl-ex/0607012

Djordjevic, Phys. Lett. B632, 81

Armesto, Phys. Lett. B637, 362

Measured suppression of non-photonic electrons larger than

expected

Using non-photonic electrons

Expect: heavy quarks lose less energy due to dead-cone effect

Radiative (+collisional) energy loss not dominant? E.g.: in-medium hadronisation/dissociation (van Hees, et al)

21

Heavy Quark comparison

No minimum – Heavy Quark suppression too large for ‘normal’ medium density

Armesto, Cacciari, Salgado et al.

22

Charm/bottom separation

Combine rB and RAA to extract RAA for charm and bottom

arXiv:0903.4851 hep-ex

Bottom/charm ratio in p+p agrees with theory expectations (FONLL)

23

I: Djordjevic, Gyulassy, Vogt and Wicks, Phys. Lett. B 632 (2006) 81; dNg/dy = 1000II: Adil and Vitev, Phys. Lett. B 649 (2007) 139III: Hees, Mannarelli, Greco and Rapp, Phys. Rev. Lett. 100 (2008) 192301

pT > 5 GeV/c

RAA for c e and b eB

.Biritz Q

M09

Combined data show:electrons from bothB and D suppressed

Large suppression suggestsadditional energy loss mechanism

(resonant scattering, dissociative E-loss)

24

Summary so far

• Large suppression of light hadrons parton energy loss

• Rough estimates: E/E ~ 0.2, or 80% absorption

• QCD predicts broad distribution P(E)

• Geometry: many small path lengths– Effectively black/white NB: means no sensitivity to dynamics!

• IAA vs RAA: L2 preferred – radiative dominates

• RAA vs reaction plane: modulation in data larger than expected?

• Heavy quarks: suppression larger than expected

25

JetsBasic idea: recover radiated energy – parton energy before E-loss

Out-of cone radiationSuppression of jet yield

RAAjets < 1

In-cone radiation: Softening of fragmentation function and/or broadening of jet structure

Alternative: use recoil photon in -jet events (low statistics)

Sa

lga

do

, Wie

de

ma

nn

, P

RL

93

, 04

23

01

Early prediction: out-of-cone radiation small effect

26

Fragmentation functions

Qualitatively: )()()( zDEPzD vacmed

Dashed lines: include gluon fragments (assuming 1 gluon emitted)

Fragmentation functions sensitive to P(E)Distinguish GLV from BDMPS?

27

Are FF sensitive to P(E) ?Toy model curves

P(E) toy model Fragmentation function ratio

Fragmentation functions are sensitive to P(E) – Somewhat

28

-hadron results

STAR Preliminary

Large suppression for away-side: factor 3-5

Results agree with model predictions

Would like to see z-dependence, uncertainties still large

A. H

am

ed

, ST

AR

, QM

09

8 < ET, < 16 GeV

PH

EN

IX, P

RC

80

, 02

49

08

29

Jet finding in heavy ion events

η

p t p

er g

rid c

ell [

GeV

]

STAR preliminary~ 21 GeV

FastJet:Cacciari, Salam and Soyez; arXiv: 0802.1188http://rhig.physics.yale.edu/~putschke/Ahijf/A_Heavy_Ion_Jet-Finder.html

Jets clearly visible in heavy ion events at RHIC

Use different algorithms to estimate systematic uncertainties:• Cone-type algorithms

simple cone, iterative cone, infrared safe SISCone

• Sequential recombination algorithmskT, Cambridge, inverse kT

Combinatorial backgroundNeeds to be subtracted

30

p+p Au+Au central

Jet spectra

Note kinematic reach out to 50 GeV

• Jet energy depends on R, affects spectra• kT, anti-kT give similar results

Take ratios to compare p+p, Au+Au

31

Jet RAA at RHIC

Jet RAA >> 0.2 (hadron RAA)

Jet finding recovers most of the energy loss measure of initial parton energy

M. P

loskon, ST

AR

, QM

09

Some dependence on jet-algorithm? Under study…

32

Radius dependence

RAA depends on jet radius:Small R jet is single hadron

Jet broadening due to E-loss ?

M. P

losko

n, S

TA

R, Q

M0

9

33

Fragmentation functions

STAR Preliminary

pt,rec(AuAu)>25 GeV20<pt,rec(AuAu)<25 GeV

Use recoil jet to avoid biases

Recoil suppression in reconstructed jets small

E. B

runa, ST

AR

, QM

09

34

Di-jet suppression

34

Ele

na B

run

a fo

r the S

TA

R C

olla

bora

tion

- Q

M0

9STAR PreliminarySTAR Preliminary

E. B

runa, ST

AR

, QM

09

Jet IAA

Away-side jet yield suppressed partons absorbed

... due to large path length(trigger bias)

35

Emerging picture from jet results

• Jet RAA ~ 1 for sufficiently large R – unbiased parton selection

• Away side jet fragmentation unmodified – away-side jet emerges without E-loss

• Jet IAA ~ 0.2 – Many jets are absorded (large E-loss)

Study vs R, E to quantify P(E) and broadening

Ongoing developments of event generators for modified fragmentation important for measurement, interpretation

JEWEL, q-PYTHIA, YaJEM, PYQUEN, MARTINI

36

RHIC future

• Machine upgrades– Stochastic cooling to increase luminosity– Several polarisation upgrades

• Experiment upgrades– Vertex detectors for charm, bottom– Forward calorimeters– STAR: TOF for PID– DAQ upgrades to deal with rates

Increased luminosity: -hadron, jets, charmonia ()

37

Delivered Integrated Luminosity

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Weeks into run

Nu

cleo

n p

air

lum

inos

ity

LN

N [

pb-1

]

d-Au2008

Au-Au2007

Cu-Cu2005

Au-Au2004

d-Au2003

Au-Au2001/02

0

10

20

30

40

50

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Weeks into runN

ucl

eon

pai

r lu

min

osit

y L

NN

[p

b-1]

2003P=34%

2005P=46%

2006P=60%

2008P=45%

Nucleon-pair luminosity allows comparison of luminosities of different species

Inte

grat

ed n

ucl

eon

-pai

r lu

min

osit

y L

NN [

pb

-1]

Inte

grat

ed n

ucl

eon

-pai

r lu

min

osit

y L

NN [

pb

-1]

Collider luminosity increases as experience grows– Often beyond original design!

Heavy ion runs Polarized proton runs

38

-hadron luminosity projection

Gradual increase in p+p and Au+Au luminosity reduces measurement uncertainties

39

Inner tracking upgrades at RHIC

STAR PHENIX

Goals:Charm flow, spectra, RAA

b-tagging for bottom RAA

40

Heavy-light ratios

Armesto plot

Armesto et al, PRD71, 054027

light

M.Djordjevic PRL 94 (2004)

Wicks, Horowitz et al, NPA 784, 426

Heavy-light RAA ratios directly sensitive to dead-cone effect

Effect sizeable, should be measurable with vertex detectors

41

ALICE

2010: p+p collisions @ 7-10 TeV2010: Pb+Pb collisions @ ? TeV

3 Large general purpose detectorsALICE dedicated to Heavy Ion Physics, PID p,K, out to pT > 10 GeV

Large Hadron Collider at CERN

ATLAS

CMS

ATLAS, CMS: large acceptance, EM+hadronic calorimetry

42

From RHIC to LHC

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

- Energy dependenc of E-loss with high-energy jets

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)

43

Energy loss distribution

BrickL = 2 fm, E/E = 0.2E = 10 GeV

Typical examples with fixed L

E/E> = 0.2 R8 ~ RAA = 0.2

Significant probability to lose no energy (P(0))

Broad distribution, large E-loss (several GeV, up to E/E = 1)

Broad distribution; typical energy loss ~5 GeV

RHIC: E ~ Ejet, LHC: Ejet > E sensitivity to P(E)

44

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

RHICRHIC

45

Heavy-to-Light ratios at LHC

Heavy-to-light ratios: )()()( )(/)( t

hAAt

BDAAthBD pRpRpR

gq EE mass effect

For pT > 10 GeV charm is ‘light’ RD/h probes colour-charge dep. of E lossRB/h probes mass dep. of E loss

Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027

Colour-charge and mass dep. of E loss

46

TDpp

TDAA

collT

DAA dpdN

dpdN

NpR

/

/1)(

Tepp

TeAA

collT

eAA dpdN

dpdN

NpR

/

/1)(

ALICE heavy flavour performance

mb = 4.8 GeV

D0 K B e + X

1 year at nominal luminosity(107 central Pb-Pb events, 109 pp events)

Alice can measure charm from 0 < pT < 20 GeV and bottom (semi-leptonic decays) 3 < pT < 20 GeV

47

ALICE performance: heavy-to-light

1 year at nominal luminosity(107 central Pb-Pb events, 109 pp events)

)()()(/ ThAAT

DAAThD pRpRpR

)()()( D from eB from e/ tAAtAAtDB pRpRpR

48

=ln(EJet/phadron)

pThadron ~2 GeV for

Ejet=100 GeV

Borghini and Wiedemann, hep-ph/0506218

Medium modification of fragmentation MLLA calculation: good approximation for soft fragmentationextended with ad-hoc implementation medium modifications

Recent progress: showering with medium-modified Sudakov factors, see Carlos’s talk and arXiv:0710.3073

Trends intuitive: suppression at high z, enhancement at low z

z 0.37 0.14 0.05 0.02 0.007

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

Jet shapes

51

Jet shapes in ATLAS

52

Conclusion• Parton energy loss at RHIC is large

E ~ E Limited sensitivity to E-loss dynamics• Di-hadron suppression indicates L2 dependence radiative

dominates• Heavy flavour more suppressed than expected• Jet broadening significant• Fragmentation function modification difficult to measure –

expected ?• Future at RHIC

– Direct measurements of charm, heavy/light ratios– Increase -jet statistics– Improve understanding of jet results

• Future at LHC– Verify/test our understanding in a new regime– May reach into E > E regime– Abundant jets E ~ 100 GeV

Sensitivity to E-loss dynamics

We still have 4+ formalisms – Need to devise tests of validity

53

Thanks for your attention!

54

Heavy quark fragmentation

Light quarks Heavy quarks

Heavy quark fragmentation: leading heavy meson carries large momentum fraction

More handle to extract P(E)?

55

Direct photons: no interactions

PHENIX

Direct spectra

Scaled by Ncoll

PHENIX, PRL 94, 232301

ppTbin

AuAuTAA dpdNN

dpdNR

/

/

Direct in A+A scales with Ncoll

Centrality

A+A initial state is incoherent superposition of p+p for hard probes

56

Geometry III

Brick isolines from left to right:R7 = 0.45, 0.25, 0.15, 0.05

57

Determining the medium densityPQM (Loizides, Dainese, Paic),Multiple soft-scattering 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• Typical energy loss per L: 0

• Coupling constant S

q̂

PHENIX, arXiv:0801.1665,J. Nagle WWND08

58

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

Data constrain model parameters to 10-20%

Method extracts medium density given the model/calculation Theory uncertainties need to be further evaluated

e.g. comparing different formalisms, varying geometry

But models use different medium parameters– How to compare the results?

59

Some pocket formula results

Large differences between models

GLV/WHDG: dNg/dy = 1400

2

1)(

Rdy

dN g

3

0 fm4.12)fm1( 32

202.116T

T(0) = 366 MeV

PQM: (parton average) /fmGeV2.13ˆ 2q

32202.172

ˆ Tq s

T = 1016 MeV

AMY: T fixed by hydro (~400 MeV), s = 0.297

60

Naive picture for di-hadron measurements

PT,jet,1

PT,jet,2

Fragment distribution(fragmentation fuction)

Out-of-cone radiation:PT,jet2 < PT,jet1

jetT

hadrT

P

pz

,

,

dzdN

Ref: no ElossIn-cone radiation:PT,jet2 = pT,jet1

Softer fragmentation

Naive assumption for di-hadrons: pT,trig measures PT,jet

So, zT=pT,assoc/pT,trig measures z

61

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 fromaway-side suppression

and single hadron suppression agree

Theory: ZOWW, PRL98, 212301

Data: STAR PRL 95, 152301

8 < pT,trig < 15 GeV

zT=pT,assoc/pT,trig

(Experiment and theory updates in the works)

62

Heavy Quark Fragmentation II

Significant non-perturbative effects seen even

in heavy quark fragmentation

63

• Use e-K invariant mass to separate charm and bottom

• Signal: unlike-sign near-side correlations

• Subtract like-sign pairs to remove background

• Use Pythia to extract D, B yields

arXiv:0903.4851 hep-ex

D/B from e-K correlations

B → e + D D → e + KD → e + K

64

Luminosity projections

Also: -hadron correlations

Projection of uncertainties in Upsilon(1S) RAA for two sets of integrated luminosity.

Heavy Flavor signalsstudy color screening with quarkonia

J/Ψ

65

Testing Ncoll scaling II: Charm

PRL 94 (2005)

NLO prediction:m ≈ 1.3 GeV, reasonably hard scale at pT=0

Total charm cross section scales with Nbin in A+A

Scaling observed in PHENIX and STAR – scaling error in one experiment?

66

B

DX.Y. Lin, hep-ph/0602067

e h rBe hB (1 rB )e h

D

rB eB /(eD eB )

Charm/bottom separation

Idea: use e-h angular correlations to tag semi-leptonic D vs B decay

D → e + hadrons

B peak broader due to larger mass

Extract B contribution by fitting:

67

Charm-to-Bottom Ratio

PHENIX p+p measuments agree with pQCD (FONLL) calculation

arXiv:0903.4851 hep-ex

68

STAR TOF – now fully installedTOF 1/β cut rejects hadrons providing nearly

complete and accurate electron identification for di-lepton program.

Large statistics for di-hadron, fragmentation studiesIdentification of decay products from charm, bottom

69

STAR inner tracker upgrade (HFT)

2 30.5

~ 30 microns pointing resolution at 0.7 GeV/c

~ 30 microns secondary vertex resolution (large p)

3 Layers: SSD: existing double sided strip detector IST: intermediate strip layer PIXEL: 2 inner layers of high resolution

Pixel (MAPS) (18*18 m) and thin 0.4% Xo per layer

Main goal: heavy flavour spectra and v2 at low and high pT

70

PHENIX Silicon Vertex Detectors

• VTX: silicon VerTeX barrel tracker– Fine granularity, low occupancy

• 50m×425m pixels for L1 and L2• R1=2.5cm and R2=5cm

– Stripixel detector for L3 and L4• 80m×1000m pixel pitch • R3=10cm and R4=14cm

– Large acceptance• ||<1.2, almost 2 in plane

– Standalone tracking

• FVTX: Forward silicon VerTeX tracker – 2 endcaps with 4 disks each– pixel pad structure (75m x 2.8 to

11.2 mm)FVTX endcaps1.2<||<2.7 mini strips

VTX barrel ||<1.2

71

Physics Projections with HFT+TOF

Charm collectivity Medium properties, light flavor thermalization

Charm energy loss Energy loss mechanisms, Medium properties

72

PHENIX VTX Performance

Large suppression of heavy flavourCurrent result mix of b and cVTX can separate b and c

Expected with VTX (0.4/nb ~3 weeks in RUN11)

73

ALICE charm performancepp, s = 14 TeV

charm (D0 K) beauty (B e+X)

1 year at nominal luminosity(109 pp events)

A. DaineseAlice can measure charm from 0 < pT < 20 GeV

and bottom (semi-leptonic decays) 3 < pT < 30 GeVin p+p events