Hard Probes of the Quark Gluon Plasma

Lecture II: heavy flavour, geometry

Marco van Leeuwen, Nikhef and Utrecht University

Lectures at: Quark Gluon Plasma and Heavy Ion Collisions

Siena, 8-13 July 2013

2

So what are we trying to do...

• First: understand (parton) energy process– Magnitude, dominant mechanism(s)– Probability distribution

• Lost energy• Radiated gluons/fragments

– Path length dependence*– Flavour/mass dependence*– Large angle radiation?

• Goal: use this to learn about the medium

High-energy

parton(from hard scattering)

Hadrons

* Topics of today’s lecture

3

)/()( , jethadrTjetshadrT

EpDEPdEdN

dpdN

`known’ from e+e-knownpQCDxPDF

extract

Parton spectrum Fragmentation (function)Energy loss distribution

This is where the information about the medium isP(E) combines geometry with the intrinsic process

A simplified approachThis is the simplest ansatz – most calculation to date use it (except some MCs)

From yesterday’s lecture: Absolute calibration unknown Always need to compare 2 or more measurements

4

Heavy quarks

Definition: heavy quarks, m >> QCD

Charm: m ~ 1.5 GeVBottom: m ~ 4.5 GeVTop: m ~ 170 GeV

M. Cacciari, CTEQ-MCNet summer school 2008

Complications exist: QCD, EW corrections; quark mass defined in different ways

‘Perturbative’ hadronisation

Heavy quarks are (mostly) produced in hard scattering, no (or small)contribution from hadronisation and thermal production in QGP

Heavy quarks: m >> TQGP

5

Dead cone effect

Radiated wave front cannot out-run source quark

Heavy quark: < 1

Result: minimum angle for radiation Mass regulates collinear divergence

6

Heavy quark suppressionM

.Djordjevic P

RL 94

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

light

Wicks, H

orowitz et al, N

PA

784, 426

Calculated energy loss

Most pronounced for bottom

Interference effect:Radiated wave front cannot out-run source quark

Heavy quark: < 1

Result: minimum angle for radiation Mass regulates collinear divergence

7

Heavy-to-Light ratio expectations at LHC

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

hAAt

BDAAthBD pRpRpR

gq EE mass effect

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

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

Colour-charge and mass dep. of E loss

8

Charm decay reconstruction

KD 0 softsoft KDD 0*

D meson mass ~ 1.8 GeV/c2

Decay length ~ 100 mmBranching ratios ~ 4%

9

Heavy vs light comparison

Charm meson and light hadron RAA similar at

LHC

No dead cone effect?or

Not sensitive?

Jury still out...

Experiment: measure BeautyChallenging, but not impossible,

see Andrea Beraudo’s talk

10

D meson RAA

RAA < 1: charm also loses energyAgrees with (most) model calculations

However: some models have no prediction for light quarks. Calibration?

11

So how about beauty?

Would like to measure beauty – Heavier, so larger dead cone effect

:Very few all-hadron decays; tiny branching ratios:

Experimentally very challenging

Current techniques:- Semi-leptonic decays; displaced electrons- Electron-hadron correlations ‘partial reconstruction’ of the decay- Secondary J/- Beauty in jets (displaced electrons, muons)

0DB BR: 0.48%

KJB / BR: 0.1%

12

Heavy quark fragmentation

Light quarks Heavy quarks

Heavy quark fragmentation: leading heavy meson carries large momentum fraction

(Dead cone in vacuum)

Can we use this to get a handle on parton pT? To be studied...

13

Part II: Path length dependence

14

Geometry

Density profile

Profile at ~ form known

Density along parton path

Longitudinal expansion dilutes medium Important effect

Space-time evolution is taken into account in modelling

15

RAA vs and elastic elossT

. Renk, P

RC

76, 06

4905, J. Auvinen et al, P

RC

82, 051901

Elastic E-loss givessmall v2

Data require L2 or stronger path length

dependence

However, also quite sensitive to medium density evolution

In Plane

Out of Plane

16

Path length dependence: RAA vs PHENIX, arXiv:1208.2254

In Plane

Out of Plane

Suppression depends on angle, path length

Not so easy to model: calculations give different results

17

RAA vs : heavy flavour

In- vs out-of-plane difference also seen for charm Additional constraint for models?

18

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

19

pT

assoc > 3 G

eVp

Tassoc >

6 GeV

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

Suppression of away-side yield in Au+Au collisions: energy loss

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

Di-hadrons at high-pT: recoil suppression

20

Di hadron yield suppression

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?

Near sideassociated

trigger

Away side associated

trigger

21

Path length II: ‘surface bias’

Near side trigger, biases to small E-loss

Away-side large L

Away-side suppression IAA samples longer path-lengths than inclusives RAA

22

Di-hadron modelingT

. Renk, P

RC

, arXiv:1106.1740

L2 (ASW) fits dataL3 (AdS) slightly below

Modified shower generates increase at low zT

L (YaJEM): too little suppresionL2 (YaJEM-D) slightly above

Model ‘calibrated’ on single hadron RAA

23

Di-hadrons and single hadrons at LHC

Need simultaneous comparison to several measurements

to constrain geometry and E-loss

Here: RAA and IAA

Three models:ASW: radiative energy lossYaJEM: medium-induced virtualityYaJEM-D: YaJEM with L-dependent virtuality cut-off (induces L2)

24

Di-hadron with high-pT trigger

pttrig > 20 GeV at LHC: strong signals even at low pT

assoc 1-3 GeVCMS-PAS-HIN-12-010

19.2 - 24.0 GeVpTtrig (GeV): 14.0 - 28.8 GeV 28.8-35.2 GeV 35.2-48.0 GeV

25

CMS di-hadrons: near side19.2 - 24.0 GeV

pTtrig (GeV):

14.0 - 28.8 GeV 28.8-35.2 GeV 35.2-48.0 GeV

Transition enhancement → suppression @ pT ~ 3 GeV

also compatible with IAA=1 at pT > 3 GeV?

peripheral 50-60%

central 0-10%

CM

S-P

AS

-HIN

-12-010

26

CMS di-hadrons: away side

Transition enhancement → suppression @ pT ~ 2 GeV

CM

S-P

AS

-HIN

-12-010

19.2 - 24.0 GeVpT

trig (GeV):14.0 - 28.8 GeV 28.8-35.2 GeV 35.2-48.0 GeV

peripheral 50-60%

central 0-10%

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Part III: Low, intermediate pT

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Identified hadron RAA

M. Ivanov, A. Ortiz@QM2012

Low-intermediate pT (1-6 GeV):Large baryon/meson ratio

Probably due to:1) radial flow2) parton recombination

M. Ivanov, A. Ortiz@QM2012

Baryon, meson RAA similar at pT > 8 GeV

As expected from parton energy loss

29

Identified hadron RAA (strangeness)

Kaon, pion RAA

similar

: RAA~1 at pT~3 GeV/cSmaller suppression,/K enhanced at low pT

pT ~8 GeV/c:All hadrons similar

partonic energy loss + pp-like fragmentation?

30

Hadronisation by recombination

fragmenting parton:

ph = z p, z<1

recombining partons:

p1+p2=ph

Fries, Muller et alHwa, Yang et al

MesonpT=2pT,parton

Recombination of thermal (‘bulk’) partons

produces baryons at larger pT

Hot matter

Baryon pT=3pT,parton

Expect large baryon/meson ratio associated with high-pT trigger

(Hwa, Yang)

Baryon pT=3pT,parton

MesonpT=2pT,parton

Hard parton

Hot matter

‘Shower-thermal’ recombination

31

Di-hadrons: p/ in jets

associated

trigger

Jet peak

Background/Bulk region(v2, v3 peak here)

5 < pTtrig < 10 GeV

Use TOF+dE/dx to identify particles

32

p/ bulk vs jetsM

. Veld

hoen, HP

p/ ratio in Bulk regionagrees with inclusive

p/ ratio in jet* agrees with Pythia

*after background subtraction

No effect of shower-thermal recombination and/or modified color flow observed

33

Near-side ridge in AA – Flow

d+Au, 200 GeV

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

Au+Au 0-10%STAR preliminary

trigger

d+Au: ‘jet’-peak, symmetric in ,

Au+Au: extra correlation strengthat large ‘Ridge’

First seen at RHIC – Unexplained for a while

Most likely: flow, v3

34

Di-hadron correlations and flow at low pT

ALIC

E, P

RL 107, 032301

Alver and

Roland

,P

RC

81, 054

905

Low pT <~ 3 GeV: di-hadron correlations dominated by flow

Important contributions from v3, v4

Also NB: v1 can mimick jet (near or away-side)

35

Low pT di-hadron shapes at LHC

pT

0-10% 60-70% pp

2 <

pT

,tri

g <

31

< p

T,a

sso

c <

24

< p

T,t

rig <

82

< p

T,a

sso

c <

3

36

• The lowest pT bin shows a structure with a flat top in

• This feature is reproduced by AMPT

•

• Qualitative and quantitative agreement of peak shapes with AMPT compatible with hypothesis of interplay of jets with the flowing bulk

Departure from Gaussian

AMPTData

0-10%2 < pT,t < 3 GeV/c1 < pT,a < 2 GeV/c

37

Peak Deformation

PRL 93,242301 (2004)

dNch/d

rms (calc.)

rms (calc.)

Calculation + STAR prel:

2 < pT,t < 3 1 < pT,a < 2 GeV/c4 < pT,t < 8 2 < pT,a < 3 GeV/c

Centrality | 100 = pp

,

(f

it)

Significant increase of towards central events

– > (eccentricity ~ 0.2)Armesto, Salgado, Wiedemann

Longitudinal flow deforms jet shape

38

AMPT Comparison

• AMPT (A MultiPhase Transport Code) describes collective effects (e.g. v2, v3, v4) in HI collisions at LHC

– Here version with string melting (2.25) is shown

• RMS of the near-side peak reasonably described by AMPT– Detailed mechanism not known; Interplay of jet and flow ?

Centrality | 100 = pp

(fit

) (r

ad.)

(fit

) (r

ad.)

Centrality | 100 = pp

Lines: AMPT 2.25 and Pythia P-0 (for pp)

2 < pT,t < 3 1 < pT,a < 2 GeV/c3 < pT,t < 4 2 < pT,a < 3 GeV/c4 < pT,t < 8 2 < pT,a < 3 GeV/c

39

Integrated vs differential

• Inclusive hadron suppression RAA

– Overall magnitude + pT dependence: limited dynamical information

– Only useful when the energy loss mechanism is understood

• Di-hadrons; IAA

– Two ‘degrees of freedom’

– Adds constraints when compared to RAA; mostly geometry?

• Low pT, shape info– More differential, but also more difficult to model

40

Summary

• Heavy flavour– Expect dead cone effect: reduced energy loss– No clear evidence of dead cone effect for charm in data

• Path length dependence– RAA vs

– Di-hadron suppression

Favours: L2 or stronger

• Low, intermediate pT < 6-8 GeV/c– Large B/M ratio: flow or recombination?– B/M in jets ~0.2– Jet-like correlations:

• Large effect of bkg flow

• Shapes change (broadening in )

41

Extra slides

42

Comparing single- and di-hadron @ RHIC

Armesto, Cacciari, Salgado et al.

RAA and IAA fit with similar density

Confirms ~L2 dependence

Calculations with elastic loss give too little suppression

43

D mesonsQM2012, Zaida CdelV

44

Jet Quenching

1) How is does the medium modify parton fragmentation?• Energy-loss: reduced energy of leading hadron – enhancement of yield

at low pT?

• Broadening of shower?• Path-length dependence• Quark-gluon differences• Final stage of fragmentation

outside medium?

2) What does this tell us about the medium ?• Density• Nature of scattering centers? (elastic vs radiative; mass of scatt. centers)• Time-evolution?

High-energy

parton(from hard scattering)

Hadrons

45

Heavy Quark Fragmentation II

Significant non-perturbative effects seen even

in heavy quark fragmentation

46

Di-hadron correlations

associated

trigger

Background

Di-hadron correlations:• Simple and clean way to access di-jet

fragmentation• Background clearly identifiable• No direct access to undelying kinematics

(jet energy)

Compare AA to pp

After background subtraction

Energy loss+fragmentation

Quantify/summarise: IAA

Near side: yield increases

Away side: yield decreases

ALIC

E, arX

iv:1110.0121

47

Comparing single- and di-hadron @ RHIC

Armesto, Cacciari, Salgado et al.

RAA and IAA fit with similar density

Confirms ~L2 dependence

Calculations with elastic loss give too little suppression

48

Spectra at intermediate pT

Low-intermediate pT (1-6 GeV):Large baryon/meson ratio

Probably due to:1) radial flow2) parton recombination

Schukraft et al, arXiv:1202.3233

49

Parton energy loss – main questions

• Understand production rates• Understand parton energy loss process

– Energy loss as a function of density– Path length dependence

• Elastic, radiative, other?

– Mass dependence– Interplay between vacuum and

medium radiation– Broadening of shower:

• Out-of-cone radiation

– Leading hadron vs softening of FF

• Use as a probe to determine medium density (and other properties)

High-energy

parton(from hard scattering)

Hadrons

50

Di-hadron yields at LHCA

LICE

, PR

L 108,092301

Near side Away side

8 < pTtrig < 15 GeV

Near side: ~20% yield enhancement

Away side: suppressionby factor ~2

Fragmentation after energy loss Recoil parton energy loss