Jets and high-pT probes in ALICE

Marco van Leeuwen,

Nikhef & Utrecht University

ALICE junior meeting, Wuhan

17 October 2013

2

Introduction

Data/results in order of experimental difficulty level:

• Inclusive spectra

• Di-hadrons

• Jets

Goal of the talk: discuss understanding of energy loss

in terms of physical mechanisms and theory/modelling

Interpretation aspects:

- Mean energy loss

- Geometry

- Energy loss distrubution

For a list of ongoing analyses and new ideas in ALICE, see my talk from Tuesday

3

Seeing quarks and gluons

In high-energy collisions, observe traces of quarks, gluons (‘jets’)

4

Singularities in pQCD

Divergence of perturbative calculation is absorbed in hadronisation effects

(massless case)

Soft divergence Collinear divergence

p

rad

1This is why jets are collimated:

5

Hard processes in QCD

• Hard process: scale Q >> LQCD

• Hard scattering High-pT parton(photon) Q ~ pT

• Heavy flavour production m >> LQCD

Cross section calculation can be split into

• Hard part: perturbative matrix element

• Soft part: parton density (PDF), fragmentation (FF)

Soft parts, PDF, FF are universal: independent of hard process

QM interference between hard and soft suppressed (by Q2/L2 ‘Higher Twist’)

Factorization

c

chbbaa

abcd

ba

T

h

pp

z

Dcdab

td

dQxfQxfdxdxK

pdyd

d

0

/22

2)(

ˆ),(),(

parton density matrix element FF

Probably not strictly true in Pb+Pb; take as working hypothesis for now

6

Fragmentation and parton showers

large Q2 Q ~ mH ~ LQCD mF

Analytical calculations: Fragmentation Function D(z, m) z=ph/Ejet Only longitudinal dynamics

High-energy

parton

(from hard scattering)

Ha

dro

ns

MC event generators

implement ‘parton showers’ Longitudinal and transverse dynamics

7

Soft QCD matter and hard probes

Hard-scatterings produce ‘quasi-free’ partons Initial-state production known from pQCD Probe medium through energy loss

Heavy-ion collisions produce QCD matter

Dominated by soft partons p ~ T ~ 100-300 MeV

‘Hard Probes’: sensitive to medium density, transport properties

8

Nuclear geometry: Npart, Ncoll

b

Two limiting possibilities:

- Each nucleon only interacts once, ‘wounded nucleons’

Npart = nA + nB (ex: 4 + 5 = 9 + …)

Relevant for soft production; long timescales: Npart

- Nucleons interact with all nucleons they encounter

Ncoll = nA x nB (ex: 4 x 5 = 20 + …)

Relevant for hard processes; short timescales: Nbin

Transverse view

Eccentricity

Density profile r: rpart or rcoll

22

22

xy

xy

x

y

L

9

Testing volume (Ncoll) scaling in Au+Au

PHENIX

Direct g spectra

Scaled by Ncoll

PHENIX, PRL 94, 232301

ppTcoll

AuAuT

AAdpdNN

dpdNR

/

/

Direct g in A+A scales with Ncoll

Centrality

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

10

Nuclear modification factor RAA

p+p

Au+Au

pT

1/N

bin d

2N

/d2p

T

‘Energy loss’

Shifts spectrum to left

‘Absorption’

Downward shift

Measured RAA is a ratio of yields at a given pT

The physical mechanism is energy loss; shift of yield to lower pT

ppTcoll

PbPbT

AAdpdNN

dpdNR

/

/

11

Getting a sense for the numbers – RHIC

Oversimplified calculation:

- Fit pp with power law

- Apply energy shift or relative E loss

Not even a model !

Ball-park numbers: DE/E ≈ 0.2, or DE ≈ 3 GeV

for central collisions at RHIC

0 spectra Nuclear modification factor

PH

EN

IX, P

RD

76, 0

51106, a

rXiv

:0801.4

020

12

From RHIC to LHC RHIC: 200 GeV

LHC: 2.76 TeV

per nucleon pair

Energy ~24 x higher

LHC: spectrum less steep,

larger pT reach

n

T

TT

pdp

dN

p

2

1

RHIC: n ~ 8.2

LHC: n ~ 6.4

Fractional energy loss:

2

1

D

n

AAE

ER

RAA depends on n, steeper spectra, smaller RAA

13

From RHIC to LHC

RHIC LHC

RHIC: n ~ 8.2 LHC: n ~ 6.4

20.023.012.6

32.023.014.4

Remember: still ‘getting a sense for the numbers’; this is not a model!

With same medium density/energy loss, expect less suppression at LHC

Similar/slightly lower RAA indicates significant increase in medium density!

14

Towards a more complete picture

• Energy loss not single-valued, but a distribution

• Geometry: density profile; path length distribution

• Energy loss is partonic, not hadronic

– Full modeling: medium modified shower

– Simple ansatz for leading hadrons: energy loss followed by

fragmentation

– Quark/gluon differences

Many model calculations exist:

some are more realiable/realistic than others!

15

RAA at LHC and JEWEL

Suppression factor 2-6

Significant pT-dependence

JEWEL energy loss model agrees with measurements

(tuned at RHIC, LHC ‘parameter-free’)

JEWEL: Monte Carlo event generator with radiative+collisional energy loss

- Modified showers with MC-LPM implementation

- Geometry: expanding Woods-Saxon density

16

Dihadron correlations

associated

D

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

Combinatorial background

17

Path length: ‘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

NB: ‘surface bias’ is not a sharp effect; it’s a shifted distribution

18

Di-hadron modeling T. R

en

k, P

RC

, arX

iv:1

10

6.1

74

0

L2 (ASW) fits data

L3 (AdS) slightly below Modified shower

generates increase at low zT

L (YaJEM): too little suppresion

L2 (YaJEM-D) slightly above

Model ‘calibrated’ on single hadron RAA

19

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 loss

YaJEM: medium-induced virtuality

YaJEM-D: YaJEM with L-dependent

virtuality cut-off (induces L2)

Note also: IAA > RAA

Compatible with expected

‘L2’ dependence

however, many details

20

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 fragmentation

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

21

Jets at LHC ALICE

h

Transverse energy map of 1 event

Clear peaks: jets of fragments

from high-energy quarks and gluons

And a lot of uncorrelated ‘soft’ background

22

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 h,jet = <h,>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 algorithm

If parameters are right, may approximate parton

23

Collinear and infrared safety Illu

stra

tion b

y G

. Sa

lam

Jets should not be sensitive to soft effects

(hadronisation and E-loss)

- Collinear safe

- Infrared safe

24

Collinear safety

Note also: detector effects,

such as splitting clusters in calorimeter (0 decay)

Illustra

tion b

y G

. Sa

lam

25

Infrared safety

Infrared safety also implies robustness

against soft background in heavy ion collisions

Illustra

tion b

y G

. Sa

lam

26

Background jets Raw 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

27

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

28

PbPb jet spectra Charged jets, R=0.3

Jet spectrum in Pb+Pb: charged particle jets

Two cone radii, 4 centralities

M. Verweij@HP, QM

RCP, charged jets, R=0.3

Jet reconstruction does not

‘recover’ much of the radiated energy

29

Pb+Pb jet RAA

Jet RAA measured by

ATLAS, 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 agreement

between experiments

Despite different methods:

30

Particles and jets model comparison

M. Verweij@HP, QM2012

JE

WE

L: K

. Zapp e

t al, E

ur P

hys J

C69, 6

17

U. Wiedemann@QM2012

Charged particle RAA

Jet RAA

Schukra

ft et a

l, arX

iv:1

202.3

233

At least one model calculation reproduces the observed suppression

Understand mechanism for out-of-cone radiation?

31

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

32

Remove background by

subtracting spectrum with

lower pTtrig:

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

33

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

34

Hadrons vs jets II: recoil

PR

L108 0

92301

Hadrons Jets

Hadron IAA = 0.5-0.6

In approx. agreement with models;

elastic E-loss would give larger IAA

Jet IAA = 0.7-0.8

Jet IAA > hadron IAA

Not unreasonable

NB/caveat: very different momentum scales !

35

Summary

• Jet quenching is a large effect

• Goal: understand energy loss mechanism and learn about medium density/geometry – Need close contact with theory

– Need good and controlled models

• Need multiple measurements to constrain models/theory – Inclusive and di-hadron measurements constrain path length

dependence (+geometry)

• Jets show large suppression (with limited cone size R=0.2, 0.3 in ALICE) – Indicates large out-of-cone radiation

Work in progress – plenty of room to get involved

36

Four formalisms

• Hard Thermal Loops (AMY)

– Dynamical (HTL) medium

– Single gluon spectrum: BDMPS-Z like path integral

– No vacuum radiation

• Multiple soft scattering (BDMPS-Z, ASW-MS)

– Static scattering centers

– Gaussian approximation for momentum kicks

– Full LPM interference and vacuum radiation

• Opacity expansion ((D)GLV, ASW-SH)

– Static scattering centers, Yukawa potential

– Expansion in opacity L/l (N=1, interference between two centers default)

– Interference with vacuum radiation

• Higher Twist (Guo, Wang, Majumder)

– Medium characterised by higher twist matrix elements

– Radiation kernel similar to GLV

– Vacuum radiation in DGLAP evolution

Multiple gluon emission

Fokker-Planck

rate equations

Poisson ansatz

(independent emission)

DGLAP

evolution

See also: arXiv:1106.1106

37

RAA at RHIC: ‘calibrating’ the models B

ass e

t al, P

RC

79

, 02

49

01

ASW:

HT:

AMY:

/fmGeV2010ˆ2q

/fmGeV5.43.2ˆ2q

/fmGeV4ˆ2q

Large density:

AMY: T ~ 400 MeV

Transverse kick: qL ~ 10-20 GeV

Large uncertainty in

absolute medium density

PH

EN

IX, a

rXiv

:12

08

.22

54

All calculations describe the data

Not very sensitive to

energy loss distribution

38

RAA at LHC & models

ALICE: arXiv:1208.2711

CMS: arXiv:1202.2554

Broad agreement

between models and

LHC RAA

Extrapolation from

RHIC

tends to give too much

suppression at LHC

Many model curves: need more constraints and/or selection of models

39

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)

Ha

dro

ns

40

pTa

sso

c > 3

GeV

p

Ta

sso

c > 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

41

Dihadron 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 side associated

trigger

Away side associated

trigger

42

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)

43

Nuclear modification factor

ppTcoll

PbPbT

AAdpdNN

dpdNR

/

/

Suppression factor 2-6

Significant pT-dependence

Similar at RHIC and LHC?

So what does it mean?

PHENIX at RHIC ALICE at LHC

√sNN = 200 GeV √sNN = 2760 GeV

44

0 RAA – high-pT suppression

Hard partons lose energy in the hot matter

g: no interactions

Hadrons: energy loss

RAA = 1

RAA < 1

0: RAA ≈ 0.2

g: RAA = 1

45

g, hadrons, jets compared g, hadrons Jets

Suppression of hadron (leading fragment) and jet yield similar

Is this ‘natural’? No effect of in-cone radiation?