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High-pT probes of QCD matter

Marco van Leeuwen, Utrecht University

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Outline of lectures

I. QCD in p+p collisions

II. Energy loss in Heavy Ion Collisions at high-pT

III. Intermediate pT

Extracting medium parameters

IV. Jet reconstructionOutlook to LHC

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Part I: perturbative QCD in p+p collisions

• A few words on experiments

• What do we know/understand in p+p– Jet production– Direct photons– Hadron production– Heavy flavour

• Experimental notes: luminosity and triggering

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Particle Data Group topical reviews http://pdg.lbl.gov/2004/reviews/contents_sports.html

QCD and jets: CTEQ web page and summer school lectures http://www.phys.psu.edu/~cteq/

Handbook of Perturbative QCD, Rev. Mod. Phys. 67, 157–248 (1995)http://www.phys.psu.edu/~cteq/handbook/v1.1/handbook.ps.gz

QCD and Collider Physics, R. K. Ellis, W. J. Sterling, D.R. Webber, Cambridge University Press (1996)

An Introduction to Quantum Field Theory, M. Peskin and D. Schroeder, Addison Wesley (1995)

Introduction to High Energy Physics, D. E. Perkins, Cambridge University Press, Fourth Edition (2000)

General QCD references

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Heavy Ion references

RHIC overviews:P. Jacobs and X. N. Wang, Prog. Part. Nucl. Phys. 54, 443 (2005)B. Mueller and J. Nagle, Ann. Rev. Nucl. Part. Sci. 56, 93 (2006)

RHIC experimental white papersBRAHMS: nucl-ex/0410020PHENIX: nucl-ex/0410003PHOBOS: nucl-ex/0410022STAR: nucl-ex/0501009

LHC Yellow Reports

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Experimental facilities: accelerators

Centre-of-mass energies √s:SPS < 20 GeVRHIC 200 - 500 GeVTevaTron 1.9 TeVLHC 5.5 - 14 TeV

Note also:SppS 630 GeV

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Example detector for DIS

Here: forward electromagnetic calorimeters to measure scattered electron in Deeply Inelastic Scattering

Detectors are designed for specific measurements

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STAR and PHENIX at RHIC

PHENIXSTAR

Large acceptance at mid-rapidity:TPC tracking(coarse) EMCalSome forward Calorimeters

General purpose detector

Central tracking/calo arms(partial coverage, finely segmented calo)Forward muon arms

Focus on rare probes(electrons/photons)

STAR

(PHOBOS, BRAHMS even more specialised)

PHENIX

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Barrel: tracking + secondary vertices + PID– Charged particles || < 0.9– Excellent momentum resolution up to 100 GeV/c

(p/p < 6%)– Tracking down to 100 MeV/c– Excellent Particle ID and heavy flavor tagging

EMCal for jet reconstruction– Pb-scintillator, 13k towers– = 107, || < 0.7– Energy resolution ~10%/√E

– Trigger capabilities

PHOS: small acceptance, High granularity EMCal

– High resolution PbWO4 crystals– || < 0.12, 220 < < 320– Energy resolution: E/E = 3%/E

Hard probes in ALICE

Forward muon arm

‘STAR+PHENIX in one’ at LHC

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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

Confinement, asymptotic freedom are unique to QCD

At high energies, quarks and gluons are manifest

gqqee

Theory only cleanly describes certaint limits

Study ‘emergent phenomena’ in QCD

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Perturbative QCD: a controlled approximation

c

chbbaa

abcdba

T

hpp

z

Dcdab

td

dQxfQxfdxdxK

pdyd

d

0

/222

)(ˆ

),(),(

Parton density functionNon-perturbative: distribution of partons in protonExtracted from fits to DIS (ep) dataRelatively well-known

Matrix elementPerturbative componentCalculated at NLO (s

2) or betterOften need resummed logs (e.g. FONLL)

Fragmentation functionNon-perturbativeMeasured/extracted from e+e-

Convolution: most observables integrate over x, Q2

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Perturbative QCD processes

• Hadron production• Heavy flavours• Jet production

– e+e- → jets – p(bar)+p → jets

• Direct photon production

Measurem

ent difficulty

The

ory

diff

icul

ty

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Resolved kinematics inDeep Inelastic Scattering

small x

large x

x = partonic momentum fraction

DIS: Measured electron momentum fixes kinematics

),(2

),(2

14 2

22

2

2

4

2

2

2

QxFy

QxFy

yxQdxdQ

dL

eXep

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Differential kinematics in p+pExample: 0-pairs to probe low-x

p+p simulation

hep-ex/0502040

Forward pion

Second pion

Resulting x-range

Need at least two hadrons to fix kinematics in p+p

211

ees

px T

15

p+pbar dijet at Tevatron

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Testing QCD at high energy

small x

large x

x = partonic momentum fraction

Dominant ‘theory’ uncertainty: PDFs

Theory matches data over many orders of magnitude

Universality: PDFs from DIS used to calculate jet-production

Note: can ignore fragmentation effects

CDF, PRD75, 092006

DIS to measure PDFs

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Testing QCD at RHIC with jets

Jets also measured at RHIC

However: signficant uncertainties in energy scale, both ‘theory’ and

experiment

STAR, hep-ex/0608030

Note: kinematic range up to 50 GeV

NLO pQCD also works at RHIC

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Direct photon basics

Production processes– Direct (LO): Compton and

annihilation – Fragmentation (NLO)

direct

fragment

Gordon and Vogelsang, PRD48, 3136 (1993)

Small Rate: Yields

Fragmentation:

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R

Experimental challenge: 0

Below pT=5 GeV: decays dominant at RHIC

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Direct photons: comparison to theoryP. Aurenche et al, PRD73:094007

Good agreement theory-experiment From low energy (√s=20 GeV at CERN) to highest energies (1.96 TeV TevaTron)

Exception: E706, fixed target FNAL deviates from trend: exp problem?

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(fragment) / (inclusive)

Experimental access to fragmentation • Two Methods in p+p 200GeV

– Isolation cut ( 0.1*E > Econe(R=0.5) ): identifies non-fragmentation photons– Photons associated with high-pT hadron: fragmentation

PHENIX, PRL98, 012002 (2007)

R E Triggering leadinghadron

Look at associatedphotons

(Isolated)/(all direct)

Only ~10% of show significant associated hadronic activity

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QCD NLO resources

• PHOX family (Aurenche et al)http://wwwlapp.in2p3.fr/lapth/PHOX_FAMILY/main.html

• MC@NLO (Frixione and Webber)http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/

You can use these codes yourself to generate the theory curves!

And more: test your ideas on how to measure isolated photons or di-jets or...

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0 at lower energies

0 production at lower energies (ISR, fixed target FNAL) not

well described

Good description at RHIC energies

Soft jets at lower √s not well controlled?

C. Bourelly and J. Soffer, hep-ph/0311110

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Light hadron production at RHIC

NLO calculations: W. Vogelsang

Star, PRL 91, 172302Brahms, nucl-ex/0403005

0 and charged hadrons at RHIC in good agreement with NLO pQCD

PRL 91, 241803

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Baryon production in p+p @ √s=200 GeV

Albino, Kniehl, Kramer, Nucl Phys B725, 181

Several sets of fragmentation functions (from e+e-) give large differences for baryon production at RHIC

Need to keep track of uncertainties in FF

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Uncertainty analysis of Fragmentation functionHirai, Kumano, Nagai, Sudo, PRD75:094009

z=pT,h / 2√s z=pT,h / Ejet

Full uncertainty analysis being pursuedUncertainties increase at small and large z

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Global analysis of FFproton anti-protonpions

De Florian, Sassot, Stratmann, PRD 76:074033, PRD75:114010

... or do a global fit, including p+p dataUniversality still holds

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Hadron spectraAKK S. Albino, B. A. Kniehl and G. Kramer, Nucl. Phys. B 725 (2005) 181

KKP B. A. Kniehl, G. Kramer and B. PotterNucl. Phys. B 597 (2001) 337

DSS D. de Florian, R. Sassot, and M. Stratmann, Phys.Rev.D75 (2007) 114010 Kretzer S. Kretzer, Phys. Rev. D 62

(2000) 054001

New data up to pT~14 GeVLatest FF fits describe data reasonably well

Extra lesson: don’t ‘just take something’ if you want to do serious physics

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~factor 2

D0

Non-Photonic Electrons

hep-ex/0609010

CD

F,

PR

L 91

, 24

1804

(2

003)

Theoretical Uncertainty Band

Non-photonic electrons measure charm+bottom

Reasonable agreement with theoryNote: large scale uncertainties

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Charm from non-photonic electrons

• PHENIX electrons from heavy-flavor decays agree with FONLL pQCD calculation

• STAR electrons, D mesons, muons disagree with FONLL

hep-ex/0609010

Theoretical Uncertainty Band

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Summary: QCD in p+p collisions

• Testing universality of PDFs, FFs• Jets, photons mainly depends on PDFs: works well• Baryon fragmentation uncertain

– Hadron spectra agree with theory, if you take right FF

• Heavy flavour – Large discrepancy between RHIC experiments– Theory also large uncertainty?

Spectra measurements integrate over large ranges in x, Q2

Di-hadron measurements can be used to select low-x

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Intermezzo: Luminosity and all that

100 x 100 GeV pp RUN-6 integrated Luminosity (final)

0

5

10

15

20

25

30

35

40

45

50

12-F

eb

4-M

ar

24-M

ar

13-A

pr

3-M

ay

23-M

ay

12-J

un

delivere

d lu

min

osit

y (

pb

-1)

PHENIX pb-1

STAR pb-1

Inte

gra

ted

de

liv

ere

d l

um

ino

sit

y (

pb

-1)

2006 pp @ s = 200 GeV

Improved Collision Luminosity 2006-8

10

50

40

30

20

0

Run7 RHIC AuAu Integrated Luminosity for Physics(singles corrected)

0

500

1000

1500

2000

2500

3000

3500

25-Mar

1-Ap

r

8-Ap

r

15-Ap

r

22-Ap

r

29-Ap

r

6-May

13-May

20-May

27-May

3-Jun

10-Jun

17-Jun

24-Jun

date

Inte

gra

ted

Lu

min

os

ity

[m

b^

-1]

STAR

PHENIX

Lmax

Lmin

MONOp

LARP

Time during runIn

teg

rate

d d

eli

ve

red

lu

min

os

ity

(mb

-1)

2007 Au+Au @ sNN = 200 GeV

Simple question: what do these plots mean?(in practical terms)

L = 45 pb-1 inel = 42 mb

45 1012* 42 10-3 = 1.9 1012 collisions!

L=3200 mb-1 hadr = 7b

2.2 1010 collisions

From S. Vigdor, QM2008:

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Event rates

L = 45 pb-1 inel = 42 mb

45 1012* 42 10-3 = 1.9 1012 collisions

L=3200 mb-1 hadr = 7b

2.2 1010 collisions

Examples from RHIC

p+p Au+Au

Note <Ncoll> ~ 200

Interaction rate 500-1000 kHz

Interaction rate 5-20 kHz

Recording rates: STAR 100 Hz, PHENIX 5 kHz (?)

Need to trigger, i.e. select ‘interesting’ events

Rate reduction: 1000-10000 for p+p, 10-100 for Au+Au

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High-pT triggers

Fast detectors (measure up to a few MHz)– Obvious choice: (EM) calorimeter

For example: Keep all events with a photon > 7 GeV, rate few Hz at RHIC

Two strategies:– Small fiducial, trigger photons– Larger fiducial, trigger 0, ‘jet’ energy

Very suitable for high-pT/hard physics

Trigger sees all 1012 events

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End of lecture I

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0 in p+p, d+Au

M. Russcher

2005 p+p

STAR gearing up , 0 in p+p, d+Au

Good agreement with NLO pQCD and PHENIX

PHENIX, B. Sahlmüller

RdA centrality dependence

Measures Cronin, initial state effects

nucl-ex/0610036

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Trigger example

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fiber optic links

mwave links

Plan to Implement and Test Stochastic Cooling of Heavy Ion Beams at RHIC

(submitted to DOE 12/31/07) Test combined effect of long. &

transverse stochastic cooling for one beam in 2009 run.

If results follow detailed simula-tions, full implementation by 2011.

Simulations long. + trans. stochastic cooling + 56 MHz SRF for both beam goes ~2/3 way (with present bunch intensity) to RHIC-II projected L at order of magnitude less cost, ~5 years quicker than e-cooling.

“RHIC-II” goal

Simulated effects of beam cooling for full-energy Au+Au

Present intensity, no cooling

Present intensity, full stochastic

Present intensity,

e-cooling

Intensity x 1.5, full stochastic

Intensity x 1.5, e-cooling

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What Are Jets ?

Colored partons from the hard scatter evolve via soft quark and gluon radiation and hadronization process to form a “spray” of roughly collinear colorless hadrons → JETS

The hadrons in a jet have small transverse momenta relative to their parent parton’s direction and the sum of their longitudinal momenta roughly gives the parent parton momentum Keep in mind that there are particles in a jet originating from other partons in the event

Jets manifest themselves as localized clusters of energy

Jet

outgoing parton

Fragmentation process

Hard scatter

colorless states - hadrons -

R ( ) ( ) 2 2coneR

Jets are the experim

ental signatures of q

uarks and gluons

They are expected to re

flect k

inematics and to

pology of parto

ns

Really, je

ts are what je

t algorit

hm defines th

em to be!

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Probing differential kinematicsExample: 0-pairs to for low-x

FMS plots

hep-ex/0502040p+p simulation d+Au simulation

Sensitive down to xg ~ 10-3 (few 10-4 in CGC scenario)

Measure gluon density at low x in cold nuclear matter Can distinguish shadowing (suppression of yields) and CGC (‘monojets’)

FMS group

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Direct at high-pT

T. Isobe

p+p year-5

RHIC is accumulating p+p stats

0-10% Au+Au

Nuclear effects + E-loss (frag )

Quark- in-medium conversions

No enhancement in Au+AuAgrees with NLO pQCD

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Particle Ratios

• Ratios extended to 15 GeV/c• The effect of the jet trigger on the ratios is negligible (comparison

between Pythia minimum bias and Pythia jet triggered data)

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π and p Spectra

• Spectra compared to NLO pQCD calculations

AKK S. Albino, B. A. Kniehl and G. Kramer, Nucl. Phys. B 725 (2005) 181

KKP B. A. Kniehl, G. Kramer and B. PotterNucl. Phys. B 597 (2001) 337

DSS D. de Florian, R. Sassot, and M. Stratmann, Phys.Rev.D75 (2007) 114010 Kretzer S. Kretzer, Phys. Rev. D 62 (2000) 054001