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High-p T probes of QCD matter. Marco van Leeuwen, Utrecht University. Part III: intermediate p T. Di-hadron correlations at intermediate p T Near-side: the ridge Away-side: double-bump Coalescence and identified associated hadron yields. Energy loss in a QCD medium.

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high p t probes of qcd matter

High-pT probes of QCD matter

Marco van Leeuwen, Utrecht University

part iii intermediate p t
Part III: intermediate pT
  • Di-hadron correlations at intermediate pT
    • Near-side: the ridge
    • Away-side: double-bump
  • Coalescence and identified associated hadron yields
energy loss in a qcd medium
Energy loss in a QCD medium

A more complete picture

Energy loss and fragmentation

Or in-medium fragmentation

Unmodified fragmentation after energy loss


Fragmentation in the medium completely modified

Time-scales matter

In-medium energy loss


Hadron formation time

Lower pTassoc: measure radiation fragments

Lower pTtrig: explore timescale

lowering p t gluon fragments bulk response
Lowering pT: gluon fragments/bulk response



Long. flow

Long. flow

d+Au, 200 GeV

Au+Au 0-10%

STAR preliminary

Jet-like peak


3 < pt,trigger < 4 GeV

pt,assoc. > 2 GeV

J. Putschke, M. van Leeuwen, et al

`Ridge’: associated yield at large 

dN/d approx. independent of 

Strong - asymmetry suggests coupling to longitudinal flow

near side ridge shape in more detail
Near-side/ridge shape in more detail

pt,assoc. > 2 GeV

STAR Preliminary

STAR preliminary

yield(in  window

 window center

Ridge yield approx. independent of deta for 1< Dh < 2

Ridge also visible for larger pTtrig ( 4-6 GeV and 6-12 GeV )

dh broadening at lower p t
Dh broadening at lower pT

Correlation peak widths

pt,assoc. > 2 GeV

Dh width

Dj width

L. Gaillard

Gradual increase of Dh width over Dj for pT,trig < 3

At low pT, ridge and jet merge  broadened peak

Are these still ‘jets’?

separating jet and ridge p t spectra
Separating jet and ridge: pT-spectra

Jet spectra

Ridge spectra

Yield (pt,assoc > pt,assoc,cut)‏

Yield (pt,assoc > pt,assoc,cut)‏

J. Putschke, M. van Leeuwen, et al





Ridge yield and spectra independent of pT,trig

Slope of spectra similar to inclusives

Jet (peak) spectra harden with pT,trig

Peak dominated by jet fragmentation

Radiated gluons ‘thermalise’ in the medium?

Jet and ridge  separate dynamics

baryon enhancement
Baryon enhancement

M. Konno, QM06

High pT: Au+Au similar to p+p

 Fragmentation dominates

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

p/ ~ 1, /K ~ 2

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

hadronisation through coalescence
Hadronisation through coalescence

Fries, Muller et al

Hwa, Yang et al

fragmenting parton:

ph = z p, z<1

recombining partons:


Recombination of thermal (‘bulk’) partonsproduces baryons at larger pT

Baryon pT=3pT,parton


Recombination enhancesbaryon/meson ratios

Hot matter

associated yields from coalescence
Associated yields from coalescence

Recombination of thermal (‘bulk’) partons

‘Shower-thermal’ recombination

Baryon pT=3pT,parton


Baryon pT=3pT,parton


Hot matter

Hot matter

Hard parton

(Hwa, Yang)

Hard parton

No associated yield with baryons from coalescence:

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

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

associated baryon meson ratios
Associated baryon/meson ratios

STAR Preliminary

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

STAR Preliminary

Jet-like peak:(Λ+Λ)/2K0S≈0.5

Ridge:(Λ+Λ)/2K0S≈ 1

Note: systematic error due to v2 not shown

Similar to p+p inclusive ratio

No shower-thermal contribution?

Baryon/meson enhancement in the ridge?

associated baryon meson ratios1
Associated baryon/meson ratios

STAR Preliminary

STAR Preliminary

pTtrig > 4.0 GeV/c

2.0 < pTAssoc< pTtrig


Ridge region

p/p ratio in jet-peak < inclusive

p/p ratio in ridge > inclusive

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

more medium effects away side
More medium effects: away-side

Mach Cone/Shock wave

T. Renk, J. Ruppert

Gluon radiation+Sudakov

Stöcker, Casseldery-Solana et al

A. Polosa, C. Salgado

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

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

3.0 < pTtrig < 4.0 GeV/c

1.3 < pTassoc < 1.8 GeV/c

Trigger particle

Au+Au 0-10%


M. Horner, M. van Leeuwen, et al

Near side:

Enhanced yield in Au+Au consistent with ridge-effect


Strong broadening in central Au+Au

‘Dip’ at  = 

away side shapes
Away-side shapes


3.0 < pTtrig < 4.0 GeV/c

4.0 < pTtrig < 6.0 GeV/c

6.0 < pTtrig < 10.0 GeV/c

1.3 < pTassoc < 1.8 GeV/c

Au+Au 0-12%


M. Horner, M. van Leeuwen, et al

Low pTtrig: broad shape, two peaks

High pTtrig: broad shape, single peak

Fragmentation becomes ‘cleaner’ as pTtrig goes up

Suggests kinematic effect?

note i large backrgounds
Note I: Large backrgounds





STAR, Phys Rev Lett 95, 152301

STAR, Phys Rev Lett 95, 152301

Not quite so bad for the “Double hump” region: S/B~1/20

note ii background also has a shape
Note II: background also has a shape

After subtraction

Assoc hadron distribution

C. Pruneau, QM06

Flow background


‘Ad hoc’ approach: Zero (jet) Yield at Minimum (ZYAM)

Is it a good approximation?

Could background (flow) be modified by jet?

energy loss in action
Energy loss in action

Near side yield

Away side yield

Away side yield ratio

Near side yield ratio



8 < pT < 15 GeV

8 < pTtrig < 15 GeV

Au+Au / d+Au

Au+Au / d+Au

8 < pT < 15 GeV



M. Horner, M. van Leeuwen, et al






Lower pTtrig

Lower pTtrig


M. Horner, M. van Leeuwen, et al


Near- and away-side show yield enhancement at low pT

Away-side: gradual transition to suppression at higher pT

Possible interpretation:

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

‘primordial process’

Near side: ridge

Away-side: broadening

High-pT fragmentsas in vacuum

intermediate p t summary
Intermediate pT summary
  • Three unexpected phenomena:
    • Large baryon/meson ratio
    • Near-side ‘ridge’, peak broadening
    • Away-side: double-hump

Low-pT yields enhanced

Is there a connection?

Many ideas proposed, but difficult to model accurately

part iv quantitative interpretation
Part IV: Quantitative interpretation

What can we learn about energy loss from experiment?

  • Again P(DE)
    • Sensitivity of RAA, IAA
  • Fragmentation bias
    • Case study: di-trigger correlations (3-hadron)
  • g-jet and jet measurements
radiation spectrum p d e
Radiation spectrum P(DE)

Salgado and Wiedemann, RD68, 014008

Radiation spectrum calculated in pQCD

Subject to approximations, uncertainties

Can we measure this in experiment?

Broad distribution, expect large fluctuations in energy loss

p d e in a collision
P(DE) in a collision

~15 GeV

Renk, Eskola, hep-ph/0610059

Radiation spectrum

Di-hadron emission points

Hydro profile

Box density

In a nuclear collision model, P(DE) integrates over geometry

  • P(DE) is a very broad distribution:
  • Need large kinematic reach to measure the distribution
  • Width dominated by intrinsic process ‘surface bias’ not such a useful concept
raa insensitive to p de
RAA insensitive to P(DE)

Input energy loss distribution

Resulting RAA

T. Renk, PRC 74, 034906

Use very different (hypothetical)P(DE) distributions

All ‘fit’ RAA, except DE/E = const

RAA folds geometry, energy loss and fragmentation

Need more differential probes to constrain energy loss distribution

i aa insensitive to p d e
IAA insensitive to P(DE)

T.Renk, PRC

Away-side slope: some sensitivity to medium density model

(black core model deviates)

Still limited sensitivity to P(DE)

fragmentation bias
Fragmentation bias

PHENIX PRD74: 072002


Quarks: D(z) ~ exp(-8.2 z)

Gluons: D(z) ~ exp(-11.4 z)

For exp(-b z) fragmentation:

Small difference in dN/dxE or dN/dzT

from large difference in D(z) slopes

Shape determined by power-law exponent n

Explains similarity of zT-slopes in d+Au and Au+Au

In other words: di-hadron correlations do not constrain the parton energy

 Limited sensitivity to P(DE)

For exponential fragmentation

summary so far
Summary so far

Best achievable goal: determine P(DE) experimentally

(Or at least some features of it)

Difficult in practice:

RAA (at RHIC) not sensitive

IAA limited sensitivity (fragmentation bias)

comparison to model s including systematic errors
Comparison to Model(s) Including Systematic errors



also consistent with:

Many models explain RAA. All have different assumptions about nuclear overlap geometry, medium expansion, parton propagation, etc, and use a parameter to characterize the medium. For example, we give a fit to the PQM model, Dainese, Loizides,Paic, EPJC38, 461 (2005)

The derived transport coefficient , the mean-4-momentum transfer2/mean free path, is strongly model dependent and under intense theoretical debate, e.g. see Baier,Schiff JHEP09(2006)059.

Fit by PHENIX including systematic errors arXiv:0801.1665

zhang owens wang wang model
Zhang, Owens Wang, Wang Model

Zhang, Owens, Wang and Wang, PRL 98 (2007) 212301 found in their model, 0=1.6-2.1 GeV/fm



Fit by PHENIX including systematic errors arXiv:0801.1665

Again a precision of 20-25% (1)

a very interesting new formula for the x e distribution was derived by phenix in prd74
A very interesting new formula for the xE distribution was derived by PHENIX in PRD74

Relates ratio of particle pT

Ratio of jet transverse momenta


Can be determined

If formula works, we can also use it in Au+Au to determine the relative energy loss of the away jet to the trigger jet (surface biased by large n)

exponential frag fn and power law crucial
Exponential Frag. Fn. and power law crucial

Fragment spectrum given pTt is weighted to high zt by ztn-2

Incomplete gamma function

Bjorken parent-child relation: parton and particle invariant pT spectra have same power n, etc.