Aspect s of jet production with phenix
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Aspect(s) of “Jet” Production with PHENIX. OR Can we really learn about QCD from heavy-ion collisions?. Matthew Nguyen Moriond 2009. PHENIX @ RHIC. Colliding Au+Au, p+p, etc. @ a modest of 200 GeV. PHENIX Central Arms. | h | < 0.35.

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Aspect(s) of “Jet” Production with PHENIX

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Aspect(s) of “Jet” Production with PHENIX


Can we really learn about QCD from heavy-ion collisions?

Matthew Nguyen

Moriond 2009


Colliding Au+Au, p+p, etc. @ a modest of 200 GeV

PHENIX Central Arms

|h| < 0.35

PHENIX specializes in rare probes: Photons and Leptons

Matt Nguyen -- Moriond 2009

Jet Tomography

“The medium”: A dense, thermalized, effectively deconfined, strongly coupled Quark Gluon Plasma formed in high energy nuclear collisions

Parton beam

QGP Brick


Matt Nguyen -- Moriond 2009

By modeling energy loss of hard partons can we determine properties of medium and understand QCD in the soft, collective phase?

Can also turn the question around.

Can we learn something about hard scattered partons by investigating how they interact with dense matter?

The Hard Probe Paradigm

In practice we have neither parton beams nor QGP bricks

Lifetime, System size order 10 fm

Instead use hard scattered partons

“Medium modified FF”:


The question of energy loss of fast partons led to the consideration of destructive interference, the QCD LPM effect, previously unsolved


A fundamental test of QCD radiation

Path-length through the medium

Scattering strength [GeV2/fm]

Matt Nguyen -- Moriond 2009

The Baseline: NLO vs. p+p Collisions

200 GeV

Inclusive Direct g




PHENIX can identify p0 by direct reconstruction out to > 20 GeV

Measure both inclusive and isolated direct photon cross sections

NLO pQCD works at RHIC!

Matt Nguyen -- Moriond 2009

Nuclear Modification

RAA = Observed yield over expected yield

= Yield in A+A / yield in p+p scaled by the number of binary collisions

Head-on Collisions

Direct g


Strong jet quenching, yield of high pT fragments reduced by factor of 5!

To first approx. direct g are unmodified as expected for color neutral objects

Matt Nguyen -- Moriond 2009

Jet Reconstruction in HI Collisions

~ 21 GeV

STAR preliminary

pt per grid cell [GeV]




Reconstructed Jet

Out-of-cone area

Jet Reconstruction in Heavy-Ion Collisions is an extremely active topic

Backgrounds from soft collisions are non-trivial even at the LHC, particularly at small z

Out-of-cone area

Matt Nguyen -- Moriond 2009


Two-Particle Correlations

  • Essential features of jet production are evident via azimuthal correlations between particle pairs

  • In p+p collisions:

  • Approximate UE as a flat pedestal

  • Fit double Gaussian + constant to remove pedestal – may be taken as definition

  • In A+A collsions:

  • Two-source model: Jet + combinatorial background from soft collisions

  • Estimate background by event-mixing

  • Background has it’s own azimuthal correlations due to collective behavior of medium : It flows!

Per-Trigger (Conditional) Yield:

Yield of Associated Particles Per Trigger

Per-Trigger Yield

Underlying Event =

Combinatorial Bknd +

Elliptic Flow

Modified Jet Shapes: The Cone


  • At intermediate pT, away-side peaks are displaced in central collisions

  • Collective phenomenon, e.g., shock wave or modification to QCD bremsstrahlung in medium?



Matt Nguyen -- Moriond 2009

Modified Jet Shapes: The Ridge

Jet shape also modified on the near-side Dh

Limited to pT < 4 GeV, same region as away-side cone structure

PHOBOS: Ridge correlation extend out to Dh > 4!

Matt Nguyen -- Moriond 2009

Modified Jet Shapes from Modified pQCD

Polosa and Salgado, PRC75, 041901 (2007)

  • Uses standard perturbative methods (Sudakov Form Factors) calculates shower evolution

  • Introduce modified splitting functions to account for multiple scattering in medium

  • Large angle scattering is enhanced, reproducing conical emission

Matt Nguyen -- Moriond 2009

Modified Leading Log Approach


Borghini and Wiedemann

MLLA -- Resummation of interference effects in shower evolution,

used to calculate D(z) using LPHD


0.37 0.14 0.05 0.02 0.007 0.002

Very different than “fractional energy loss”, may be Q2 dependent

Di-hadron measurements introduce a trigger bias which makes their calculation difficult.

For quantitative comparisons we need to look to full jet reconstruction or direct photon correlations where the parton energy is determined

Matt Nguyen -- Moriond 2009

Direct g Correlations

200 GeV p+p

7 < pTg < 9 GeV, 3 < pTh < 5 GeV

Inclusive g-h

Decay g-h

Direct g-h

Inclusive (direct + decay) photon correlations are measured

Decay correlations are estimated from measured p0-hadron and h-hadron yields

Direct g-h hadrons are obtained by statistical subtraction of subtraction of decay correlations from inclusive:

Isolated Direct g Distributions

Zhang, Owens, Wang, Wang: arXiv:0902.4000

Yield of hadrons per isolated, direct g

Relation to the FF:

1/Ng dNg-h/dzT


To first order p+p baseline well described by NLO:

Work being done to quantify scale uncertainties,

sensitivity to kT effect, etc.

Matt Nguyen -- Moriond 2009

Direct g-h in HI Collisions

  • Expectation in HI depends on model

  • Surface bias for single hadrons

  • Tangential Bias for high pT di-hadrons

  • Diffuse medium/few scattering model:

  • Punch-through

  • g-jet surface biased at large zT, but probe progressively further into medium as zT decreases

Black Core / Corona vs. Diffuse Medium

Single hadron




ZOWW  Model of energy loss using effective FF’s


+ ref’s therein

Thesis Defense

Jet Suppression Opposite Direct g

Ratio of Au+Au Yields to p+p expectation

At intermediate zTg-h consistent with p0 RAA dominated by surface bias

Just getting started, new higher statistics data soon,

Will help us push to lower values of zT

No isolation cut applied

Nuclear Modification

Matt Nguyen -- Moriond 2009


  • QCD at high density/temperature accessible at RHIC

  • Qualitatively new features in jet correlations attest to strong medium modifications to vacuum jet fragmentation

  • Modeling such effects is challenging and requires better hard probe observables, jet reconstruction and direct photon correlations with high luminosity data from RHIC and large collision energy data from the LHC

  • g-h data are now available from RHIC and will enable precision studies of jet fragmentation in medium as more statistics are accumulated

Matt Nguyen -- Moriond 2009

Backup Slides

Matt Nguyen -- Moriond 2009

Away-side: head vs shoulder

Matt Nguyen -- Moriond 2009

Di-hadron IAA

Df (rad)

Matt Nguyen -- Moriond 2009

Two Component Fits

Matt Nguyen -- Moriond 2009

The Ridge and the Cone

Matt Nguyen -- Moriond 2009

Jet Hadro-chemsitry

Matt Nguyen -- Moriond 2009

Near and away: ridge vs shoulder

Spectra and yields for ridge and shoulder are similar, and show same trend with centrality

Matt Nguyen -- Moriond 2009

pT Dependence of Di-hadron Corrleations

partner pT

Dip develops

Yield suppressed

Yield enhanced

trigger pT

Jet reemerges

Matt Nguyen -- Moriond 2009

Photon Sources in Au+Au

Matt Nguyen -- Moriond 2009

World direct g data (p+p)

Aurenche et al. Phys.Rev. D73 094007 (2006)

  • PHENIX bridges the gap between Tevatron and fixed target data

  • Highest energy data that removes decay background by direct reconstruction rather than calorimeter response

Matt Nguyen -- Moriond 2009

Isolated Direct g Cross Section

Matt Nguyen -- Moriond 2009

Direct g x-sec, Data/Fit

Matt Nguyen -- Moriond 2009

g-jet in HI Cartoon

Matt Nguyen -- Moriond 2009

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