Aspect(s) of “Jet” Production with PHENIX

Aspect(s) of “Jet” Production with PHENIX

Matthew NguyenMoriond 2009

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

PHENIX @ RHICColliding Au+Au, p+p, etc. @ a modest of 200 GeVNNs

2Matt Nguyen -- Moriond 2009

PHENIX specializes in rare probes: Photons and Leptons

PHENIX Central Arms

|| < 0.35

Jet Tomography

QGP Brick

Parton beam

q


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

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?

In practice we have neither parton beams nor QGP bricksLifetime, System size order 10 fmInstead use hard scattered partons

The Hard Probe Paradigm

),'('

)(),( 2/

1

0

2/ QzD

z

zPdQzD qh

mediumqh

“Medium modified FF”:


Dmed(z)

QGP

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

2ˆE qL

'1

zz

E E

Scattering strength [GeV2/fm]

Path-length through the medium

A fundamental test of QCD radiation

The Baseline: NLO vs. p+p Collisions


PHENIX can identify 0 by direct reconstruction out to > 20 GeVMeasure both inclusive and isolated direct photon cross sections NLO pQCD works at RHIC!

Pions

InclusiveIsolated

Inclusive Direct 200 GeV

Nuclear ModificationRAA = Observed yield over expected yield = Yield in A+A / yield in p+p scaled by the number of binary collisions


Strong jet quenching, yield of high pT fragments reduced by factor of 5!To first approx. direct are unmodified as expected for color neutral objects

Head-on Collisions

Direct

0

Jet Reconstruction in HI Collisions

ϕ Out-of-cone area

ϕη

pt p

er g

rid

cel

l [G

eV]

STAR preliminary~ 21 GeV

Reconstructed Jet

Out-of-cone area

Jet Reconstruction in Heavy-Ion Collisions is an extremely active topicBackgrounds from soft collisions are non-trivial even at the LHC, particularly at small z


PHENIX

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!

Two-Particle Correlations

8

Underlying Event =Combinatorial Bknd +Elliptic Flow

Per-Trigger (Conditional) Yield: Yield of Associated Particles Per Trigger

Per-

Trig

ger Y

ield

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?

Centrality

Di-hadron Correlations

Modified Jet Shapes: The Ridge

Limited to pT < 4 GeV, same region as away-side cone structurePHOBOS: Ridge correlation extend out to > 4!


Jet shape also modified on the near-side

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

Modified Leading Log Approach

Matt Nguyen -- Moriond 2009 12

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

0.37 0.14 0.05 0.02 0.007 0.002

CERN-PH-TH-2005-100

MLLA -- Resummation of interference effects in shower evolution, used to calculate D(z) using LPHD

z

Borghini and Wiedemann

Direct Correlations

13

200 GeV p+p

Inclusive (direct + decay) photon correlations are measuredDecay correlations are estimated from measured 0-hadron and -hadron yieldsDirect -h hadrons are obtained by statistical subtraction of subtraction of decay correlations from inclusive:

7 < pT < 9 GeV, 3 < pT

h < 5 GeV

Inclusive -hDecay -hDirect -h

1

1direct inclusive decayY R Y YR

1 hdNY

N d

inclusive

decay

NR

N

Isolated Direct Distributions

To first order p+p baseline well described by NLO: Work being done to quantify scale uncertainties, sensitivity to kT effect, etc.


Relation to the FF:

1

h h JetT T T T T

T

-h

T

z p /p p /p

Neglecting k Smearing:

dND(z)

N dz

Yield of hadrons per isolated, direct

CTEQ + NLO + KKP

Zhang, Owens, Wang, Wang: arXiv:0902.4000

hadron γT T Tz p /p

1/N

dN

-h/

dzT

Direct -h in HI Collisions

1/22/2009 Thesis Defense 15

Black Core / Corona vs. Diffuse Medium

ZOWW

ZOWW Model of energy loss using effective FF’s arXiv:0902.4000+ ref’s therein

Single hadron di-hadron -hadron

Expectation in HI depends on model Surface bias for single hadrons Tangential Bias for high pT di-hadronsDiffuse medium/few scattering model:Punch-through -jet surface biased at large zT, but probe progressively further into medium as zT decreases

Jet Suppression Opposite Direct

At intermediate zT -h consistent with 0 RAA dominated by surface biasJust getting started, new higher statistics data soon,Will help us push to lower values of zT


Nuc

lear

Mod

ifica

tion

Ratio of Au+Au Yields to p+p expectation

No isolation cut applied

Conclusions

• 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

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


Backup Slides


Away-side: head vs shoulder


Di-hadron IAA

(rad)


Two Component Fits


The Ridge and the Cone


Jet Hadro-chemsitry


Near and away: ridge vs shoulder

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


pT Dependence of Di-hadron Corrleationstr

igge

r pT

partner pT

Dip develops

Jet reemerges

Yield enhanced

Yield suppressed


Photon Sources in Au+Au


World direct data (p+p)

• PHENIX bridges the gap between Tevatron and fixed target data

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


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

Isolated Direct Cross Section


Direct x-sec, Data/Fit


-jet in HI Cartoon


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Aspect(s) of “Jet” Production with PHENIX

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