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Exploring the Early Parton Momentum Distribution using the Ridge Phenomenon

Cheuk-Yin Wong Oak Ridge National Laboratory

QM2008, February 5, 2008

• Introduction• The momentum kick model• Use the momentum kick model & STAR ridge data t

o extract the early parton momentum distribution • Other momentum kick model predictions• Conclusions

C.Y.Wong, Phy.Rev.C76,054908(’07)C.Y.Wong, arXiv:0712.3282(‘07)

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Introduction• How does the ridge phenomenon occur?• What is the momentum distribution of the early medium part

ons?• What is the dominant mechanism of jet momentum loss?

These questions are linked together and can be answered by the momentum kick model:

Ridge particles are medium partons kicked by the jet. The

kicked partons carry direct information on the medium parton momentum distribution and the magnitude of the momentum kick (or equivalently, the jet momentum loss).

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Ridge particles are medium partons kicked by the jet

• (i) Ridge yield correlated with N_participants• (ii) Ridge yield nearly independent of pt trigger, flavor, baryon, meson characters of the jet• (iii) Tjet>>Tridge > Tinclusive

• (iv) Δφ ~ 0 implies that the ridge particles acquire their azimuthally properties from the jet

The most likely explanation:

~

ridge particles are medium partons kicked by the jet and they acquire a momentum kick q along the jet direction

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The ridge distribution and the momentum kick

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•The kicked final partons subsequently materialize as hadrons by parton-hadron duality•The ridge particle distribution depends on the initial parton momentum distribution and the momentum kick q.

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Parametrization of initial parton momentum distribution

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Ridge yield is a maximum at Δφ~0

.0at maximum a is yield particle ridge theTherefore,

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The width in Δφ depends on the magnitude of q.

at pt=2 GeV

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initial parton dN/dy ~ (1-x)a

The shape in Δη around Δη=0 depends weakly on a

The shape in Δη around large Δη depends strongly on a

The ridge shape in Δη

at pt=2 GeV

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The observed distribution in the momentum kick model

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We need the pp near-side jet data

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pp near-side jet data (open blue circles)

Data fromPRL95,152301(05) & J. Phy. G34, S679 (07)

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The pp near-side jet data can be described by

GeV 1.1 ,5.0 ,

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This set of parameters describe well the pp near-side jet data for pt<4 GeV, |Δφ|<1.5, and |Δη| < 1.4.

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.GeV 1 GeV, 0.5 , 5.0 ,5.2 GeV, 0.1

find we, , Assuming

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The initial parton momentum distribution

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AA near-side data (black solid points) described wellby the momentum kick model around Δη~0

Data fromPRL95,152301(05) & J. Phy. G34, S679 (07)

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AA near-side data (black solid points) described wellby the momentum kick model around |Δη|~3.3

Data fromF. Wang et al. arXiv:0707.0815 (‘07)

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Centrality dependence of ridge yield

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Parton momentum distribution at the moment of jet-parton collision

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Possible evolution scenario of medium partons

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Two-particle correlations

jetridgeJR d

dNddN

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)()()(

particle associated One

)()(21

)()()()()()(),(:particles associated Two

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212121212

JJ

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PP

PPPPPPP

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Two associated particle correlations

Momentum Kick Model Predictions

|Δφ|<0.7, |ηjet|<1, 1<pt<3 GeV

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Forward Rapidity Distributions for PHOBOS Measurements

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Conclusions• The ridge particles can be described as medium

partons kicked by the jet, and they carry information on the early parton momentum distribution and the momentum kick.

• The parton momentum distribution at the moment of jet-parton collision is relatively flat in rapidity with a thermal-like transverse momentum distribution and sharp kinematic boundaries.

• The magnitude of the momentum kick gained by the parton is 1 GeV, which is also the momentum loss by the jet in a jet-parton collision.