# Presentation: 500756

## 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 to 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)

## Introduction

- How does the ridge phenomenon occur?
- What is the momentum distribution of the early medium partons?
- 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).

## 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**

## The ridge distribution and the momentum kick  

- 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.

## Parametrization of initial parton momentum distribution

## Ridge yield is a maximum at Δφ~0

- _Ridge yield is a maximum at __Δφ__~0_

## The width in Δφ depends on the magnitude of q.

- at pt=2 GeV

##                       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

## The observed distribution in the momentum kick model

- We need the pp near-side jet data

## pp near-side jet data (open blue circles)

- _pp near-side jet data (__open blue circles__)_

- Data from
- PRL95,152301(05)  & J. Phy. G34, S679 (07)

## The pp near-side jet data can be described by 

- _The pp near-side jet data can be described by _

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

## The initial parton momentum distribution

- _The initial parton momentum distribution_

## AA near-side data (black solid points) described well

- _AA near-side data (__black solid points__) described well_
- _by the momentum kick model around  __Δη__~0 _

- Data from
- PRL95,152301(05)  & J. Phy. G34, S679 (07)

## AA near-side data (black solid points) described well

- _AA near-side data (__black solid points__) described well_
- _by the momentum kick model around  |__Δη__|~3.3_
- _                                       _

- Data from
- F. Wang et al. arXiv:0707.0815 (‘07)

## Centrality dependence of ridge yield

## Parton momentum distribution 

- _Parton momentum distribution _
- _at the moment of jet-parton collision_

## Possible evolution scenario of medium partons 

## Two-particle correlations

- _Two-particle correlations_

##  Two associated particle correlations

- _Momentum Kick Model Predictions_

- |Δφ|<0.7,  |ηjet|<1,  1<pt<3 GeV

##  Forward Rapidity Distributions for PHOBOS Measurements

## 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.