# Presentation: 232248

## A NLO Analysis on Fragility of Dihadron Tomography in High-Energy Nuclear Collisions

**Enke Wang**
**Institute of Particle Physics, Central China Normal University**
**Collaborator:  Hanzhong Zhang (CCNU)  **
**                        ****Joseph. F. Owens (Florida)**
**                    ****Xin-Nian Wang (LBNL****)**
**PRL 98 (2007) 212301   (nucl-th/0701045 )**

##                          Outline

**Introduction**
**Dihadron Production in NLO pQCD**
**Nuclear Modification Factor of Hadron-trigged Fragmentation Function**
** ****Summary**

## I.    Introduction

- Hard-scattering between partons in pp.
- Fragmentation of partons produce back-to-back jets of hadrons.
- Jets are clustered in angle and rich in high-pt particles.

- Jets produced in AA traverse and interact with the medium, lose energy and thus carry information of the medium.

**I.    Introduction**

**Jet Quenching**

**Notes:**
partons mean quarks and gluons

## Leading Particle Suppression

**Nuclear Modification Factor:**

**E. Wang, X.-N. Wang, Phys. Rev. C 64(2001) 034901**

**Leading Particle Suppression**

**No Medium Effect**

**Medium Effect**

## Experimental Observation of Jet Quenching

**away-side particles **
**suppressed at high pT**

**leading  particle **
**suppressed**

**Experimental Observation of Jet Quenching**

**Notes:**
partons mean quarks and gluons

## Motivation:

**Surface Emission**

- In surface emission RAA depend on the thickness of the outer corona  which varies very slowly with the initial gluon density

**Motivation:**

**R****AA****  from single hadron spectra is  insensitive to the initial gluon density**

**What is the sensitive probe to initial gluon density ?**

**outer corona**

## II.  Dihadron Production in 

**      ****NLO pQCD**

## Parameterization of Energy Loss

**Energy loss parameter **
**proportional to the initial gluon density**

**Parameterization of Energy Loss**

**Enke Wang and Xin-Nian Wang, PRL87(2001)142301**

## LO analysis of jet quenching in AA

**Jet quenching in 2****→****2 processes**

**LO**** analysis of jet quenching in AA**

**2→2 processes (tree level)**

**A factor K=1.5-2 was put by hand  to account for higher order corrections**

## NLO (Next to Leading Order ) corrections:

**2→3**** ****processes (tree level)**

**NLO**** (Next to Leading Order ) corrections:**

**One-loop corrections**

**J. Owens , PRD65 (2002) 034011; **
**B.W. Harris and J. Owens , PRD65 (2002) 094032**

## Single hadron production in p+p

**Single hadron production in p+p**

## Modified Fragmentation Function in A+A

**Modified Fragmentation Function in A+A**

**KKP**

**(****X. -N. Wang , PRC70(2004)031901)**

**averaged scattering number (opacity)**

## Single hadron production in Au+Au

**Single hadron production in Au+Au**

**Single hadron spectra and R****AA**** at different     (initial gluon density)**

## Dihadron Production in Au+Au 

**Invariant mass:**

**d+Au (no jet quenching): **

**Dihadron Production in Au+Au **

**Hadron-triggered fragmentation function**

## Centrality Dependence of DAA

**If no jet quen-****ching,**

**Centrality Dependence of D****AA**

## III. Nuclear Modification Factor of Hadron-trigged Fragmentation Function

## Surface Emission of Single Hadron Production

**Color strength ****=**** ****single hadron**** ****yield**** from partons in the square**

**completely suppressed**

**Surface Emission of Single Hadron Production**

## Color strength = dihadron yield from partons in the square

**Color strength ****=**** ****dihadron**** ****yield**** from partons in the square**

**Surface Emission + Punch-through jet  in Dihadron Production**

## Nuclear Modification Factor of Hadron-trigged Fragmentation Function

## Centrality Dependence of IAA

**Centrality Dependence of I****AA**

## Sensitivity of IAA to Initial Gluon Density

**Sensitivity of I****AA ****to Initial Gluon Density**

**At LHC energy dihadron spectra have more contribution from dijet with higher initial energy, they are less suppressed than at RHIC energy, R****AA**** become very flat (insensitive) with initial gluon density.**

## VI.    Summary

**the single hadron is dominated by ****vertical surface emission;**
**the dihadron is from ****tangential surface emission + punch-through jets.**

**The dihadron          is more sensitive to the initial gluon density**
**than the single hadron         . When         becomes insensitive in **
**higher energy A+A collision,          ****is a sensitive probe of dense matter.**

**     ****-fit to both single and dihadron spectra can be achieved with a narrow range of the energy loss parameter **
**at RHIC energy, it provide convincing evidence for the jet quenching description. **

**1)**

**2)**

**3)**

**VI.    Summary**

## Thank you for your attention!

## Slide 24