# Presentation: 645535

## Understanding Jet Energy Loss with Angular Correlation Studies in PHENIX

**Ali Hanks**
***for the PHENIX Collaboration***
**24****th**** Winter Workshop on Nuclear Dynamics**
**   ****South Padre Island, TX**
**   ****April 8th, 2008**

## Outline

- Motivation and some background

- Testing various predictions

- The Near-side "ridge"

- Correlations with photons

- Conclusions

- Understanding the Away-side

- High pT and geometric bias

- ?

- April 8, 2008

## Jet fragmentation in Au+Au

- Near-side
- jet + ridge (medium response?)
- Away-side
- head region (suppressed - jet modification)
- shoulder region (medium response)

- April 8, 2008

## pT evolution of correlations

- pT evolution of correlations

- Increase trigger pT

**Yield enhanced**

**Shoulder structure remains**

**Away-side**
**jet reemerges**

** ****arXiv:0801.4545**

- April 8, 2008

## pT evolution of correlations

- pT evolution of correlations

- Increase partner pT

**Yield suppressed**

**Away-side jet reemerges**

**Yield enhanced**

**Shoulder drops off**

** ****arXiv:0801.4545**

- April 8, 2008

## Away-side head vs shoulder

- define head region (HR) as |– | < /6
- and shoulder region (SR) as /6 < | - | < /2

- Shoulder region:
- enhanced for pT< 4GeV/c
- pT spectra appear softer
- Head region:
-  suppressed at high pT
- pT spectra steeper at low pT only

- April 8, 2008

## Away-side head vs shoulder

- Competition between “head” (suppression) and “shoulder” (enhancement)
- Shoulder is important up to ~4 GeV/c

** ****arXiv:0801.4545**

- April 8, 2008

## Away-side composition

- Shape similar for mesons and baryons
- provides constraint on models describing modification of away-side

- Baryon to Meson ratio similar to the bulk
- inconsistent with vacuum fragmentation
- consistent with jet induced medium excitation

**arXiv:0712.3033**

- April 8, 2008

## Predictions from theory

- April 8, 2008

## Away-side peak positions

- Fit Method 1:
- 2 guassian away-side seperated by D
- Fit Method 2:
- 3 guassian away-side
- one fixed by p+p centered at 
- 2 seperated by D

- Weak pT dependence rules out simple gluon radiation models,
- but is consistent with mach-cone model predictions

** ****arXiv:0801.4545**

- April 8, 2008

## Peak position vs Reaction plane

- Shoulder peak position independent of reaction plane angle
- => insensitive to path length
- consistent with Mach cone scenarios
- poses challenge for models involving jet-flow coupling

**Recall from D. Winter's talk**

- April 8, 2008

## Near-side 

- Broadening in  up to pT ≈ 4GeV/c
- Subtracting p+p distribution leaves flat background
- consistent with ~flat ridge + unmodified jet

** ****arXiv:0801.4545**

- April 8, 2008

## The ridge and shoulder connection

- ridge and shoulder pT spectra are softer in Au+Au than in p+p
- Shoulder is close to inclusive
- ridge may be harder than shoulder
- similar centrality dependence

- J. Chen QM08 poster

- April 8, 2008

## Going to high pT - the away-side

- Modified jet yield vs pT
- enhancement at low pT
- suppression at high pT
- Shoulder is ehanced up to pT~ 4GeV/c
- Agrees well with expected jet energy loss and induced gluon radiation

- Away-side

** ****arXiv:0801.4545**

- April 8, 2008

## Away-side widths at high pT

- Widths unmodified across centrality bins for high trigger pT
- But broadening is predicted

- A. Adare QM2008

- April 8, 2008

## Going to higher pT - the near-side

- Modification of  IAA disappears at high trigger pT
- Widths unmodified at high trigger pT

- A. Adare QM2008

- April 8, 2008

## Geometric Bias?

- -jet
- photon non-interacting so no bias effect on trigger
- possible associated hadron surface bias at high pT
- 2+1 correlations: additional condition of high pT particle on away-side
- reduce surface bias
- still susceptible to tangential bias

- intermediate pT

- high pT

- Solutions (?):

- April 8, 2008

## -jet correlations in p+p

- -jet correlations in p+p

- Photon momentum balances with away-side jet
- sensitive to the FF
- Statistical subtraction
- Technique works over large pT range in p+p
- Rγ = N(inclusive γ)/N(decay γ)

- M. Nguyen QM2008

- April 8, 2008

## -jet correlations in Au+Au

- Suggests away-side suppresion in Au+Au
- sensitive to energy loss of jet

- inclusive -h

- Direct photon S/B
- 0 peak S/B
- pQCD accuracy

- Near/Away-Side ≈ 1

- Near/Away-Side > 1

- M. Nguyen QM2008

- April 8, 2008

## Fragmentation photons

- Medium induced modifications highly sensitive to energy loss of jets
- Direct measurement will improve experimental sensitivity
- p+p measurement important test of pQCD

- April 8, 2008

## Fragmentation photons

- First measurement of its kind at RHIC
- next step measure jet shape properties
- ultimately apply procedure to Au+Au

- Nfrag/Ninc ≈ 0.1 at intermediate pT

- A. Hanks QM08 poster

- April 8, 2008

## Conclusions

- Modification to two particle correlations seen in Au+Au collisions on both the near and away-side
- away-side pT dependence consistent with mach cone scenario while challenging gluon radiation and deflected jet models
- modification at intermediate pT suggests dominance of jet induced medium excitations
- high pT dominated by jet fragmentation
- away-side suppresion consistent with energy loss
- unmodified near-side and no observation of broadening in away-side suggests geometric bias
- -jet and h- correlations can provide new insight

- April 8, 2008

## Backup Slides

- April 8, 2008

## 2+1 correlations

- April 8, 2008

## head vs shoulder in centrality

- April 8, 2008

## Near-side modification

- Enhancement at low pT
- "Suppression" at high associated pT and low trigger pT
- dillusion effect?

- Broadening in  at low trigger and associated pT
- Agrees with p+p at high pT

- April 8, 2008

## Near side composition

- April 8, 2008

## Ridge vs Shoulder cont'd

- Near-side

- Away-side

- April 8, 2008