# Presentation: 089817

## Study of the away side jet in Au + Au collisions in PHENIX

-   pT evolution of the away side correlation
-   Away side at moderate high pT
-   Away side at Intermediate pT.

- Jiangyong  Jia for the PHENIX Collaboration

## Evolution of away-side jet shape

**low p****T**

**Moderate high p****T**

- What is the picture?

- 4-6 x 2-4 GeV/c
- pT,assoc  2 GeV/c

- pT,assoc  0.2 GeV/c

**Phys. Rev. Lett. 90,  (2003)**

**Phys.Rev.Lett.95,2005**

**STAR**

**STAR**

## Di-jet correlation at moderate high pT

**Jet quenching!**

- Di-jet correlation at moderate high pT

**Number of pairs**

****

**0o**

**180o**

- 4-6 x 2-4 GeV/c
- pT,assoc  2 GeV/c

**STAR**

**Notes:**
Consistent with surface emission picture

## What happens to the lost energy?

- Moderate high pT → Away-side suppression

- How the medium responds to the jet?

- 4-6 x 2-4 GeV/c
- pT,assoc  2 GeV/c

**STAR**

**STAR**

## How does the medium respond to the jets?

- 0-5%

**PHENIX preliminary**
**2.5-4 x 1-2 GeV/c**

- Other possible mechanisms:
- Cherenkov radiation, bending jet,
- Gluon radiation...

## Di-jets at high pT

- Near side jet yield is constant with centrality.
- Clear away side peak
- Suppression of away-side peak increases with centrality

**Notes:**
Start off with 2 assoc, then move to 6 assoc

## My speculative picture

- Trigger and associated pT

- Away jet IAA

- Thermallized
- gluon radiation

- Shock wave or
-  cherenkov?

- Punch through jets or tangential contribution?

- Questions
- Region III : Disappearance of away side? Where is the 		        tangential/punchthrough contribution?
- Region II  : Is the dip real ?

- ?

- T

- 3T

- Interplay between jet quenching, medium response and remaining jets!?

- ?

- I

- II

- III

- IV

**Notes:**
Tangential contribution? The fraction should be 20%, relatively insensitive to pT. Except the width is wider

## Does the away side jet disappear at moderately high pT?

- Tangential/punchthrough fraction are there?

- or

**Phys. Rev. Lett. 90,  (2003)**

**Phys. Rev. Lett. 95, 2005**

- STAR 4-6 x 2-4 GeV/c

## Jet yield in different regions

- Three regions:
- Disentangle “Mach Cone” and “normal jet fragmentation” components.

**Near |********/3.**
**Away dip |********/6****, **
**Away peaks ******** <********/6, 7******** <********/2**** **

## Jet yield: 3 regions, vs pT,assoc

**Trigger: 2.5 –  4 GeV/c**

- Slope ~ 500MeV/c

## The away side Icp

- Ratio of per-trigger yield: Icp

- _Away side_
-  dip: always below 1 and decrease towards high pT
-  shoulder: enhancement at low pT,assoc which decreases towards high pT
-  The full away side didn’t disappear! Suppressed and broader

##  Away side jet shape at intermediate pT?

- CF = J() + (1+2v2tv2a cos2)

## How well do we control the subtraction?

**Issues**
**Underlying event level : ********.**
**Non-flow effects in the RP v2**
**Two source model assumption: **
**	****	****	****<**v2tv2a**>=? <**v2t**><**v2a**>**
**Systematic errors on the v2.**

- CF = J() + (1+2v2tv2a cos2)

## 1) Determine 

- 1) Determine 

- Opposite/same sign correlation
- Identical away side strength
- Different near side strength
- v2 contributions are identical

- +-  -+

- ++  --

- 0-5%
- 2.5-4 x 1-2.5 GeV/c

## 2) Non-flow effect due to jets

- HIJING events are randomly assigned a RP direction and particles are weighted according to the experimental measured v2(pT,)
- Embed pythia di-jet events (trigger >6 GeV/c) into the HIJING with flow
- The jet bias will leads to fake v2 for the embed dijet

- Jet bias should be small at BBC (3<|<4), we can confirm it

- Away side  distribution from pythia
-  (leading particle =0)

- Event

- Jet

- 

- The perturbation in Reaction Plane is correlated with the jet direction. This leads to a fake v2 (non-flow) of the jet (leading particle) at mid-rapidity
- This fake v2 depends on the multiplicity and eccentricity.

- Large bias

- small bias

- Fake v2 generated!

## Fake v2 of the high pT triggers 

- Note, the true v2 of trigger is 0.

- 0.42.8

- Fake Trigger v2 need to be corrected by RP resolution

## 3)&4) RP dependence

- Study (di)jet correlations vs angle of trigger h relative to reaction plane
- J. Bielcikova *et al*,   **Phys. Rev. C69:021901, 2004**

- a is proportional to the triggers in the bite

## Slide 19

## Shape can be constrained by the RP dependence

- Shoulder and dip seen in all bins.
**Direct constrains **
**	**
**The dip is significant for bin 4 where the v2 systematic is small.**

## Summary

- Evolution of away-side jet shape
- Discussed four regions : low pT, **intermediate pT**, moderately high pT, **high pT**
- Different physics modification at different regions:

- Away side jet does not disappear at moderate high pT.
- Away side jet shape in 0-5% central Au + Au is robust
- Jet shape is not sensitive to the background level .
- The non-flow contribution from jet is negligible.
- RP dependence results is consistent with the two source model, and helps constrain the v2 systematic.

- Trigger and associated pT

- Away jet

- Thermallized
- gluon radiation

- Low pT

- Intermediate pT

- Moderate high pT

- high pT

- Shock wave or
-  cherenkov?

- Punch through jets or tangential contribution?

## Back up

## Dependence on away side jet multiplicity

- We know the away side yield seems enhanced at low pT.
- We estimate by doubling the Di-Jet multiplicity.

- The fake v2 is below 4% as long as low>1, and is negligible at BBC.

- Icp

## Comparison Au + Au and Cu + Cu

- Npart and Ncoll between the two are close
- The comparison between the two could provide constrains on the collision geometry dependence of the modification

- 30-40% Au+Au
- Npart = 114

- 0-10% Cu+Cu
- Npart = 98

## PHENIX: Cu+Cu high pT Jet Correlations

- Di-jet signal persists even for the most head-on Cu+Cu collisions.
- May allow better determination of matter properties!