# Presentation: 122328

## Aoqi Feng 

**               ****Relative to Reaction Plane **
**              ****at 200 GeV Au+Au Collisions**

-   QM2008,  Jarpur, India                                                          Feb. 5th, 2008

**Aoqi Feng**** **
**for the STAR Collaboration**

**                    ****Purdue University**
**Institute of Particle Physics, Wuhan, China**
**            ****Lawrence Berkeley Lab, Berkeley**

## Outline

-   Motivation
-   Di-hadron correlation wrt reaction plane
-   Summary

**   **Previous key measurements of di-hadron corr.
-    Path-length effect study via di-hadron corr.

**   **Away-side discussion.
-    Near-side  discussion.

## Motivation: the Away-side Modification

- High pT di-hadron suppression
-                      partonic energy loss.

-   Low pT di-hadron correlations
-                     strong jet-medium interaction

-   High pT di-hadron correlations (w.r.t RP)
-                      path-length dependent jet quenching.

**PRL 90 (2003) 082302**

**PRL 95 (2005) 152301**

**PRL 93 (2004) 252301**

**Jet quenching:**   energy loss is **path-length** dependent.

## Motivation: the Near-side Ridge

**Motivation: the Near-side Ridge**

- In-plane

- Out-of-plane

- Non-central collision (20-60%):
- overlap region like almond.
- select trigger particle direction relative to reaction plane.

- Ridge (long range correlation in )is observed on the near-side.

- To gain more insights into the away-side modification and near-side ridge, we study RP dependence.

**Au+Au 0-10%**
- STAR preliminary

- The underlying physics is not understood yet!

## Flow Background Subtraction

- Ref: Phys. Rev C 69, 021901, 2004

- (1)

- (2)

**,**

- The contribution from v4 terms is about 10%, can not be neglected!

- VnR is the trigger flow                    in the angular slice R.

**Notes:**
vnR is the average nth harmonic flow for trigger particles. The average is taken in a region R. The region R has the same shape of our pies

## Results: Correlations v.s. Reaction Plane 

- Away-side:
- Evolves from single- to double-peak.
- Near-side:
- Amplitude drops.

**3<p****T****trig****<4GeV/c, 20-60%**

**STAR Preliminary**

- 0.15

- 0.5

- 1.0

- 1.5

- 2.0

- 3.0

- GeV

- Histograms:
-            v2 uncertainty.
- Red curves:
-           dAu data

**Notes:**
curves:  red lines: questionmark

## Mid-Central v.s. Central Collisions Comparison

**top 5%**

**3<p****T****trig****<4GeV/c & 1.0<p****T****asso****<1.5GeV/c**

**20-60%**

** ****In 20-60%, away-side evolves from single-peak (****φ****S**** =0) to double-peak (****φ****S**** =90****o****).**
** ****In top 5%, double peak show up at a smaller ****φ****S****.**
** ****At large ****φ****S****, little difference between two centrality bins.**

**STAR Preliminary**

**STAR Preliminary**

**Notes:**
V2 and dAu

## Focus On Away-side: Broadness

**3<p****T****trig****<4,1.0<p****T****asso****<1.5GeV/c**

- Slice 1: remains constant. not much broader than dAu.
- Slice 6: higher than slice1.
-             increase with pTasso.

- Double peak: strongest when  more out-of-plane and associate particle is harder.

**3<p****T****trig****<4GeV/c**

**RMS**

**STAR Preliminary**

**v****2****{4}**

**v****2****{RP}**

- v2 sys. error

## Focus On Away-side: Amplitude

- top 5%

**πregion**:  drops with φs, similar between the two centrality bins.
**double peak region:**  constant over φs.
- 	                     top 5%  >  mid-central.

- 20-60%

**3<p****T****trig****<4,1.0<p****T****asso****<1.5GeV/c**

**STAR Preliminary**

- πregion

- double peak

## Focus On Near-side

- Amplitude seems to change, whereas naively little modification is expected.

- 3<pTtrig<4, 1.5<pTtrig<2.0 GeV/c

-    Raw(|      |<0.7) - C×Raw(|      |>0.7)
-    Correlation in      .

**Ridge part:  **|      |>0.7, flow background subtracted.
**Jet part:   **

**acceptance factor**

##  Jet and Ridge Yield

-  Jet and Ridge Yield

- 20-60%

- top 5%

- jet part, near-side
- ridge part, near-side

- jet part, near-side
- ridge part, near-side

- Ridge: seem to decrease with φs . More significant in 20-60% than top 5%.
- Jet: seem to slightly increase with φs .

- Strong near-side jet-medium interaction in reaction plane, generating sizable ridge?
- Minimal near-side jet-medium interaction perpendicular to reaction plane?

**STAR Preliminary**

- 3<pTtrig<4, 1.5<pTtrig<2.0 GeV/c

**Notes:**
Yield instead of Amplitude. In this plot: ridge are from cx|dEta|>0.7.

## Ridge In Two Centralities

- 3<pTtrig<4GeV/c

- 4<pTtrig<6GeV/c

- Collision geometry?    Gluon density?

- At φS=0o: Ridge yields are similar in two centralities.

## Summary

-  Both near- and away-side are modified. The modification depends on the trigger particle direction relative to RP.
-  Away-side:
**                           ****==>   ****path-length dependence of jet quenching.**
-  Near-side:
**                      ****==> **** near-side strong jet-medium interaction in-plane.**
**                                  ****collision geometry? Gluon density effect?**

-    In 20-60%, it evolves from single peak (φs =0o) to double peak(φs =90o).
-    In top 5%, double peak shows up at a small φs .
-    At large φs , little difference between the two centralities.

-    Ridge drops with φs , Jet slight increase.
-    At φs =90o, there appears small or no ridge in 20-60%.
-    At φs =0o, strong ridge generation.

## Thank you!

## backup

## Flow Background Estimation

- (1)

- (2)

- (3)

**Notes:**
vnR is the average nth harmonic flow for trigger particles. The average is taken in a region R. The region R has the same shape of our pies

## Something Relative to the Analysis

**  ****Determination of Event Plane:****  **
**     **modified reaction plane**               **reduce non-flow effect;
-      associate pT range excluded              avoid auto-correlations.
-   **Corrections to raw correlation function:**
-      tracking efficiency is corrected for the associated particles;
-      2-particle acceptance is corrected for by the event-mixing technique.** **
**   ****Systematic errors:**
**      **v2:  average v2 as default results, v2_{4} and v2_{RP} as sys. estimation.
-       resolutions:  random sub-event and charge sign sub-event.
-       B: from 3 different fitting methods.

## Systematics Errors

-  From v2
-      use v2_{EP}, average v2 and v2_{4} to estimate.
-  From event plane resolution
-     it’s smaller than that from v2.
-  From B
-       2, 4 and 6 lowest data points are used to get 3 B values.

## Fitting Method 

**J:**** jet signal**
**F:**** [1+2v****2****trig,R****v****2****asso****cos(2****Δφ****)]**
**Real Flow:**** B*F = B* ****[1+2v****2****trig,R****v****2****asso****cos(2****Δφ****)]**
**Raw:**** raw signal = J+B*F**

- Find 2(4/6) continuous lowest points as the fitting range.

## 2 points

- 6 points

- 4 points

- Raw signal/(1+2*v2*v2*cos(2*dphi))

## 4<pTtrig<6 GeV/c, 20-60%

## 3<pTtrig<4GeV/c, top 5%

## 4<pTtrig<6GeV/c, top 5%

## Two Methods: Consistent

## Ridge Comparison

- 4<pTtrig<6, 1.5<pTasso<2.0GeV/c

- 3<pTtrig<4, 1.5<pTasso<2.0GeV/c

## dPhi x dEta and Projection

## Jet width

## Details

- Near-side amplitude:
-           |Δφ|<0.52     (-30o,30o)
- πregion:
-    2.75<Δφ<3.53  (180o-22.5o,180o+22.5o)
- Double-peak region:
-    1.44<Δφ<2.49 and 3.80< Δφ<4.84
-       (82.5o,112.5o) and (217.5o,277.5o)