# Presentation: 545479

## Study Jet Energy Loss with 2-particle Correlations

-   How back-to-back jets lose energy
-   Different pT region of jet energy loss
-   Changing surface bias: 2+1 particle correlation

- Hua Pei
- Iowa State University
- For PHENIX at RHIC

## We know Jet Quenches in AA relative to p+p, and we measure per-trigger yield

**More central**

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## A Jet Function Can Be Divided into Regions

- We define “HR”, the head region, as  **|******** – ********| < ******** /6**
- This **HR** is where we expected the away-side jet appears.
- For region “SR”, given in _parallel talk_ M. McCumber

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## We Measure the Central Au+Au Jet Spectrum at Head Region and Compare to p+p

- Each panel shows a different **p****T****a** (trigger), and plotted as function of **p****T****b** (assoc)

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## We use IAA to quantify the change of jet spectrum

- IAA is defined as the modification of per-trigger yield Y**jet_ind**, of AA relative to p+p.

## The Modification of per-trigger Yield (IAA) has strong assoc pT Dependence

- At high **p****T****a** and **p****T****b**, I**AA** ≈ R**AA** (inclusive) at head region.
- Quantified suppression of far-side jet => further constraint on e-loss.

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## Compare Jet Spectrum (mean pT) of Head Region to Inclusive Spectrum

- At most central collisions, the mean pT of Head Region approaches the inclusive spectrum (gray line near bottom of each panel)

- “Punch-through” of jet at highest pT?

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## JAA is used to measure the change of particle pairs induced by di-jet

-    JPY is the number of di-hadron pairs (both particles from jet) per event.
-   J**AA** is the ratio of JPY of AA to p+p, normalized by N**coll**
-   Thus by definition, J**AA** = I**AA** ***** R**AA**

## JAA pair suppression

-   J**AA** is suppressed at high p**T** at all centralities.
-   At all centralities, J**AA**  ≈ R**AA****2** (dashed line) at highest p**T****a,b**, indicating both trigger/assoc are suppressed.

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## Relative energy loss back-to-back jet

- Ratio of MEAN parton transverse momentum

- Be fitted from spectrum

***PHYSICAL REVIEW D 74, 072002 (2006)***

## The spectrum is fitted to get xh

- ^

## Compare Au+Au x to p+p x

- ^

**Shown by Michael J. Tannenbaum DNP07**

- ^

## Change the surface-bias of near-side?

- Select events that have **both** _a high-pt ____0_ **and **_a back-to-back hadron_ 		                                              (back-hemisphere of 0 )

- Shift distribution of hard scattering _towards center of medium_.  Near-side parton travels through more medium

_**Removes**_ some events where
- hard-scattering occurs near surface but not tangential (large difference between path lengths)

- Path lengths **comparable** in dense medium.
- A.k.a., 2+1 correlations

## This 2+1 changes near-side jets of both p+p and Cu+Cu

- Per-trigger yield of p+p on near side increase with conditional particle pT.
- Expected in p+p! Higher** ****Q****2**** **comes with** **higher pT away-side particle.
- In Cu+Cu the yield also increases but not same slope as in p+p.

## Centrality dependence of near-side yields

- Cu+Cu yield increases from central (left) to peripheral (right) in each bin and approaches p+p (most right point in each bin)
- The fact that Cu+Cu yield is reduced at central is possibly due to
**1) **weaker surface-bias**, 2) **more “+1” particles from underlying event

## Another handle on path-length: Reaction plane dependence

- Evolution of “Head Region” is observed.
- Need more work to quantify that

## Conclusion

- Jets are affected by medium, on both near and far side.
- Medium effect on jets vary on centrality and pT.
- Thus, we quantify the medium effects as the suppression of jet, using per-trigger-yield, I_AA, J_AA. This suppression shows strong indication of jet particle sources at different kinetic region.
**2+1** correlation brings another method of controlling jet source via the surface-bias, especially on exploring the *near side jet suppression*.
- PHENIX has the brand new 2007 Au+Au data and we are showing many more results in this QM08 and near future.

## Back up

## The Modification of per-trigger Yield (IAA) has strong assoc pT Dependence

- At high **p****T****a** and **p****T****b**, IAA ≈ RAA (inclusive) at head region.
- Quantifies suppression of far-side jet => further constraint on e-loss.

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## The IAA of both “Head” and “Head + Shoulder”

- At **high p****T****a**** and p****T****b****, I****AA**** ≈ R****AA**** (inclusive) at head region **

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## The mean pT as a function of pTB range

- _[arXiv:0801.4545v1](http://arxiv.org/abs/0801.4555v1)_
**[nucl-ex]**

## Source of formula on xh

***PHYSICAL REVIEW D 74, 072002 (2006)***