# Presentation: 923488

## Back-to-back Jets Tagged via 2+1 Correlation

- Hua Pei
- Iowa State University

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Outline

- Motivation
- Method
- Current Analysis Result
- Summary

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## 2-Particle Correlations Indicate Suppression

- We have been studying jets using the 2-particle correlation.
- Clear jet signal is observed on near-side even in the most central AA collisions.
- Strong suppression is observed on the away-side.

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Theory Models Are There to Explain

-  We all agree:
-  Jet-quenching is due to partons lose energy in the hot and dense QCD medium
-  We don’t agree on: energy loss mechanisms
- Mach cone  (Stoecker, Casalderrey-Solana, Shuryak, Teaney, Ruppert, Muller, Renk)
- Cherenkov gluon radiation (Dremin, Koch, Majumder, Wang)
- Large angle gluon radiation (Vitev, Salgado)
- Flow-induced deflection (Armesto, Salgado, Wiedemann)
- Parton multiple scattering (Chiu, Hwa)
- ....
** ****Important to study the back-to-back jets simultaneously: use di-jets events.**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## First Observations of Di-jets at RHIC

- PRL 97 (2006) 162301

**8<p****T****(trig)<15 GeV/c**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Analysis Method

-  Select only events with 2 back-to-back (within azimuthal space) high-pT hadrons.
-  Then in the 2-D space, plot the azimuthal distribution of lower-pT particles around one of the two high-pT hadrons.
-  The flow background and underlying events are taken into consideration.

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Advantage of Using Di-jets

- Select events that have **two**_ high-pT hadrons back-to-back_

- 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

- vs.

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## First 2+1 / T1T2_A Measurement by STAR

**	****Di-jets in d+Au**
*  **200 GeV Minimum bias data*

**T1: p****T****>5 GeV/*****c******, *****T2: p****T****>4 GeV/*****c******, *****A: p****T****>1.5 GeV/*****c***

**	**
- Difference in associated yield
- Di-triggers sample higher energy jets

**From QM08 talk of**
**Olga Barannikova**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Di-jets Correlation in Most Central Au+Au

**From QM08 talk of**
**Olga Barannikova**

- One high-pT trigger only: Away-side modification.
**Di-jet trigger: jet peaks on both near and away side!**

**T1: p****T****>5 GeV/*****c, *****T2: p****T****>4 GeV/*****c, *****A: p****T****>1.5 GeV/*****c***

*200 GeV ZDC central 12% Au+Au*

**From QM08 talk of**
**Olga Barannikova**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Di-jets Correlation of Au+Au vs. d+Au

**T1: p****T****>5 GeV/*****c, *****T2: p****T****>4 GeV/*****c, *****A: p****T****>1.5 GeV/*****c***

-  We see similar shapes between d+Au and most central Au+Au
-  The suppression is less comparing to “normal” 2-particle correlation.
-  Was this due to tangential di-jets (surface-bias) or real punch-through jets, without much energy loss?

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## And Where does the Ridge Go?

**T1: p****T****>5 GeV/*****c, *****T2: p****T****>4 GeV/*****c, *****A: p****T****>1.5 GeV/*****c***

-  Ridge is naturally considered as the energy lost (deposited) to the medium by jets.
-  di-jets statistically remove those surface-biased jets, and keep either tangential di-jets or those punch-through from center of medium, in either case two partons travel comparable path-lengths.
-  Lack of ridge, shall this remain us only tangential di-jets?

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Tangential Bias of Di-jets

- T1: pT>5GeV/*c*
- T2: pT>4GeV/*c*

- If the triggers are from tangential surface of medium, we expect a term related to the surface: ~ R2 ~ Npart2/3

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Model Calculation

- Renk, Phys. Rev. C 75, 054910 (2007)

- <E> deposition for back-to-back jets

*Thorsten Renk, private comm.*

- 2 density models

- T1  & T2   energy deposition by T2 

**T1**

**T2**

**From QM08 talk of**
**Olga Barannikova**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Measurement: Di-jets Still Lose Energy

-  PHENIX h-h correlation. This JAA is calculated on per-event basis instead of per-trigger.
-  At right-end of top-left panel (most central) , the JAA (pTA  pTB) approaches the inclusive RAA2 (dash line), indicating an evident energy loss.
-  We need to further quantize the di-jets.

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

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Di-jets Correlation Shape Evolution with Centrality in Cu+Cu 200GeV

- We show the near-side widths in di-jet correlation as function of centrality.
- Inside each bin, it goes from most central Cu+Cu (most left) towards peripheral until p+p (most right)
- Each bin represents a different pT region of “conditional” particle, or “T1”.

**QM08 talk of**
**Hua Pei**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Di-jets Correlation Yield Evolution with Centrality in Cu+Cu 200GeV

- We show the near-side yields in di-jet correlation as function of centrality.
- Inside each bin, it goes from most central Cu+Cu (most left) towards peripheral until p+p (most right)
- Each bin represents a different pT region of “conditional” particle, or “T1”.

**QM08 talk of**
**Hua Pei**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Discussion on Di-jets Correlation Centrality Dependence

-  Cu+Cu yields are higher than p+p at “non-required” conditional pT bin, is consistent to the Au+Au h-h correlation result (_**arXiv:0801.4545**_)
-   Cu+Cu or p+p yield increase with conditional pT is possibly due to the Q2 shift. That is, if we fixed the trigger pT and increase the away-side conditional pT, we are increasing the Q2 of di-jets, thus the parton energy on near-side, and naturally increase the yields. A similar discussion was given in PHENIX paper _**Phys. Rev. D 74, 072002 (2006)**_
-   On the other hand, the fact that Cu+Cu yields increase slower than p+p with conditional pT, indicates possible medium effect such as jet absorption. Existence of underlying events (random combination of T1 and T2) can also lower the yields especially in central AA.
-   Far-side yields was not shown here since I am still in development of removing the bias introduced by this “conditional” particle.

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Outlook

- People at RHIC are analyzing their Run 2007 Au+Au data with much more statistics (e.g., PHENIX has more than 3-times of what it got in 2004 Au+Au, and better flow measurement with the new RXNP detector).
- Geometry dependence of di-jets (yields, ridge shapes, etc) can be extend to a broader range then.
- Methods of removing underlying events will be improved.
*Long-term scope: *
*(nearly) full di-jet reconstruction (STAR and PHENIX Upgrade)?*
*Direct ********jet correlation to ultimately remove the surface bias? Can be the silver bullet.*

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008

## Summary

- We already know from 2-particle correlation that jets are modified by medium, on both near and far side. Medium effect shows as jet suppression and varies on centrality and pT.
**New analysis of 2+1 correlations:****a new method of controlling jet source via the surface-bias, especially on exploring the **_***jet suppression***_**.**
**Preliminary results from both PHENIX and STAR show a di-jet structure, and statistically shift our observation to either tangential jets or punch-through (both sides) jets.**
**New technique allows further exploration of the “surface bias”, and will probe the medium in a more controlled way.**

- RHIC-AGS Annual User Meeting 2008

- 05/27/2008