# Presentation: 666919

## Hot Quarks 2004, Taos Ski Valley, New Mexico

**Di-Hadron Correlations at High pt**

- Kirill Filimonov

- Lawrence Berkeley National Laboratory

**Di-hadron correlations in pp,dAu,AuAu at 200 GeV**
**What should we expect in AuAu at 62 GeV?**

## How it all started...

**STAR, PRL 90:082302,2003**** **
**Au+Au @ 200 GeV/c**
**0-5%  most central**
**4 < p****T****(trig) < 6 GeV/c**
**2 < p****T****(assoc.) < p****T****(trig)**

**STAR, PRL 90:03201,2003**** **
**Au+Au @ 130 GeV/c**
**0-10%  most central**
**4 < p****T****(trig) < 6 GeV/c**
**2 < p****T****(assoc.) < p****T****(trig)**

**Reconstructing jets in Au+Au on a statistical basis:**

## Suppression of back-to-back jets

**Central Au + Au**

**Peripheral Au + Au**

-  Near-side well-described
-  Away-side suppression in central collisions

- Away side jets are suppressed!

- near side

- away side

- STAR

**STAR, PRL 90:082302,2003**

## Back-to-back jet suppression is a final state effect

- STAR, Phys.Rev.Lett.91:072304,2003

- Back-to-back jets
- are strongly suppressed
- in central Au+Au collisions
- but not in d+Au collisions

## Di-hadron correlations wrt reaction plane

-  **STAR, nucl-ex/0404007****        ****p****T****trigger****=4-6 GeV/c, 2<p****T****associated****<p****T****trigger****, |********|<1**

**Flow pattern is shifted by ********/2:**
**J. Bielcikova, S.Esumi, KF, S.Voloshin, and J.P.Wurm**
**Phys.Rev.C69:021901,2004****  **

- Back-to-back suppression depends on the reaction plane orientation:
- energy loss dependence on the path length

## Theoretical challenges...

**Lowering the trigger threshold increases the near-side jet correlation strength**

**STAR, PRL 90:082302,2003**

**F.Wang, STAR Preliminary**
**4 < pt(trig) < 6 GeV/c**

- Near

## Going down to 62 GeV: Inclusive Spectra

## RCP and RAA at 62 GeV

**Inclusive spectra of charged particles and ********0 **** are suppressed in central collisions relative to the scaled peripheral AuAu and pp collisions**

**What to expect from di-hadron measurements at 62 GeV?**

## Di-hadron azimuthal correlations in Pythia

- Pythia 6.131, pp at 200 and 62 GeV
- Kinematic selection: CKIN(3)=4.0 GeV
-    -minimum pt for hard 2→2 processes
- Initial state radiation on/off
- Final state radiation on/off
- Primordial kt=1GeV on/off

## Parton spectra in pp at 200 and 62 GeV

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=0,FS=0,KT=0**

## Di-hadron azimuthal distributions

- +,-,K+,K-,p,pbar only, ||<0.9
- pTtrig=4-6 GeV/c, 2 GeV/c<pTasso<pTtrig

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=0,FS=0,KT=0**

**Near-side peak is “suppressed” by about a factor of 3**

## Near and away side parton pt-distributions

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=0,FS=0,KT=0**

**NEAR**

**AWAY**

**Near - |********|<********/2**
**Away- |********-********|<********/2**

**NEAR: <p****T****>=11.4 GeV/c**
**            ****<p****T****>=  9.2 GeV/c**
**AWAY: <p****T****>= 9.2 GeV/c**
**             ****<p****T****>= 7.0 GeV/c**

## “Suppression” of near-side from parton spectra 

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=0,FS=0,KT=0**

**Yield ratio: <Near>/<away>~0.3 @200 GeV**
**	****        ****	****	****	****   ****~0.1 @62 GeV**

**Factor of ~3! **

## KT- effect on the di-hadron distribution

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=0,FS=0,KT=0**

## KT- effect on the di-hadron distribution

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=0,FS=0,****KT=1**

## Final State radiation effect

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=0,****FS=1****,KT=0**

## Initial State radiation effect

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=1****,FS=0,KT=0**

## All effects combined

**Pythia 6.131**
**pp 200 GeV**
**pp 62 GeV**
**IS=1****,****FS=1****,****KT=1**

## pp-reference at 62 GeV

**pt trigger>****2.5 ****GeV/c, pt associated>****1.0 ****GeV/c**

**pt trig, asso may be too low for comparison with Au+Au**** **

## What to expect in Au-Au at 62 GeV?

- Much weaker near-side correlations than for Au-Au at 200 GeV
- “Similar” strength of away-side correlations in peripheral collisions at both energies
- Away-side peak should still be suppressed in central Au-Au collisions at 62 GeV
- Measurements are much more statistics hungry... but you’ll see it very soon ;-)