# Presentation: 773305

## Leading Hadron Correlation Analysis at RHIC 

**Leading Hadron Correlation Analysis at RHIC **

**Fuqiang Wang**
**Purdue University**

## Physics motivation

**Physics motivation**

- To create and study QGP –
- a state of deconfined, thermalized quarks and gluons predicted by QCD at high energy density.

**Lattice ****Q****C****D**** prediction:**

- T**C **~ 1708 MeV
- **C** ~ 0.5 GeV/fm3

- _Penetrating probe:_
- High pT and jets

- Can be calculated from pQCD thus are well calibrated in vacuum.
- Are expected to be modified in nuclear medium hence provide information on medium properties.

- F. Karsch, hep-lat/0401031.

## Jet quenching – final state effect

- STAR, PRL 91, 072304 (2003).

- Vitev, Gyulassy, PRL 89, 252301 (2002); X.-N. Wang, PLB 595 (2004) 165.
- pQCD model calculations of partonic energy loss:
- x30 gluon density, x100 energy density in central Au+Au,
- =10-20 GeV/fm3, well above the predicted critical C.

- 	“surface”emission

- 	strongabsorption

- d+Au ~ pp.
- near side Au+Au ~ pp.
- away side Au+Au ~ 0.

## But... Baryon/meson puzzle!

- PHENIX, PRL 91, 172301 (2003)

- QM’04

- Identified jet correlations!
- PHENIX – nucl-ex/0408007.
- STAR – preliminary results

- Fries et al, PRC 68 044902 (2003);
- Greco et al, PRC 68, 034904 (2003);
- Hwa et al, PRC 70, 024905 (2004).

- QM 04

## State of Affair

- Experimental evidence of jet quenching well established.
- But baryon/meson puzzle!

- Key questions to ask:
- Where does the energy go?
- Amount of energy loss? medium contribution?
- How is energy distributed? thermalization?

## Where did the energy go?

- need to measure the entire jet including low pT.

- We can do it but with some care...
- angular correlation with a leading hadron – combinatorial background from mixed-events.

## Reconstructing low pT associated particles

-  Take trigger particle.
-  Form  and   correlations of other   hadrons with trigger.
-  Background from   mix-events. Add v2    modulation.   Normalize in  0.8<||<1.2.
-  Efficiency correction       on  associated     particles.
-  Take difference and  normalize per trigger.

- Systematic uncertainties: background normalization and v2 correction.

## Correlation functions

**Correlation functions**

- STAR, nucl-ex/0501016.

- Enhanced and broadened distribution at low pT.
- Away side suppression at high pT.

## Re-appearing at low pT

**Re-appearing at low p****T**

- High pT

- Low pT

- _Definition:_
- near side: ||<1, ||<1.4 away side: ||>1, ||<1

- Separation at 1 radian.

## Associated multiplicity and “energy”

**Associated multiplicity and “energy”**

- Same pT final trigger particle appears to select larger energy jet in central AA than in pp.

- p+p

- 	Is this the amount of energy loss?

## Jet quenching model

- X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036

- with energy loss

- without energy loss

## Vector momentum balance?

- TPC acceptance of away side partner.

- Total scalar pT:Initial parton energy + medium contribution

- Medium contribution to the away side associated energy?

## Medium contribution to associated energy?

- Shower-thermal recombination:
- Hwa, Yang, PRC 70, 024905 (2004): TS largest contribution to high pT particles.
- Mach shock waves:
- Stoecker, nucl-th/0406018.Casalderra, Shuryak, Teaney, hep-ph/0411315.

- Excess of energy on the away side most prominent at  1.

- 	How does medium energy become correlated with the trigger?

## Correlations vs centrality

**Correlations vs centrality**

## Correlations vs pT

## Shapes varies with pT

- 	Shape varies with pT, becomes broader and appears doublepeaked at large pTassoc. Effect is more pronounced for lower pTtrig !
- 	Vitev, hep-ph/0501255: should not have peak structure.

- Preliminary

## Associated particles pT distributions

- syst. error

**Associated particles p****T**** distributions**

_**Near side:**_
- overall
- enhancement
- from pp to AA
- larger initial
- parton energy
- (and modest
- energy loss)?

_**Away side:**_
_** **_
- softening of
- spectra from pp
- to central Au+Au
- energy from
- initial parton
- converted to
- low pT particles.
- energy loss in
- medium!

- 4 < pTtrig < 6 GeV/c

## Away side <pT>

- 	Two sources of particles:hard: jet fragmentation products.soft: bulk medium decay products.
- 	Peripheral: <pT> very differentCentral:      <pT> not much different
- 	Gradual decrease with centrality. Similar for two trigger pT windows.
- 	Whatever interaction mechanisms, away-side jet products approach equilibrium with the bulk medium traversed  thermalization of the bulk itself quite plausible.

- <pT> from away jets

- <pT> from medium decay

## Novel behavior of away <pT>

- <pT> more robust than correlation functions.
- Novel dip structure observed in central AA.
- Energy loss effect? Mach shock wave?

## Away <pT> vs centrality

- Away core <pT> drops with centrality faster than corona <pT>.
- Core hadrons almost identical to medium in central collisions.
- Energy loss pattern: path-length effect?
- Mach shock wave manifestation?

- Preliminary

## Mach cone and/or deflected jets

- Deflected jets

- Mach cone

## Summary and open questions

- 

- 

- ?

- Statistical reconstruction of jets in pp, dA and AA. Connection between high pT and low pT physics.
- Same trigger pT appears to select larger energy jets in AA than in pp. Near side associated multiplicity is enhanced.
- Away side correlation disappears at high pT, and reappears at low pT. Correlations functions broadened. Interplay between energy loss and Mach shock wave?
- Significant softening of spectra from pp to central AA. Partial thermalization between jets and bulk medium. Imply high degree of thermalization in medium itself.

- To create and study
- QGP – a state of
- deconfined
- thermalized
- quarks and gluons
- predicted by QCD at
- high enerdy density

## ---Backup slides---

- Backup Slides

## Measuring the lost energy?

- possible... by going to low pT.

- Pal, Pratt, PLB 574 (2003) 21

- How is energy distributed?
- amount of energy loss?
- contribution from medium?

- S. Pal, S. Pratt, PLB574 (2003) 21.
- X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036.
- C.A. Salgado, U.A. Wiedemann, hep-ph/0310079.
- M. Gyulassy, I. Vitev, X.-N. Wang, B.-W. Zhang, nucl-th/0302077.
- ......

## Jet quenching model

- X.-N. Wang, PLB 579 (2004) 299, nucl-th/0307036

- with energy loss

- without energy loss

- Caution: cannot be readily           compared to data yet.

## Thermalization at work?

- yes... by putting two sources of particles together:
-   one from jet fragmentation that are initially hard.
-   the other from bulk medium that are soft.

- going to low pT.

- medium

- jet

## Existing pp, pA results

- G. Boca *et al.* ZPC49, 543 (1991)

- M.G. Albrow *et al.* NPB145, 305 (1978)

- B. Alper, NPB 87 (1975) 41

- p+p @ ISR

## IAA – new vs old

- IAA

- Centrality dN/d

- Old
- New

- Line = new with old v2.