# Presentation: 449470

## STAR HIGH PT

- Oana Catu, Yale University,
-            for the STAR Collaboration

## Motivation

- Use high-pT particles and jets to *probe the hot medium*

-  produced in the early stage of the collisions and interact with the produced medium
-  can be calculated using pQCD – theoretical handle
-  can be calibrated by reference measurements ( p-p )

- measure medium properties
- Density, temperature, number of degrees of freedom
- Dynamical properties  ( e.g. viscosity )

-  Many interesting ( sometimes surprising ) experimental results already – promising topic
-  Results stimulated new theoretical ideas – experiment driven area

- Oana Catu, Yale University

## Outline

**Hadron Correlations:**
- Charged and identified
- Near-side - Interaction of jet with bulk –”ridge”
- Baryon/meson composition of jet, ridge
- Dihadron fragmentation functions
- Away-side
- Suppression of dihadron fragmentation functions – determine the energy loss parameter
- modification of shape at lower pT assoc
- Closer look using 3 particle correlations
- new analysis pin-pointing di-jet axis
******-h correlations in p+p** – Calibrated parton energy

- Oana Catu, Yale University

- pTtrig=3-6 GeV/c,
- 1.5 GeV/c <pTassoc< pTtrig

-  Au+Au central @ 200 GeV

- Au+Au: long-range  correlations at near side (“the ridge”)

- Lesson: The near-side jet does interact with the medium

- Fragmentation and energy loss - near-side

- Di-hadron correlations

**v****2 ****+ away-side peak**

- Oana Catu, Yale University

## Extracting the ridge and jet yields

- 

- Oana Catu, Yale University

## Centrality, pT trigger dependence of near-side yield

- pt,assoc. > 2 GeV

- 3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV

- Ridge yield ~ constant as function of trigger pT ( slightly decreasing in central collisions)

- Ridge yield increases sharply with centrality
- Jet yield independent of centrality

- Au-Au at s=200GeV

- Oana Catu, Yale University

## Ridge and jet pT- spectra 

**p****t,assoc,cut**

- Ridge and jet pT- spectra

- Ridge pT-spectra are ‘bulk-like’
- Jet spectra – much harder, harder for higher pT trigger

- Ridge spectra slopes - independent of pT trigger

- Jet spectra

- Ridge spectra

** ****Yield (p****t,assoc ****> p****t,assoc,cut****)**

** ****Yield (p****t,assoc ****> p****t,assoc,cut****)**

- STAR Preliminary

- STAR Preliminary

- Oana Catu, Yale University

## Particle composition of the ridge

- |Δη|<0.7

- /K0S ratio:
-    in the ridge: ~ 1.0   similar to that from inclusive pT spectra
-    in the jet:     ~  0.5   consistent with p+p

- STAR preliminary

- By studying correlations with identified associated particles

- Ridge – medium response
-    - several models on the market – need more quantitative predictions to pin down mechanism

- Oana Catu, Yale University

## -h correlation

- R. C. Hwa et al., nucl-th/0602024

- -h correlation

- Near-side yield similar for , ,  triggered correlations

- Initial expectation:  dominantly from TTT recombination, no associated yield

- Revisited (at QM06): possible large contribution from reheated medium

- Experimental tests pending

- Oana Catu, Yale University

## Near-side zT Distributions: “Jet”

- to dAu
-         *no 50% dilution from *
*thermal coalescence triggers?*

- R. Hwa, C.B. Yang - Phys.Rev. C70 (2004) 024905

- Measure hadron triggered fragmentation functions:
*D**h1,h2**(z**T**)*
*z**T**=p**T assoc**/p**T trigger*
- Ratio Au-Au/d-Au

- Near-side zT Distributions: “Jet”

**  ****   ****8<p****T****trig****<15 GeV/c**
**      ****STAR PRL 97 162301 **

- Oana Catu, Yale University

## Away-side suppression at high pT

- T. Renk and Eskola, hep-ph/0610059

- Dihadron suppression: smaller surface bias, potentially better probe of the medium density

- Emission pointsHydro profile

- Theory: H. Zhang, J.F. Owens, E. Wang, X-N. Wang
- nucl–th/0701045

- 2.8 ± 0.3 GeV2/fm

- Extracted transport coefficient

- Input from other theory groups ?

- Oana Catu, Yale University

## Away-Side: shape modification

**0-12%**

- 1.3 < pTassoc < 1.8 GeV/c

- 4.0 < pTtrig < 6.0 GeV/c

- 6.0 < pTtrig < 10.0 GeV/c

- Away-side:
- Structures depend on pT.
- becomes flatter with increasing pTtrig
- yield increases

- 3.0 < pTtrig < 4.0 GeV/c

**AuAu 0-12%**

**A central contribution “filling in” the dip at higher p****T****trig  ****?**

- Away side

- Oana Catu, Yale University

## 3-particle correlations

- 13

- 12

- 0

- 

- 

- 

- 

- 13

- 12

- 0

- 

- 

- 

- 

**Event by event **
**deflection of jets**

**Cone like structure **
**in each event**

- 3-particle - probes away-side structure:
- Distinguish event-by-event deflection vs conical (Mercedes) emission pattern

- However: Large backgrounds, background shapes not simple

- Oana Catu, Yale University

## 3-particle results

- Au+Au 0-12%

- (12-13)/2

- (12+13)/2-

- 13

- Cumulant analysis:
- Model-independent
- Non-zero 3-particle structure

- Jet+background analysis:
- Model-dependent, more sensitive
- Off-diagonal peaks consistent with conical emission

- 12

- 3 < pT,trig < 4 GeV/c
- 1 < pT,assoc < 2 GeV/c

- Used two approaches to extract the truly correlated 3-particles component

- Need more clear
- understanding
- of the 3-particle structure

## 2+1 Correlations: Selecting di-jet events

- Aim of analysis – “pin” the jet axis
- Require that the 2 highest pT particles are back-to-back in 
*Assume *this defines the jet-axis
- Correlate all particles with this axis

- T1: pT>5GeV/*c*
- T2: pT>4GeV/*c*
- A:   pT>1.5GeV/*c*

- 

- Oana Catu, Yale University

## 2+1 Correlations - Results

- STAR preliminary

- STAR preliminary

- Correlation of associates with:
- T1 (also requires a T2)
- T2
- Detailed shape of near- and away-side is the same

- Di-jet trigger - no modification on away-side
- Surface dominated?
- No energy loss for di-jets?

- Oana Catu, Yale University

## High ET Clusters - trigger particles
     ETtrig >ETthreshold1
(6.5-8.5GeV/c, 8.5-10.5 GeV/c ...)
Associated tracks:
     ETtrig> pT > pTthreshold2 
        (> 1GeV/c)
3x3 tower Charged-track-Veto for pure photon candidates

-      ETtrig >ETthreshold1
- (6.5-8.5GeV/c, 8.5-10.5 GeV/c ...)
- Associated tracks:
-      ETtrig> pT > pTthreshold2
-         (> 1GeV/c)
- 3x3 tower Charged-track-Veto for pure photon candidates

******0**

****

**associated h******

**trigger ******

**incoming partons**

- Towards -jet: (,0)-h  Correlations

- Oana Catu, Yale University

**Notes:**
Cluster o single tower? Le cose sono diverse (vedi simulation of sub) -> correzioni  di pt

## Selection of photon-rich samples

******Bin**

**************Bin**

**Mixed**

- 2D Selection regions varied to get optimum results

**2D shower shape analysis**

- Oana Catu, Yale University

## (,0)-h  Correlations – p-p collisions

- (,0)-h  Correlations – p-p collisions

## Spectra for -tagged Events

**Promising for future ****γ****-jet studies in heavy ions with increased statistics**

- Spectra for -tagged Events

- Use -enriched sample and subtracting the fraction of the π0 contribution :
- Extract the  away-side pT spectra for photon-tagged events
- Matches charged hadron spectra in direct photon events from HIJING.

- Oana Catu, Yale University

## (,0)-  Correlations in heavy ions

**STAR Preliminary**

- Similar Away-side
-  for 0 and 

- Reduction in the
- near-side for 
- compared to 0.

- Reduction is more noticeable at higher ET_trig and also at higher centrality bins.

- (,0)-  Correlations in heavy ions

- Y axis: 1/Ntrg dN/d(ΔΦ)

- Blue - 0 bin
- Black -  bin

- Oana Catu, Yale University

## Summary

**Hadron correlations:**
**Near Side**: Broadening along Δη. (“Jet”+”Ridge”)
**“****ridge” – bulk-like** spectra, baryon/meson ratio
- after subtraction of long range Δη, D(zT) similar to d-Au.
**Away side:**
- Suppression of dihadron fragmentation function – extract** energy loss parameter** using NLO calculations
- Shape modification - contribution to *Δ**Φ**=**π* increases with higher pTtrig, fills dip
- 3-particle correlations showing promise for a more detailed look
**γ**** - hadron correlations** analysis in p-p -> reference, analysis in place to look into heavy ion collisions when enough data available
- A wealth of results for different system sizes – helped stimulate new theoretical ideas
- Ready for more!

- Oana Catu, Yale University

- Oana Catu, Yale University

- Oana Catu, Yale University

## What is the ridge?

- Proposed explanations so far:

- Radiated gluons, broadened by
- Longitudinal flow, Armesto et al, PRL 93 (2004)
- QCD magnetic fields, Majumder et al, hep-ph/0611035
- Anisotropic plasma, P. Romantschke, PRC,75014901 (2007)
- Medium heating + recombinationChiu & Hwa, PRC72, 034903
- Radial flow + trigger bias S. Voloshin, nucl-th/0312065, Nucl. Phys. A749, 287

- All models in qualitative agreement  quantitative calculations needed !

- Oana Catu, Yale University

## Two Analysis Techniques

- Measure 1-, 2-, and 3-Particle Densities

- 3-particle densities = superpositions of ***truly*** correlated 3-particles, and combinatorial components.
- We use ***two approaches*** to extract the truly correlated 3-particles component

- Cumulant technique:

- Jet+Flow Subtraction Model:

- Simple Definition
- Model Independent.

- Intuitive in concept
- Simple interpretation in principle.

**PROs**

**CONs**

- Not positive definite
- Interpretation perhaps difficult.

- Model Dependent
- v2 and normalization factors systematics

- .

- Oana Catu, Yale University

## LBNL 21-05-07

**LBNL 21-05-07**

- Clear structure for the
- two showers in  and 0
- at moderate energy.

- 0 shower at high
- energy is still wider
- than the single photon
- shower.

- Could be used to distinguish 0/

- Clear sensitivity to
- the halo region.

- Oana Catu, Yale University

## Extraction of  away-side charged particle pt spectra for “photon-tagged” events for pp collisions  (Et,trig : 8.5 – 10.5 GeV/c)

_**Method:**_
**Evaluate ************ fraction (f) from reduction in near side yield in ****“photon bin”**** compared to ****“ ********0**** bin” **
**Take  ****away-side p****T**** spectra for ********0**** bin**** and scale down by f**
**Subtract  away-side pt spectra for “****scaled ********0**** bin”**** from “****photon-bin****” and make a (1-f) scaling.**
**Compare with Hijing for extracted  spectra from ********0**** and photon bins.**

- Oana Catu, Yale University

## pTtrigger dependence of jet and ridge yields   

-  both unidentified and identified ( neutral strange ) hadrons
-  near-side associated yield is ridge dominated at intermediate pT

-  jet yield:
-              - increases steeply with p**T trigger**
- 	- jet/ridge ratio increases with p**T trigger**

- Oana Catu, Yale University

## Centrality dependence of near-side yield

-  steep increase of near-side yield with centrality in Au+Au
-  ratio of yields in central Au+Au/d+Au ~ 4-5

-         -> “jet” yield is independent of centrality and agrees with d+Au

-      -> “ridge” yield increases with centrality

- Oana Catu, Yale University

##    Baryon/meson anomaly at intermediate pT

-  intermediate pT :
-   baryon/meson splitting independent 	of strangeness content
-  high pT (pT > 5 GeV/c) :
-   all particles have similar RCP
-   and appear to show similar
-   suppression
- large enhancement of baryon/meson
-    ratio in central Au+Au relative
-               to  p+p collisions
-    - reaches maximum at pT~3 GeV/c
-   jet fragmentation is not a dominant
-    source of particle production

-    Baryon/meson anomaly at intermediate pT

- Put ridge and jet ratios on it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

- Oana Catu, Yale University

## Measure hadron triggered fragmentation functions:
Dh1,h2(zT)
zT=pTassoc/pTtrig
Similarity between AuAu and dAu after ridge subtraction

- Measure hadron triggered fragmentation functions:
*D**h1,h2**(z**T**)*
*z**T**=p**T**assoc**/p**T**trig*
- Similarity between *AuAu* and *dAu *after ridge subtraction

- Near-side zT Distributions: “Jet”

- Oana Catu, Yale University

## What is the origin of the ridge ? 

- Oana Catu, Yale University

## What is the origin of the ridge ? 

- 1) Parton radiation and its coupling
-     to the longitudinal flow
- gluon bremsstrahlung of hard-scattered parton
-              parton is shifted to lower pT
- radiated gluon contributes to broadening
- Longitudinal broadening of quenched jets in turbulent color
-  fields - A. Majumder, B. Mueller, S.A.Bass, hep-ph/0611135
- plasma instabilities in expanding medium   broadening of jet cone
- 2) Medium heating + parton recombination
-                           Chiu & Hwa Phys. Rev. C72:034903,2005
- hard parton passing through the medium  enhanced thermal parton distribution (ΔT=15 MeV)  recombination of thermal partons forms a pedestal (ridge)
- enhanced baryon/meson ratio
- 3) Radial flow + trigger bias
-                           S.Voloshin, nucl-th/0312065,  Nucl. Phys. A749, 287 (2005)

- Armesto et al, PRL 93 (2004)

- Oana Catu, Yale University

## High zT for hard triggers shows suppression (~0.2)

Larger yields seen at low zT or low pTtrig 
bulk response

Deviation from suppression depends on pTtrig

- High zT for hard triggers shows suppression (~0.2)
- Larger yields seen at low zT or low pTtrig
- bulk response
- Deviation from suppression depends on pTtrig

- Away-side: zT Distributions

- 2.5 < pTtrig < 4.0 GeV/c never reaches the 0.250.06 IAA  away-side suppression for pTtrig>8 GeV/c (STAR PRL 97, 162301)

- Oana Catu, Yale University