# Presentation: 506913

##   Medium properties and jet-medium interaction from STAR

- 	                     Jana Bielcikova
-                                         for the STAR Collaboration
-                         NPI ASCR and Center for physics of ultra-relativistic
-                            heavy-ion collisions, Prague, Czech Republic

- XLIII Rencontres de Moriond “QCD and High Energy Interactions”

## Outline:

Introduction
“Jet”-medium interaction via 2- and 3-particle correlations 
             - conical emission ?
             - long range pseudo-rapidity correlations (“ridge”)
Summary

- XLIII Rencontres de Moriond

- Outline:
- Introduction
- “Jet”-medium interaction via 2- and 3-particle correlations
-              - conical emission ?
-              - long range pseudo-rapidity correlations (“ridge”)
- Summary

## Probing QCD matter with jets

- What happens to high-pT particles/jets
- which pass through the medium?
- Are they similar to p+p or modified by the medium?

- p+p

- Au+Au

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## Jet-like correlations at RHIC

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**4 <p****T****(trig)<6 GeV/c **

**2 GeV/c <p****T****(assoc)<p****T****(trig) **

-                          central Au+Au collisions @ 200 GeV:
- disappearance of away-side correlations observed at intermediate p**T**
- d+Au and p+p similar -> jet suppression is a final state effect
-  lowering associated pT resurrects correlated yield
-           - enhanced yield at near and away side + shape modification

*STAR, PRL 91 (2003) 072304*

## Conical emission in A+A collisions?

- Jana Bielcikova

- 2.5 < pTtrig< 4 GeV/c and 1< pTassoc < 2.5 GeV/c

-  Mach cone in heavy-ion physics introduced
-      in1970’s (Hofmann, Stöcker, Heinz, Scheid, Greiner)
-  a supersonic parton creates shock waves:
-       - hydrodynamics
*                            **Stöcker et al., NPA750 (2005) 121*
*                        **Casalderrey-Solana et al., NPA774 (2006) 577*
*                        **Renk, Ruppert, PRC73 (2006) 011901*
-       - colored plasma
-                         *Ruppert, Mueller, PLB618 (2005) 123*
-        -  AdS/CFT
-                         *Gubser, Pufu, Yarom, PRL100, (2008) 012301*
-  Čerenkov gluon radiation by a superluminal parton
*                       **Dremin, NPA750 (2006) 233*
*                       **Koch et. al., PRL96 (2006) 172302*

-   To distinguish from other
-     mechanisms 3-particle
-  correlation studies needed

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- M. Horner (STAR), J.Phys.G34, S995,2007

## Conical flow or deflected jets? (I)

- Jana Bielcikova

- deflected jets
- (by the collective movement, “flow”, of the expanding medium)

*Armesto , Salgado, Wiedemann, *
*PRL 93, (2004) 242301 *

- conical emission

- di-jets

- cartoons of 3-particle
- azimuthal correlations
- (1 trigger + 2 associated particles)

## Conical flow or deflected jets? (II)

- 

- STAR uses 2 methods:
- 1. Jet+flow background
- - model dependent analysis
-  evidence for conical emission
- 2. Cumulant method
*C. Pruneau (STAR),**J.Phys.G34** (**667**)**,2007**;*
*C. Pruneau, PRC 74 (2006) 064910*
-  unambiguous evidence
-   for 3-particle correlations
-  strength and shape of away-side
-   structures depend on magnitude
-   of flow coefficients
-  no conclusive evidence for
-   conical emission

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

*J. Ulery (STAR), arXiv:0704.0224*

- STAR Preliminary

- d+Au

- central Au+Au

- Note: Large and complicated
-          backgrounds

- STAR Preliminary

- STAR Preliminary

## Mach cone or Čerenkov gluons?

- Mach cone or Čerenkov gluons?

- Mach cone
- cone angle independent  of pTassoc
- Čerenkov gluon radiation:
- cone angle decreases with pTassoc

- Jana Bielcikova

- XLIII Rencontres de Moriond

- pT dependence of cone angle favors Mach cone over Čerenkov gluon emission

*Koch, Majumder, Wang,  PRL****96**** 172302 (2006*)

## Momentum conservation in correlation analyses

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- Calculation by N. Borghini:
- momentum conservation  sizable correlation between pairs or triplets
- of high-pT particles in central Au+Au collisions at RHIC
-            C3(pT) ~ C3(flow)
-            jet “distorts” the event
-              needs to be evaluated in the data

*N. Borghini, PRC *75 (2007) 021904

-  2-particle correlation
- after v2 subtraction

- Calculation for:
- <pT,trig > = 3.2 GeV/c
- <pT,assoc >= 1.2 GeV/c

*N. Borghini, arXiv:0710.2588*

## A closer look at the near-side peak ...

- Jana Bielcikova

- Additional near-side
- correlation in 
- observed in central Au+Au collisions

- pTtrig=3-6 GeV/c, 2 GeV/c <pTassoc< pTtrig

**“****jet”**

**ridge**

- d+Au, 200 GeV

- Au+Au, 200 GeV

- What is the ridge?
- 1) Medium heating and parton recombination
-       *Chiu & Hwa PRC 72  (2005) 034903 *
- 2) Radial flow + high-pT trigger particle
-       *Shuryak, **Phys.Rev.C76** (2007) **047901*
-       *Voloshin, nucl-th/0312065  NPA 749,  (2005) 287 *
- 3) Parton radiation and its coupling
-     to the longitudinal flow
*     **Armesto, Salgado, Wiedemann, PRL 93 (2004) 242301 *

- 4) Momentum broadening in anisotropic QGP
-       *Romatschke, PRC 75 (2007) 014901 *
- 5) Longitudinal broadening of quenched
-     jets in turbulent color fields
-       *Majumder, Mueller, Bass, **PRL** **99** (2007) **042301*
- 6) Momentum kick imparted on partons in medium
-        *Wong, PRC**  **P**R**C** **76** (2007) **054908** *
*    *

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- STAR preliminary

## Centrality and system size dependence of near-side yield 

-  “jet” yield independent of colliding system,
-           Npart and trigger particle type
-  ridge yield increases with Npart

*Au+Au:  J. B. (STAR), QM 2006*
*Cu+Cu: C. Nattrass (STAR), QM2008*

- Jana Bielcikova

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## “Jet” and ridge: pT dependence 

- Jana Bielcikova

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- Jet:
-       T(jet) > T(bulk)
-       T(jet) increases with pTtrig
- Ridge:
-        T(ridge) ~ T(bulk)
-        T(ridge) ~ independent of pTtrig
-        ridge persists up to pTtrig ~ 10 GeV/c

- Ridge: solid symbols
- Jet: open symbols

- “jet” slope
- ridge slope
- inclusive slope

- h-h correlations
- pTassociated>2GeV/c

- pT assoc > 2 GeV/c

- J. Putschke (STAR), J.Phys.G34:S679 (2007)

- J. Putschke (STAR), J.Phys.G34:S679 (2007)

**“****jet” slope**
**  ****ridge slope**
**  ****inclusive slope**

## Particle composition in “jet” and ridge

- Baryon/meson ratios:
- “jet”:   smaller than inclusive
-  ridge: similar to inclusive

- Particle composition in “jet” and ridge

* **Cu+Cu: C. Nattrass (STAR), QM2008*
* **Au+Au:  J.B. (STAR), WWND07*

- Jana Bielcikova

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## Near-side “jet” and ridge w.r.t. event plane

- ridge: decreases its magnitude
-                       with φS
- “jet”:  slight increase with φS
-          magnitude consistent
- 	      with d+Au

- 3< pTtrig < 4 GeV/c,  pTassoc =1.0- 1.5 GeV/c

- Jana Bielcikova

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*A.Feng (STAR), QM2008 *

## 3-particle xcorrelations

- Jana Bielcikova

- 1) Jet fragmentation
-     in vacuum

- 3-particle xcorrelations

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*P. Netrakanti (STAR), QM 2008*

## Summary

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- Strong modification of correlation patterns (not present in d+Au collisions) observed in central A+A collisions at RHIC:
-  broadening of away-side peak with angular substructure
- inconsistent with Čerenkov gluon radiation
- 	possible evidence for conical emission ?
-        (need to evaluate effects of momentum conservation)
-  medium responds through “ridge” formation in pseudo-rapidity
-         bulk-like properties (spectra, particle composition)
-         medium density/path length effects (dominated in the event plane)
-         further studies of 3-particle correlations to understand ridge underway
-                                             THANK YOU!

## BACKUP

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## Di-hadron correlations w.r.t. event plane

- 3< pTtrig < 4 GeV/c, pTassoc=1.0-1.5 GeV/c

**RMS**

- Jana Bielcikova

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## Di-hadron correlations: pathlength effects

- similar to d+Au in 20-60%
- broader than d+Au in central collisions
- out-of-plane:
- small difference between the two
- centralities

- Away-side features reveal
-       pathlength effects

- 3< pTtrig < 4 GeV/c, pTassoc=1.0-1.5 GeV/c

- Au+Au 200 GeV

*A.Feng (STAR), QM2008 *

- 20-60%

- 0-5%

- Jana Bielcikova

- XLIII Rencontres de Moriond

**Notes:**
The RMS increases with phiS. 

## Extracting near-side “jet” and “ridge” yields

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-           **J = “jet”,    R= “ridge”**

- pTtrig=3-4 GeV/c, pTassoc>2 GeV/c

- J. Putschke (STAR),QM’2006

## What is the origin of the ridge? (I) 

- What is the origin of the ridge? (I)

- 1) Medium heating and parton recombination
-                           Chiu & Hwa Phys. Rev. C72:034903,2005
- hard parton enhances thermal parton distribution
-                             (ΔT=15 MeV)
-        recombination of thermal partons forms
-                        a pedestal  (ridge)
- enhanced baryon/meson ratio
- 2) Parton radiation and its coupling
-     to the longitudinal flow
- gluon bremsstrahlung of hard-scattered parton
- radiated gluon contributes to broadening

- Bormio 2008

## What is the origin of the ridge?   (II) 

- What is the origin of the ridge?   (II)

- 3) Longitudinal broadening of quenched
-     jets in turbulent color fields
- 		     A. Majumder, B. Mueller, S.A.Bass, hep-ph/0611135
- plasma instabilities in expanding medium
-  		     non-thermal color fields
-               broadening of jet cone
-               wide ridge in rapidity at low pTassoc

- Bormio 2008

## What is the origin of the ridge? (III) 

- What is the origin of the ridge? (III)

- 4) Correlations between jet and radial flow* *
-        S. Voloshin, nucl-th/0312065, Nucl.Phys. A749, 287 (2005)
-        E. Shuryak, arXiv: 0706.3531 [nucl-th]
-  radial expansion + jet quenching  correlation
-  ridge is independent of jet
-  particle spectra in ridge:
-       points of origin are biased towards surface
-         ‘a bit’ stiffer slope than that of bulk
- 5) Momentum kick* *
-                C.-Y. Wong , arXiv: 0707.2385, arXiv:0712.3282
-  medium partons acquire ‘kick’ from propagating jet
-  T=470 MeV,  q1 (mom. kick)  and σy (rapidity distribution)
-  narrow peak in depends mainly on momentum kick
-  ridge in depends on initial parton  y distribution

- Bormio 2008

- More quantitative theoretical
- predictions are needed!