# Presentation: 337479

##   Intermediate and high-pT physics from STAR

- 	                       Jana Bielcikova
-                                           Nuclear Physics Institute ASCR
-                                                                and
- 	         Center for physics of ultra-relativistic heavy-ion collisions
-                                                  Prague, Czech Republic

- for the STAR Collaboration

## central Au+Au collisions produce 
dense and rapidly thermalizing matter:

it behaves almost like 
     a ‘perfect liquid’  
     (elliptic flow well described 
      by ideal hydrodynamics)

the relevant degrees of freedom seem to be partonic (constituent quark scaling)

it is opaque to jets

   It is not the asymptotically free plasma of quarks and gluons.

   strongly coupled plasma (sQGP) ?

- central Au+Au collisions produce
- dense and rapidly thermalizing matter:
- it behaves almost like
-      a ‘perfect liquid’
-      (elliptic flow well described
-       by ideal hydrodynamics)
- the relevant degrees of freedom seem to be partonic (constituent quark scaling)
- it is opaque to jets
-    It is not the asymptotically free plasma of quarks and gluons.
-    strongly coupled plasma (sQGP) ?

- What did we find at RHIC so far?

- STAR Collaboration, NPA 757,102 (2005)

*See e.g. STAR White Paper : Nucl.Phys. A757, 102 (2005)** *

- Bormio 2008

## Outline:

STAR experiment at RHIC
Particle production at intermediate and high-pT 
        - nuclear modification factors, baryon/meson ratios:
                   recombination vs pQCD, energy loss
        - two- and three-particle correlations: 
                   Mach cone, ridge
Summary

- Bormio 2008

- Outline:
- STAR experiment at RHIC
- Particle production at intermediate and high-pT
-         - nuclear modification factors, baryon/meson ratios:
-                    recombination vs pQCD, energy loss
-         - two- and three-particle correlations:
-                    Mach cone, ridge
- Summary

## Solenoidal Tracker At RHIC

- Solenoidal Tracker At RHIC

- Detectors:
-  Time Projection Chamber
-   (TPC): particle tracking, PID
-  Electromagnetic Calorimeter
-   (EMCal): trigger, 0,e PID
-  Time of Flight (TOF): PID
-  Silicon Vertex Tracker (SVT)
-  Forward Time Projection
-   Chamber (FTPC):2.7<||<3.9
*PID = particle identification*
***= pseudorapidity*

- Magnetic field: solenoidal,  B=0.5 T
- Acceptance: full azimuth, ||<1

- Bormio 2008

##       Au+Au collision as seen by STAR 

-       Au+Au collision as seen by STAR

- TPC  + EMC

- Jana Bielcikova

- Bormio 2008

- ~ 1500 charged hadrons
-     and leptons in central
-     Au+Au collisions

## Particle identification

- Identification of strange particles
- from V0-decay vertices:
**   **Λ     p  + -	BR=64%
-    K0S  + + -	BR=69%

****

**K****0****S**

- cuts on dE/dx of daughters
- topological cuts

**TPC**

**TOF**

**              ****mass (GeV/c****2****)                                    mass (GeV/c****2****)**

- Bormio 2008

## Probing QCD matter with high-pT particles and jets

- Probing QCD matter with high-pT particles and jets

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

- p+p

- Au+Au

- Bormio 2008

## High-pT particle production

- Bormio 2008

- High-pT particle production

## Radiative energy loss in QCD

-  found to be dominant for light quarks
-   independent of parton energy but
-    depends on the path length L in the medium
-  two example approaches:
-  medium properties can be characterized by a single constant:
-  static medium: E  L2 due to interference effects, expanding medium:  E  L

- Jana Bielcikova

- Bormio 2008

## How well does pQCD describe p+p?

- Do we understand our p+p baseline?

- Depends on particle species and choice of fragmentation function.

- p + p  0 + X

**NLO pQCD calculations by W. Vogelsang**

- STAR: Phys Lett B, 637 (2006) 161

- Jana Bielcikova

- Bormio 2008

## STAR measurement of strange particles in p+p constrained AKK FF

AKK fragmentation functions agree well with both mesons and baryons at mid-rapidity.
						KKP (Kniehl-Kramer-Potter): NPB 582 (200)
						AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006)
                                                                                                       	DSV (DeFlorian-Stratmann-Vogelsang): PRD57, 58111 (1998)

- Bormio 2008

- STAR measurement of strange particles in p+p constrained AKK FF
- AKK fragmentation functions agree well with both mesons and baryons at mid-rapidity.
*	**	**	**	**	**	**KKP (Kniehl-Kramer-Potter): NPB 582 (200)*
*	**	**	**	**	**	**AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006)*
*                                                                                                       **	**DSV (DeFlorian-Stratmann-Vogelsang): PRD57, 58111 (1998)*

- Strange particle production in p+p

- STAR, PRC 75 (2007)

## Nuclear modificiation factor in Au+Au 

- Bormio 2008

- Nuclear modificiation factor in Au+Au

- T. Isobe (PHENIX), QM’06

** ****Direct photons:**** **
**      ****- ****not suppressed **
**      ****do not interact strongly**
**      ****- binary scaling works**
** ****Light hadrons:**
**      ****- ****strongly suppressed**
**        ****(factor 5x) ****in central      **
**        ****Au+Au collisions **

**Large energy loss of light partons in the formed nuclear matter at RHIC**

## What do we learn from RAA ?

- What do we learn from RAA ?

- Use RAA to extract medium density :
- transport coefficient:

-  large values of  q = 5-15 GeV2/fm compatible with the data
-  suppression only supplies a lower bound on medium density 

-  ^

- Bormio 2008

**Notes:**
BDMPS

## Limited sensitivity of RAA

- Bormio 2008

- Limited sensitivity of RAA

- Energy loss distributions very different
-      for BDMPS and GLV formalisms
-               BUT!    RAA is similar

- Renk, Eskola, PRC 75, 054910 (2007)

- thick plasma
-  (‘BDMPS’)

- thin plasma
- (‘GLV’)

- Wicks,Horowitz,Djordjevic,Gyulassy
- NPA 784, 426 (2007)

- Reason: leading hadrons come preferentially
-                           from the surface
-   RAA has limited sensitivity to the region
-      of the highest energy density
-                    More differential probes needed: JETS** **

## Do baryons and mesons behave similarly?

- Intermediate-pT (pT =2-5 GeV/c): baryon/meson splitting 				                         	RCP(meson)<RCP(baryon)
- High-pT (pT > 5 GeV/c):
-  			RCP()~RCP(p)
-  			RCP(K)~RCP()
- Does it mean similar energy loss of  quarks and gluons ?

*STAR, PRL 97, 152301 (2006)*

## Gluon vs quark energy loss 

- Gluon jet contribution factor increases from , K, p towards:
-    e.g.  pT = 8 GeV/c:  50% for 
-                                    90% for p
- If
- and
- At high pT for same beam energy, system and centrality:
-                    RCP() > RCP(p)
-                    RCP(K) > RCP()

- Gluon vs quark energy loss

- AKK = particle + anti-particle

*AKK (Albino-Kniehl-Kramer): NPB 734, 50 (2006)*

##    Baryon/meson ratios at RHIC

- Bormio 2008

-  large enhancement of B/M ratio in 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 ratios at RHIC

*STAR, PRL 97 (152301) 2006*

- Au+Au:		p/ ~ 1		   Λ/K0S ~ 1.8
- p+p:			p/ ~ 0.3		   Λ/K0S ~ 0.6
- e++e-:		p/ ~ 0.1-0.2

## Parton recombination at intermediate pT

- Bormio 2008

- Parton recombination at intermediate pT

-  in vacuo fragmentation of a high momentum quark to produce hadrons competes with in medium recombination of lower momentum quarks to produce hadrons
- 6 GeV/c particle via :
-  fragmentation from high pT
-  meson
-     - 2 quarks at  pT~3 GeV/c
-  baryon
-    - 3 quarks at  pT~2 GeV/c

- Recombination produces more baryons than mesons at intermediate pT

*R.J. Fries et al., PRL 90 (202303) 2003*
*V. Greco et al.,  PRL 90 (202302) 2003*

**baryon**

**meson**

## Towards more differential probes ...

- Bormio 2008

## “Jet-like” correlations 

- Bormio 2008

- “Jet-like” correlations

- Full jet reconstruction in A+A
- collisions at RHIC difficult
- due to underlying background:
-       use azimuthal correlations
-            of high-pT particles

- near-side

- away-side

- Correlated yield is related to ratio of
-  di-hadron to single hadron
-  fragmentation functions:

-  * **z**T**=p**Tassoc**/p**Ttrig** *

*R**AA*

## Jet-like correlations at RHIC

-                          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
- Is there a punch through? YES

*STAR, Phys. Rev. Lett. 97 (2006) 162301*

*STAR, Phys Rev Lett 91, 072304*

- Bormio 2008

##   Fragmentation: near vs away side 

-   Fragmentation: near vs away side

- Widths unchanged with centrality:
- seeing those partons that
- fragment in vacuum?

- Away-side yield strongly suppressed
- 	to level of RAA
-  No dependence on zT in measured range

-     Near side:
- no suppression

-  Away side:
-  - strongly suppressed
-        to level of RAA
-   - no dependence on zT

- 8 < pT(trig) < 15 GeV/c

- Bormio 2008

*STAR, Phys. Rev. Lett. 97 (2006) 162301*

## Away-side di-hadron suppression at high pT

- Bormio 2008

- Away-side di-hadron suppression at high pT

-  di-hadrons have a smaller surface bias
-     a “better” differential probe
- 2-minimum narrower for di-hadrons
-     stronger constraint on density
-  extracted medium properties:
- 0 = 1.68 GeV/fm
-        q = 2.8±0.3 GeV2/fm

- Zhang,Owens,Wang,Wang, PRL 98 212301 (2007)
- NLO pQCD + KKP FF + expanding medium

- ^

- 2 (IAA)

- 2(RAA)

## System size dependence of the away-side suppression

-  IAA ratio calculated relative to d-Au
-  model comparisons
-  modified fragmentation:
-       - Npart scaling, but misses shape
-  Parton Quenching Model (PQM):
-       - wrong relationship between
-         Cu+Cu and Au+Au
-         (wrong q evolution or geometry?)

-   H.Zhang, J.F. Owens, E. Wang, X.N. Wang,
-   based on PRL 98 (2007), private communication

-  C. Loizides, Eur. Phys. J. C 49, 339-345 (2007)

- agreement with models better
-               at higher pTtrig

- Jana Bielcikova

- Bormio 2008

- 6< pT trig < 10 GeV

- STAR preliminary

- STAR preliminary

- 4< pT trig < 6 GeV

- Data: O. Catu (STAR), DNP 2007

-  suppression scales with Npart

-  ^

## Away-side di-hadron fragmentation functions

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

- zT=pTassoc /pTtrig

-  fragmentation functions more suppressed for higher Npart
-  theory curves using parameters from the fit to previous
-    Au+Au data at higher-pT match the Cu+Cu data

- Jana Bielcikova

- Bormio 2008

- O. Catu (STAR), DNP 2007

- Model: X.N.Wang private communication
- see also  H.Zhang, J.F. Owens, E. Wang, X.N. Wang, PRL 98 (2007)

## Where did the energy go?

-   lowering associated pT :
-     - resurrects correlated yield at away side
-     - near and away-side yields are enhanced with respect to d+Au
-     - shape of the away-side peak is not Gaussian

*STAR, PRL 95 (2005) 152301*

- Bormio 2008

*STAR, Phys Rev Lett 91, 072304*

##  Azimuhal correlations: pT dependence 

- Jana Bielcikova

- Bormio 2008

-  Azimuhal correlations: pT dependence

- increased statistics (year4):
-      full exploration of trigger        and associated pT range
- Medium response:
-       at lower pT,assoc
- away side enhanced
- and shape modified:
-    broad peak with
- angular substructure

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

## Away-side peak: Mach cone?

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

-  Stoecker, Casalderry-Solana et al,
-  Muller et al.; Ruppert et al., ...

- vs~0.33 c

- 3-particle correlation
-     studies needed!

- Bormio 2008

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

## Conical flow vs deflected jets

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

- STAR uses two methods:
- 1. Cumulant
- unambiguous evidence for 3-particle correlations, although not definitive about conical emission.
- 2. Jet-flow background (shown)
- Model dependent analysis.
- Evidence for conical emission.

- Note: Large and complicated backgrounds.

- J. Ulery (STAR), arXiv:0704.0224

- Jana Bielcikova

- C. Pruneau  (STAR), **J.Phys.G34****,****S667,2007**
- J. Ulery (STAR), **arXiv:0704.0224** [nucl-ex]

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

- Bormio 2008

- Additional near-side correlation in      pseudo-rapidity () observed

-   The near-side jet interacts
-         with the medium!

## Centrality dependence of near-side yield (I)

- Bormio 2008

-  jet yield independent of Npart and consistent with d+Au
-  ridge yield increases ~ linearly with Npart

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

## Centrality dependence of near-side yield (II)

- Bormio 2008

-  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
-                        ridge for K0S trigger < ridge for Λtrigger

*J.B. (STAR), QM’2006 **J.Phys.G34:S929** (**2007**)*

## “Jet” and ridge: pT dependence 

- Bormio 2008

- “Jet” and ridge: pT dependence

- 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)

## Particle composition in “jet” and ridge

- Bormio 2008

- A hint?  Ridge: B/M ratio closer to bulk
-              Jet:      B/M ratio ~ p+p
-                                                           More data needed !

*J.B. (STAR),** arXiv:0707.3100 [nucl-ex]** *

## Is there ridge at forward rapidity?

- Bormio 2008

*L. Molnar (STAR), **J.Phys.G34**, **S593** (**2007**)  *

- pTassoc = 0.2-2.0 GeV/c: no near-side peak within systematic errors
- pTassoc > 1 GeV/c:        : data suggest a non-zero correlation at near side (?)

## 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!

## Summary

- Bormio 2008

-                                                 Modification of jets:
-  inclusive measurements: large suppression of yields,
-         but RAA has only limited sensitivity to the medium density 
-  di-hadron measurements:
-          away-side associated jet-yield suppressed to the level of RAA
-   but shape of fragmentation function unmodified (pT, assoc > 3 GeV/c)
-                                                 Medium response:
-  away-side: broadening, angular substructure
-                     possible evidence of conical emission
-  near-side  “ridge”:  bulk-like properties
- 	             coupling of jet to the medium
- Stay tuned for the upcoming Quark Matter Conference in India which is just around the corner ...                                                        THANK YOU!

## BACKUP

- Bormio 2008

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

- Bormio 2008

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

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

- J. Putschke (STAR),QM’2006

## Testing recombination with  and 

- Bormio 2008

- Testing recombination with  and 

- The production of  and Ω is almost exclusively from thermal
- s-quarks even out to pT = 6-8 GeV/c
- (shower = “jet” contributions are
- strongly suppressed)

- Prediction:
- 1. Ω/ratio should rise linearly with pT
- 2. There should be no Ω or  di-hadron correlations at near side!

*R. Hwa, C.B. Yang , nucl-th/0602024*

- S = shower (“jet”)
- T = thermal

## Test 2: correlations with multi-strange particles 

- Bormio 2008

- Test 2: correlations with multi-strange particles

- There is a near-side peak for  and -triggered correlations and
-  its magnitude is independent of strangeness content !
- This is in disagreement with the recombination picture.

*             **B. Abelev (PhD 2007, Yale), J.B. (STAR) QM2006*

##  Baryon/meson enhancement: model comparisons

- Bormio 2008

-  Baryon/meson enhancement: model comparisons

-        Both, soft+quench and recombination type of models,
-        predict a peak in Λ/K ratio, however they can’t reproduce
-        the peak location in pT, its magnitude and shape in detail.

*SOFT+QUENCH: Gyullassy, Levai, Vitev,*
*                             **PRL 85 (2000),*
*                             **Nucl.Phys. A 715, 779 (2003) *

*TEXAS: Greco, Ko, Levai, PRL 90 (2003), *
*DUKE: Fries, Mueller, Nonaka, Bass, PRC 68 (2003)*
*OREGON: Hwa, Yang, PRC 67, 034902 (2003)*

## Elliptic flow: constituent quark scaling

- Bormio 2008

## Di-hadron correlations: near-side peak

- Bormio 2008

- Near-side jet peak
- Near-side ‘ridge’
- Modified away-side (+ v2)

- 3 GeV/c < pT trigger < 4 GeV/c, pT,assoc. > 2 GeV/c

- The near-side jet interacts
-        with the medium!

-  J. Putschke (STAR), HP2006, QM2006

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

- 4) Momentum broadening in an anisotropic QGP
-       *Romatschke, PRC 75, 014901 (2007)*
- 5) Longitudinal broadening of quenched
-     jets in turbulent color fields
-       *Majumder, Mueller, Bass, hep-ph/0611135*

## System size dependence of ridge yield

- Bormio 2008

- Relative ridge yield comparable in Au+Au and Cu+Cu at same Npart

- pTassoc. > 2 GeV/c

- STAR preliminary

- STAR preliminary

- relative ridge yield

- relative ridge yield

- relative ridge yield = ridge yield / jet()

- Au+Au @ 200 GeV
- Cu+Cu @ 200 GeV

- 3 GeV/c <pTtrigger<4 GeV/c

- Au+Au @ 200 GeV (30-40 %)
- Cu+Cu @ 200 GeV (0-10 %)

*J. Putschke (STAR), QM2006*

##       Correlations at near-side

- Bormio 2008

-       Correlations at near-side

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

-    Near-side yield   : Δɸ (J+R) = Δɸ (J) + Δɸ (R)
-    Jet yield	 : Δɸ (J)      = Δɸ (|Δη| < 0.7) - Δɸ (|Δη| > 0.7)
- 	               :  Δη (J)
-    Ridge yield	 : Δɸ (R)     = Δɸ (J+R) - Δɸ(J)

## Energy dependence: anti-particle/particle ratios

- Bormio 2008

*STAR, nucl-ex/0703040*

*STAR, PRL 97, 152301 (2006)*

*STAR, nucl-ex/0703040*

## Relativistic Heavy Ion Collider

- Relativistic Heavy Ion Collider

**RHIC**

- BRAHMS

- PHOBOS

- PHENIX

- STAR

**AGS**

**TANDEMS**

**Long Island, NY**

**1 km**

**v = 0.99995********c**

- 3.8 km circumference
- 2 concentric rings of 1740 superconducting magnets
- counter-rotating beams of ions from *p* to Au
- √sNN = 20, 62, 130, 200 GeV

- BOOSTER

- Bormio 2008

## Energy content in the ridge

- Bormio 2008

- STAR, Phys. Rev. Lett. 95 (2005) 15230,
- J. Putschke (STAR),J.Phys.G34:S679 (2007)

- 0.15 < pt,assoc < 4 GeV/c

- 4 < pt,trigger < 6 GeV/c
- 6 < pt,trigger < 10 GeV/c

-  near-side modification in published results also due to ridge
-  energy content deposited in the ridge is few GeV