# Presentation: 527887

## Energy Loss in Heavy Ion CollisionsJana Bielcikova (Yale University)

## Outline:

introduction to energy loss
p+p reference data vs pQCD
gluon/light quark energy loss       
heavy-flavor energy loss
direct photons
   Observables: RAA, particle ratios, two-particle correlations
                        vs  system size, energy, rapidity

- introduction to energy loss
- p+p reference data vs pQCD
- gluon/light quark energy loss
- heavy-flavor energy loss
- direct photons
-    Observables: RAA, particle ratios, two-particle correlations
-                         vs  system size, energy, rapidity

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## High-pT particle production in p+p

-  scattering of partons followed by
-   fragmentation  jet
-  can be calculated in perturbative QCD
-  collinear factorization

- High-pT particle production in p+p

- Parton distribution function

- Matrix element

- Fragmentation
-  function

- Measured in DIS

- e+e-

- pQCD

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Parton energy loss in A+A (“jet quenching”)

- A+A collisions:
-  initial state: similar to p+p
-  final state: hard partons traverse medium
-                     and lose energy
-       - gluon radiation
-       - elastic collisions with surrounding partons
-      softening of particle spectra at high pT
-  energy loss different for gluon, light and heavy quarks
-                 (color factor, dead cone effect)
- Goal: Use in-medium energy loss to measure medium properties

**c**

**d**

**hadrons**

**hadrons**

**b**

**a**

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Radiative energy loss in QCD

- 4 jet quenching schemes:
- higher twist expansion
*          **Qiu, Sterman, Wang, Wang, Zhang, Majumder,...*
- finite temperature field theory
*          **Arnold, Moore,Yaffe  (AMY)*
- opacity expansion:
-   - thin medium/single hard scattering
-       *Gyulassy, Levai, Vitev,Djordjevic,... (GLV)*
*   *- thick medium/multiple soft scatterings
-       *Baier, Dokshitzer, Mueller, Peigne, Schiff (BDMPS)*
-       *Armesto, Salgado, Wiedemann (ASW)*
-  medium properties can be characterized by a single constant:
-  static medium: E  L2 due to interference effects, expanding medium:  E  L

- energy loss

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## p+p reference data

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## p+p reference @ 200 GeV vs pQCD

- Compilation by D. d’Enterria nucl-ex/0611012

- Good agreement with theory

- pT-differential cross section:
-  - jets
-  - charged hadrons
-  - 0                                                        ~0
-  - direct photons
-  - D,B (non-photonic e-)
-  - negative charged hadrons     =3.2
-  9 orders of magnitude (10mb-10pb)
-  broad range in pT

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Proton and pion production in p+p

**p+p  -> p + X**

**p+p  -> ******** + X**

- Pion and proton spectra agree with NLO pQCD using the latest AKK fragmentation functions
- (Note: p is more sensitive to gluon fragmentation – KKP does not work!)

- STAR, PLB 637 (2006)

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## 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)

- 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)

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Hadron production in p+p at y~3

- BRAHMS, hep-ex/0701041, submitted to PRL

-  NLO pQCD describes production of pions and kaons well at y~3, but fails to account for large proton yields and small p/p ratio even with AKK FF.

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Nuclear modification factorsParticle ratios

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## RAA in Au+Au @ 200 GeV 

- Direct photons:
-  measured p+p reference
-  RAA() < 1 for pT > 12 GeV/c
-       Isospin effect seen?
- Ongoing study at √sNN=62 GeV
- 0 and 
-  Run 5: extended reach in pT
-   out to 20 GeV/c for0 and
-   15 GeV/c for 
-  both have a common RAA~0.2

- T. Isobe (PHENIX), QM’06

- binary collision scaling

- p+p reference

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Energy dependence of RAA 

- New!
- Measured p+p reference at √sNN = 62 GeV instead of previously used ISR data
-              RAA at √sNN=62 GeV is now very close to √sNN=200 GeV !

- Energy dependence of RAA

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Comparing RAA() to models

-  minimization fit to obtain the probability of a given parameter
- values given for probability > 10%
-                   RAA shows only a small sensitivity to model parameters

*B. Sahlmueller (PHENIX), QM’06*

*C. Loizides, hep-ph/0608133*

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## What do we learn from RAA?

- ~15 GeV

- Energy loss distributions very different for BDMPS and GLV formalisms

- BUT!    RAA is similar

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

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

- thick plasma  (‘BDMPS’)

- thin plasma (‘GLV’)

-   More differential probes needed!

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## 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)*

## RCP of identified particles 

- 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)*

- RCP of identified particles

## Energy dependence: anti-particle/particle ratios

*STAR, nucl-ex/0703040*

- -/+: independent of pT
- p/p: model calculations with/without
-        Eloss do not describe data
*	**X.-N. Wang et al., PRC 70 (2004) – E**loss*
-  Baryon junction and coalescence models
-  describe data at intermediate pT
*                 **I. Vitev et al.. NPA 715 (2003)  - baryon junction *
*                **V. Greco et al., PRC 71 (2005) - coalescence*

*STAR, PRL 97, 152301 (2006)*

*STAR, nucl-ex/0703040*

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Rapidity dependence of RAA 

- RAA independent of rapidity for produced mesons

- Au+Au at 200 GeV

- I. Bearden (BRAHMS), QM2006

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Why is RAA independent of rapidity?

-  forward y = a unique information about medium
-      in the longitudinal direction in the early stage
-  competing effects: shadowing, multiple scattering,
-                                 energy loss (GLV), geometry
-  opacity decreases ~ linearly with y
-                        BUT!
-  shadowing stronger at forward y
-             effects compensate each other
-             RAA  independent of y

- Barnafoldi, Levai, Papp, Fai, EJP C49 (2007)

- L/opacity

- pT (GeV/c)

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## RAA of non-photonic electrons 

-  *Note: ~ agreement between STAR and PHENIX  *
*  **(disagreement is common to p+p/Au+Au and cancels out in R**AA**)*

-     Models have difficulties to describe
-                 the measured RAA:
- radiative energy loss with typical gluon densities is not enough      (I)
-       *M. Djordjevic et al., PLB 632 (2006) 81*
- models involving a very opaque medium agree better                  (II)
-       *N. Armesto et al., PLB 637 (2006) 362*
- collisional energy loss/resonant elastic scattering                  (III,IV,V)
-     	  *S. Wicks et al., NPA 784, (2007) 426                                           *
*      **H. v. Hees, R. Rapp, PRC 73 (2006) 034913*
- heavy quark fragmentation and dissociation in medium → strong suppression for c and b             (VI)
-        *A. Adil, I. Vitev, PLB 649, (2007) 139*

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Towards more differential probes ...

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## RAA vs reaction plane

- RAA vs reaction plane

*Au+Au collisions at 200GeV*

*PHENIX, nucl-ex/0611007, submitted to PRC*

-  factor 2 suppression out-of-plane
-    than in-plane
-  in plane emission shows no
-   energy loss in peripheral bins.

- 3<pT<5 GeV/c

**In Plane**

**Out of Plane**

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Path length dependence of energy loss

- L =  matter thickness calculated in Glauber model

-  RAA is universal function of L for all centrality classes and both pT ranges
-  Little/no energy loss for L< 2 fm  Formation time effect? Surface emission zone? v2?       *Pantuev, hep-ph/0506095,*
*	**	**	**	**	**	**	**Shuryak+Zahed, hep-ph/0307267*

- 0-10%

- 50-60%

*PHENIX, nucl-ex/0611007, submitted to PRC*

- Au+Au √sNN = 200GeV

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

**Notes:**
The pathlength weighted integral over the density is done on a Glauber model by assuming that the density goes like the Ncoll density. 

The data seem to imply that we don’t reach a regime that matches the energy dependence of purely radiative energy loss until high pT. Is this a signal that at lower pT’s collisional losses are significant? This is all in pretty good agreement with Magdelena’s calculations (see backup slide). 

## Jet-like correlations at high-pT

- disappearance of away-side correlations observed at intermediate p**T**
**       **in central Au+Au collisions
- Run 4 statistics: a punch through observed at high pT
-          - away-side yield is suppressed: RAA ~ IAA
-          - suppression without angular broadening or medium modification
-          - RAA ~ IAA in Cu+Cu as well

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

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## 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, nucl-th/0701045
- NLO pQCD + KKP FF + expanding medium

- ^

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Di-hadron correlations: near-side peak

- 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*

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Near-side yields

- M. Horner (STAR), QM2006

- C. Zhang

-  increase of near-side yield
-   at low pTassoc (zT) observed by
-   STAR and PHENIX
-  subtraction of -independent
-   ‘ridge-yield’ recovers
-   centrality-independent jet yield

- zT = pT,assoc/pT,trig

- STAR preliminary

- STAR preliminary

- J. Jin (PHENIX), QM2006

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Ridge properties

- Ridge yield:
-  increases with centrality
-  ~ independent of pTtrigger
-  pT -spectra are ‘bulk-like’
-  particle composition in the ridge 
-        see talk of M. Daugherity

- Ridge: solid symbols
- Jet: open symbols

- jet yield

- jet+ridge

- “jet” slope
- ridge slope
- inclusive slope

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

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Energy content of the ridge

- STAR, Phys. Rev. Lett. 95 (2005) 15230, J. Putschke (STAR), QM2006

- 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

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## The golden channel: -jet

- The golden channel: -jet

- hard process: pQCD calculations describe well measured data
- no surface bias
- calibrated probe: ET, = pre-quenched ET,jet
- monochromatic source  differential measurement of jet-quenching

** ****has no energy loss in medium**

**But D(z) is softened in opposing jet**

**Medium  size ******

*X.-N. Wang, Z. Huang, PRC 55, 3047, F. Arleo et al JEHP 0411, 009, T. Renk, PRC 71, 034906*

- PHENIX

**hadrons**

**b**

**a**

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## First results on -hadron correlations

-  p+p: yields consistent with expectations from Pythia/HIJING
-  Au+Au: a hint of away-side modification (?)
-  ongoing studies in Cu+Cu (STAR/PHENIX)
-  statistical and systematic uncertainties are large
-   We have the tools, we just need more statistics 

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)

## Summary

- THANKS to BRAHMS, PHENIX and STAR for their input to this talk !!!

-  high-statistics Run4/Run5 enable exploration of in-medium
-   energy loss in detail:
-     integrated  (RAA)  differential  (RAA vs reaction plane, correlations)
-     hadrons  identified particles (non-strange, strange, heavy-flavor)
-    ‘the golden channel’ – -jet is emerging
-  data trigger lots of interest in theoretical community
-  upgrades to PHENIX and STAR will bring us even more exciting results

- 2007 RHIC&AGS Annual Users' Meeting

- J. Bielcikova (Yale)