# Presentation: 794332

##   Near-side di-hadron correlations at RHIC

-   Near-side di-hadron correlations at RHIC

-                     Jana Bielcikova (Yale University)

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Outline:
Introduction
Properties of ridge and jet-like correlations at near side
Where does the ridge come from?
Summary

- Introduction
- Properties of ridge and jet-like correlations at near side
- Where does the ridge come from?
- Summary

- STAR Preliminary

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Jet-like correlations at RHIC

- disappearance of away-side correlations observed at intermediate pT
- lowering associated pT :
-     - resurrects correlated yield at away side
-     - near and away-side yields are enhanced with respect to p+p/d+Au
-     - shape of the away-side peak is not Gaussian     Mach cone? Deflected jets?
-                                                                                                          Cherenkov radiation?

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

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

- STAR, PRC73 (2006) 064907,
-            J.Phys.G32 (2006) 37

- pT < 2 GeV/c

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

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

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## The ridge properties

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

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

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

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

- J. Putschke (STAR),QM’2006

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

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

-  jet yield independent of centrality (Npart)
-  ridge yield increases ~ linearly with Npart

- J. Putschke (STAR),QM2006

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

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

-  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

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## pT spectra of associated particles at near side

- pT spectra of associated particles at near side

- M. Horner (STAR), QM2006

-  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 = pTassoc/pTtrig

- STAR preliminary

- STAR preliminary

- ||<0.35

- ||<1.0

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## pT spectra of associated particles 

- STAR preliminary

**Ridge/Jet yield (p****T****assoc ****> p****T****assoc,cut****)**

- pT spectra of associated particles

-  jet spectrum  harder than inclusive pT-spectrum
-                     T(jet) increases with pTtrigger
-                                         x
-  ridge spectrum similar to particles from bulk
-                     T(ridge) ~ independent of pTtrigger

**p****T****assoc.cut ****(GeV/c)**

- J. Putschke (STAR),QM’2006

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## pTtrigger dependence of ridge

- pTtrigger dependence of ridge

- pT assoc > 2 GeV/c

- STAR preliminary

- Ridge yield:    persists up to pTtrig ~ 10 GeV/c
-                     Ridge is independent of  jet energy

- J. Putschke (STAR),QM’2006

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Particle composition in jet and ridge

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

*J.B. (STAR), WWND 2007*

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## System size dependence of ridge yield

- 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*

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Energy content in 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

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Is there ridge at forward rapidity?

*L. Molnar (STAR), QM’06, nucl-ex/0701061*

- 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 (?)

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Where does the ridge come from?

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## 
                          C. Chiu & R. Hwa, Phys. Rev. C72 (2005) 034903
hard parton passing through the medium enhances 
          thermal parton distribution (ΔT=15 MeV) 
       recombination of thermal partons forms a pedestal (‘ridge’) 
enhanced baryon/meson ratio

-                           *C. Chiu & R. Hwa, Phys. Rev. C72 (2005) 034903*
- hard parton passing through the medium enhances
-           thermal parton distribution (ΔT=15 MeV)
-        recombination of thermal partons forms a pedestal (‘ridge’)
- enhanced baryon/meson ratio

- Parton recombination

**ridge**

**jet**

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Parton radiation and its coupling to longitudinal flow

*                                     **Phys. Rev. Lett. 93 (2004)*
-  parton energy loss is sensitive
-   to energy density of medium +
-   collective flow
-  medium induced gluon radiation
-  radiated gluon contributes
-   to broadening in 

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## 

           A. Majumder, B. Mueller, S.A.Bass
                     hep-ph/0611135

plasma instabilities in longitudinally
      expanding medium
 	  non-thermal color fields
      broadening of jet cone in 
but not in
      wide ridge in rapidity 
                  at low pTassoc  

-            *A. Majumder, B. Mueller, S.A.Bass*
*                     **hep-ph/0611135*
- plasma instabilities in longitudinally
-       expanding medium
-  	  non-thermal color fields
-       broadening of jet cone in 
- but not in
-       wide ridge in rapidity
-                   at low pTassoc

- Longitudinal broadening of quenched jets in turbulent color fields

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Momentum broadening in anisotropic QGP

- 	*P. Romatschke, Phys.Rev. C75, 014901 (2007)*
-  momentum broadening in a homogeneous
-   but locally anisotropic system
-  calculation: eccentricity ~ √8/3
-                            z/T ~ 3
-  caveats:
-     - calculation done only for charm quark and
-             collisional energy loss
-      - requires large anisotropy (ξ ~ 10)
-             large shear viscosity (ξ~ 10/sT)
-             but data suggest low shear viscosity (~0.1)

- z/T ≃ Δη/Δ > 1

- data: D. Magestro (STAR),
-      Hard Probes 2004

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Correlations between jet and radial flow

- 	                       *S. Voloshin, nucl-th/0312065, Nucl.Phys. A749, 287 (2005) *
*                **E. Shuryak, nucl-th/0706.3531*
- transverse radial expansion in central collisions
- correlation appears due to jet quenching
- averaging over jet origin and 1
-              width broader than that in data
- ridge is independent of jet
- particle spectra in ridge:
-       points of origin are biased towards surface
-         ‘a bit’ stiffer slope than that of bulk
- particle composition: N(baryons)>N(mesons)

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Momentum kick model

*                                                 **C.-Y. Wong , hep-ph/0707.2385*
-  partons in medium acquire momentum ‘kick’ from propagating jet
-  fix T = 470 MeV,  vary q1 (mom. kick)  or σy (rapidity distribution)
-  narrow peak in depends mainly on momentum kick
-  ridge structure in depends on initial parton rapidity distribution

- q1=0.6 GeV/c

- y= 5

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)

## Summary

- Existence of ridge-like correlations at near side
- is well established and measured:
-    - particles associated with the ridge are similar to bulk
-                        (pT spectra, particle composition)
-    - more data are becoming available for identified particles
-    - studies with 3-particle correlations ongoing (2+1, 1+2)
-    - studies at forward rapidity
-          These are important measurements to pin down
-                         the physics origin of the ridge
-                                             BUT
-      More quantitative theoretical predictions are needed as well!

- ISMD 2007, Berkeley

- Jana Bielcikova (Yale)