# Presentation: 550539

## Intra-jet correlations inhigh energy nuclear collisions

**Intra-jet correlations in****high energy nuclear collisions**
- Jörn Putschke
*Yale University** *

- Weisshorn (4505m), Switzerland

## Outline

**Outline**

## Reminder: Jet-quenching in heavy-ion collisions

-  direct photons scale with Nbin
-  hadrons are suppressed with respect Nbin scaled p+p
-  recoil jet is quenched in dense matter (final state effect)
****** ****jets lose energy in dense colored matter**

- 4 < pt,trig < 6 GeV and pt,assoc > 2 GeV

- Near-side

- Away-side

## Reminder: Di-hadron correlations

- 8 < pTtrig < 15 GeV

- pTassoc > 3 GeV

- Use di-hadron correlations to probe the jet-structure in p+p, d+Au

**Near side**

**Away side**

- and Au+Au

## Di-hadron azimuthal correlations

**low p****t****: ****4 < p****t,trigger ****< 6****  and p****t,assoc**** > 0.15 GeV**

**high p****t ****: 8 < p****t,trigger ****< 15**

**Away-side yield is suppressed but ****finite and measurable at high p****t,trigger ****and p****t,assoc ****in central Au+Au collisions**

**Away-side (and near-side) yield is enhanced for low p****t,assoc**** in Au+Au**

- Recoil distribution soft and broad, angular substructure,
- Mach cone? (not topic of this talk)

## Di-hadron associated spectra 

- Surviving pairs at high pt seems to favor conditions with small energy loss:
- tangential (halo) emission, finite probability for no energy loss (energy loss fluctuations) or dilution due to the expanding system (*T. Renk, Hard Probes 06 and PQM: Dainese, Loizides and Paic*)
-  limited sensitivity to energy loss in di-hadron fragmentation on the away-side ?
** ****study near-side modification**

- Low pt  : Strong modification of near-side and away-side observed
- High pt : Near-side unmodified, away-side suppressed but shape unmodified

## Ridge “observation”

**Ridge “observation”**

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

**Au+Au 0-10%**

**d+Au minbias **

**Additional near-side long range corrl. in ******** in central Au+Au (“ridge like” corrl.) observed**** **Dan Magestro, Hard Probes 2004, STAR, nucl-ex/0509030, Phys. Rev. C73 (2006) 064907  and P. Jacobs, nucl-ex/0503022

- STAR preliminary

- STAR preliminary

## Components of  correlations

**Near-side jet-like corrl.****+ ridge-like corrl. ****+ v****2 ****modulated bkg.**** **
**Ridge-like corrl. ****+ v****2 ****modulated bkg.**** **
**Away-side corrl.****+  v****2 ****modulated bkg.**

- _Strategy:_ Subtract  from  projection to isolate the ridge-like correlation

**Au+Au 0-10%**
- STAR preliminary

- How to extract the ridge yield ?

## Extracting near-side “jet-like” yields

**Extracting near-side “jet-like” yields**

- Au+Au 20-30%

## Analysis methods cont.

**v****2**** and systematic error estimation in Au+Au:**
- a) Used v2 values = mean between v2 RP and v2{4}
- b) Systematic errors mainly due to uncertainties
- in v2; use v2 RP and v2{4} as upper and lower limit

**Use event-mixing to account for pair acceptance ****and use eff. correction for ass. particles**
**Background:**
- Subtract constant background for (J)
- Subtract v2 modulated background for (J+R)(ZYAM method)
**Assume Gaussian correlation shape:****yield(********) = gaus integral / bin counting******** (********) = gaus width**

- 2.5 < pt,trigger < 3 GeV and 0.3 < pt,assoc. < 0.8 GeV

- 

- v2{4}

- <v2{RP}+v2{4}>

- v2{RP}

- ZYAM norm.

## Near-side  shape in central Au+Au

**Near-side ******** shape in central Au+Au**

- STAR preliminary

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

- 

- 

- STAR
- preliminary

- yield)

## Jet-like peak width in central Au+Au

**Jet-like peak width in central Au+Au**

- 4 < pt,trigger < 5 GeV and pt,assoc. > 2 GeV

- pt,assoc. > 2 GeV

-  Jet peak symmetric in and for pt,trig > 4 GeV and comparable to d+Au
-  Jet peak asymmetric in  for pt,trig < 4 GeV and significantly broader than d+Au

## Extracting the ridge yield

- relative ridge yield := ridge yield / Jet()

- STAR preliminary

**Jet+Ridge (********) ****Jet (********)**
**Jet********)**

- yield,)

- Npart

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

- Jet yield independent
- of Npart and consistent
- with d+Au reference measurements !

- _Definition of “ridge yield”:_
- ridge yield := Jet+Ridge(  Jet()

## Near-side:  component picture

- Components:
**Near-side jet peak****comparable to d+Au**
**Near-side ********independent ridge**
**Away-side (and v****2****)**

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

**Au+Au 0-10%**
- STAR preliminary

**New phenomenon in****Au+Au “The ridge”:**

**What is it ? ****‘something’ coupling to long flow ?  ****Can this quantify E-loss ?**
**How to deal with it?****Need to subtract for near-side studies?**

**Near-side: ******** component picture**

## Ridge yield in Au+Au

- Ridge yield persists to highest trigger pt  correlated to jet production

- Ridge only in Au+Au (not present in p+p or d+Au or peripheral Au+Au)

## Ridge/Jet pt,assoc spectrum in central Au+Au

-  Jet pt-spectra harder and increasing with pt,trig, as expected from jet fragmentation
-  Ridge pt-spectra are ‘bulk-like’ and approx. independent on pt,trig

## Ridge/Jet pt,assoc spectrum in central Au+Au

- preliminary

- Au+Au 0-10%

- preliminary

**Ridge / Jet yield**

- STAR preliminary

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

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

## Subtracting the ridge

**Subtracting the ridge**

- Subtraction of -independent ‘ridge-yield’ recovers centrality-independent jet yield

- p+p like (vacuum) fragmentation after energy loss?

- zT = pT,assoc/pT,trig

- zT = pT,assoc/pT,trig

- STAR preliminary

- STAR preliminary

**Jet+Ridge**

**Jet in d+Au**

**“****Jet” in Au+Au**

- M. Horner (STAR), QM’2006

## “Jet”/Ridge energy

*STAR, Phys. Rev. Lett. 95 (2005) 15230*

- 0.15 < pt,assoc < 4 GeV

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

## Trigger PID Ridge/Jet

-   (ridge for K0S trigger < ridge for Λ trigger ?)
-  jet yield is independent of centrality and agrees with d+Au

**Trigger PID Ridge/Jet**

## Trigger PID ridge/jet pt,assoc spectrum 

-  Trigger PID ridge spectra   comparable to h correlations   and ~ bulk
-  Trigger PID Jet spectra   comparable to h correlations   and slightly harder ~ 50 MeV

**Trigger PID ridge/jet p****t,assoc**** spectrum **

- J. Bielcikova (STAR), QM’2006

## Particle composition in Ridge/Jet

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

- |Δη|<2.0

- |Δη|<0.7

- STAR preliminary

- STAR preliminary

**Particle composition in Ridge/Jet**

- Outlook: similar measurement will be pursued using dE/dx ID to look at p/π ratio in ridge/jet

- J. Bielcikova (STAR), QM’2006

## Particle composition in Ridge/Jet vs. inclusive

- Ridge: B/M ratio closer to bulk
- Jet    :  B/M ratio ~ p+p

*J. Bielcikova (STAR), WWND 2007*

**Particle composition in Ridge/Jet vs. inclusive**

- More statistics needed ...

## Ridge yield in Au+Au and Cu+Cu

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

- pt,assoc. > 2 GeV

- STAR preliminary

- STAR preliminary

- relative ridge yield

- relative ridge yield

- relative ridge yield

- relative ridge yield

- relative ridge yield := ridge yield / Jet()

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

- 3<pt,trigger<4 GeV

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

## Ridge characteristics

**STAR Au+Au 0-10%, RHIC, US (~0m)**
- preliminary

-  ridge approx. independent on 
-  ridge persists up to highest trigger pt    correlated to jet production
-  ridge spectrum ~ “bulk-like”
-  ridge energy roughly a few GeV
-  jet di-hadron fragmentation function   after subtracting the ridge contributions  comparable to d+Au
-  PID trigger dependence (?)
-  /K0S ratio in ridge ~ inclusive B/M ratio
-  ridge comparable in Au+Au and Cu+Cu at ~ Npart

**Do we see vacuum fragmentation after **
**energy loss on the near-side in central **
**Au+Au collisions with the lost energy**
**deposited in the ridge ?**

## 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 + recombination**Chiu & Hwa, PRC72, 034903
**Radial flow + trigger bias**** **S. Voloshin, nucl-th/0312065, Nucl. Phys. A749, 287E. Shuryak, nucl-th/0706.3531
**Momentum-kick model**C.-Y. Wong , hep-ph/0707.2385

## Discussion

-  ridge spectrum slightly harder (?) than inclusive h+,-  (tens of MeV) consistent with medium heatingparton recombination (T~15 MeV) ?
-  agreement with radial flow + jet quenching ?
-  ridge spectrum qualitatively in agreement with parton energy loss and coupling to longitudinal flow

**Unfortunately all models are in qualitative agreement ******** quantitative calculations needed !**

## What else can we measure ...

-  3-particle  near-side correlations  test longitudinal flow picture
-  How far does the ridge extend ?  forward rapidity measurements
-  Is there a ridge on the away-side ?  di-hadron triggered correlations				           (3-particle x correlations)
-  Full jet reconstruction  E-by-E jet-shape modifications
-  			   (radiation spectrum)
-  Geometry effects (pathlength)  ridge vs. reaction plane 				    (system size; Cu+Cu)

**More ? Predictions ? Ideas ? ...**

## 3-particle  near-side correlations

**Radial flow + trigger bias **S. Voloshin, nucl-th/0312065, Nucl. Phys. A749, 287

**Toy model **

- Analysis started, first results soon ...

- associated

- trigger

- 

- 

- 

**Long. flow picture**Armesto et al, PRL 93 (2004)

## Is there a ridge at forward rapidity ?

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

**Is there a ridge at forward rapidity ?**

-           |trig| <1
- 2.7 < |assoc| < 3.9

- Data are not conclusive; larger  acceptance  PHOBOS Caveat: no momentum measurement at high rapidity

## Is there a ridge on the away-side ?

- T1: “near-side trigger” > 6 (8) GeV

- T2: “away-side trigger” > 4 (6) GeV
- (and < near-side trigger)
- within 180 deg. +- 30 deg.

- A: assoc. particles > 2 GeV

- disappearing bkg.

- 8<pt,trigger<15 GeV

**Use “away-side trigger” for 2-particle ******** correlations **

- Caveat: probably favor small energy loss of the surviving di-hadron pair
- 	 3-particle correlation needed

## Di-jets in Au+Au

**Di-jets in Au+Au**

- Surviving (di-jet) pairs at high pt seems to favor conditions with small energy loss

- Di-jet measurements suggest that neither
- the widths in  and (ridge/mach cone) are modified (challenge radial flow picture?) nor the yields are suppressed and comparable to d+Au

- Caveat: Non-trivial bkg. subtraction

*STAR,O. Baranikova, ISMD (07)*

- 

- 

## Fundamental quantity P(E)

- ~15 GeV

- Renk, Eskola, hep-ph/0610059

**Can we constrain this by experiment?**

- Salgado and Wiedemann, Phys. Rev. D68, 014008

- Energy loss distribution P(E) depends on

- Radiation spectrum

- Geometry, time evolution of matter

- High-pt di-hadron correlations: favor cases with small/no energy loss
- Low-pt di-hadron correlations: modification of near-side jet-part and complex away-side structure

**Need an unbiased measure of the parton energy ******** full jet reconstruction**

## Jet reconstruction in heavy ion collisions

**Heavy ions: large background **
**from underlying event**

**Control background by limiting jet cone radius R, track p****T**** cut**
****** ****measure a fraction of partonic energy **

**Unbiased jet reconstruction: parton kinematics ******** hadrons**

## Outlook: Full jet reconstruction in HI @ RHIC ?

** ****Jets > 10-15 GeV should be ****reconstructable in Cu+Cu @ RHIC **

- Full jet reconstruction in p+p up to 40 GeV

- Back of the envelope calculation ...

- (should be sufficient statistic on tape ...)

## Summary

**Near-side jet modification “ridge” due to energy loss ?****If so, do we have a direct measurement of energy loss and medium response ?**
**Many measurements characterizing the ridge properties are already available. Quantitative calculations are needed now ****to understand the underlying mechanism of the ridge !**
**Additional measurements possible (on the way) to further**
**address the origin of the ridge. Extend sensitivity with**
**full jet-reconstruction to constrain radiation spectrum.**

## “Ridgeologie” (R. Hwa ISMD 07)

-    Azimuthal anisotropy is mainly a ridge effect. No fast thermalization or hydrodynamical flow are needed.
-    Hydrodynamics may still be applicable after some time, but it is not needed for v2, for which the relevant physics at <1 fm/c is crucial --- semi-hard scattering at qT<3.
-    For pT<1.5 GeV/c, the analysis is simple, and the result can be expressed in analytic form that agrees with data.
-    For pT>1.5 GeV/c, shower partons must be considered. Jet dominance (>3GeV/c) will saturate v2.
**   ****No part of the study suggests that the medium behaves like a perfect fluid.**

- “Ridgeologie” (R. Hwa ISMD 07)

## Backup slides

## Pion vs. Proton relative ridge yield

- Proton content of ridge larger than of jet part

**Assoc. Protons**
**Assoc. Pions**
**Assoc. h******

## Particle composition near/away-side: p/π

- Preliminary

- Preliminary

- Near side

- Away side

**Baryon/meson assoc**

**Baryon/meson assoc**

**B/M singles**

- Trigger h±: 2.5 < pT < 4.0GeV/c

- Near side: increase with centrality
- Measure particle composition of ridge?

- Away-side B/M increases strongly with centrality

- Need comparisons to theory for interpretation

*A. Sickles, PHENIX (QM06)*

**Particle composition near/away-side: p/π**

## What are we measuring at high-pt ?

*K. Werner, **Phys.Rev.Lett.98:152301,2007*

**Do we have a black/white picture ?**
**High-p****t**** particles who survive do not loose energy ?**
**Can this explain the p/π ratio at high-p****t****,**
**R****AA****(e) ~ R****AA****(π) and no modification of the FF ?**

*PHENIX, QM06*

*STAR, QM06*

- RAA(e)RAA(π)

- p/π(Au+Au)
-  p/π(d+Au)

## Extracting the transport coefficient “via suppression”

**Extracting the transport coefficient “via suppression”**

- Di-hadrons provide stronger constrain on density

- Extracted transport coefficient from singles and di-hadrons consistent

- Di-hadron suppression

- Inclusive hadron suppression

- Caveat: Different models extract different transport coefficient values ...

## Limitations of di-hadron measurements

- T. Renk, Hard Probes 06: Di-hadron fragmentation not very sensitive to medium density profile

**Limited discirminating power in di-hadron fragmentation for different energy loss/medium density model**

- PHENIX hep-ex/0605039: Di-hadron fragmentation not very sensitive to D(z)

****** ****full jet reconstruction is necessary**

## Jet modification at LHC

- Radial profile

- Fragmentation function

- PQM with fragmentation of radiated gluons (A. Morsch)

- Energy loss depletes high-zand populates low-z

- Low-z fragments from gluon radiation at large R

- In-medium energy loss redistributes momenta in jets

- Measure these modifications to extract P(E), medium properties

- Expectations from QCD+jet quenching

- Jet reconstruction

- Ejet = 125 GeV

- =ln(EJet/phadron)

- z

- 0.37

- 0.14

- 0.05

- 0.02

- 0.007