# Presentation: 652459

## The “Shoulder” and the “Ridge” in PHENIX:	Medium Response to Fast Partons in 	Heavy Ion Collisions via Di-hadron 	Correlations

- for the PHENIX Collaboration
- Quark Matter 2008
- Jaipur, India
- 5 February 2008

_***The “Shoulder” and the “Ridge” in PHENIX:***_***	******Medium Response to Fast Partons in ******	******Heavy Ion Collisions via Di-hadron ******	******Correlations***

## Heavy Ion Collisions

- _Jet Suppression:_
-   - Fast partons lose energy in
-     the medium
-   - Lost energy should be
-     deposited locally in the
-     medium
- ► Where does the energy go?
- ► Does anything of the jets
-     remain?
- ► Does the lost jet energy elicit
-     a response from the medium?

_***Heavy Ion Collisions***_

-  - Jet reconstruction difficult in
-    heavy ion collisions
-  - Jet physics can still be studied
-    via two-particle correlations

- Δφ

- Δφ

- _Two Particle Correlations:_

## Medium Response

- p+p, peripheral Au+Au

- central Au+Au

- _Typical:_
-   - Near-side Jet
-   - Away-side Jet – “Head”

- _New:_
-   - Near-side Modification – “Ridge”
-   - Away-side Modification – “Shoulder”

- PHENIX poster (Chin-Hao Chen)

- Near-side Ridge theories:  Boosted Excess, Backsplash, Local Heating,...
- Away-side Shoulder theories:  Mach, Jet Survival + Recom, Scattering,...

## Away-Side Decomposition

- _Bin Method:_
-    - Model independent
-    - Measures physics of
-      dominant contribution
-    - Contamination

- _Fit Method:_
-    - Model dependent
-    - Needs higher statistics
-    - Less contamination

- data from arXiv:0801.4545

## Away-side Contributions

-  - Suppressed relative
-    to p-p baseline
-  - Dominated by
-    shoulder at low pT
-  - See Hua Pei’s talk
-    in Section IX
- _Away-side Shoulder:_
-  - Strongest at lower
-    pT ( < 4 GeV/c )
-  - Δφ position largely
-    independent of pT
-      ( ~ π ± 1.1 )

_***Away-side Contributions***_

**increasing p****T**

- arXiv:0801.4545

## Away-side Shoulder Position

-    separately
-  - Position largely
-    independent of both
-    trigger and partner pT
-    selection

_***Away-side Shoulder Position***_

- arXiv:0801.4545

- Fit Method

## Away-side Composition

- arXiv:0712.3033

- arXiv:0712.3033

- _Shapes:_
-   - Similar shape in away-side
-     mesons and baryons

- _Ratios:_
-    - Away-side baryon/meson
-      ratios approach inclusive
-      values
-    - Incompatible with in-vacuum
-      fragmentation

## Away-side Shoulder Spectra

- arXiv:0801.4545

- Bin Method

- _p-p baseline:_
-  - Spectral shape depends
-    on trigger pT selection
-    (0.55 → 0.73 GeV/c)
- _Mid-Central __→__ Central Au-Au:_
-  - Medium response dominates
-    the shoulder bin (>50 Npart)
-  - Softer than p-p away-side
-  - Little dependence on trigger
-    pT selection (~0.45 GeV/c)

## Away-side by Geometry

**30-40%**

_***Away-side by Geometry***_

- _Reaction-Plane Binned Triggers from Run 7 with RXPN Detector:_
-   - Reaction-plane resolution sys errors – Black lines (correlate in-out)
-   - BBC-RXPN v2 sys errors – Red lines (anti-correlate in-out)

**in-plane**

**Cent **
**0-5%**

**out-of-plane**

- in

- in

- out

- out

## Near-side Enhancement

-    at low partner pT
-  - No enhancement or
-    suppression at highest
-    pT
-  - Apparent “suppression”
-    at low pT triggers and
-    high pT partners

_***Near-side Enhancement***_

- arXiv:0801.4545

## Near-side Composition & Spectra

- data from: arXiv:0712.3033

-  - Near-side Baryon/Meson
-    ratio increases in central
-    collisions

- arXiv:0801.4545

-  - Near-side spectra are
-    softer than p-p baseline
-    at intermediate pT

## Near-side Ridge

-  - Broad Δη near-
-    side enhancement
-    measured in
-    Au+Au collisions
-    at intermediate pT
-  - High pT near-
-    sides are similar
-  - Intermediate pT
-     p+p near-side is
-    narrower in Δη
-    than central
-    collisions

- arXiv:0801.4545

-  - At intermediate pT, little p-p jet beyond Δη > 0.5

## Connections - Centrality

- PHENIX poster (Chin-Hao Chen)

-  - Away-side
-    shoulder and
-    near-side ridge
-    share a common
-    centrality
-    dependence
-  - Scale similarity
-    here is largely a
-    factor of pT
-     selection

## Connections - Balance

- 0.0 < |Δη| < 0.1

- 0.5 < |Δη| < 0.7

-  - Jet & Ridge balances Shoulder & Head
-  - Ridge & Shoulder balance seperately!

## Connections - Spectra

- PHENIX poster (Chin-Hao Chen)

-  - Near-side ridge and
-    away-side shoulder are
-    both softer than p-p
-    counterparts
-  - Near-side ridge is
-    possibly harder
-    than away-side shoulder
-  - Away-side shoulder is
-    closest to inclusive
-    hadron slope

## Triggering on Medium Response

- arXiv:0801.4545

-  - 120 deg is a special angle
-  - Two-sided shoulder
-    mechanisms could create
-    structures at Δφ = 0
-    and Δφ = π ± 1.1

-  - IAA trigger-partner anti-
-    symmetry indicates not
-    all triggers are jet
-    fragments

## Summary

-  - PHENIX is measuring both _**Ridge and Shoulder**_
-  - Shoulder & Head variation consistent with contributions of both
-    medium response and suppressed in-vacuum jet fragmentation
-  - Ridge and Shoulder measurements consistent with _**medium**_
**   **_**response**_, inconsistent with in-vacuum jet fragmentation
-  - Ridge & Shoulder share much of the _**same behavior**_
-       - appear at similar pT
-       - similar centrality dependence
-       - softer than p-p counterparts
- 	  - baryon-meson ratios larger than jet fragmentation
-       - balance pT
-  - At low enough pT, some triggers must come from medium response
- _Further:_
- 	PHENIX posters on intermediate pT correlations:
- 		Inc γ-h correlations – Chin-Hao Chen (PHENIX)
- 		h-h correlations – Jiangyong Jia (PHENIX) – P165

## Backups

## Cent 0-20%

## Cent 60-92%

## Reaction-Plane Dependence – Full

## Fit Method Discussion

## Away-side Shoulder Enhancement

- Away-side enhancement over p-p baseline is limited to lower partner pT
- Away-side suppressed below p-p values at high pT
- Head and Shoulder suppressions similar at high pT

- arXiv:0801.4545

- Head region also shows enhancement at low pT due to contamination from Shoulder region

## Inclusive γ-h Correlations

## PHENIX Detector

**Central Arms**

-  |η| < 0.35

- Charged tracking from DC, PC1, and PC3 at mid rapidity, η.
- Centrality and Reaction-plane from BBC and ZDC at large |η|.

## Background Subtraction

- Jet + Flow                                   Jet

## Acceptance Correction

- 0

- π

- Acc(Δφ)

- ΔΦAB

- The two-arm acceptance effects are removed by building a correction from event mixing.

**A**

**B**

**A**

**B**

**A**

**B**

**A**

**B**

**A**

**B**

- Event N

- Event N-1

- Event N-2

- Event N-3

- Event N-4

- Rolling buffer mixing technique
- Pooled by Event type:
-    5cm zvertex
-    5% centrality

## 2D Acceptance

## Large Angle Scattering -- Vitev

- hep-ph/0501255

- Angles are typically smaller than observed
- Average scattering angle falls with path length
- Contrasts our centrality dependence

## Large Angle Scattering – Polosa, Salgado

- Claim that the similar pT windows restrict signal to a     small number of splittings

- PRC **75**, 041901(R) (2007)

## Mach Cones

***D ~ 1.0-1.1 ******→ c******s****** ~ 0.45 - 0.54***

- Casalderray,Shuryak,Teaney hep-ph/0411315
- Stöcker, Nucl. Phys. **A750** (2005) 121

- θM = 1.2 → cs ~ 0.33
- (0.57 in QGP, 0.2 in hadron gas)

- Reproduces large angles seen in the data
- Expectation of little pT dependence
- Expectation of a beam energy dependence

## Mach Cones – Jorge & Edward

_***Mach Cones – Jorge & Edward***_

- hep-ph/0511263

- First-order phase transition would reflect some Mach Cone

## Mach Cones – Renk & Ruppert

- HP 2006

- Longitudinal flow improves the predicted Δφ signal from a Mach Cone
- Match to the data requires large fractional energy loss to Mach cone

- Away

- Near→

## Mach Cones – Chaudhuri & Heinz 

- Assume lost energy is instantaneously thermalized in their hydro simulations
- Extract no Mach cone signal
- Large energy loss scenarios give “splash-back” signal

- PRL **97, **062301 (2006)

## Cherenkov – Majumder, Wang

- Cherenkov production has “a strong dependence on the gluon momentum”
- 	“disappear for high-energy gluons”
- Predict D values will shrink as associated momentum is increased.

- data from: Phys. Rev. Lett. 98, 232302  (2007)

- PHENIX

- PRL **96, **172302 (2006)

## Testing Thickness Scaling

*peripheral*

- thicker via centrality
- selection

- thicker via trigger to reaction plane selection

*central*

*in*

*out*

*mid*

- Cu+Cu                             Au+Au

## System and Energy Scan

- nucl-ex/0611019

- Away-side structure vs. beam species, beam energies, and centrality
- _All cases:_
- ● Peripheral similar to p-p
- ● Central shows development of “lobe”-like structure

## Npart Shape Scaling

- nucl-ex/0611019

**J(****Δφ****)**

- ● Shape saturates above 100 Npart