# Presentation: 079974

## Medium Response Measurements

*An experimental overview of interactions with the medium at RHIC*

- Michael Daugherity – University of Texas

## Jet-Medium Interactions

- Daugherity - Medium Response

**Creation of a nuclear or partonic medium**

**?**

- Use jet interactions to probe the medium

## Jet Correlations 101

- What do jet correlations look like?  Use proton-proton as a reference...

## Revisiting An Old Story

- _Overview of this talk_

- PRL 91 072304

- η1 - η2

- Φ1 - Φ2

- proton-proton jet

## Act I – Same-Side

## “Ridge” Observation

- “Ridge” Observation

**An Important Discovery**
- The symmetric jet cone becomes extremely broad along η in central Au-Au forming a ridge-like structure

**STAR Data**

## Ridge Components

- Daugherity - Medium Response

- Components
**Same-side jet peak**
**Same-side ridge**
**Away-side (and v****2****)**

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

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

**nucl-ex/0701074**

## Ridge Properties

**Ridge Properties Overview**
- Ridge yield increases linearly with centrality
- same-side yield dominated by ridge in central collisions
- Ridge persists at very high pt
- Ridge spectra and composition much more like bulk medium than jets
- Relative ridge yield comparable at same Npart in Au+Au and Cu+Cu

- Daugherity - Medium Response

******/K~1**

******/K~0.5**

## Minimum-bias Correlations

- All results so far have used a high-pt triggered jet analysis, but:
- we need to search lower momentum range for more jet-medium interactions and medium response
- we may be missing effects due to high-pt trigger
- Solution:  remove the trigger particle to make minimum-bias correlations
- or, if you prefer, trigger on *every* particle

- _High-pt Triggered Jet Analysis_
-  clean separation of jets from other correlations (except flow)

- _Minimum-bias Analysis_
-  will see all correlation sources on equal footing, can use maximal information to isolate one from another

-  jets in vacuum above ~2 GeV are well described theoretically with pQCD

-  angular correlations described by a pedestal + Gaussian

-  dominated by soft particles, *nonperturbivate* regime of QCD

-  will have to fit data with simplest model functions possible

## Minbias – Intermediate pt

- Minbias – Intermediate pt

- Daugherity - Medium Response

- 0.8 < pt < 4.0

- Au-Au 0-5% (background subtracted)

**STAR Preliminary**

- PRC 75 (2007) 034901, nucl-ex/0702008

-  η width increases by 3x
-  Φ width ~ constant

- Similar η broadening seen in minbias as in trigger particle correlations.
- Next we study inclusive pt range

-  Dominant feature is a jet-like peak at the origin
-  Other structures are flow and instrumental effects removed with high-precision fit

## Minbias – Inclusive pt

- Minbias – Inclusive pt

-  no backgrounds removed: large spike from HBT and e+e- pairs, flow signal evident
-  new structure observed: correlation on η in peripheral events, more prominent at low energies; *what is it?*
-  90-100% Au-Au is very similar to proton-proton, let’s look for answers there...

- Most jet analyses used a mixed event reference to remove background and report the ratio
*same / mixed *
- This correlation measure uses
*(same-mixed) / √mixed*
- to achieve **per-particle** normalization

## Two-component Model

**proton-proton collisions  **
- With high-precision STAR p-p data, we can test power-law fit and improve upon it with two-component soft + (semi)hard model
- PRD 74, 032006 (nucl-ex/0606028)
**First **use correlations on transverse momentum to isolate these components
**Next **see what the η-φ correlations look like

## Proton-Proton Correlations

- hep-ph/0506172

**y****t1**

**y****t2**

**What happens in Au-Au collisions?**

**STAR Preliminary**

## Isolating the Components

- Au-Au 90-100%

- Directly related to v2,
- precise separation of flow from non-flow
- hep-ph/0704.1674

**STAR Preliminary**

-  minijet peak becomes extremely large and broad
-  string fragmentation correlation disappears
-  flow correlation measured

## Energy and Centrality Dependence

- Minbias – dominated by low pt
-  width on η increases 4x
-  width on Φ *decreases*

- Jet Amplitude

- Jet η Width

- Jet φ Width

**Au-Au 200, ****130,**** ****62**

**STAR Preliminary**

- 100%

- 0%

- 100%

- 0%

- 100%

- 0%

- with PRC 73 (2006) 064907

## Charge-Dependent Correlations

- Daugherity - Medium Response

- proton-proton

-  Evidence for charge-ordering moving from one-dimensional string to a surface
-  The 1-D signal becomes ***symmetric*** on ηΔ and ΦΔ in central Au-Au
-  Suggests a new geometry of hadronization in RHIC collisions

**Charge-ordering:  **a particle produced from fragmentation tends to have the *opposite electric charge *as its neighbors
- In the Lund model, one-dimensional strings fragment on η, should produce charge-ordering signal.
- Look for Charge-Dependent correlation = (Like sign pairs) – (Unlike sign pairs)

- see PLB 634, 347

## PHOBOS

- Foreground

- Mixed-event Background

**proton-proton**

-  Minbias correlations by necessity
-  Exciting prospect of ridge measurements at large pseudorapidities

- correlation
- function
- (uncorrected):

## PHOBOS Cu-Cu

-  After flow subtraction will have great potential for ridge measurements at large η

## A New Correlation

- Daugherity - Medium Response

**Number Correlation**

-  backgrounds subtracted

- PRC 73 064907

- Instead of asking *how many pairs* in a bin, we can count *how much p**t* and find momentum correlations

- Momentum Correlation shows jet peak with two new structures

## Momentum Correlations

- Au-Au 20-30%

- Daugherity - Medium Response

- When the away-side jet is absorbed, where does the momentum go?
- Medium recoil?  Momentum red-shift?

- Why does the jet peak sit inside a valley?

- plotted in cylindrical coordinates

- peak

- J Phys G 32 L37

## The story so far...

- Same-side correlations reveal *broad structures on η *that evolve dramatically with pt, centrality and energy.

- High pt:  “bulk-like” ridge + p-p-like jet
- Minbias:  minijet broadening
- Can a single theoretical model describe both of these regimes?

- Low pt studies revealed string fragmentation correlations

- Why is the dominant hadronization mechanism in p-p not present in central Au-Au?
- Related to change in charge-ordering geometry?

- Momentum correlations reveal qualitatively new information about the medium response to minijets

## Act II – Away-Side

## Away-Side Minbias

- Explore the away-side from low to high pt
- In minbias analyses, we expect a global momentum conservation term of cos(φ)
- Surprisingly, the structure from away-side jets is *indistinguishable* from this term
- Measured amplitude of cosine increases ~ linearly with multiplicity, whereas momentum conservation alone scales as 1/N

- Daugherity - Medium Response

- cos φ

## Away-Side Shapes - PHENIX

- Empirical observation:
- Minbias analyses fit away-side jet with cosine, high-pt triggered analyses use a Gaussian.  Here there is a smooth evolution between the two...

-  In proton-proton we see a clear away-side jet (“head” region)

- nucl-ex/0705.3238
- submitted to PRL

-  In Au-Au we see a dip after flow subtraction at π defining the “shoulder” regions

-  The dip grows as the away-side jet is suppressed, finally reappears at very high momentum

## Away-Side Shapes - STAR 

- Away-Side Shapes - STAR

- 7

**increase trigger p****T**

**increase associated  p****T**

- Similar story from STAR:
-  dip increases
-  jet suppressed
-  high pt punch-through
- Unique results:

-  Large away-side yield for high trigger pt

-  Same shape in jet region as in ridge region (blue vs green points)

## Away-Side Dependencies

- Daugherity - Medium Response

- The away-side dip:
-   increases with energy

-   increases with centrality

-   disappears at high trigger pt

-   increases with associated pt

## Modeling the Shape 

- Modeling the Shape

**PHENIX**
**nucl-ex/0611019**
**submitted to PRL**

- Model away-side jet as two symmetric Gaussians equally displaced from π

-   Parameters scale with Npart only, not energy or momentum
-   Disfavors Cherenkov models where the opening angle depends on the velocity of the particle through the medium

## Path Length Dependence

-  Hint of stronger suppression out-of-plane than in-plane, consistent with energy loss picture.
-  Hint of stronger double-peak structure out-of-plane than in-plane.

- J. Jia (PHENIX), nucl-ex/0510019.

- In plane

- Out of plane

-  New Cu-Cu results show no path-length dependence within errors
-  Need precise determination of flow background

## Reappearance at High pt

- Reappearance at High pt

- clear away-side peak – dijets
- away-side yield suppressed, but shape is unchanged
- likely contributions from tangential jets

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

- STAR, PRL 97 (2006) 162301.

- J. Jia (PHENIX), nucl-ex/0510019.

## Conditional 2-Particle Distributions

## Conditional 2-Particle Distributions

- trigger 3<pT<4, associated 1<pT<2

**Au-Au 200 GeV raw data**

- Model background as having hard (jets) and soft (flow) components, need to remove all possible cominbations

- Cone angle = 1.45 radians (~83°)
- much larger than expected

**Final signal**

- Subtract all backgrounds

## “True” 3-particle correlations

- “True” 3-particle correlations

- Particles are labeled 1, 2, 3.
- In this analysis
- Particle 1: Used as a jet tag/trigger,    3 < pt < 4 GeV/c.
- Particles 2 & 3: associated 1 < pt < 2 GeV/c.
- Measure 1-, 2-, 3- particle densities normalized per event
- Calculate
-   2-Cumulant
-   3-Cumulant

**Au+Au 0-10%**

- As before, subtract two-particle correlations

- Conditional analysis is more intuitive and easier to interpret, but requires model-dependent background subtraction
- We can also find the true 3-particle correlation which is model-independent, but more difficult to interpret

## 3-particle Correlations

- Daugherity - Medium Response

- Au+Au 0-10%

- Subtract backgrounds

- Final Nominal Cumulant

- v2 and v4 from Yutin Bai

- Subtract additional
- v2v2v4 flow term:

- Clear evidence for *finite 3-Particle Correlations*
- Observation of flow like and jet like structures.
- Finite Sensitivity: No evidence for Mach cone

## PHENIX Two-particle Conditional Distributions polar coordinates

- Daugherity - Medium Response

- PHENIX Two-particle Conditional Distributions **polar coordinates**

**Back-to-back Jet**

**assoc. pTs (2,3)**

- * : trigger-associated
-       particle polar angle
- * : *2assoc -*3assoc

- defines
- z-axis

**Deflected jet **
- * ~ 0
- * ~ D

**Mach Cone**
- * ~ R

## PHENIX Data

- _PHENIX Data_
- _With harmonic_
- _subtraction_

- Simulated
- Deflected jet

- Evidence for Mach Cone signal, but does not rule out contributions from other topologies.

**True 3P jet correlations**

## Away-Side Summary

- At lower pt, minbias analysis suggests jet momentum dissipation
- For high-pt trigger analysis a dip appears at Δφ=π (after v2 subtraction) which motivates studies of cone emission.
- Three-particle correlation analyses are complex, requiring meticulous background subtractions.
- Consensus has not yet been reached among the three analyses on the question of cone emission.

## Conclusions

- Detailed and differential correlation analyses access many areas of RHIC physics
- Same-side jet evolution:
- momentum dissipation of medium
- parton-medium interaction and/or modification of fragmentation
- evolution of hadronization geometry.
-  Away-side:
- energy loss and momentum dissipation
- possible collective modes of the bulk.
- The challenge ahead is to describe these interactions with the medium into a single theoretical framework