# Presentation: 652506

## Particle correlations at RHIC

- Jiangyong Jia
- Stony Brook University & BNL

## Using single jets and dijets as probe

- 

- 

- Single hadron yield : vary pT
- Correlated hadron pairs shape and yield:
- 	vary , , pT1, pT2.

- Jet1

- Jet2

- p+p

## Using single jets and dijets as probe

- 

- 

- 

- 

- 

- Jet1

- Jet2

- 

- 

**away**

- p+p

- Au+Au

## Four components

- Near-side : surface jet + medium (ridge)
- Away-side : suppressed jet + medium (the cone)

- Jet fragmentation dominates high pT
- Medium response dominates low pT (<4 GeV/c)

- High pT

- Low pT

- Competition of jet quenching and medium response

**Notes:**
Both the near and away-side can be interpreted to consist of a jet fragmentation component and medium response component.

## pT Scan: evolution of jet quenching and medium response

- Result of the variation of the four components with pT

## arxiv. 0801.4545, Phys. Rev. C 78, 014901 (2008) 

## pT Scan: evolution of jet quenching and medium response

- Study the pTa , pTb dependence of the four components

- Head region (suppressed jet)
- Shoulder region (Cone)

- Near region (jet+ridge) project in 

-  RHS(pTa,pTb), D(pTa,pTb), (pTa,pTb)
-  Yield(pTa,pTb), IAA(pTa,pTb) in
-    three regions

**Notes:**
Many different ways of mapping out the transition

## Yield(pTa,pTb) in three  regions

- Head

- Shoulder

- Near

## IAA(pTa,pTb) in three  regions

- High pT jet like.
- Low pT enhancements are due to ridge and cone.

- Detailed interplay between jet and medium

- Near-side

- Away-side

## Jet and medium

- Example: Near-side  correlation

- Jet strength depends on both hadron pT, pTa+pTb
- Boundary region pTa+pTb~5-7 GeV/c

## Jet quenching

- Pair yield = trigger yield x

## In central collisions and high pT: away-side IAA~RAA~0.25.
Associated hadron spectra is much flatter than inclusive hadron spectra

For same suppression, flatter spectra need more energy loss.

- RAA

- IAA

- In central collisions and high pT: away-side IAA~RAA~0.25.
- Associated hadron spectra is much flatter than inclusive hadron spectra
- For same suppression, flatter spectra need more energy loss.

- Single spectra

- Per-trig spectra (5-10 GeV trigger)

- Pair yield = trigger yield x

- Ridge and cone systematics

## Ridge and cone important to pTa,pTb~4 GeV/c. 

- Near-side

- Away-side

## Away-side shoulder spectra slope

- Shoulder slope ~0.45 GeV/c, independent of trigger pT and centrality, and harder than inclusive.

**2<p****T****a****<3**

**3<p****T****a****<4**

**4<p****T****a****<5**

- Mean-pT at intermediate pT (1<pTb< 5)

**arxiv:0705.3238 [nucl-ex] Phys.Rev.C77:011901,2008**

**Notes:**
small variation due to the ridge

## Near-side ridge spectra slope

- J. Putschke QM06

- Jet
- Ridge

- Ridge slope is universal and slightly harder than the shoulder

## PID composition of ridge and shoulder

- Ridge and cone important up to pTa,pTb~4 GeV/c
- They have similar slope (ridge is slightly harder)
- Their particle compositions are similar to bulk.

- Near-side

- Away-side

- Mechanisms for ridge and shoulder are related?

- Mach cone

- ridge

## What constitutes the triggers? P+P

- Jet fragmentation is important to rather low pT
- pQCD calculation describe hadrons spectra done to 2 GeV
- Large fraction of soft pairs show jet-like correlation.

- 2-3 x 0.4-1 GeV/c

## What constitutes the triggers? Au+Au

- Intermediate pT particles are not jet-like.
- Less suppression and large v2.
- Strong dependence on flavor

- Not simple jet fragmentation:
- Soft hadrons, jet medium response etc

## Per-trigger yield 

- Per-trigger yield

- Per-trigger yield is useful if triggers comes fragmentation,
- But origins of triggers are complicated at pT < 4 GeV/c.
- Per-trigger yield can’t be compared with p+p directly.
- Low pT triggers maybe emitted from the whole overlap

**Notes:**
Many 2- and 3-p analyses are done in this region in this way

## Dilution for soft triggers

- Low pT trigger

**Near side**

- High pT trigger

- IAA

- Non-jet triggers dilute the high pT per-trigger yield

## Reaction Plane dependence.

- We often plot per-trigger yield as a function of angle between trigger and RP.
- These triggers may not come from jet fragmentation
- They have their own large v2, much bigger than what is given by jet quenching, thus per-trigger yield may not be good for jet tomography.

**3<p****T****trig****<4GeV/c & 1.0<p****T****asso****<1.5GeV/c 20-60%**

**STAR**

- The pair yield vs angle is more appropriate.

## Pair yield modification: JAA

- Pair yield modification: JAA

- Pair yield in Au+Au normalized by Ncoll scaled p+p pair yield.

- JAA =RAA IAA

## Pair yield modification: JAA

- Pair yield modification: JAA

- Energy scale is controlled by pTsum=pTa+pTb
- Low pT pair yield is not suppressed!
- Pair yield scale faster than Ncoll at low pTa+pT, remnants of quenched jets!
- Medium response important up to pTsum~7 GeV/c

- Near-side

- Away-side

## Source of pairs at intermediate pT

- Most models include only jet-jet and jet-medium contribution.
- The medium-medium contribution (triggering on the medium) could be large, in the limit of RAA --> 0, it dominates.

- jet-jet

- jet-medium

- medium-medium

- Nucl-ex/0806.1225

- We investigate the role of medium-medium pairs

**Notes:**
symmetric

## Simulation set up

- Glauber modeling of Au+Au (Woods-Saxon). Generate dijets according to Ncoll (x,y) with random direction, then swim the dijet through a medium with density  Npart (x,y)

- Jet absorption picture: loose all energy in a single interaction

- Chose =0.7 to reproduce <f> =RAA = 0.22 in 0-5% Au+Au collisions.

- nucl-th/0310044

**Notes:**
First we use Glauber modeling of Au+Au collision to calculate two density profiles (npart, Ncoll), then generate dijets according to ......

## Reproduce the centrality dependence

- Can reproduce the centrality dependence of RAA and IAA

- nucl-th/0310044

## Simulation setup

- The quenched jets are converted into jet-induced medium particles.

- Medium response to
- quenched jet

- Surviving jet

- We consider 1<pT<4 GeV/c as a single bin and choose
- Jet multiplicity        <Nj> =1  (roughly corresponds to 6 GeV/c jet).
- Medium multiplicity <Nm>=2  (account for enhancement)

## Centrality dependence

- Jet-Jet decreases with Npart
- Medium-medium increases with Npart
- Jet-medium first increase then decrease with Npart.

- jet-jet < jet-med < med-med

**Notes:**
Next can it describe the angular correlation?

## Fix jet kinematics

- Jet fragmentation width 0.3 rad,  jet kT smearing width 0.4 rad. accept 30% away-side jet pairs in .
- Medium response hadrons appear at D = 1.1, with a width of 0.3 rad.

- nucl-ex/0806.1225 for details

## Jet-Jet and Jet-Medium term

- Kinematics are chosen to qualitatively match the data.
- Concentrated at 0, , D.

- Mach cone

- This step is trivial,
- but what does it imply for
- medium-medium contribution?

**Notes:**
emphasize

## Medium-Medium term

- Both jets are converted into hadrons emitted at angle D from original jet direction. Pairs peaks at: 0, , 2D,   2D.

**Notes:**
Cone jet:  pairs from same jet split roughly equally at 0 and 2D, 
Deflected jet: All hadrons bend in same direction a large near side peak

## Connecting the ridge and cone

- If only consider jet-medium contribution, pairs at the near-side and away-side are different.
- But medium-medium pairs are symmetric between the two sides (no surface bias).
- In this calculation, ridge also comes from Mach cone (which is  elongated in  on both sides.

- jet-medium

- medium-medium

- Mach cone

- ridge

## How about 3-p correlation?

- jet-med-med+jet-jet-med appears in
- off-diagonal and along-diagonal
- But med-med-med triplets do not show a
- clear off-diagonal component.
- (kinematics are different from jet-med-med)

- The interpretation of  3-p is complicated when triggering on the medium response!

## Pair v2 

- Quenched jets should have negative v2.
- But large emission angle w.r.t the jet direction smears and changes the sign of v2.

- Jetall= Jetsurv+Jetquen

- 0=v2all= v2surv+v2quen

- V2quen = - v2surv

- No jet tomography when triggering on medium response?

## Conclusion

- Correlation patterns are consistent with competition between jet fragmentation and medium response.
- Similarity between the near-side ridge and the away-side cone.
- Understanding the medium response requires knowledge of the origins of the particles involved in the pair.
- Both the spectra and correlation results suggest we are triggering on medium response at intermediate pT.
- The correlation among medium response particles may dominate the pair yield at intermediate pT, while preserving the dihadron correlation pattern in  and .

**Notes:**
The v2 of the correlated pairs depends on the emission pattern. 

## Few passing comments

##  Flow background subtraction

- C() =  J() + (1+2<v2tv2a>cos2)

- Two source model assumption
- C() = Jet + flow + jet-flow-cross-term. (“Jet”=fragmentation+medium response)
- Thus J()=Jet + cross-term (the only experimentally feasible way)
- Which v2 should be used? How about non-flow?
- Essentially C() = Jet + flow + non-flow (other than jets)
- Other Non-flow and fluctuation contribute to C(), so should be included v2 subtraction.
- Non-flow due to jet can be suppressed with large  gap (PHENIX BBC).
- How to fix  (underlying event level)?
- ZYAM is the simplest approach
- Other approaches attempt correct for the over-subtraction, but with new set of assumptions.
- Theoretical calculation should be adjusted to the subtraction scheme (similar to different jet reco. scheme).

**Notes:**
(note in p+p jet reconstruction, =0)

## Is there a away-side dip?

- Dip is neither a sufficient nor necessary condition for Mach cone.
- Competing scenarios.
- Fill up due to punch-through jet, or the jet wake
- Also depends on the variable used to make the plot.
- No dip in number distribution, but dip in energy flow?

## Modified fragmentation vs. medium response

- Modified fragmentation involves New particle creation:
- Radiative energy loss, cerenkov gluons, etc
- Treated perturbatively
- Medium response involves Local heating:
- bending jet, momentum kick, Mach cone, backsplash, Glasma bending, coupling to transverse or longitudinal flow etc.
- Largely non-perturbative.
- Easier to generate large yield by pickup from the bulk (since no particle production is required)
- We know the pair yield is enhanced at low pt.
- Supported by the property of the ridge/cone (PID, slope etc)

## Energy dependence for ridge & shoulder

- CM<0.7

- Head200 GeV          Head17.2 GeV
- Shoulder200 GeV  2x Shoulder17.2 GeV
- Near200 GeV          8 x Near17.2 GeV

- ||<0.35

- 	At SPS
- Smaller jet quenching+Stronger Cronin
- 	-> Less suppression in Head
- Smaller medium component
- 	-> Smaller ridge/ Shoulder

- RAA at SPS is totally different, dominated by Cronin effect
- Ridge is almost gone, the shoulder is still significant (kT broadening)?

- Energy scan is important!

- 

## Jet correlation vs jet reconstruction

- Particle correlation is important complementary tool for jet reco at LHC.
- Help to understand the jet shape, and tune the jet reco-algorithm.
- Very important for low Et jets (<50 GeV).

- ATLAS

## Dihadron correlation vs. single spectra 

- % of singles from jets?
- Jet fragmentation contribution
- Fraction determined by the surface volume.
- Or medium response
- Can estimate if the jet multiplicity is known.
- Flow+recombination picture consistent with Mach cone or ridge?

## Backup

**Notes:**
17+8+10+5
10+10+10+6

## V2 of survived jets

- Jet quenching give positive v2, but underestimate the data by factor of 2 (woods-saxon)

- nucl-th/0310044

- Jetall= Jetsurv+Jetquen

- 0=v2all= v2surv+v2quen

- V2quen = - v2surv

## V2 of survived jets

- Finite near-side width reduce the pair v2.

## Dilution to ridge

- Scale up the Au+Au by 1/IAA(pTa), then subtract pp

- Near-side

- ~consistent with pp jet + roughly flat ridge

- *

## Sources of “jet” pairs

- One usually find only one “jet” pair per event.
- “Jet” is statistical sum of different types of signals

- 0

- 