Interaction between jets and dense medium in

heavy-ion collisions

Rudolph C. HwaUniversity of Oregon

TsingHua University, Beijing, China

May 4, 2009

Outline

1. Introduction

2. Jets at high transverse momentum pT

3. Back-to-back jets

(effect of medium on jets)

4. Ridges

(effect of jets on the medium)

5. Conclusion

1. Introduction

Creation of hot, dense matter at RHICT > 170 MeV ~ 1012 K

> 5 GeV/fm3 ~ 50 normal nuclear density

Deconfined quarks and gluons

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Collision geometry

azimuthal angle φ

φ

transverse momentum pT

pT

pseudorapidity

η =ln(cotθ / 2)

θ

Non-central collisionx

yz

N participant

(Npart)

Azimuthal variation in non-central collisions

x

y

p

patan=φ

px

py

pT

For good resolution we need << L L ~ size of system

In nuclear collisions the transverse size of collision zone is about 10 fm (10-12cm).For << 1 fm, we need p = h/ >> 1 GeV

At RHIC cm energy of a nucleon is 100 GeV, but it is the momentum-transfer scale that measures the small-distance resolution:

Δp :h

Δx

We can’t shoot a probe through the dense medium, as in X-ray diagnostic.

pT

It must come from within.

p+p dijet

nucleon nucleonparton

jet

Au+Au 2000 particles

Jet quenching

In the transverse plane a hard scattering can occur anywhereIf the hot medium is sQGP, the partons that traverse it lose energy.

pT

pp

AA

So the pT of the detected

jet in AA collision is lower than a similar jet in pp collision.That is a suppression effect

2. Jets at high pT

Ncoll

A more revealing way to see its properties is to examine the azimuthal dependence of jet production

Δφtrigger

associated particle

How can we be sure that the suppression is due to parton interaction with QGP as the medium?

Can it be due to some initial state interaction?

Dihadron correlations

PRL 91, 072304

trigger in-plane

trigger out-of-plane

STARSTARpreliminary20-60% central

Striking final state effects

Dihadron correlations in Δ

Δφ

If there is severe damping on the away side, then most observed jets are produced near the surface.

to detector

undamped

absorbed

3. Back-to-back jets

Measurable: trigger momentum pt

associated particle (same side) pa associated particle (away side) pb

Not measurable: initial parton momenta k, k’parton momenta at surfaces q, q’

centrality c=0.05c=0.5

nearaway

Hwa-Yang 0812.2205, PRC (2009)

Yield per trigger

Near

Away

Suppression factor

Γnear (pt ) = e−βt

Γaway (pt , pb ) = e−β (L− t )

tL-t

Energy loss 1-Γ

More energy loss on the away side

Much less energy loss on the near side

if we fix the length L

The problem is that the path length L cannot be fixed experimentally.

It is only possible to fix the centrality c.

Data integrates over all points of interaction.

Some paths are long

Some are short

Tangential jets dominate.

Au+Au centrality comparison

12% Central40-60% MB60-80% MB

Δ-1-2 0 1 2 3 4 5

1

_d

N_

Ntr

ig d

Δ

)

2

STAR Preliminary

0

T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c

Δ projection: no significant centrality dependence• No modification of away-side jet

T2A1_T1

STAR has recent data on Di-jets

associates

associates

primary

trigger (T1)

“jet-axis”

trigger (T2)

Dominance by tangential jets!

Very hard to probe the interior of dense medium

--- if the thickness cannot be controlled.

That’s about the effect of dense medium on dihadron correlation in jets.

Interaction between jets and medium

• Effect of medium on jets. trigger

direction

distribution of particles associated with the trigger

4. Ridges

• Effect of jets on medium.

A ridge is discovered on the near side.

ridgeJet

ΔηΔηTrigger

ΔΔ

Trigger

24

Trigger: 3 < pT < 4 GeV/c

Associated: 1.5 < pT < 2 GeV/c

Not hard enough for pQCD to be reliable, too hard for hydrodynamics.

We have no reliable theoretical framework in which to calculate all those subprocesses.

Physical processes involve:

• semihard parton propagating through dense medium

• energy loss due to soft emission induced by medium

• enhancement of thermal partons

• hydro flow and hadronization

• ridge formation above background

hard parton

SS

trigger

ST

peak (J)

TT ridge (R)

η

associated particles

These wings identify the Ridge

We focus below on mainly the Δ distribution.

A very quick explanation of ridge formation in the recombination model of partons

Hwa-QM08

Dependence of ridge yield on the trigger azimuthal angle

ΔηΔηTrigger

ΔΔ

Trigger

restrict |Δη|<0.7

What is the direction of the trigger T?

irrelevant

very relevant

Quark Matter 2008 -- A. Feng (STAR)Dependence on trigger azimuthal angle

1

43

2

56

in-plane

out-of-plane

φs = φT − Ψ RP

top 5%

20-60%

in-plane S=0 out-of-plane S=90o

• In 20-60%, away-side evolves from single-peak (φS =0) to double-peak (φS =90o).• In top 5%, double peak show up at a smaller φS.• At large φS, little difference between two centrality bins.

STAR Preliminary

STAR Preliminary

STAR Preliminary

in-plane S=0 out-of-plane S=90o

Rid

ge

Jet

3<pTtrig<4, 1.5<pT

trig<2.0 GeV/c

φs = φT − Ψ RP

In-plane

Out

-of-

plan

e

1

43

2

56

After separating Ridge from Jet --

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Chiu-Hwa PRC(09)

Strong ridge formation when trigger and flow directions match. probe medium

Correlated emission model (CEM)

s>0

In CEM we found an asymmetry in the Δ distribution

trigger pt=3-4 GeV/c

Jet

Ridge

s|

CEM model

STAR Preliminary

Ridge: assoc pt=1-1.5 GeV/cRidge: assoc pt=1.5-2 GeV/cJet: assoc pt=1.5-2 GeV/c

Netrakanti

QM09

R only

s<0

Δ

Trigger jet

Away side jet

Heating

Sound wave

Recoil jet on the away-side direction

This is an active area of current research.

Do you believe it?

Shock wave?

Correlation among hadrons reveals that quarks interact strongly with QGP, not weakly (as initially suspected).Interaction at intermediate pT cannot be treated by either hydrodynamics or perturbative QCD.

But that is where most of the data exist, and they provide information that we need to understand.

Conclusion

We have discussed jet-medium interaction at intermediate pT.

• Effect of jets on medium:

Semi-hard parton -> energy loss to medium -> Ridge. Our interpretation is that the ridge is formed by the recombination of thermal partons enhanced by jet. The prediction on asymmetry has also been verified by data.

• Effect of medium on dijets:

Energy loss to medium -> strong correlation between jets.

It is hard to probe the medium interior by dijets because of dominance by tangential jets --- it has been verified by data on 2jet+1 correlation.

Thank you!