Transcript Tsinghua - University of Oregon

Interaction between jets and
dense medium in
heavy-ion collisions
Rudolph C. Hwa
University 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 RHIC
T > 170 MeV ~ 1012 K
 > 5 GeV/fm3 ~ 50 normal nuclear density
Deconfined quarks and gluons
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Collision geometry
pT


pseudorapidity
azimuthal angle
transverse momentum
  ln(cot  / 2)

pT
Azimuthal variation in
non-central collisions
pT

z
y
x
Non-central collision
N participant
(Npart)
  atan
py
px
For good resolution we need  << L
In nuclear collisions the transverse size of
collision zone is about 10 fm (10-12cm).
L ~ size of
system
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
h
measures the small-distance resolution: p :
x
We can’t shoot a probe through the dense medium,
as in X-ray diagnostic.
It must come from within.
pT
jet
parton
nucleon
nucleon
Au+Au  2000 particles
p+p  dijet
2. Jets at high pT
Jet quenching
In the transverse plane
a hard scattering can
occur anywhere
If the hot medium is sQGP,
the partons that traverse it
lose energy.
So the pT of the detected jet in
AA collision is lower than a
similar jet in pp collision.
That is a suppression effect
pp
AA
pT
N coll
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?
A more revealing way to see its properties
is to examine the azimuthal dependence of
jet production
trigger

associated
particle
Dihadron correlations
Dihadron correlations in 

PRL 91, 072304
Striking final state effects
trigger
out-of-plane
trigger
in-plane
STAR preliminary
20-60%
central
If there is severe damping on the away side, then
most observed jets are produced near the surface.
absorbed
undamped
to detector
3. Back-to-back jets
Hwa-Yang 0812.2205, PRC (2009)
away
near
Not measurable:
initial parton momenta k, k’
parton momenta at surfaces q, q’
Measurable: trigger momentum pt
associated particle (same side) pa
associated particle (away side) pb
centrality c=0.05
c=0.5
Yield per trigger
Near
Away
Suppression factor
1-
 near ( pt )  e t
 away ( pt , pb )  e
Energy loss
  (L t )
Much less energy loss
on the near side
if we fix the
length L
L-t
t
More energy loss on the
away side
The problem is that the path length L
cannot be fixed experimentally.
It is only possible to fix the centrality c.
Some paths are long
Some are short
Data integrates over all
points of interaction.
Tangential jets dominate.
STAR has recent data on Di-jets
Au+Au centrality comparison
1
_dN_
Ntrig d( )
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
12% Central
40-60% MB
60-80% MB
T2A1_T1
associates
2
“jet-axis”
trigger (T2)
0
STAR Preliminary
-2
-1
0
1
2

3
associates
4
5
  projection: no significant centrality dependence
• No modification of away-side jet
Dominance by tangential jets!
primary
trigger (T1)
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.
4. Ridges
Interaction between jets and medium
• Effect of medium on jets.
• Effect of jets on medium.
trigger direction
Trigger

Trigger

A ridge is
discovered on
the near side.
distribution of particles
associated with the trigger
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.
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
We have no reliable theoretical framework
in which to calculate all those subprocesses.
24
A very quick explanation of ridge formation
in the recombination model of partons
Hwa-QM08
hard parton
SS
associated
particles
trigger
peak (J)
ST
TT ridge (R)

We focus below on mainly
the  distribution.
These wings
identify the Ridge

Dependence of ridge yield on the
trigger azimuthal angle
Trigger
Trigger


restrict ||<0.7
What is the direction of the trigger T?
irrelevant
very relevant
out-of-plane
Quark Matter 2008 -- A. Feng (STAR)
6 5
4
3
2
1 in-
Dependence on trigger azimuthal angle
 s  T   RP
plane
in-plane S=0
out-of-plane S=90o
20-60%
STAR Preliminary
top 5%
STAR Preliminary
• 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.
Out-ofplane
 s  T   RP
6
5
4
3
2
1
After separating Ridge from Jet -in-plane S=0
In-plane
out-of-plane S=90o
Ridge
3<pTtrig<4, 1.5<pTtrig<2.0 GeV/c
STAR Preliminary
Jet
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Correlated
emission
model
(CEM)
Chiu-Hwa
PRC(09)
Strong ridge formation when trigger and flow directions match.
medium
probe
In CEM we found an asymmetry in the  distribution
R only
s>0
s<0
Netrakanti
trigger pt=3-4 GeV/c
STAR
Preliminary
CEM model
Ridge
QM09
Ridge: assoc pt=1-1.5
GeV/c
Ridge: assoc pt=1.5-2
GeV/c
Jet: assoc pt=1.5-2
GeV/c
Jet
|s|
Recoil jet on the away-side direction

Sound
wave
Away
side jet
Heating
Trigger
jet
Shock
wave?
Do you believe it?
This is an active area of current research.
Conclusion
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.
We have discussed jet-medium interaction
at intermediate pT.
• 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.
• 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.
Thank you!