Download How does one probe dense matter at 1012 K ?

Probing dense matter at
extremely high temperature
Rudolph C. Hwa
University of Oregon
Jiao Tong University, Shanghai, China
April 20, 2009
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Outline
1. High temperature. How high?
1000K?
106?
109?
1012K
2. How do we get there?
3. How do we probe it?
4. What do we know so far?
5. What are new very recently?
6. What can be expected in the future?
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RHIC
Relativistic Heavy Ion Collider
100 +100 GeV (Au+Au)
LHC
Large Hadron Collider
2.75 + 2.75 TeV (Pb+Pb)
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PHOBOS
BRAHMS
PHENIX
STAR
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STAR detector
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QuickTime™ and a
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QuickTime™ and a
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lumpy initial conditions and a QGP expansion
collision evolution
particle
detectors
expansion and cooling
kinetic
freeze-out
distributions and
correlations of
produced particles
hadronization
lumpy initial
energy density
QGP phase
quark and gluon
degrees of freedom
collision
overlap zone
 ~ 0 fm/c
quantum
fluctuations
0~1 fm/c
 ~ 10 fm/c
 ~ 1015 fm/c
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Collision geometry
pT


pseudorapidity
azimuthal angle
transverse momentum
  ln(cot  / 2)

pT
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Collision geometry
centrality
Au + Au sNN = 200 GeV
very peripheral
very central
c=0-0.05
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Azimuthal variation in
non-central collisions
pT

z
y
x
Non-central collision
N participant
  atan
py
px
(Npart)
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How can we probe
such a medium?
z
y
x
Non-central collision
We need a
penetrating probe.
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Example: X-ray
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For good resolution we need  << L
In nuclear collisions the transverse size of
collision zone is about 10 fm (10-12cm).
L ~ size of
biological
molecules
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
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low
soft
intermediate
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semi-hard
relativistic
hydrodynamics
high
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no reliable
theory
Jet production
in pp collision
pT
hard
perturbative Quantum
Chromo Dynamics (quarks,
gluons) -- partons
jet
parton
nucleon
nucleon
jet
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What do we see at high pT?
Au+Au  0 + anything
pT (GeV/c)
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N coll
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Jet quenching
In the transverse plane
a hard scattering can
occur anywhere
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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
p
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How do we know that the suppression is due to
parton interaction with QGP as the medium?
A more revealing way to see its properties
is to examine the azimuthal dependence of
jet production
trigger

associated
particle
Dihadron correlations
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Dihadron correlations in 

PRL 91, 072304
Striking final state effects
trigger
out-of-plane
STAR preliminary
20-60%
central
trigger
in-plane
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If there is severe damping on the away side, then
most observed jets are produced near the surface.
absorbed
undamped
to detector
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Back-to-back jets
Hwa-Yang 0812.2205
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
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Yield per trigger
Near
Away
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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
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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.
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STAR has recent data on Dijets
Au+Au centrality comparison
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_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
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“jet-axis”
trigger (T2)
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STAR Preliminary
-2
-1
0
1
2

3
primary
trigger (T1)
associates
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  projection: no significant centrality dependence
• No modification of away-side jet
Dominance by tangential jets!
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Very hard to probe the interior
of dense medium
--- if the thickness cannot be
controlled.
That’s the problem with jet-jet correlation.
So let’s move on to the medium
response to jets.
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Jet-medium interaction
1. Effect of medium on jets.
2. Effect of jets on medium.
trigger direction
Δφ
Trigger
Assoc.
Δη
A ridge is discovered on
the near side.
distribution of particles
associated with the trigger
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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
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Out-ofplane
New data presented at QM08
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3
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A. Feng (STAR):
Dependence of
ridge yield on
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 s  T   RP
in-plane S=0
In-plane
out-of-plane S=90o
assoc
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
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probe
What is 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.
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At LHC, cm energy is increased over RHIC by factor of 27.
Energy density is expected to increase by < 10.
Tinitial
……………………………………………………….
< 2.
It is hard to hold the dense matter together for long to thermalize.
Large pT range will increase by > 20.
Good ground to test pQCD.
There are wide variations in extrapolation to higher energies.
Ex. Most people predict p/ < 0.5 for 10<pT<20 GeV/c.
We (RH & CBYang) predict 5 < p/ < 20.
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Most significant advance will be
either to confirm conventional wisdom
or to validate unconventional ideas.
I hope that I can tell you which
next time.
Thank you!
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