Transcript A Pedestrian's Guide to RHIC and Its Experiments

Recreating the
Birth of the Universe
T.K Hemmick
University at Stony Brook
14-Jan-01
W.A. Zajc
1
The Beginning of Time

Time began with the Big Bang:


The universe expanded and cooled up to the
present day:




All energy (matter) of the universe concentrated at a
single point in space and time.
~3 Kelvin is the temperature of most of the universe.
Except for a few “hot spots” where the expanding matter
has collapsed back in upon itself.
How far back into time can we explain the
universe based upon our observations in the Lab?
What Physics do we use to explain each stage?
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Thomas K Hemmick
Evolution of the Universe
Too hot for quarks to bind!!!
Quark Plasma…Standard Model Physics
Too hot for nuclei to bind
Hadronic Gas—Nuclear/Particle Physics
Nucleosynthesis builds nuclei up to Li
Nuclear Force…Nuclear Physics
Universe too hot for electrons to bind
E-M…Atomic (Plasma) Physics
Universe Expands and Cools
Gravity…Newtonian/General Relativity
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Thomas K Hemmick
Decoding the Analogy
Sport
Force
Exchange
Particle
Strength Range
Calculable?
FRISBEE
ElectroMagnetic
(QED)
Photon
Moderate
Infinite
Most
accurate
theory ever
devised
CHESS
Weak Force
(unified w/
EM)
W+, W-, Z0
Weak
Short
Perfect
LOVE
Strong Force
(QCD)
8 gluons
Strong
Infinite
Nearly
incalculable
except for
REALLY
VIOLENT
COLLISIONS!
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Electric vs. Color Forces

Electric Force


The electric field lines can be
thought of as the paths of virtual
photons.
Because the photon does not
carry electric charge, these lines
extend out to infinity producing a
force which decreases with
separation.,


Color Force


The gluon carries color
charge, and so the force lines
collapse into a “flux tube”.
As you pull apart quarks, the
energy in the flux tube
becomes sufficient to create
new quarks.
Trying to isolate a quark is as
fruitless as trying to cut a string
until it only has one end!
CONFINEMENT
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Thomas K Hemmick
What about this Quark Soup?




If we imagine the early state of the universe, we imagine
a situation in which protons and neutrons have
separations smaller than their sizes.
In this case, the quarks would be expected to lose track
of their true partners.
They become free of their immediate bonds, but they do
not leave the system entirely.
They are deconfined, but not isolated

similar to water and ice, water molecules are not fixed in
their location, but they also do not leave the glass.
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Thomas K Hemmick
Phase Diagrams
Nuclear Matter
Water
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Thomas K Hemmick
Making Plasma in the Lab

Extremes of temperature/density are
necessary to recreate the Quark-Gluon Plasma,
the state of our universe for the first ~10
microseconds.

Density threshold is when protons/neutrons overlap
4X nuclear matter density = touching.
 8X nuclear matter density should be plasma.


Temperature threshold should be located at
“runaway” particle production.
The lightest meson is the pion (140 MeV/c 2).
 When the temperature exceeds the mc 2 of the pion,
runaway particle production ensues creating plasma.
 The necessary temperature is ~10 12 Kelvin.


Question: Where do you get the OVEN?

Answer: Heavy Ion Collisions!
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Thomas K Hemmick
RHIC


RHIC = Relativistic Heavy Ion Collider
Located at Brookhaven National Laboratory
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Thomas K Hemmick
RHIC Specifications


3.83 km circumference
Two independent rings




120 bunches/ring
106 ns bunch crossing time
Can collide
~any nuclear species
on
~any other species
6
1’
4
2
Top Center-of-Mass Energy:
 500 GeV for p-p
 200 GeV/nucleon for Au-Au

Luminosity


5
3
Au-Au: 2 x 1026 cm-2 s-1
p-p : 2 x 1032 cm-2 s-1
(polarized)
10
1
RHIC’s Experiments
STAR
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Thomas K Hemmick
RHIC in Fancy Language

Explore non-perturbative “vacuum”
by melting it
Temperature scale T ~  /(1 fm ) ~ 200 MeV
 Particle production
 Our ‘perturbative’ region
is filled with

c
c
Perturbative Vacuum
gluons
 quark-antiquark pairs

 A Quark-Gluon Plasma (QGP)

Experimental method:
Energetic collisions of heavy nuclei

Experimental measurements:
Use probes that are


c
Auto-generated
Sensitive to all time/length scales
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c
Color Screening
Thomas K Hemmick
RHIC in Simple Language

Suppose…










You lived in a frozen world where water existed only as ice
and ice comes in only quantized sizes ~ ice cubes
and theoretical friends tell you there should be a liquid phase
and your only way to heat the ice is by colliding two ice cubes
So you form a “bunch” containing a billion ice cubes
which you collide with another such bunch
10 million times per second
which produces about 1000 IceCube-IceCube collisions per
second
which you observe from the vicinity of Mars
Change the length scale by a factor of ~1013
You’re doing physics at RHIC!
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Nature’s providence
How can we hope to study such a complex system?
1 ~  a
L  i D  Fa F   Mˆ 
4
g, e+e-, +
p, K, h, r, w, p, n,
f, L, D, X, W, D, d, J/Y,…
PARTICLES!
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Thomas K Hemmick
Deducing Temperature from Particles

Maxwell knew the answer!

Temperature is proportional to mean Kinetic Energy
Particles have an average velocity (or momentum)
related to the temperature.
 Particles have a known distribution of velocities
(momenta) centered around this average.


All the RHIC experiments strive to measure the
momentum distributions of particles leaving
the collision.


Magnetic spectrometers measure momentum of
charged particles.
A variety of methods identify the particle species
once the momentum is known:
Time-of-Flight
 dE/dx

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Thomas K Hemmick
Magnetic Spectrometers

Cool Experiment:



Hold a magnet near the screen of a B&W TV.
The image distorts because the magnet bends the
electrons before they hit the screen.
Why? :

dp e  
 vB
dt c

e
| p | B  R,
c
e
0.3 GeV / c

c Tesla  meter
1 meter of 1 Tesla field deflects p = 1 GeV/c by ~17O
a
x
z
qin
qout
s
By(z)
STAR
y
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Thomas K Hemmick
Particle Identification by TOF

The most direct way

Measure b by distance/time
Typically done via scintillators
read-out with photomultiplier tubes
Time resolutions ~ 100 ps

Exercise: Show


e
p
K
p
2
2

 p

t

s
   
4 
  g      

  
 m   p 
 t   s  
 m 

2
2
Performance:
t ~ 100 ps on 5 m flight path
P/K separation to ~ 2 GeV/c
K/p separation to at least 4 GeV/c

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Thomas K Hemmick
Particle Identification by dE/dx

Elementary calculation of energy loss:
Charged particles traversing material give impulse to
atomic electrons:

E (t )
b
Ze

x=bt
2
2
Ze
e
p y  e  E y ( t )dt  e  E y ( t )

bb
e 2b
( py )
1
Energy transfer 
~ 2
2m e
b
dx
dE/dx:
STAR
The 1/ b2 survives
integration over impact
parameters
 Measure average
energy loss to find b
 Used in all four
experiments

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K
p
p
e
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Thomas K Hemmick
Measuring Sizes

Borrow a technique from Astronomy:



Two-Particle Intensity Interferometry
Hanbury-Brown Twiss or “HBT”
Bosons (integer spin particles like photons, pions,
Kaons, …) like each other:

Enhanced probability of “close-by” emission
1
X
Source
y
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19
Momentum difference can
be measured in all three
directions:
Conventional wisdom:


The “Long” axis includes the
memory of the incoming
nuclei.
The “Out” axis appears
longer than the “Side” axis
thanks to the emission time:
X-Axis
Beam
Axis
ZAx
is

So

P1
K
P2
qSIDE
“Long” (along beam)
 “Out” (toward detector)
 “Side” (left over dimension)
e
This yields 3 sizes:
ur
c

Y-Axis

q
Measuring Shapes
qLONG
Source
qOUT
2
2
  ROut
 RSide
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Run-2000





First collisions:15-Jun-00
Last collisions: 04-Sep-00
RHIC achieved its First Year
Goal (10% of design
Luminosity).
Most of the data were
recorded in the last few
weeks of the run.
 Recorded
~5M events
The first public
presentation of RHIC
results took place at the
Quark Matter 2001
conference.


January 15-20
Held at Stony Brook
University
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Thomas K Hemmick
How Do You Detect Plasma?

During a plenary RHI talk at APS about
10 years ago, I wound up seated among
“real” plasma physicists who made
numerous comments:

“These guys are stupid…”
 Always

a possibility.
“…why don’t they just shoot a laser through
it and then they’d know if its plasma for
sure!”
 Visible
light laser…bad idea.
 Calibrated probe through QGP…good idea…
 …but not new. (Wang, Gyulassy, others…)
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Thomas K Hemmick
The “Calibrated” Plasma Probe


Many Many results (concentrate on one).
Hard scattering processes (JETS!) :


Occur at short time scales.
Are “calculable” (even by experimentalists) in simple
models (e.g. Pythia) with appropriate fudging:
Intrinsic kT
 K scaling factor.





Find themselves enveloped by the medium
Are “visible” at high pT despite the medium
Promise to be our laser shining (or not) through the
dense medium created at RHIC.
We can measure the ratio of observed to
expected particle yield at large momentum and
it should drop below 1.0.

Scaled proton-proton collisions provide reference.
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Thomas K Hemmick
Particle Spectra Evolution
“Peripheral”
Particle
Physics
Nuclear
Physics
“Thermal”
Production
Hard
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Scattering 24
“Central”
Thomas K Hemmick
Raa

We define the nuclear
modification factor as:
1 d 2 N A A
N evt dpT dh
RAA ( pT ) 
 N binary  d 2 N  N
N N
 inel
dpT dh



By definition, processes
that scale with Nbinary will
produce RAA=1.
RAA is what we get
divided by what we
expect.
RAA should be ~1.0
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RAA is below 1 for both charged hadrons
and neutral pions.
The neutral pions fall below the charged
hadrons since they do not suffer proton
contamination
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Thomas K Hemmick
Away-side Jets Missing!





STAR Experiment
reconstructs
azimuthal
correlations.
Peak Around 0 are
particles from “same
side jet”.
Peak at +/- p is the
away-side jet.
In central collisions
the away-side jet
disappears!!!
Medium is black to
jets.
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Thomas K Hemmick
Quantifying the away-side.



Near-side jet/pp data ~1.0.
Away-side jet/pp falls to ~0.2 in central collisions.
Simple jet-quenching confirmed?

Not so fast…
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Thomas K Hemmick
“Jet” Particle Composition



Composition of jets violates normal pQCD!
How could jet fragmentation be affected?
Puzzles Puzzles Puzzles…
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Thomas K Hemmick
Other Bizarre Results:


Azimuthal asymmetries beyond the “black
almond” scenario.
The HBT interferometric technique for
determining the lifetime of the particle
source. Theory: 
 R2  R2
emission

side
Experim ent: Rout  Rside ???


out
The theoretical community simply can’t
explain the data.

PS—This is the good news 
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Thomas K Hemmick
Another Surprise!

Rout<Rside!!!!!


Normal theory cannot account for this
Imaginary times of emission!!
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Thomas K Hemmick
Possible Explanation??

Stony Brook theory
student Derek Teaney
(advisor E. Shuryak)
calculated an exploding
ball of QGP matter.



The exploding ball drives
an external shell of
ordinary matter to high
velocities
Rout is the shell thickness
Rside is the ball size
Plasma
Shells of ordinary matter
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Thomas K Hemmick
Is it Soup Yet?

RHIC physics in some reminds me of the
explorations of Christopher Columbus:





He had a strong feeling that the earth was round
without having detailed calculations to back him up.
He traveled in exactly the wrong direction, as
compared to conventional wisdom.
He discovered the new world…
But he thought it was India!
Our status:


We see jet quenching for the first time.
We see results which defy all predictions
Hard proton production exceeds pion production
 Imaginary emission time


We could be in India (QGP), the New World, or just a
place in Europe where the customs are VERY strange.
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Thomas K Hemmick
Summary

RHIC is more exciting than we dared hope:


We see jet quenching for the first time.
We see results which defy all predictions
Hard proton production exceeds pion production
 Imaginary emission time



Even the hard physics “reference” fails in the
face of our new matter.
2002 run:



d-Au collisions to finalize nuclear effects that could
fake jet suppression.
p-p results for nucleon spin measurements.
2002-2003 run:


Au-Au … for high statistics.
Electromagnetic Probes!!
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Thomas K Hemmick
Summary


Extreme Energy Density is a new frontier for
explorations of the state of the universe in the
earliest times.
The RHIC machine has just come on line:



The machine works
The experiments work
The data from signatures of QGP as well as
outright surprises…
It’s not your Father’s Nuclear Matter anymore!

The real look into the system will come in the
next run (May 2001):



Electrons, Photons, Muons
We dream of India as our glorious destination
But maybe….
We’ll find the new world instead.
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Thomas K Hemmick
Electron Identification


E/p matching for
Problem: They’re rare
p>0.5 GeV/c tracks
Solution: Multiple methods
 Cerenkov
 E(Calorimeter)/p(tracking)
matching
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All tracks
Electron enriched
sample
(using RICH)
35
Thomas K Hemmick
Why electrons?

One reason: sensitivity to heavy flavor production
D0
D0
D0
B0
B0
B0
D0D0
D0D0
D0D0

Dalitz and conversions
K- p+
K - e+  e
K- + 
charm
e-
beauty eDrell-Yan
D- p+
D- e+ e
D- + 
+- K+ K- 
e+e- K+ K- ee
+e- K+ K- e
e-
e-
Study by Mickey Chiu, J. Nagle
Other reasons: vector mesons, virtual photons  e+e-
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Thomas K Hemmick
p0 Reconstruction



A good example of a “combinatoric” background
Reconstruction is not done particle-by-particle
Recall: p0  gg and there are ~200 p0 ‘s per unit rapidity

So:
p0 1  g1A  g 1B
p0 2  g2A  g 2B
p0 3  g3A  g 3B
p0
N  gNA  g NB
PHENIX
p0 reconstruction
pT > 2 GeV/c
Asymmetry < 0.8
 .Unfortunately, nature doesn’t use subscripts on photons
N correct combinations: (g1A g 1B), (g2A g 2B), … (gNA g NB),
N(N-1)/2 – N incorrect combinations (g1A g 2A), (g1A g 2B), …
 Incorrect combinations ~ N2 (!)
 Solution: Restrict N by pT cuts
use high granularity, high
resolution detector
37
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Thomas K Hemmick
BRAHMS
An experiment with an emphasis:


Quality PID spectra over a broad range
of rapidity and pT
Special emphasis:
Where do the baryons go?
 How is directed energy transferred to
the reaction products?


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Two magnetic dipole spectrometers in
“classic” fixed-target configuration
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Thomas K Hemmick
PHOBOS
An experiment with a
philosophy:

Global phenomena
large spatial sizes
small momenta

Minimize the number
of technologies:
All Si-strip tracking
 Si multiplicity
detection
 PMT-based TOF


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Unbiased global look
at very large number
of collisions (~109)
Thomas K Hemmick
PHOBOS Details

Si tracking elements





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15 planes/arm
Front: “Pixels”
(1mm x 1mm)
Rear: “Strips”
(0.67mm x 19mm)
56K channels/arm
Si multiplicity detector

22K channels

|h| < 5.3
Thomas K Hemmick
PHOBOS Results
First results on dNch/dh
Hits in SPEC
Tracks in SPEC
Hits in VTX


for central events
At ECM energies of
56 Gev
 130 GeV

(per nucleon pair)
To appear in PRL
130 AGeV
(hep-ex/0007036)
X.N.Wang et al.
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Thomas K Hemmick
STAR

An experiment with a challenge:

Track ~ 2000 charged particles in |h| < 1
Time
Projection
Chamber
Magnet
Coils
Silicon
Vertex
Tracker
TPC
Endcap &
MWPC
FTPCs
ZCal
ZCal
Endcap
Calorimeter
Vertex
Position
Detectors
Barrel EM
Calorimeter
Central
Trigger
Barrel or
TOF
RICH
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Thomas K Hemmick
STAR Challenge
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STAR Event
Data Taken June 25, 2000.
Pictures from Level 3 online display.
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Thomas K Hemmick
STAR Reality
45
PHENIX


An experiment
with something
for everybody
A complex
apparatus to
measure




High
resolution

Muon Arms
West Arm
Hadrons
Muons
Electrons
Photons
South muon
Arm
High
granularity
University at Stony Brook
Coverage (N&S)
-1.2< |y| <2.3
-p < f < p
DM(J/ )=105MeV
DM(g) =180MeV
3 station CSC
5 layer MuID (10X0)
p()>3GeV/c
Executive
summary:

Global
MVD/BB/ZDC
East Arm
Central Arms
Coverage (E&W)
-0.35< y < 0.35
30o <|f |< 120o
DM(J/ )= 20MeV
DM(g46
) =160MeV
North muon
Arm
Thomas K Hemmick
PHENIX Design
47
PHENIX Reality
48
January, 1999
Thomas K Hemmick
PHENIX Results
(See nucl-ex/0012008)
 Multiplicity grows significantly faster than N-participants
 Growth consistent with a term that goes as N-collisions
(as expected from hard scattering)
dN dh h 0  A  N part  B  N coll
A  0.88  0.28
B  0.34  0.12
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Thomas K Hemmick
Summary

The RHIC heavy ion community has


Constructed a set of experiments designed for the
first dedicated heavy ion collider
Met great challenges in
Segmentation
 Dynamic range
 Data volumes
 Data analysis



Has begun operations with those same detectors
Quark Matter 2001 will


See the first results of many new analyses
See the promise and vitality of the entire RHIC
program
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Thomas K Hemmick