Transcript A Pedestrian's Guide to RHIC and Its Experiments
14-Jan-01
Recreating the Birth of the Universe
T.K Hemmick University at Stony Brook
W.A. Zajc 1
The Beginning of Time
Time began with the Big Bang:
All energy (matter) of the universe concentrated at a single point in space and time.
The universe expanded and cooled up to the present day:
~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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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 3
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Standard Model (simplified)
Imagine a college campus on a warm summer day
Students are uniformly distributed in an open field.
Now introduce a FRISBEE into the system!
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Standard Model II
5
Students who interact with the FRISBEE form a group.
These students are “charged” Other students don’t interact with the FRISBEE.
neutral or “nerds”
Now introduce CHESS into the campus!
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Standard Model III
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Some charged and some neutral students decide to play chess
Very short range interaction More than one type of exchange particle
Finally, introduce LOVE into the college campus
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Standard Model IV
All the remaining students form into tightly bound pairs
(and triples) If you break up with one partner, you immediately find another (confinement) Force grows stronger with separation 7
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Decoding the Analogy
Sport FRISBEE Electro Magnetic (QED) CHESS Force Weak Force (unified w/ EM) Exchange Particle Photon W + , W , Z 0 Strength Range Calculable?
Moderate Infinite Weak Short Most accurate theory ever devised Perfect LOVE Strong Force (QCD) 8 gluons
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8 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.
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Trying to isolate a quark is as
CONFINEMENT
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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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Phase Diagrams
Nuclear Matter
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11 Water
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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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RHIC
RHIC = Relativistic Heavy Ion Collider Located at Brookhaven National Laboratory
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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 1’ 3 6 4 5 2
Top Center-of-Mass Energy:
500 GeV for p-p
200 GeV/nucleon for Au-Au Luminosity
Au-Au: 2 x 10 26 p-p : 2 x 10 ( polarized ) 32 cm -2 cm -2 s -1 s -1 14 1
RHIC’s Experiments
STAR
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RHIC Video
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How is RHIC Different?
It’s dedicated to High Energy Heavy Ion Physics
Heavy ions will run 20-30 weeks/year It’s a collider
Detector systematics independent of ECM
(No thick targets!) It’s high energy
Access to non-perturbative phenomena
Jets (very violent calculable processes in the mix)
Non-linear dE/dx Its detectors are comprehensive
~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons
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RHIC in Fancy Language
Explore non-perturbative “vacuum” by
melting it
Temperature scale
T
~
Particle production Our ‘perturbative’ region is filled with /( 1 f
m
) ~ 200 MeV
gluons quark-antiquark pairs
A Quark-Gluon Plasma (QGP) Experimental method: Energetic collisions of heavy nuclei Experimental measurements: Use probes that are
Auto-generated Sensitive to all time/length scales 18
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c c Perturbative Vacuum c c Color Screening
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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 ~10 13
You’re doing physics at RHIC!
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Nature’s providence
How can we hope to study such a complex system?
L
i
D
1 4 ~
F a
F a
University at Stony Brook g
, e+e-,
+
p,
K
, h, r, w,
p
,
n
, f, L, D, X, W,
D
,
d, J/Y,… PARTICLES!
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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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q in
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? :
d p
e dt c v
B
|
p
|
e c B
R
,
e c
0 .
3
GeV Tesla
/
c
meter
1 meter of 1 Tesla field deflects p = 1 GeV/c by ~17 O x
a
z
q out
s B y (z) 22
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z
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
m m
2
p p
2
g
4
t t
2
s s
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 e
p
K p 23
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Particle Identification by dE/dx
Elementary calculation of energy loss: Charged particles traversing material give impulse to atomic electrons:
E
(t )
p y e
e
E y
(
t
)
dt
e
E
Ze b x=
b
t Energy transfer
(
y p
(
t y e dx
) ) 2
b
~ 2
m e
b
2 1 2
Ze
b
b
2
dE/dx:
The 1/
b
2 survives integration over impact parameters
Measure average energy loss to find b
Used in all four experiments
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p
K p
STAR
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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 Source X y 2 25
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Measuring Shapes
Momentum difference can be measured in all three directions:
This yields 3 sizes:
“Long” (along beam)
“Out” (toward detector)
“Side” (left over dimension) 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: Beam Axis
S o ur ce
K P 2 P 1 q q
Source
LONG q OUT
2
R Out
2
R Side
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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.
The first public presentation of RHIC results took place at the Quark Matter 2001 conference.
January 15-20
Held at Stony Brook University
Recorded ~5M events 27
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Jet Quenching
At RHIC energies, some of the processes are calculable from first principles
Hard scattering
Jets
Violent collisions between quarks and gluons.
Excess yield at high momentum.
One effect of Plasma is the “quenching” of these jets.
They lose their energy while crossing the plasma.
They “cool” down to the soup temperature.
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Jet Quenching Observed
Stony Brook Postdoc Federica Messer , presented PHENIX spectra of charged particles.
( should be dominated by pions).
BNL scientist (formed SB student) Gabor David pions.
presented measurements of neutral, IDENTIFIED What??? The all charged and neutral pions DIVERGE!!
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Identified Particle Spectra
Stony Brook Postdoc Julia Velkovska presented identified charged particle spectra at high momentum
The proton production EXCEEDS the pion production at high momentum
NOONE PREDICTED THAT!
This causes the divergence between “all-charged” and neutral pions.
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Where are the Jets?
Expectation
Charged particle production falls below the expectations by about a factor of two despite the proton contamination.
Neutral pion production is a factor of 10 below predictions.
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Another Surprise!
R out
Normal theory cannot account for this
Imaginary times of emission!!
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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 R out R side is the shell thickness is the ball size Plasma Shells of ordinary matter 33
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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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Next Steps
Simple Language:
After the icecubes collide and melt, fragments leave which are frozen by the time they reach us, masking the true nature of the early state.
Lesson: Don’t look at the fragments of frozen water which leave the collision, take a picture using light while the system is melted!
Sophisticated Language:
Since hadrons are made of quarks, they reform and thereby lose information from the early stage.
Photons and leptons leave the plasma directly and give detailed information from the center of the collision!
Photons and leptons are rare and require more RHIC running.
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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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Electron Identification
Problem: They’re
rare
Solution: Multiple methods
Cerenkov
E(Calorimeter)/p(tracking) matching E/p matching for p>0.5 GeV/c tracks All tracks Electron enriched sample (using RICH)
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Why electrons?
One reason: sensitivity to heavy flavor production D D D 0 0 0 K K K -
p
+ e +
e
+
Dalitz and conversions e charm e beauty e Drell-Yan e-
B B B 0 0 0 D D D -
p
+ e +
e
+
D D D 0 0 0 D D D 0 0 0
+
e
+ + e e K + K K + + K K K -
e
e
e
Study by Mickey Chiu, J. Nagle
Other reasons: vector mesons, virtual photons
e + e -
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p
0
Reconstruction
A good example of a “combinatoric” background Reconstruction is
not
done particle-by-particle Recall:
p
0
gg
and there are ~200
p
0
So:
p
0
p
0
p
0 1
g 1A g
2
g 2A g
3
g 3A g 1B 2B 3B
‘ s per unit rapidity PHENIX
p
0 reconstruction p T > 2 GeV/c Asymmetry < 0.8
p
0 N
g NA g NB .
Unfortunately, nature doesn’t use subscripts on photons N correct combinations: ( N(N-1)/2 – N incorrect
g 1A
Incorrect combinations ~ N 2
g
(!)
1B
), (
g 2A g 2B
combinations (
g 1A g
), … (
g NA g 2A
), (
g 1A g NB 2B
), ), …
Solution: Restrict N by p
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T cuts use high granularity, high resolution detector 39
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BRAHMS
An experiment with an emphasis:
Quality PID spectra over a broad range of rapidity and p T Special emphasis:
Where do the baryons go?
How is directed energy transferred to the reaction products?
Two magnetic dipole spectrometers in “classic” fixed-target configuration
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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 Unbiased global look at very large number of collisions (~10 9 ) 41
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PHOBOS Details
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Si tracking elements
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
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Hits in SPEC Tracks in SPEC Hits in VTX
PHOBOS Results
130 AGeV
First results on dN ch /d
h
for central events
At E CM
energies of 56 Gev
130 GeV (per nucleon pair) To appear in PRL ( hep-ex/0007036)
X.N.Wang et al.
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Magnet Coils
STAR
An experiment with a challenge:
Track ~ 2000 charged particles in |
h
| < 1 TPC Endcap & MWPC ZCal Endcap Calorimeter Barrel EM Calorimeter
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44 Time Projection Chamber Silicon Vertex Tracker FTPCs Vertex Position Detectors Central Trigger Barrel or TOF ZCal RICH
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STAR Challenge
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STAR Event
Data Taken June 25, 2000.
Pictures from Level 3 online display.
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STAR Reality
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An experiment with something for everybody A complex apparatus to measure
Hadrons
Muons Electrons Photons Executive summary:
High resolution South muon Arm
High granularity
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PHENIX
Global MVD/BB/ZDC West Arm Muon Arms Coverage (N&S) -1.2< |y| <2.3
-
p
<
f
<
p D
M(J/
)=105MeV
D
M(
g
) =180MeV 3 station CSC 5 layer MuID (10X 0 ) p(
)>3GeV/c East Arm Central Arms Coverage (E&W) -0.35< y < 0.35
30 o <|
f
|< 120 o
D
M(J/
)= 20MeV
D
M(
g
) =160MeV North muon Arm
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PHENIX Design
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PHENIX Reality
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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 d
h h
0
A
N part
B
N coll
A
0 .
88 0 .
28
B
0 .
34 0 .
12
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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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