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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(simplified)

Imagine a
college campus
on a warm
summer day


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Students are
uniformly
distributed in
an open field.
Now introduce a
FRISBEE into
the system!
Thomas K Hemmick
Standard Model II
Students who
interact with the
FRISBEE form a
group.


Other students
don’t interact with
the FRISBEE.



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These students are
“charged”
neutral or “nerds”
Now introduce
CHESS into the
campus!
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Standard Model III

Some charged
and some neutral
students decide
to play chess



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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



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(and triples)
If you break up
with one partner,
you immediately
find another
(confinement)
Force grows
stronger with
separation
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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Thomas K Hemmick
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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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
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)
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 ~  /(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
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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 ~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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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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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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Thomas K Hemmick
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
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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Thomas K Hemmick
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 presented
measurements of
neutral, IDENTIFIED
What??? The allpions.
charged and neutral
pions DIVERGE!!
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Thomas K Hemmick
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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Thomas K Hemmick
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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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
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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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)
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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
39
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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
47
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(g48
) =160MeV
North muon
Arm
Thomas K Hemmick
PHENIX Design
49
PHENIX Reality
50
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