2
Back to 1964
people were trying to understand origins of so many discovered particles
Gell-Mann and Zweig propose quarks as underlying structure
quark concept focuses on kinship relations among particles
quarkshadrons
mesons baryons
pions,kaons,
...
protons,neutrons,
...
3
While others were at Woodstock Feynman, in 1969,
proposes “partons” as way to explain experimental results from SLAC at Stanford
parton concept focuses on dynamics, the way things behave when then interact
With advent of QCD in 1973 partons are identified with quarks and gluons
hadron
pL
pT
hadron
4
quarkshadrons
mesons baryons
pions,kaons,
...
protons,neutrons,
...
nucleons
protons neutrons
partons
quarks gluons
Some more terminology
pentaquark
,–,...
Free quarks?
Strong nuclear force has some very unusual properties doesn’t get weaker with
distance! So what happens when
you try to send two quarks flying apart?
anti-quark
quark
EM force decreases with distance
6
Force between two quarks
Compare to gravitational force at Earth’s surface
Quarks exert 16 metric tons of force on each other!
quark quarkgluons
7
A directed “spray” of particles as connection between
quarks breaks up, most of the motion stays close to direction of the original quarks
the fragmented bits appear as normal subatomic particles pions, kaons, protons, ...
kaon
pion
pion
pion
8
Origin of the word “jet”
“sprays” of particles had been seen in experiments before
original term “core”, came from cosmic ray experiments
first use of “jet” seems to be by Bjorken in 1970
high-energyproton
14N
corecore
9
Jet properties
cone-like spray of particles surrounding direction of each outgoing parton
quark-quark scattering leads to back-to-back structure
high-energy parton-parton interaction can be calculated with precision
10
Many sources of low pT particles many ways to create
particles in electron-positron or proton-proton collisions that don’t involve jets
typical transverse momentum (pT) few hundred MeV/c
13
First evidence for jets was subtle By 1975 at SLAC (DESY
too) energy of electron-positron collisions high enough for jets to appear ... statistically
As collision energy was raised, average “sphericity” decreased
Gradual appearance of back-to-back jets in Mark I experiment
collision energy
sphe
ricity
15
Jets & proton-antiproton collisions e+e- one thing; hadron
collisions another incoming partons vary
International conference on high-energy physics, Paris, 1982
Results from CERN experiment UA2 really convinced everyone that jets in hadron-hadron collisions had been seen
16
Very selective timeline
quarks1964
partons1969
“jets”1970
jets in e+e–
1975QCD1973
Fermilab(NAL)CERN ISR
CERN SPS
UA2jets inp +p1982
17
Jets and the period 1969-1982 It took time for suitable facility to be available
high enough energy for jets to stand out It took time to design and build the right sort
of experiment Fundamental theory was developed part-way
through the period
18
RHIC Physics Program
RHIC proposed 1983 One of the main emphases is study of
properties of matter under extreme conditions huge energy densities enormous temperatures (over 1 trillion C)
To achieve these conditions we collide heavy nuclei at very high energies
Extremely useful to have probes with known properties
20
Jets at RHIC
Not starting at “square one” properties of jets in electron-positron, proton-
proton, proton-antiproton collisions well-measured relying on over 30 years of jet physics results
Energy high enough that jets not too rare Experiments designed with jets in mind The plan in a nutshell
show that RHIC experiments can “see” jets look for changes in expected jet properties
PHENIX
STAR
Each collaborationabout 400 physicists
and engineersMuch of the research
driven by students
25
Finding jets using correlations Method 1
find a high-momentum charged particle and look nearby for others
Method 2 main particle in jet is very
often a pion a 0 usually decays into
photons find a high-energy photon
and look nearby for others
pion
photons
pion, kaon, ...
triggerparticle
26
An algorithm
a way to locate the running of the bulls in Pamplona, Spain:
1. start by finding one high-momentum bull
2. look others moving in roughly the same direction
3. if the bull density is high, you might reconsider the place you’ve chosen to stand
The next step is simple: just replace “bull” by “particle”
30
RHIC proton-proton collisions We haven’t discovered jets; we’ve shown that
the experiments can detect the expected jets Provide a solid baseline for studying nucleus-
nucleus collisions Fine-tune techniques and the understanding
of detectors to prepare for finding jets in nuclear collisions
34
Collision geometry
gold nucleus
gold nucleus
gold is a large nucleus, lots of
partons (quarks, anti-quarks and gluons)
“pancake” thin due to special
relativity
38
Case study: opacity of fog
“is this thing on?” if you detect one beam, at least know the source is on
intensity of the “other” beam tells you a lot
39
High-energy physics in vacuum
parton parton
parton
parton
focus is on understanding interactions between elementary particles
40
High-energy physics in medium
parton parton
parton
parton
hot, dense systemof quarks, gluons
focus is on understanding nature of created medium
41
High-energy physics in cold nuclear matter
quark quark
jet?
jet?
deuteron(bound state of
neutron and proton)gold
provides a crucial “control” experiment–what does a parton do passing through normal nuclear matter?
42
Pedestal&flow subtracted
angle away from initial high momentum particle
inte
nsity
same direction opposite direction
43
Quantifying the effect
jet “
stre
ngth
”
glancing head-ontype of collision
STAR results: PRL 90, 082302 (2003)
44
Interpreting the observation
quark quark
jet!
The “stuff” created during collision is very unusual, very unlike normal nuclear matter. Energy loss of parton ~5 GeV/fm. Compare with results from e+A at HERA: 0.3 GeV/fm.
45
A (very) loose comparison
accelerator RF cavity: 10 MV/m
parton in hot, dense “QCD matter”: 5 GeV/fm
factor of 500 quadrillion different
46
Determining the reaction plane
x
z
y
Initial asymmetry in coordinate space leads to asymmetry in pressure leads to asymmetry in
momentum space–which can be detected
STAR Preliminary
pTtrigger=4-6 GeV/c, 2<pT
associated<pTtrigger, ||<1
Aihong Tang, January 16, 2004 at Quark Matter conference
48
Exploring different territories
PETbiological tissues
RHICdense mix of quarks, gluons
γ
γ
particles
particles
brain nuclearcollision
49
Parallels with 1970’s high-energy RHIC is creating nuclear collisions at particle
physics energies The experiments have been designed with
the benefit of previous efforts acceptance, resolution, calorimetry, particle
identification Very active exchanges between experiment
and theory; active development of theory
50
Fin
RHIC experiments detect jets convincingly Jet measurements contributing to very lively
interplay between theory and experiment quark-gluon plasma, color-glass condensate, color-flavor
locked matter, color superconductivity Jets used as sophisticated probe of very complex
environment of nuclear collision only the more straightforward jet analyses have been
published so far di-hadron correlations, γ–jet studies, high pT identified
particles, many more to come!
52
We’d like to “X-ray” nuclear collisions However, analogy is
slightly flawed Medical X-ray source is
(usually) outside the system being studied
Properties of X-ray beam can be prepared precisely
Can study X-ray photos with and without a sample in place
55
What is a nucleus?
+
Depends how you ask the question. If you ask what it looks like to a not too high energy electron, it’s a heavy blob of positively charged matter
-
56
What is a nucleus?
If you ask a medium energy proton, it’s largely a collection of protons and neutrons
+