David Morrison Jet Physics at RHIC Focusing high-energy tools on nuclear collisions.
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Transcript of David Morrison Jet Physics at RHIC Focusing high-energy tools on nuclear collisions.
David Morrison
Jet Physics at RHIC
Focusing high-energy tools on nuclear collisions
David Morrison 2
The BNL lecture last month …
PET Imagingby
Dave Schlyer
electron positron
photonE = 500,000 eV
photon
2 mm
David Morrison 3
The BNL lecture this month …
quark quark
photonE = 500,000 eV
photon
2 mm
David Morrison 4
The BNL lecture this month …
quark quark
quarkE = 500,000 eV
quark
2 mm
David Morrison 5
The BNL lecture this month …
quark quark
quarkE > 5,000,000,000 eV
quark
2 mm
David Morrison 6
The BNL lecture this month …
quark quark
quarkE > 5,000,000,000 eV
quark
0.000000000002 mm
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Exploring different territories
PETbiological tissues
RHICdense mix of quarks, gluons
particles
particles
brain nuclearcollision
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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,
...
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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
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
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Fragmentation
q q
quarks, anti-quarks appear, break
original connection into more and more and more particles
q qqq
qqqqq q
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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
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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
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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
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Many sources of low pT particles many ways to create
particles in electron-positron or proton-proton collisions that don’t involve jets e.g., create an unstable
particle that then decays typical transverse
momentum (pT) few hundred MeV/c
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At low energy, jets hard to discern
?
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At higher energy, jets stand out
!
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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
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Jets in electron-positron collisions
qqee
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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
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Very selective timeline
quarks1964
partons1969
“jets”1970
jets in e+e–
1975QCD1973
Fermilab(NAL)CERN ISR
CERN SPS
UA2jets inp +p1982
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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
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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
1984 BNL note about RHIC physics
Jets in nuclear collisions
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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
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STAR
PHENIX
PHOBOS BRAHMS
you are here
PHENIX
STAR
Each collaborationabout 400 physicists
and engineersMuch of the research
driven by students
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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
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Correlations outside particle physics One way to find “el
encierro” (the running of the bulls) in Pamplona, Spain:
1. start by finding one high-momentum bull
2. look near that bull for others moving in the same direction
3. if the bull density is high, you’re likely standing in the middle of the bull run
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PHENIX
See a pattern consistent with jets!
part
icle
trac
k de
nsity
angle of track away from photon
+-
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Candidate jets in RHIC p+p collisions
PHENIX
STAR
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Collisions of larger objects
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Hiding in plain sight
gold ion
gold ionAu+Au at full RHIC energy
in STARdetector
jets can be difficult to
find,even when you know
they’re there
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What is a nucleus?
If you ask a very high energy proton, it’s a huge collection of partons
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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
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A “peripheral” (glancing) collision
a bit like a proton-proton
collision
quark quark
jet
jet
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A “central” (head-on) collision
quark quark
jet?
jet?
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Single pions and energy losspr
obab
ility
of c
reat
ing
0
PHENIX Preliminary
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A “lighthouse” of parton fragments
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Using this tool to study QCD “fog” seeing one beam, know
the other should be there collide two fog banks,
wait for spontaneous appearance of working lighthouse
look for changes intensity wavelength angular spread
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High-energy physics in vacuum
parton parton
parton
parton
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High-energy physics in medium
parton parton
parton
parton
hot, dense systemof quarks, gluons
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What about the other jet?
jet “
stre
ngth
”
glancing head-ontype of collision
STAR results: PRL 90, 082302 (2003)
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Interpreting the observation
quark quark
jet!
strongly suggests that “stuff” created during collision is very unusual, very unlike normal
nuclear matter
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A (very) loose comparison
accelerator RF cavity: 10 MV/m
parton in hot, dense “QCD matter”: 5 GeV/fm
factor of 500 quadrillion different
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Parallels with 1970’s high-energy RHIC is creating nuclear collisions at particle
physics energies C-AD runs a machine with unprecedented
capabilities 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
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Fin
RHIC experiments detect jets convincingly Jets used as sophisticated probe of very
complex environment of nuclear collision study of jets also important for spin physics!
Only the more straightforward jet analyses have been published so far
Jet measurements contributing to very lively interplay between theory and experiment
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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
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STAR
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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
-
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What is a nucleus?
If you ask a medium energy proton, it’s largely a collection of protons and neutrons
+
David Morrison 56
What about the other jet?
jet “
stre
ngth
”
glancing head-on
type of collision
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Yes, almost completely.
jet “
stre
ngth
”
glancing head-on
type of collision
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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