Nuclear Physics at Jefferson Lab

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Nuclear Physics at Jefferson Lab
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Matter is made of atoms which, in turn, consist
of nuclei surrounded by electrons. Nuclei
contain 3 quarks. The quarks and electrons
are fundamental cannot be subdivided.
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The science at Jefferson Lab aims to
understand in detail the structure of matter by
probing the quarks in nuclei. To perform
this study we use an atom smasher and
detectors. Together, these form a kind of
microscope.
Picture of 3 quarks in a proton.
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How can we obtain experimental
information about nuclei ?
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Step 1 Produce an electron beam
Electrons are pushed to higher energy by
microwaves (electromagnetic waves)
acceleration
microwave
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Step 2 Smash the electrons into a target.
Its the quarks in the target that we are
studying.
Target
Electron Beam
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Step 3 Detect the scattered debris in
Spectrometers
Spectrometer 1
Spectrometer 2
Target
Electron Beam
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Spectrometers at Jefferson Lab
These machines are microscopes for looking at
quarks.
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me
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Spectrometers Measure
Magnets measure momentum Detectors
identifies particles Direction of pointing
outgoing angle
Detector System
Target
Beam
Magnets
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Step 4 Detect the Particles
Particle Detectors determine what kind of
particles exist in the scattered debris.
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Artists view of detector stack
Consists of several kinds of detectors. The
technology is always improving and sometimes has
spinoffs (e.g. medical imaging)
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Step 5 The Physics
A measurement results in publishable new
information which can be compared to other
measurements and to theories. Results may
end up in textbooks someday.
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Specific example of research
Weve known for 30 years that there are 3 quarks
in a proton. uud (2 up, 1
down) What else is there ? e.g. anti-quarks ?
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How can you tell ?
probes
1. Electric Force 2. Magnetic Force
1. Up quark
in proton
2. Down quark
3. Other ?
Need another force

• Use the weak nuclear force
• (couples to electrons to protons )

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How to isolate the weak interaction
Do 2 experiments that are mirror images Weak
interaction looks different. EM interaction
looks same.
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The weak interaction changes with mirror
imagingwhich allows to isolate it.
Incident electron
Positive spin
Target
momentum
spin
Mirror Image
A proton in target
Negative spin
So, we have to flip spin of electrons
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Experiment Flip spin and count
scattered electrons in each spin state
Detector 2 (count electrons)
Detector 1 (count electrons)
Proton target
electrons
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Result

• Less than 1 of the proton
• (or neutron) has exotic stuff
• quark - antiquark pairs.

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How to make electrons that spin
Pockel Cell flips spin
GaAs Crystal
Laser
-
e beam
spinning

• Must be careful when flipping spin
• Cant change anything else (e.g. position)
• Need position control to 1 nm !!
• Requred technological breakthroughs