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In addition to colliding
heavy ions, RHIC will also be able to collide single protons.
While these collisions won't produce quark-gluon plasma, they're
interesting to physicists for other reasons. Namely, scientists
want to know more about a property of particles called spin.
Spin is the direction
a particle is spinning around an axis as it travels -- just like
the Earth spins on its axis as it travels around the sun. Each
proton has a specific spin, which helps give it a
characteristic magnetic property.
In this picture of a proton-proton collision, the spin of the particles is shown as
arrows circling the spherical particles. The red and green
particles represent reaction products from the collision which
will be "seen" and analyzed by RHIC detectors.
Proton beams in RHIC will
be "spin polarized", meaning that all the protons in one
beam will be spinning the same way, and that the other beam will
contain protons all spinning in the opposite direction. RHIC is the
first machine in the world with the capability to collide such beams
head-on.
Why is proton spin
important to understand? Astronomers studying the universe use
proton spin and magnetism as important measuring properties. Spin is
also what allows doctors to use an MRI (Magnetic Resonance Imaging)
machine to see inside the human body to diagnose disease.
For these reasons and
others, physicists want to measure and understand how different
factors influence a proton's spin. Experiments elsewhere have shown
that the spins of the quarks (and antiquarks, in some cases) inside
particles such as protons accounts for only about 30% of the
particle's overall spin.
RHIC spin experiments
should provide the first information on how much the spin of gluons
contributes to the proton's spin, a contribution which recent
theoretical work suggests may be large.
If the quark and gluon
spins together still do not account for the proton's spin, the only
remaining source available to "balance the books" is the
movement of quarks and gluons relative to one another. Thus, RHIC's
measurements of the spin substructure of the proton may lead us
beyond our current, still rudimentary understanding of how quarks
move inside protons and other particles.
On to Experimental
facilities >
To learn more about RHIC and nuclear physics in
general, see the National Research Council's new book, Nuclear Physics: The Core of the Matter, the Fuel of the
Stars. You can read it on-line for free, in either HTML
or
PDF
formats.
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