RHIC-spin is an experiment to uncover the mystery of proton
spin using spin- polarized protons. Proton is a fundemental
particle, a major constituent of our world.
It has a spin of 1/2, and naively thought to be composed of
three quarks of spin 1/2 -- two spin-up quarks and
one spin-down quark. However, past experiments tell us that
this naive picture is very poor -- spin of quarks add up
to only about 1/4 of nucleon spin. Then, where the
rest of the nucleon spin comes from? This question is called
the "proton-spin puzzle", and is what we would like to
answer by the experiment.