The surprising results of the polarized muon scattering off the polarized
proton reported by the EMC collaboration have stimulated both experimental and
theoretical works to elucidate the spin structure of the proton.
The fraction of the proton spin carried by quark, 
, was
amazingly small comparing to the canonical expectation
0.60
0.12 [1].
The current best guess is:
Current knowledge on the quark polarization in the proton is summarized in Table 1.
Table 1: Summary of the experimental results and some theoretical
predictions on the quark polarization in the proton.
As a result of extensive efforts, those works nearly converge into two possible solutions of the proton spin puzzle:
, is large,
is amazingly large and
anti-parallel to the proton spin.
, W, 
).
Furthermore, flavor decomposition
of quark and anti-quark polarization
can be confirmed in W-production.
The measurements of 
and 
will contribute much
to understanding the spin structure of the proton.
Such measurements will be discussed after a brief description of
PHENIX detector system in the next section.