  The PHENIX MPC-EX detector is a Si-W preshower extension to the existing
PHENIX Muon Piston Calorimeters (MPC). The MPC-EX will consist of eight layers
of alternating W absorber and Si mini-pad sensors and will be installed in time
for RHIC Run-15. Covering a large pseudorapidity range, 3.1 < eta < 3.8, the
MPC-EX and MPC access high-x partons in the projectile nucleon (and low-x
partons in the target nucleon) in p+A and transversely polarized proton-proton
collisions at 200 GeV. With the addition of the MPC-EX, the neutral pion
reconstruction range extends to energies > 80 GeV, a factor of four improvement
over current capabilities. Not only will the MPC-EX strengthen PHENIX's
existing forward neutral pion and jet measurements, it also provides the
necessary neutral pion rejection to make a prompt photon measurement feasible
in both p+A and p+p collisions. With this neutral pion rejection, prompt
(direct + fragmentation) photon yields at high p_T, p_T > 3 GeV, can be
statistically extracted using a double ratio method. In p+A collisions direct
photons at forward rapidities are optimally sensitive to the gluon distribution
because, unlike pions, direct photons are only produced by processes that are
directly sensitive to the gluon distribution at leading order. A measurement of
the forward prompt photon R_pA will cleanly access and greatly expand our
understanding of the gluon nuclear parton distribution functions and provide
important information about the initial state in heavy ion collisions. In
transversely polarized p+p collisions the MPC-EX will make possible a
measurement of the prompt photon single spin asymmetry A_N, and will help
elucidate the correlation of valence partons in the proton with the proton
spin.
