The preparation of the sPHENIX experiment has been underway as a next generation experiment in order to complete the research mission of the quark-gluon plasma (QGP) at the relativistic heavy ion collider (RHIC). The sPHENIX detector is a barrel type detector which covers the central rapidity region of $-1<¥eta<1$ and optimized to detect jets under high multiplicity circumstances of heavy ion collisions at high energy. In addition to the QGP studies, the medium energy nuclear physics such as a hadron structure study can be pursued using the sPHENIX detector. With the high DAQ rate capability and the high hermeticity of the sPHENIX, further improvements in the precision of the gluon polarization measurements can be expected assuming a few year extension of sPHENIX running period beyond it's scheduled one 2023 - 2025. The physics opportunity can be drastically enriched if the jet detection capability is extended to the forward region during the sPHENIX running period. . The latest design of a forward detector (fsPHENIX) is composed of a high momentum resolution tracking using the magnetic field provided by a sPHENIX magnet, calorimetry and particle identification covering the pseudo-rapidity region of $1 < ¥eta < 4$.
In combination with sPHENIX acceptance, the long rapidity range correlation can be studied in various collision species which is considered to give us an access to the early stage of the QGP formation. The forward region is also crucial for the spin program. Transverse spin asymmetries from Collins and Sivers effects in jets and Drell Yan yields in the unexplored large Feynman momentum $x_{¥rm F}$ region, where these effects are larger, would be accessible as well as small momentum fraction gluon contribution to the proton spin $¥Delta G$). The fsPHENIX detector is designed ultimately to be the hadron detector of the future ePHENIX in the electron ion collider (EIC) at RHIC era.