We mini-review the role of fundamental spin-0 bosons as bosonic coherent motion (BCM) in the Universe. The fundamental spin-0 bosons have the potential to account for the baryon number generation, cold dark matter (CDM) via BCM, dark energy, and infl
ation. Among these, here we focus on the CDM possibility because it can be experimentally tested with the current experimental techniques. We also comment briefly on the panoply of the other roles of spin-0 bosons.
Current upper bounds of the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle $|bartheta| lesssim 10^{-11}$. Since QCD explains vast experimental data from the 100 MeV scale to the TeV scale,
it is better to explain this smallness of $|bartheta|$ in the QCD framework, which is the strong CaPa problem. Now, there exist two plausible solutions to this problem, one of which leads to the existence of the very light axion. The axion decay constant window, $10^9 {gev}lesssim F_alesssim 10^{12} gev$ for a ${cal O}(1)$ initial misalignment angle $theta_1$, has been obtained by astrophysical and cosmological data. For $F_agtrsim 10^{12}$ GeV with $theta_1<{cal O}(1)$, axions may constitute a significant fraction of dark matter of the universe. The supersymmetrized axion solution of the strong CaPa problem introduces its superpartner the axino which might have affected the universe evolution significantly. Here, we review the very light axion (theory, supersymmetrization, and models) with the most recent particle, astrophysical and cosmological data, and present prospects for its discovery.
I discuss the essential features of the QCD axion: the strong CP solution and hence its theoretical necessity. I also review the axion and axino effects on astrophysics and cosmology, in particular with emphasis on their role in the dark matter component in the universe.
I review our recent attempts toward obtaining the MSSM from string orbifold compactification. The required constraints are the existence of three families and R parity, vectorlike exotics, one pair of Higgs doublets, and the SU(5)$$ hidden sector for
dynamical breaking of SUSY toward a GMSB scenario. We also comment on the threshold correction which are influenced by a power law evolution of gauge couplings through the KK radius in non-prime orbifolds and can be used to fit the couplings.
We present a possible explanation of the recently observed 511 keV $gamma$-ray anomaly with a new ``millicharged fermion. The new fermion is light (${cal O}({rm MeV})$) but has never been observed by any collider experiments mainly because of its tin
y electromagnetic charge $epsilon e$. We show that constraints from its relic density in the Universe and collider experiments allow a parameter range such that the 511 keV cosmic $gamma$-ray emission from the galactic bulge may be due to positron production from this millicharged fermion.
Analyzing the one-loop partition function, we discuss possible Kaluza-Klein (KK) states in the orbifold compactification of the heterotic string theory, toward the application to the threshold correction. The KK massive states associated with (relati
vely) large extra dimensions can arise only in non-prime orbifolds. The GSO projection condition by a shift vector $V^I$ is somewhat relaxed above the compactification scale 1/R. We also present the other condition on Wilson line $W$, $Pcdot W={rm integer}$. With the knowledge of the partition function, we obtain the threshold corrections to gauge couplings, which include the Wilson line effects. We point out the differences in string and field theoretic orbifolds.
