  We systematically construct gauge-mediated supersymmetry(SUSY)-breaking
models with direct transmission of SUSY-breaking effects to the standard-model
sector. We obtain a natural model with the gravitino mass $m_{3/2}$ smaller
than 1 keV as required from the standard cosmology. If all Yukawa coupling
constants are of order one,the SUSY-breaking scale $m_{SUSY}$ transmitted into
the standard-model sector is given by $m_{SUSY} \simeq 0.1 \alpha_i/(4\pi)
\Lambda$ where $\Lambda$ is the original dynamical SUSY-breaking scale.
Imposing $m_{SUSY} \simeq (10^2-10^3)$ GeV, we get $\Lambda \simeq (10^5-10^6)$
GeV, which yields the gravitino mass $m_{3/2}\simeq (10^{-2}-1)$ keV.

High Energy Physics - Phenomenology