Neutralino warm dark matter

In the supersymmetric (SUSY) standard model, the lightest neutralino may be the lightest SUSY particle (LSP), and it is is a candidate of the dark mat ter in the universe. The LSP dark matter might be produced by the non-therm al process such as heavy particle decay after decoupling of the thermal rel ic LSP. If the produced LSP is relativistic, and does not scatter enough in the thermal bath, the neutralino LSP may contribute as the warm dark matte r (WDM) to wash out the small scale structure of O(0.1) Mpc. In this Letter we calculate the energy reduction of the neutralino LSP in the thermal bat h and study whether the LSP can be the WDM. If temperature of the productio n time T-I is smaller than 5 MeV, the bino-like LSP can be the WDM and may contribute to the small-scale structure of O(0.1) Mpc. The higgsino-like LS P might also work as the WDM if T-I < 2 MeV. The wine-like LSP cannot be th e WDM in the favored parameter region, (C) 2001 Published by Elsevier Scien ce B,V.