IS NEUTRALINO DARK MATTER POSSIBLE IN THE SUPERSTRING THEORY?

Abstract

In the heterotic superstring theory, the decay constant of the QCD axion lies within the range 3×1016≲fa GeV ≲1018, the lower limit referring to the model-independent axion, while the upper limit is due to dimension-five, non-renormalizable effects first calculated by Cvetič. Consequently, the neutralino χ0, assumed to be a nearly pure B-ino, decays into the axino ã on the time scale obtained by Covi et al., [Formula: see text], which is ≲10-3 times the age of the Universe t0≈4×1017 s , but can only be made less than the time t≈1 s of the onset of Big-Bang nucleosynthesis by revising mχ0 to an unnaturally high level, mχ0≳500 TeV . Therefore, it is necessary to set the coefficient Ca YY =0, which is possible for the Kim–Shifman–Vainshtein–Zakharov invisible-axion model if the electric charge q c of the heavy-quark colour representation C vanishes. The neutralino does not then decay and can constitute some fraction of the dark matter of the Universe, depending upon the value of mχ0 (for a gaugino-dominated state, [Formula: see text] where [Formula: see text] is the SU(2) singlet slepton). The consequences of an ultra-light axion with fa≈1018 GeV are also discussed.