Is neutralino dark matter possible in the superstring theory?

摘要

In the heterotic superstring theory, the decay constant of the QCD axion lies within the range 3 x 10(16) less than or similar to f(a)/GeV less than or similar to 10(18), the lower limit referring to the model-independent axion, while the upper limit is due to dimension-five, non-renormalizable effects first calculated by Cvetic.. Consequently, the neutralino chi0, assumed to be a nearly pure B-ino, decays into the axino (a) over tilde on the time scale obtained by Covi et al., tau chi0 approximate to C-aYY(-2) (f(a)/3 x 10(16) GeV)(2)(100 GeV/mchi0)(3) x 10(11)s, which is less than or similar to10(-3) times the age of the Universe t(0) approximate to 4 x 10(17)s, but can only be made less than the time t approximate to 1s of the onset of Big-Bang nucleosynthesis by revising mchi0 to an unnaturally high level, mchi0 greater than or similar to 500 TeV. Therefore, it is necessary to set the coefficient C-aYY = 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 mchi0 (for a gaugino-dominated state, Omegachi0 approximate to (M (l) over tilde (4)(R)/Mchi(0)(2))(200 GeV)(-2) where (l) over tilde (R) is the SU(2) singlet slepton). The consequences of an ultra-light axion with f(a) approximate to 10(18) GeV are also discussed.