Reconciling Supersymmetry and Thermal Leptogenesis

Many extensions of the Standard Model predict super-weakly interacting
particles, which typically have to decay before Big Bang Nucleosynthesis
(BBN). The entropy produced in the decays may help to reconcile thermal
leptogenesis and BBN in scenarios with gravitino dark matter, which is
usually difficult due to late decays of the next-to-lightest supersymmetric
particle (NLSP) spoiling the predictions of BBN. We study this possibility
for a general neutralino NLSP. We elaborate general properties of the
scenario and strong constraints on the entropy-producing particle. As an
example, we consider the saxion from the axion multiplet and show that,
while enabling a solution of the strong CP problem, it can also produce a
suitable amount of entropy.