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No-scale supergravity and the flipped SU(5)$\times$U(1) gauge group providean ambitious prototype string-inspired scenario for physics below the stringscale, which can accommodate the Starobinsky-like inflation favoured byobservation when the inflaton is associated with one of the singlet fieldsassociated with neutrino mass generation. During inflation, the vacuum remainsin the unbroken GUT phase, and GUT symmetry breaking occurs later when a fieldwith a flat direction (the flaton) acquires a vacuum expectation value.Inflaton decay and the reheating process depend crucially on GUT symmetrybreaking, as decay channels open and close, depending on the value of theflaton vacuum expectation value. Here, we consider the simultaneouscosmological evolution of both the inflaton and flaton fields after inflation.We distinguish weak, moderate and strong reheating regimes, and calculate ineach case the entropy produced as all fields settle to their global minima.These three reheating scenarios differ in the value of a Yukawa coupling thatintroduces mass mixing between the singlets and the ${\bf 10}$s of SU(5). Thedynamics of the GUT transition has an important impact on the production ofgravitinos, and we also discuss the pattern of neutrino masses we expect ineach of the three cases. Finally, we use recent CMB limits on neutrino massesto constrain the reheating models, finding that neutrino masses and thecosmological baryon asymmetry can both be explained if the reheating is strong.

Symmetry Breaking and Reheating after Inflation in No-Scale Flipped SU(5)