Two-body decays of the lightest stop in minimal supergravity with and without R-parity Original Research Article

We study the decays of the lightest top squark in supergravity models with and without R-parity. Using the simplest model with an effective explicit bilinear breaking of R-parity and radiative electroweak symmetry breaking we show that, below the threshold for decays into charginos t̃1→b χ̃+1, the lightest stop decays mainly into third generation fermions, t̃1→b τ instead of the R-parity conserving mode t̃1→c χ̃01, even for tiny tau-neutrino mass values. Moreover we show that, even above the threshold for decays into charginos, the decay t̃1→b τ may be dominant. We study the role played by the universality of the boundary conditions on the soft supersymmetry breaking terms. This new decay mode t̃1→b τ as well as the cascades originated by the conventional t̃1→c χ̃01 decay followed by the R-parity violating neutralino decays can provide new signatures for stop production at LEP and the Tevatron.