Spontaneous lorentz breaking and massive gravity.

Theories with an extra spin-two field coupled to gravity admit a massive phase with broken Lorentz symmetry. While the equivalence principle is respected, the Newtonian potentials are in general modified, but they may be protected by a scale symmetry of the coupling term. The gravitational waves phenomenology is quite rich: two gravitons, one massive and one massless, oscillate and propagate with distinct velocities, different from the speed of light. A time of flight difference between gravitons and photons from a common source would provide a clear signal of this theory.

Leptogenesis via collisions: leaking lepton number to the hidden sector.

We propose a leptobaryogenesis mechanism in which the nonzero B-L of the Universe is produced in out-of-equilibrium, lepton number and CP-violating scattering processes that convert ordinary particles into particles of some hidden sector. In particular, we consider the processes l phi-->l(')phi('),l macro(')phi(') mediated by the heavy Majorana neutrinos N of the seesaw mechanism, where l and phi are ordinary lepton and Higgs doublets and l('), phi(') are their hidden counterparts. Such a leptogenesis mechanism is effective even if the reheat temperature is much smaller than the heavy neutrino masses. In particular, it can be as low as 10(9) GeV.