Towards a microscopic construction of flavour vacua from a space-time foam model

The effect on flavour oscillations of simple expanding background space-times, motivated by some D-particle foam models, is calculated for a toy model of bosons with flavour degrees of freedom. The presence of D-particle defects in the space-time, which can interact non-trivially (via 'particle capture') with flavoured particles in a flavour non-preserving way, generates mixing in the effective field theory of low-energy string excitations. Moreover, the recoil of the D-particle defect during the capture/scattering process implies Lorentz violation, which, however, may be averaged to zero in isotropic D-particle populations, but implies non-trivial effects in correlators. Both features imply that the flavoured mixed state sees a non-trivial flavour (Fock-space) vacuum of a type introduced earlier by Blasone and Vitiello in a generic context of theories with mixing. We discuss the orthogonality of the flavour vacua to the usual Fock vacua and the effect on flavour oscillations in these backgrounds. Furthermore, we analyse the equation of state of the flavour vacuum, and find that, for slow expansion rates induced by D-particle recoil, it is equivalent to that of a cosmological constant. Some estimates of these novel non-perturbative contributions to the vacuum energy are made. The contribution vanishes if the mass difference and the mixing angle of the flavoured states vanish.