Mr Martin Mosny

We perform a detailed study of stringy moduli-driven cosmologies between the end of inflation and the commencement of the Hot Big Bang, including both the background and cosmological perturbations: a period that can cover half the lifetime of the universe on a logarithmic scale. Compared to the standard cosmology, stringy cosmologies motivate extended kination, tracker and moduli-dominated epochs involving significantly trans-Planckian field excursions. Conventional effective field theory is unable to control Planck-suppressed operators and so such epochs require a stringy completion for a consistent analysis. Perturbation growth in these stringy cosmologies is substantially enhanced compared to conventional cosmological histories. The transPlanckian field evolution results in radical changes to Standard Model couplings during this history and we outline potential applications to baryogenesis, dark matter and gravitational wave production.

We study runaway, kination-dominated epochs in string cosmology. We show how the apparent classical decompactification runaway of the volume modulus, described by a kination epoch in the 4-dimensional EFT, can be uplifted to a classical Kasner solution in 10d in which the non-compact dimensions collapse towards a Big Crunch. We study this for both single-modulus and multi-modulus examples. We also study related behaviour for compactifications to d≠3 space dimensions, finding an interesting singling out of d=3. We conclude with some comments on how this picture is modified by quantum effects, and the need for both dynamical and kinematical Swampland constraints.

Extensions of the Standard Model with an Abelian gauge group are constrained by gauge anomaly cancellation, so that only a limited number of possible charge assignments is allowed without the introduction of new chiral fermions. For flavor universal charges, couplings of the associated hidden photon to Standard Model fermions are flavor conserving at tree level. We show explicitly that even the flavor-specific charge assignments allowed by anomaly cancellation condition lead to flavor-conserving tree-level couplings of the hidden photon to quarks and charged leptons if the Cabibbo-KobayashiMaskawa or Pontecorvo-Maki-Nakagawa-Sakata matrix can be successfully reconstructed. Further, loopinduced flavor-changing couplings are strongly suppressed. As a consequence, the structure of the Majorana mass matrix is constrained and flavor-changing tree level couplings of the hidden photon to neutrino mass eigenstates are identified as a means to distinguish the U(1)B−L gauge boson from any other anomaly-free extension of the Standard Model without new chiral fermions. We present a comprehensive analysis of constraints and projections for future searches for a U(1)B−3Li gauge boson, calculate the reach of resonance searches in B meson decays and comment on the implications for nonstandard neutrino interactions.