Evgeny I. Buchbinder

Non-Gaussianities in New Ekpyrotic Cosmology.

The new ekpyrotic model is an alternative scenario of the early Universe which relies on a phase of slow contraction before the big bang. We calculate the 3-point and 4-point correlation functions of primordial density perturbations and find a generically large non-Gaussian signal, just below the current sensitivity level of cosmic microwave background experiments. This is in contrast with slow-roll inflation, which predicts negligible non-Gaussianity. The model is also distinguishable from alternative inflationary scenarios that can yield large non-Gaussianity, such as Dirac-Born-Infeld inflation and the simplest curvatonlike models, through the shape dependence of the correlation functions. Non-Gaussianity therefore provides a distinguishing and testable prediction of New Ekpyrotic Cosmology.

Raising Anti de Sitter vacua to de Sitter vacua in heterotic M-theory

We explore the possibility of obtaining de Sitter vacua in strongly coupled heterotic models by adding various corrections to the supergravity potential energy. We show that, in a generic compactification scenario, Fayet-Iliopoulos terms can generate a de Sitter vacuum. The cosmological constant in this vacuum can be fine tuned to be consistent with observation. We also study moduli potentials in nonsupersymmetric compactifications of E{sub 8}xE{sub 8} theory with anti five-branes and E{sub 8}xE{sub 8} theory. We argue that they can be used to create a de Sitter vacuum only if some of the Kahler structure moduli are stabilized at values much less than the Calabi-Yau scale.

Fate of sound and diffusion in a holographic magnetic field

It was shown by Buchbinder, Buchel, and Vazquez [J. High Energy Phys. 12 (2008) 090] that, in the presence of the magnetic field, the sound waves in (2 + 1)-dimensional plasma disappear and are replaced by a diffusive mode. Similarly, the shear and charge diffusion fluctuations form a subdiffusive mode. However, since the limit of a small magnetic field does not commute with the hydrodynamic limit, it is not obvious whether or not these modes are stable under higher order corrections. Using AdS/CFT correspondence we show that, in the case of the M2-brane plasma, these modes do exist as we find the corresponding supergravity solutions. This allowed us to compute the conductivity and the shear viscosity to all orders in a magnetic field. We find that the viscosity to entropy ratio saturates the Kovtun-Son-Starinets bound. This extends the universality property of the shear viscosity to the case of the strongly coupled plasma in an external magnetic field.

A heterotic standard model with B − L symmetry and a stable proton

A bstractWe consider heterotic Calabi-Yau compactifications with S(U(4) × U(1)) background gauge fields. These models lead to gauge groups with an additional U(1) factor which, under certain conditions, can combine with hypercharge to a B − L symmetry. The associated gauge boson is automatically super-massive and, hence, does not constitute a phenomenological problem. We illustrate this class of compactifications with a model based on the monad construction, which leads to a supersymmetric standard model with three families of quarks and leptons, one pair of Higgs doublets, three right-handed neutrinos and no exotics charged under the standard model group. The presence of the B − L symmetry means that the model is safe from proton decay induced by dimension four operators. Due to the presence of a special locus in moduli space where the bundle structure group is Abelian and the low-energy symmetry enhances we can also show the absence of dimension five proton-decay inducing operators.

Three-form multiplet and supersymmetry breaking

A bstractRecently, a nilpotent real scalar superfield V was introduced in [1] as a model for the Goldstino. It contains only two independent component fields, the Goldstino and the auxiliary D-field. Here we first show that V can equivalently be realised as a constrained three-form superfield. We demonstrate that every irreducible Goldstino superfield (of which the Goldstino is the only independent component field) can be realised as a descendant of V which is invariant under local rescalings V → eτV, where τ is an arbitrary real scalar superfield. We next propose a new Goldstino supermultiplet which is described by a nilpotent three-form superfield Y

Superconformal field theory in three dimensions: Correlation functions of conserved currents

\mathcal{Y}

Holomorphic Yukawa couplings in heterotic string theory

that is a variant formulation for the nilpotent chiral superfield, which is often used in off-shell models for spontaneously broken supergravity. It is shown that the action describing the dynamics of Y