Non-Gaussianity of the primordial perturbation in the curvaton model

We use the delta N -formalism to investigate the non-Gaussianity of the primordial curvature perturbation in the curvaton scenario for the origin of structure. We numerically calculate the full probability distribution function allowing for the non-instantaneous decay of the curvaton and compare this with analytic results derived in the sudden-decay approximation. We also present results for the leading-order contribution to the primordial bispectrum and trispectrum. In the sudden-decay approximation we derive a fully non-linear expression relating the primordial perturbation to the initial curvaton perturbation. As an example of how non-Gaussianity provides additional constraints on model parameters, we show how the primordial bispectrum on CMB scales can be used to constrain variance on much smaller scales in the curvaton field. Our analytical and numerical results allow for multiple tests of primordial non-Gaussianity, and thus they can offer consistency tests of the curvaton scenario.Comment: 16 pages, 6 figures. V2: minor typos corrected, references added. V3: minor changes to match better with the PRD versio

Detectors for leptonic CP violation at the neutrino factory

Studies carried out in the framework of the International Design Study for the Neutrino Factory (the IDS-NF) show that the sensitivity to the CP violating phase and the last unknown mixing angle θ13 is maximised when two far detectors optimized to detect the sub-leading νe to νμ oscillation are combined. Several technologies are being discussed for these detectors: magnetised iron calorimeters; giant liquid argon TPCs; and totally active scintillating detectors. The IDS-NF baseline option, a compromise between feasibility, cost, and performance, is documented in the Interim Design Report (IDR) that has recently been completed. It consists of two magnetised iron sampling calorimeters, similar to the existing MINOS detector, but with 10-20 times more mass and improved performance. A detector of mass 100 kton is assumed at the intermediate baseline (between 2500 km and 5000 km) and a 50 kton detector at the long baseline (between 7000 km and 8000 km). The other far-detector options, which have better granularity, may be able to detect additional oscillation channels, thus improving the overall performance of the facility. However, these options are likely to be more expensive and require significant R&D

Phase-plane analysis of Friedmann-Robertson-Walker cosmologies in Brans-Dicke gravity

We present an autonomous phase-plane describing the evolution of Friedmann-Robertson-Walker models containing a perfect fluid (with barotropic index gamma) in Brans-Dicke gravity (with Brans-Dicke parameter omega). We find self-similar fixed points corresponding to Nariai's power-law solutions for spatially flat models and curvature-scaling solutions for curved models. At infinite values of the phase-plane variables we recover O'Hanlon and Tupper's vacuum solutions for spatially flat models and the Milne universe for negative spatial curvature. We find conditions for the existence and stability of these critical points and describe the qualitative evolution in all regions of the (omega,gamma) parameter space for 0-3/2. We show that the condition for inflation in Brans-Dicke gravity is always stronger than the general relativistic condition, gamma<2/3.Comment: 24 pages, including 9 figures, LaTe

QCD uncertainties at the LHC and the implications of HERA

Strong interaction physics will be ubiquitous at the Large Hadron Collider since the colliding beams consist of confined quarks and gluons. Although the main purpose of the LHC is to study the mechanism of electroweak symmetry breaking and to search for physics beyond the Standard Model, to maximise the precision and sensitivity of such anaylses it is necessary to understand in detail various perturbative, semi-perturbative and non-perturbative QCD effects. Many of these effects have been extensively studied at HERA and will be studied further at HERA II. We discuss the impact of the knowledge thus gained on physics at the LHC.Comment: Contributed to the Proceedings of DIS04, Strbske Pleso, Slovaki

Cosmology with positive and negative exponential potentials

We present a phase-plane analysis of cosmologies containing a scalar field $\phi$ with an exponential potential $V \propto \exp(-\lambda \kappa \phi)$ where $\kappa^2 = 8\pi G$ and $V$ may be positive or negative. We show that power-law kinetic-potential scaling solutions only exist for sufficiently flat ($\lambda^26$) negative potentials. The latter correspond to a class of ever-expanding cosmologies with negative potential. However we show that these expanding solutions with a negative potential are to unstable in the presence of ordinary matter, spatial curvature or anisotropic shear, and generic solutions always recollapse to a singularity. Power-law kinetic-potential scaling solutions are the late-time attractor in a collapsing universe for steep negative potentials (the ekpyrotic scenario) and stable against matter, curvature or shear perturbations. Otherwise kinetic-dominated solutions are the attractor during collapse (the pre big bang scenario) and are only marginally stable with respect to anisotropic shear.Comment: 8 pages, latex with revtex, 9 figure

Slow-roll corrections to inflaton fluctuations on a brane

Quantum fluctuations of an inflaton field, slow-rolling during inflation are coupled to metric fluctuations. In conventional four dimensional cosmology one can calculate the effect of scalar metric perturbations as slow-roll corrections to the evolution of a massless free field in de Sitter spacetime. This gives the well-known first-order corrections to the field perturbations after horizon-exit. If inflaton fluctuations on a four dimensional brane embedded in a five dimensional bulk spacetime are studied to first-order in slow-roll then we recover the usual conserved curvature perturbation on super-horizon scales. But on small scales, at high energies, we find that the coupling to the bulk metric perturbations cannot be neglected, leading to a modified amplitude of vacuum oscillations on small scales. This is a large effect which casts doubt on the reliability of the usual calculation of inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure

Slow-roll corrections to inflaton fluctuations on a brane

Quantum fluctuations of an inflaton field, slow-rolling during inflation are coupled to metric fluctuations. In conventional four dimensional cosmology one can calculate the effect of scalar metric perturbations as slow-roll corrections to the evolution of a massless free field in de Sitter spacetime. This gives the well-known first-order corrections to the field perturbations after horizon-exit. If inflaton fluctuations on a four dimensional brane embedded in a five dimensional bulk spacetime are studied to first-order in slow-roll then we recover the usual conserved curvature perturbation on super-horizon scales. But on small scales, at high energies, we find that the coupling to the bulk metric perturbations cannot be neglected, leading to a modified amplitude of vacuum oscillations on small scales. This is a large effect which casts doubt on the reliability of the usual calculation of inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure

Singularities in loop quantum cosmology

We show that simple scalar field models can give rise to curvature singularities in the effective Friedmann dynamics of Loop Quantum Cosmology (LQC). We find singular solutions for spatially flat Friedmann-Robertson-Walker cosmologies with a canonical scalar field and a negative exponential potential, or with a phantom scalar field and a positive potential. While LQC avoids big bang or big rip type singularities, we find sudden singularities where the Hubble rate is bounded, but the Ricci curvature scalar diverges. We conclude that the effective equations of LQC are not in themselves sufficient to avoid the occurrence of singularities.Comment: 5 pages, 3 figures. v2: Comments and references added. v3: Minor additions, version to appear in PR

Statefinder parameters for quintom dark energy model

We perform in this paper a statefinder diagnostic to a dark energy model with two scalar fields, called "quintom", where one of the scalar fields has a canonical kinetic energy term and the other has a negative one. Several kinds of potentials are discussed. Our results show that the statefinder diagnostic can differentiate quintom model with other dark energy models.Comment: 11 pages, including 8 figures, added reference

The Coincidence Problem in Holographic f(R) Gravity

It is well-known that $f(R)$ gravity models formulated in Einstein conformal frame are equivalent to Einstein gravity together with a minimally coupled scalar field. In this case, the scalar field couples with the matter sector and the coupling term is given by the conformal factor. We apply the holographic principle to such interacting models. In a spatially flat universe, we show that the Einstein frame representation of $f(R)$ models leads to a constant ratio of energy densities of dark matter to dark energy.Comment: 10 pages, no figure