Observational constraints on assisted k-inflation

Abstract

We study observational constraints on the assisted k-inflation models in which multiple scalar fields join an attractor characterized by an effective single field $\phi$. This effective single-field system is described by the Lagrangian $P=Xg(Y)$, where $X$ is the kinetic energy of $\phi$, $\lambda$ is a constant, and $g$ is an arbitrary function in terms of $Y=X e^{\lambda \phi}$. Our analysis covers a wide variety of k-inflation models such as dilatonic ghost condensate, Dirac-Born-Infeld (DBI) field, tachyon, as well as the canonical field with an exponential potential. We place observational bounds on the parameters of each model from the WMAP 7yr data combined with Baryon Acoustic Oscillations (BAO) and the Hubble constant measurement. Using the observational constraints of the equilateral non-Gaussianity parameter $f_{NL}^{equil}$, we further restrict the allowed parameter space of dilatonic ghost condensate and DBI models. We extend the analysis to more general models with several different choices of $g(Y)$ and show that the models such as $g(Y)=c_0+c_p Y^p$ ($p \ge 3$) are excluded by the joint data analysis of the scalar/tensor spectra and primordial non-Gaussianities.