New constraints on inflation from the cosmic microwave background

The recent data from the $\mathrm{BOOMERANG}$ and $\mathrm{MAXIMA}-1\mathrm{}$ balloon flights have marked the beginning of the precision era of cosmic microwave background (CMB) anisotropy measurements. We investigate the observational constraints from the current CMB anisotropy measurements on the simplest inflation models, characterized by a single scalar field $\phi ,$ in the parameter space consisting of scalar spectral index $n_S$ and tensor/scalar ratio r. If we include constraints on the baryon density from big bang nucleosynthesis (BBN), we show that the favored inflationary models have a negligible tensor amplitude and a red tilt, with a best fit of $n_S\simeq 0.93,$ which is consistent with the simplest small-field inflation models, but rules out large-field models at the $1\sigma$ level. Without including BBN constraints, a broader range of models is consistent with the data. The best fit (assuming negligible reionization) is a scale-invariant spectrum, $n_S\simeq 1,$ which includes large-field and hybrid scenarios. Large-field models (such as those typical of the chaotic inflation scenario) with a tilt $n_S<0.9\mathrm{}$ are strongly disfavored in all cases.