Abstract
Many extensions of the Standard Model predict large numbers of additional unstable particles whose decays in the early universe are tightly constrained by observational data. For example, the decays of such particles can alter the ratios of light-element abundances, give rise to distortions in the cosmic microwave background, alter the ionization history of the universe, and contribute to the diffuse photon flux. Constraints on new physics from such considerations are typically derived for a single unstable particle species with a single well-defined mass and characteristic lifetime. In this paper, by contrast, we investigate the cosmological constraints on theories involving entire ensembles of decaying particles --- ensembles which span potentially broad ranges of masses and lifetimes. In addition to providing a detailed numerical analysis of these constraints, we also formulate a set of simple analytic approximations for these constraints which may be applied to generic ensembles of unstable particles which decay into electromagnetically-interacting final states. We then illustrate how these analytic approximations can be used to constrain a variety of toy scenarios for physics beyond the Standard Model. For ease of reference, we also compile our results in the form of a table which can be consulted independently of the rest of the paper. It is thus our hope that this work might serve as a useful reference for future model-builders concerned with cosmological constraints on decaying particles, regardless of the particular model under study.
Highlights
Many proposals for physics beyond the Standard Model (SM) predict the existence of additional unstable particles
We review the corresponding constraints on a single unstable particle species [3]—constraints derived from a numerical analysis of the coupled system of Boltzmann equations which govern the evolution of these abundances
In order to assess whether a particular injection history is consistent with the constraints discussed in the previous section, we must evaluate the overall change δYaðtÞ ≡ YaðtÞ − Yianit in the comoving number density Ya of a given nucleus at time t 1⁄4 tnow, where Yianit denotes the initial value of Ya at the end of big bang nucleosynthesis (BBN)
Summary
Many proposals for physics beyond the Standard Model (SM) predict the existence of additional unstable particles. During and after big bang nucleosynthesis (BBN) [1–5], give rise to spectral distortions in the cosmic microwave background (CMB) [6,7], alter the ionization history of the Universe [8–12], and give rise to characteristic features in the diffuse photon background These considerations place stringent constraints on models for new physics involving unstable particles. Computational methods can certainly yield useful results in any particular individual case Another complementary approach which can provide additional physical insight into the underlying dynamics involves the formulation of approximate analytic expressions for the relevant observables—expressions analogous to those which already exist for a single particle species decaying in isolation. Our aim in this paper is to derive such a set of analytic expressions—expressions which are applicable to generic theories involving large numbers of unstable particles, but which provide accurate approximations for the relevant cosmological observables.
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