Radiative Decay of a Long-Lived Particle and Big-Bang Nucleosynthesis

TL;DR

This work investigates how radiatively decaying, long-lived particles alter big-bang nucleosynthesis via electromagnetic cascades and photodissociation. By computing the resulting photon spectra and solving a modified BBN network, the authors derive constraints on the particle's abundance and lifetime, considering two distinct $^4$He priors and incorporating $^7$Li and $^6$Li constraints. They map these constraints onto concrete models (gravitino, MSSM neutralino, modulus) to translate into limits on reheating temperature $T_R$, gravitino mass $m_{3/2}$, and modulus amplitude $\phi_0$, with implications for early-universe cosmology. The results reveal regimes where nonzero $m_X Y_X$ improves fit (low $^4$He) and regimes where standard BBN remains viable (high $^4$He), and show potential $^6$Li enhancement from photodissociation, underscoring the role of BBN as a probe of physics beyond the standard model.

Abstract

The effects of radiatively decaying, long-lived particles on big-bang nucleosynthesis (BBN) are discussed. If high-energy photons are emitted after BBN, they may change the abundances of the light elements through photodissociation processes, which may result in a significant discrepancy between the BBN theory and observation. We calculate the abundances of the light elements, including the effects of photodissociation induced by a radiatively decaying particle, but neglecting the hadronic branching ratio. Using these calculated abundances, we derive a constraint on such particles by comparing our theoretical results with observations. Taking into account the recent controversies regarding the observations of the light-element abundances, we derive constraints for various combinations of the measurements. We also discuss several models which predict such radiatively decaying particles, and we derive constraints on such models.

Figures (18)

• Figure 1: SBBN prediction of the abundances of the light elements. The solid lines are the central values of the predictions, and the dotted lines represents the 1-$\sigma$ uncertainties. The boxes denote the 1-$\sigma$ observational constraints.
• Figure 2: $\chi^2$ as a function of $\eta$, for SBBN with three degrees of freedom $(\eta, \tau_X, m_X Y_X)$. We show our results for both of the $^4$He abundances deduced from observation: low $^4$He (dashed), high $^4$He (solid).
• Figure 3: Figure 3: C.L. for BBN as a function of $\eta$ and $N_\nu$, with (a) low value of $Y$, and (b) high value of $Y$. The filled square denotes the best-fit point.
• Figure 4: Photon spectrum $f_\gamma = dn_\gamma/dE_\gamma$ for several background temperatures $T_{\gamma}^{\rm BG}$.
• Figure 5: The abundance of D/H in the $m_X Y_X$ vs. $\tau_X$ plane with (a) $\eta=2\times 10^{-10}$, (b) $\eta=4\times 10^{-10}$, (c) $\eta=5\times 10^{-10}$, and (d) $\eta=6\times 10^{-10}$.