Revisiting Big-Bang Nucleosynthesis Constraints on Long-Lived Decaying Particles

Abstract: We study effects of long-lived massive particles, which decay during the big-bang nucleosynthesis (BBN) epoch, on the primordial abundances of light elements. Compared to the previous studies, (i) the reaction rates of the standard BBN reactions are updated, (ii) the most recent observational data of light element abundances and cosmological parameters are used, (iii) the effects of the interconversion of energetic nucleons at the time of inelastic scatterings with background nuclei are considered, and (iv) the effects of the hadronic shower induced by energetic high energy anti-nucleons are included. We compare the theoretical predictions on the primordial abundances of light elements with latest observational constraints, and derive upper bounds on relic abundance of the decaying particle as a function of its lifetime. We also apply our analysis to unstable gravitino, the superpartner of the graviton in supersymmetric theories, and obtain constraints on the reheating temperature after inflation.

• The paper revisits BBN by updating reaction rates and light element measurements, refining predictions on the effects of decaying particles.
• It models interactions of energetic nucleons with background nuclei, assessing both hadronic and electromagnetic decay channels.
• The study constrains supersymmetric models by establishing stringent bounds on decaying particle yields and lifetimes during the BBN epoch.

Overview of Big-Bang Nucleosynthesis Constraints on Long-Lived Decaying Particles

The paper "Revisiting Big-Bang Nucleosynthesis Constraints on Long-Lived Decaying Particles" addresses the impact of long-lived decaying particles on the primordial abundances of light elements during the Big-Bang Nucleosynthesis (BBN) epoch. It revisits and updates constraints on such particles with an emphasis on those that decay during BBN, considering the effects on primordial deuterium (D), helium-3 ($^3$He), helium-4 ($^4$He), lithium-6 ($^6$Li), and lithium-7 ($^7$Li) abundances.

The study incorporates several significant updates over previous analyses. These include revised rates for standard BBN reactions, improved observational data on light element abundances, and updated cosmological parameters. Additionally, it assesses the interaction of emitted energetic nucleons with background nuclei, considering hadronic showers induced by high-energy anti-nucleons.

Methodology and Key Findings

The analysis commences by introducing enhancements in the theoretical treatment of particle decay. Amended reaction rates and updated observational constraints for primordial light elements form the backbone of the calculation, offering more precise predictions than previous models. Furthermore, the effects of interconversion of nucleons during inelastic scatterings and the resulting hadronic showers are meticulously modeled. These interactions often lead to photodissociation and hadrodissociation phenomena where background nuclei are broken apart by high-energy particles.

Primary findings and constraints are exhibited on $m_X Y_X$ vs. $\tau_X$ planes, where $m_X$ is the particle mass and $Y_X$ denotes the particle yield. The constraints are profoundly dependent on final decay products, with distinct decay channels like $e^{+}e^{-}$ and $b\bar{b}$ exhibiting variation in influence due to their electromagnetic and hadronic interactions, respectively.

Stringent bounds on the abundance and lifetime of decaying particles are inferred, especially when potential decay modes include hadronic components. For instance, a decay into $b\bar{b}$ places severe limits due to the copious production of hadronic showers. Conversely, channels such as $e^+e^-$ primarily introduce electromagnetic cascades, impacting light elements predominantly via photodissociation.

The implications of these findings extend to constraints on supersymmetric models, specifically in relation to the role of unstable gravitinos—superpartners to gravitons—as potential decaying relics. Applying the derived constraints, the study delineates upper bounds on reheating temperatures post-inflation to mitigate excessive gravitino production, thereby informing cosmological models incorporating supersymmetric physics.

Implications and Speculations

This paper stands as a critical re-examination and refinement of the role of long-lived decaying particles in early cosmological epochs. Practically, the derived constraints not only sharpen our understanding of particle physics in cosmological contexts but also extend implications to supersymmetry and dark matter models, wherein relic particle abundances critically hinge upon such BBN-consistent constraints.

Theoretically, the results inform our understanding of the interplay between microphysical processes and macroscopic cosmological structures, heralding a more detailed comprehension of cosmic evolution. The introduction of recent advances in reaction modeling and observational analysis fortifies the precision of these constraints.

Speculation on future developments, particularly under advancements in observational technology and theoretical models of particle physics, could further tighten these constraints, offering deeper insights into the physics governing the nascent universe. The study's rigorous approach and comprehensive modeling strategies underscore the intricate dynamics occurring in the primordial universe, paving avenues for subsequent longitudinal studies in cosmic microwave background observations and particle cosmology.

In conclusion, this research signifies a pivotal step in connecting theoretical particle physics with observable cosmic phenomena, underscoring the sustained intersection of these domains to enhance our cumulative understanding of universe formation and evolution.