Neutralino Relic Density including Coannihilations
J. Edsjo, P. Gondolo
TL;DR
This work provides a complete and precise calculation of the neutralino relic density in the MSSM by including all coannihilations between neutralinos and charginos for any mass and composition, and by employing exact cross-section integration with proper Boltzmann averaging, thresholds, resonances, and one-loop mass corrections. The authors introduce an effective annihilation framework with $W_{\rm eff}$ and $\langle \sigma_{\rm eff} v \rangle$ to streamline the multi-species dynamics into a single Boltzmann equation, enabling robust relic density predictions across the MSSM parameter space. They demonstrate that coannihilations are significant beyond light higgsino scenarios (including heavy higgsinos and mixed/gaugino-like neutralinos) and establish a condition $|\mu| \lesssim 2|M_1|$ for when coannihilations impact the relic density. A notable outcome is the persistence of a viable window for light higgsino-like dark matter and a cosmological upper bound shift on the neutralino mass from $3$ TeV to $7$ TeV, with important implications for dark matter searches and MSSM phenomenology.
Abstract
We evaluate the relic density of the lightest neutralino, the lightest supersymmetric particle, in the Minimal Supersymmetric extension of the Standard Model (MSSM). For the first time, we include all coannihilation processes between neutralinos and charginos for any neutralino mass and composition. We use the most sophisticated routines for integrating the cross sections and the Boltzmann equation. We properly treat (sub)threshold and resonant annihilations. We also include one-loop corrections to neutralino masses. We find that coannihilation processes are important not only for light higgsino-like neutralinos, as pointed out before, but also for heavy higgsinos and for mixed and gaugino-like neutralinos. Indeed, coannihilations should be included whenever $|μ| \lsim 2 |M_1|$, independently of the neutralino composition. When $|μ| \sim |M_1|$, coannihilations can increase or decrease the relic density in and out of the cosmologically interesting region. We find that there is still a window of light higgsino-like neutralinos that are viable dark matter candidates and that coannihilations shift the cosmological upper bound on the neutralino mass from 3 to 7 TeV.