Gluino coannihilation revisited

Some variants of the MSSM feature a strip in parameter space where the lightest neutralino χ is identified as the lightest supersymmetric particle (LSP), the gluino g ˜ $$ \tilde{g} $$ is the next-to-lightest supersymmetric particle (NLSP) and is nearly degenerate with χ, and the relic cold dark matter density is brought into the range allowed by astrophysics and cosmology by coannihilation with the gluino NLSP. We calculate the relic density along this gluino coannihilation strip in the MSSM, including the effects of gluino-gluino bound states and initial-state Sommerfeld enhancement, and taking into account the decoupling of the gluino and LSP densities that occurs for large values of the squark mass m q ˜ $$ {m}_{\tilde{q}} $$ . We find that bound-state effects can increase the maximum m χ for which the relic cold dark matter density lies within the range favoured by astrophysics and cosmology by as much as ∼ 50% if m q ˜ / m g ˜ = 1.1 $$ {m}_{\tilde{q}}/{m}_{\tilde{g}}=1.1 $$ , and that the LSP may weigh up to ∼ 8 TeV for a wide range of m q ˜ / m g ˜ ≲ 100 $$ {m}_{\tilde{q}}/{m}_{\tilde{g}}\lesssim 100 $$