Stop Coannihilation in the CMSSM and SubGUT Models

TL;DR

This paper investigates stop coannihilation in the CMSSM and sub-GUT models, highlighting how large A-terms and Higgs mass constraints influence the viable parameter space for supersymmetric dark matter.

Contribution

It provides a detailed analysis of stop coannihilation effects, including Goldstone mode annihilations and Sommerfeld enhancement, within the CMSSM and sub-GUT frameworks, considering Higgs mass constraints.

Findings

Large A-terms significantly split stop masses along the coannihilation strip.

Higgs mass constraints limit the extent of the coannihilation strip for certain parameter signs.

Sub-GUT models allow for heavier dark matter particles, exceeding 8 TeV.

Abstract

Stop coannihilation may bring the relic density of heavy supersymmetric dark matter particles into the range allowed by cosmology. The efficiency of this process is enhanced by stop-antistop annihilations into the longitudinal (Goldstone) modes of the $W$ and $Z$ bosons, as well as by Sommerfeld enhancement of stop annihilations and the effects of bound states. Since the couplings of the stops to the Goldstone modes are proportional to the trilinear soft supersymmetry-breaking $A$-terms, these annihilations are enhanced when the $A$-terms are large. However, the Higgs mass may be reduced below the measured value if the $A$-terms are too large. Unfortunately, the interpretation of this constraint on the stop coannihilation strip is clouded by differences between the available Higgs mass calculators. For our study, we use as our default calculator FeynHiggs 2.13.0, the most recent publicly available version of this code. Exploring the CMSSM parameter space, we find that along the stop coannihilation strip the masses of the stops are severely split by the large $A$-terms. This suppresses the Higgs mass drastically for $\mu$ and $A_0 > 0$, whilst the extent of the stop coannihilation strip is limited for $A_0 < 0$ and either sign of $\mu$. However, in sub-GUT models, reduced renormalization-group running mitigates the effect of the large $A$-terms, allowing larger LSP masses to be consistent with the Higgs mass calculation. We give examples where the dark matter particle mass may reach $\gtrsim 8$ TeV.