Lightest Neutralino in Extensions of the MSSM

TL;DR

The paper investigates how extensions of the MSSM that dynamically generate the μ-term—via Higgs singlets and optionally a new U(1)'—alter the lightest neutralino's mass and composition. By analyzing NMSSM, nMSSM, UMSSM, and the S-model, it shows that the LSP can be singlino-dominated or even massless in some limits, and that its viability is tightly constrained by LEP/Tevatron data, relic density measurements, and the muon g-2 anomaly. The work highlights how Z' physics and singlet dynamics modify annihilation channels, mass bounds, and couplings relative to the MSSM, with the extended models forming a hierarchy MSSM ⊂ nMSSM ⊂ UMSSM ⊂ S-model. These distinctions provide potential collider signatures to distinguish MSSM realizations and emphasize the importance of precise measurements of the LSP mass and composition for testing beyond-MSSM scenarios.

Abstract

We study neutralino sectors in extensions of the MSSM that dynamically generate the mu-term. The extra neutralino states are superpartners of the Higgs singlets and/or additional gauge bosons. The extended models may have distinct lightest neutralino properties which can have important influences on their phenomenology. We consider constraints on the lightest neutralino from LEP, Tevatron, and (g-2)_mu measurements and the relic density of the dark matter. The lightest neutralino can be extremely light and/or dominated by its singlino component which does not couple directly to SM particles except Higgs doublets.

Figures (5)

• Figure 1: Scatter plots of (a) the $\chi^0_1$ mass ($M_{\chi_1^0}$) versus $\tan\beta$ and (b) the $\chi^0_1$ singlino composition ($|N_{15}|^2$) versus $M_{\chi_1^0}$ for various models. The crosses in (b) represent the $Z'$-ino composition ($|N_{16}|^2$) for the UMSSM. The direct constraints of Section \ref{['direct']} are imposed. The solid curves represent a fixed set of inputs $M_2 = 250$ GeV, $\mu = 250$ GeV, $s = 500$ GeV and $\kappa = 0.5$ (for the NMSSM) without the direct constraints. The upper (lower) UMSSM singlino band in (b) corresponds approximately to moderate (large) value of $s$, with the lightest neutralino being MSSM-like for large $s$.
• Figure 2: (a) Similar plot as Figure \ref{['fig:tanb']}(a) that satisfy the $2.4\sigma$ allowed $(g-2)_\mu$ measurement with $m_L = m_E = 100$ GeV (dots) and $500$ (GeV) (squares). (b) $M_{\chi^0_1}$ versus $M_2$ with extended ranges of $M_2$ and $\mu$ up to $1000$ GeV. The solid curves are for $\mu = 250$ GeV, $s = 500$ GeV, $\tan\beta = 2$ and $\kappa = 0.5$.
• Figure 3: (a) Illustration of the massless neutralinos (dark curve) and nearby light neutralinos ($M_{\chi^0_1} < 1$ GeV) (orange region) in the $M_2$-$\mu$ plane along with constraints from $\Gamma_Z$, $M_{\chi^\pm_1}$ and $h_s$ in the nMSSM or the S-model in the decoupling limit. Here $\tan\beta = 10$ and $s = 700$ GeV are assumed. The gap in the $\Gamma_Z$ exclusion region is due to the emergence of a very light gaugino for $M_{1, 2} \sim 0$. (b) The relic density for the nMSSM or the S-model with Z-pole annihilation. The solid curve is for a fixed set of values of $\mu = 200$ GeV, $s=400$ GeV and $\tan\beta = 1.5$ with $M_2$ varying over $0 \sim 500$ GeV.
• Figure 4: Scatter plots of (a) $M_{\chi^0_1}$ and (b) $|N_{15}|^2$ versus $s$. The solid curves are for $M_2 = 250$ GeV, $\mu = 250$ GeV, $\tan\beta = 2$ and $\kappa = 0.5$. The $M_{Z'}$ bound $s> 1500$ GeV is approximate and can be evaded, as discussed in Section \ref{["sec:Z'mass"]}. The crosses in (b) are the $Z'$-ino composition ($|N_{16}|^2$) in the UMSSM.
• Figure 5: (a) An illustration of a common solution of the $\Omega_{\chi^0_1} h^2$ (dark dots, $3 \sigma$ of WMAP+SDSS) and the $(g-2)_\mu$ (cyan region, $95\%$ C.L. of E821) in the $M_2$-$\mu$ plane along with other constraints in the nMSSM or the S-model. The $Z$-pole annihilation and $m_L = m_E = 100$ GeV are used. The gap in the $\Gamma_Z$ exclusion region is where $|N_{13}|^2 \approx |N_{14}|^2$. (b) The allowed mass range of $\chi^0_1$ after applying direct constraints (model parameters, gaugino mass unification $M_{1'} = M_1 \simeq 0.5 M_2$, $M_{\chi^\pm_1}$, $\Delta \Gamma_Z$) and indirect constraints ($(g-2)_\mu$, $\Omega_{\chi^0_1} h^2$, $M_{Z'}$, domain wall). The $M_{\chi^0_1}$ bounds in the nMSSM are intended to be illustrative and are not necessarily quantitatively precise.