Lower limit on the neutralino mass in the general MSSM

TL;DR

The paper investigates the minimal neutralino mass in the general MSSM with Rp conservation and nonuniversal gaugino masses. By integrating LEP limits, WMAP relic-density constraints, and indirect observables such as $(g-2)_\mu$, $b\to s\gamma$, and $B_s\rightarrow \mu^+\mu^-$, it maps the viable parameter space for light neutralinos across heavy and light pseudoscalar Higgs regimes, showing that $m_{\tilde{\chi}_1^0}$ can be as low as about 6 GeV when $M_A$ is light and $\tan\beta$ is large, but is typically above 18–29 GeV for a heavy $M_A$. Lowering $M_A$ opens additional regions where annihilation via Higgs exchange lowers the relic density, enhancing prospects for Tevatron Higgs searches and direct detection; a 500 GeV $e^+e^-$ collider could still produce SUSY particles in many such models. The work highlights how relaxing gaugino-unification expands possible LSP masses and alters collider phenomenology relative to mSUGRA, with important implications for upcoming experiments.

Abstract

We discuss constraints on SUSY models with non-unified gaugino masses and R_P conservation. We derive a lower bound on the neutralino mass combining the direct limits from LEP, the indirect limits from gmuon, bsgamma, Bsmumu and the relic density constraint from WMAP. The lightest neutralino (mneutralino=6GeV) is found in models with a light pseudoscalar with MA<200GeV and a large value for $tanβ$. Models with heavy pseudoscalars lead to mneutralino>18(29)GeV for $\tanβ=50(10)$. We show that even a very conservative bound from the muon anomalous magnetic moment can increase the lower bound on the neutralino mass in models with mu<0 and/or large values of $\tanβ$. We then examine the potential of the Tevatron and the direct detection experiments to probe the SUSY models with the lightest neutralinos allowed in the context of light pseudoscalars with high $\tanβ$. We also examine the potential of an e+e- collider of 500GeV to produce SUSY particles in all models with neutralinos lighter than the W. In contrast to the mSUGRA models, observation of at least one sparticle is not always guaranteed.

Figures (16)

• Figure 1: a) Relic density of the LSP vs $m_{\tilde{\chi}_1^0}$ in the MSSM with $M_0=500$GeV. To guide the eye the WMAP upper bound is displayed. b) Values of $\mu$ consistent with the LEP and relic density constraints. We scan on $\tan\beta,M_2,r_{12}$ and $\mu$.
• Figure 2: Lower limit on $m_{\tilde{\chi}_1^0}$ vs $r_{12}$ after imposing the relic density constraints. The limit from LEP2 alone is also displayed. Here $M_0=500$ GeV, $\mu>0$ and we scan on $M_2,\tan\beta,|\mu|$. The region below the lines is excluded.
• Figure 3: Region allowed after imposing LEP and relic density constraints for $\tan\beta=10$, $r_{12}=0.1,\mu>0$ and $M_A=1$TeV, a) in the $M_0-m_{\tilde{\chi}_1^0}$ plane b) in the $\mu-M_0$ plane. Here we scan on $M_0,M_2,\mu$ and $A_t$. Circles (crosses) have $\Omega h^2 < 0.3 (0.128)$. Half the points in the scan are generated in the region corresponding to $M_0,\mu<150$ GeV.
• Figure 4: Lower bound on the neutralino mass vs $r_{12}=M_1/M_2$ for $\mu>0$ and a) $\tan\beta=10$, b) $\tan\beta=50$. All direct and precision measurements constraints are combined with the LEP limits. We scan over $M_2,\mu,M_0$ and $A_t$.
• Figure 5: $\delta a_\mu$ in models where $\mu<0$, $r_{12}=0.1$ with $\tan\beta=10,50$ and a-c) $M_0=150$GeV d) $M_0=1000$GeV. The constraint on $\delta a_\mu$ selects all models above the thick line. Here only LEP constraints are imposed.