Mass Bounds on a Very Light Neutralino

TL;DR

The paper investigates how light or massless the lightest neutralino can be in the MSSM when gaugino masses are non-universal, showing that a massless neutralino remains viable with current experimental and observational data. It systematically analyzes collider bounds from LEP, precision electroweak observables, rare meson decays, and astrophysical/cosmological constraints, highlighting that collider limits often dominate and can be evaded with heavy sleptons. The study finds a hot DM upper bound around $m_{\tilde{\chi}^0_1} \lesssim 0.7$ eV and a cold DM lower bound in the few-to-tens of GeV range (roughly $6$–$13$ GeV depending on assumptions), implying that while a truly massless neutralino is allowed, cosmology prefers a nonzero mass in this window. Overall, the work shows that a massless neutralino is consistent with existing data under non-universal gaugino masses, and future high-precision collider measurements could further test these scenarios.

Abstract

Within the Minimal Supersymmetric Standard Model (MSSM) we systematically investigate the bounds on the mass of the lightest neutralino. We allow for non-universal gaugino masses and thus even consider massless neutralinos, while assuming in general that R-parity is conserved. Our main focus are laboratory constraints. We consider collider data, precision observables, and also rare meson decays to very light neutralinos. We then discuss the astrophysical and cosmological implications. We find that a massless neutralino is allowed by all existing experimental data and astrophysical and cosmological observations.

Figures (13)

• Figure 1: Bino admixture of $\tilde{\chi}^0_{1}$ (left plot) and masses of charginos and neutralinos (right plot) for $M_2=200$ GeV, $\tan\beta=10$, and $M_1$ as given in Eq. (\ref{['massless-neut-condition']}), such that $m_{\tilde{\chi}^0_{1}}=0$Dreiner:2007fw. Left of the vertical lines at $\mu\approx120$ GeV, the chargino mass is $m_{\tilde{\chi}_1^\pm}< 94$ GeV. In the right panel, the dotted line indicates the kinematic reach of LEP2 ($\sqrt s= 208$ GeV) for $e^+e^- \to \tilde{\chi}^0_{1}\tilde{\chi}^0_{i}$ production ($i = 2, 3, 4$), and the dashed line indicates the mass of the Z boson, $M_Z\approx 91$ GeV. Note that $\tilde{\chi}^0_2$ is nearly mass degenerate with $\tilde{\chi}_1^\pm$ for $\mu>120\,$GeV.
• Figure 2: (a) Contour lines in the $\mu$--$M_2$ plane of the neutralino mass $m_{\tilde{\chi}^0_{2}}$. In the grey shaded area the chargino mass is $m_{\tilde{\chi}_1^\pm}< 94$ GeV. The dashed line indicates the kinematical limit $m_{\tilde{\chi}^0_{2}}= \sqrt s=208$ GeV at LEP2. Throughout we have chosen $M_1$ such that $m_{\tilde{\chi}^0_{1}}=0$. The lightest chargino is nearly mass degenerate with $\tilde{\chi}^0_2$ for $M_2\hbox{$\;>$$\sim\;$}200\,\mathrm{GeV}$ and $\mu\hbox{$\;>$$\sim\;$}125\,\mathrm{GeV}$. (b)$95\%$ confidence limit on the cross section $\sigma(e^+e^-\to\tilde{\chi}^0_{1}\tilde{\chi}^0_{2})\times {\rm BR}(\tilde{\chi}^0_{2}\to Z\tilde{\chi}^0_{1})$ at $\sqrt s = 208$ GeV (taken from Ref. Abbiendi:2003sc, Fig. 10).
• Figure 3: Feynman diagrams for neutralino production $e^{+}e^{-}\to\tilde{\chi}^0_i\tilde{\chi}^0_j$ .
• Figure 4: (a) Contour lines in the $\mu$--$M_2$ plane of the neutralino production cross section $\sigma(e^+e^-\to\tilde{\chi}^0_{1} \tilde{\chi}^0_{2})$ with $\tan\beta=10$, and $m_{\tilde{e}_R}=m_{\tilde{e}_L}= m_{\tilde{e}}=200$ GeV, at $\sqrt s = 208$ GeV. At each point, $M_1$ is chosen such that $m_{\tilde{\chi}^0_{1}}=0$. (b) Contour lines in the $\mu$--$M_2$ plane of the lower bounds on the selectron mass $m_{\tilde{e}_R}=m_{\tilde{e}_L}=m_ {\tilde{e}}$, such that $\sigma(e^+e^-\to\tilde{\chi}^0_{1}\tilde{\chi}^0_{2})=70$ fb for $m_{\tilde{\chi}^0_{1}}=0$ with $\tan\beta=10$. In (a), (b), the dashed lines indicate the kinematical limit $m_{\tilde{\chi}^0_{2}}= \sqrt s=208$ GeV, in the grey shaded areas the chargino mass is $m_{\tilde{\chi}_1^\pm}< 94$ GeV. Along the dot-dashed contour in (b) the relation $m_{\tilde{e}}=m_{\tilde{\chi}^0_{2}}$ holds.
• Figure 5: The difference of the experimental value and the theory prediction for the invisible $Z$ width, $\delta\Gamma_{\rm inv}$, (upper plot) and the total $Z$ width, $\delta\Gamma_Z$, (lower plot) in the $\mu$--$M_2$-plane, both including the process $Z\to\tilde{\chi}^0_{1}\tilde{\chi}^0_{1}$. Deviations of the theory predictions from the experimental data are indicated as $\delta\Gamma_{\textup{inv}}\equiv(\Gamma_{\textup{inv}}-\Gamma^{ \textup{exp}}_{\textup{inv}}) = (10,5,3,2,1)\times\sigma^{\textup{ exp}}_{\Gamma_{\textup{inv}}}$ (upper plot) and $\delta\Gamma_{Z}\equiv(\Gamma _{Z}-\Gamma^{\textup{exp}}_{Z}) = (20,10,3,2,1,0)\times\sigma^{\textup{exp}}_{\Gamma_Z}$ (lower plot) contours. The SUSY parameters were fixed as $\tan \beta=10$, $M_{\rm SUSY} = 250\,\, \mathrm{GeV}, A_{\tau}=A_t=A_b=m_{\tilde{g}}=M_A=500 \,\, \mathrm{GeV}$. For $M_1$ we use Eq. (\ref{['massless-neut-condition']}) (see text).