A supersymmetric solution to the KARMEN time anomaly

TL;DR

The paper investigates whether the KARMEN time anomaly can be explained by a very light neutralino LSP with mass $m_{ ilde{\ chi}^0_1}=33.9$ MeV, produced in $\'{ olinebreak π}^+$ decays via RPV coupling and decaying to a three-lepton final state through RPV operators. By allowing non-universal gaugino masses (independent $M_1$ and $M_2$), the authors identify regions in MSSM parameter space where a bino-dominated LSP with small Higgsino admixture is consistent with LEP, Z-width, oblique corrections, and cosmological bounds. They quantify the required RPV couplings and lifetimes, showing a viable but finely-tuned solution, and outline future tests at $e^+e^-$ colliders and upgraded KARMEN capabilities to discriminate this scenario from alternatives. The work highlights that a light neutralino LSP is not automatically excluded by accelerator data without the GUT gaugino-mass relation, and discusses broader RPV phenomenology and potential neutrino-mass implications.

Abstract

We interpret the KARMEN time anomaly as being due to the production of a (dominantly bino) neutralino with mass 33.9 MeV, which is the lightest supersymmetric particle but decays into 3 leptons through the violation of R-parity. For independent gaugino masses M_1 and M_2 we find regions in the (M_1, M_2, mu, tan beta) parameter space where such a light neutralino is consistent with all experiments. Future tests of this hypothesis are outlined.

Figures (7)

• Figure 1: Tree-level Feynman diagrams for pion decay via the operator $L_2Q_1D^c_1$.
• Figure 2: The solutions to the KARMEN anomaly in terms of the anomalous pion branching ratio and the $x$-particle lifetime. The hashed area denotes the experimental upper bound (\ref{['psi']}).
• Figure 3: Solutions to the KARMEN anomaly in terms of the R-parity violating couplings $\lambda'_{211}L_2Q_1D^c_1$ and $\lambda_{1\{2,3\}1}$, for different (assumed degenerate) sfermion masses. The hashed lines indicate upper limits on the couplings from perturbativity. The stars and diamonds (squares) give the upper limits on the couplings $\lambda'_{211}$ and $\lambda_{121}$ ($\lambda_ {131}$), respectively. Solutions above and to the left of the stars are excluded, as are solutions below and to the right of the squares (diamonds).
• Figure 4: Cross-section for for the production of a purely bino neutralino with mass 33.9 MeV through $e^+e^-\rightarrow\tilde{\chi}^0_1\tilde{\chi}^ 0_1\gamma$.
• Figure 5: Solutions in $(M_1,M_2,\mu,\tan\beta)$ parameter space giving a $m_{\chi^0_1}=33.9$ MeV neutralino for $\mu=300$ GeV and 2 representative values of $\tan\beta$. The width of the lines is 0.01 MeV. Below the hashed lines the chargino mass is less than 150 GeV. The dotted lines have $\Delta\rho_{\rm SUSY}<10^{-4}$ and the solid lines have $\Delta\rho_{\rm SUSY}<5 \times 10^{-4}$.