Search for chargino and neutralino production at $\sqrt{s} = 181$–184 GeV at LEP

Abstract.

A search for charginos and neutralinos, predicted by supersymmetric theories, has been performed using a data sample of 57 pb$^{-1}$ at centre-of-mass energies of 181–184 GeV taken with the OPAL detector at LEP. No evidence for chargino or neutralino production has been found. Upper limits on chargino and neutralino pair production ($\tilde\chi^+_1 \tilde\chi^-_1$, $\tilde\chi^0_1 \tilde\chi^0_2$) cross-sections are obtained as a function of the chargino mass ($m_{\tilde\chi^\pm_1}$), the lightest neutralino mass ($m_{\tilde\chi^0_1}$) and the second lightest neutralino mass ($m_{\tilde\chi^0_2}$). For large chargino masses the limits have been improved with respect to the previous analyses at lower centre-of-mass energies. Exclusion regions at 95% confidence level (C.L.) of parameters of the Constrained Minimal Supersymmetric Standard Model are determined for the case of a large universal scalar mass, $m_0$, implying heavy scalar fermions, and for the case of a small $m_0$ resulting in light scalar fermions and giving the worst-case limits. Within this framework and for $m_{\tilde\chi^\pm_1} - m_{\tilde\chi^0_1} \geq 5$ GeV the 95% C.L. lower limits on $m_{\tilde\chi^\pm_1}$ for $m_0=500$ GeV are 90.0 and 90.2 GeV for $\tan \beta = 1.5$ and 35 respectively. These limits for all $m_0$ (the worst-case) are 69.1 and 65.2 GeV for $\tan \beta = 1.5$ and 35 respectively. Exclusion regions are also presented for neutralino masses, including an absolute lower limit at 95% C.L. for the mass of the lightest neutralino of 30.1 GeV for $m_0 = 500$ GeV (24.2 GeV for all $m_0$), with implications for experimental searches for the lightest neutralino as a dark matter candidate.