Search for chargino and neutralino production at $\sqrt{s} = 189$ GeV at LEP

Abstract.

A search for charginos and neutralinos, predicted by supersymmetric theories, is performed using a data sample of 182.1 pb$^{-1}$ taken at a centre-of-mass energy of 189 GeV with the OPAL detector at LEP. No evidence for chargino or neutralino production is 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 ($m_{\tilde{\chi}^0 _2}$). Within the Constrained Minimal Supersymmetric Standard Model framework, and for $m_{\tilde{\chi}^\pm_1} - m_{\tilde{\chi}^0 _1} \geq 5$ GeV, the 95% confidence level lower limits on $m_{\tilde{\chi}^\pm_1}$ are 93.6 GeV for $\tan \beta = 1.5$ and 94.1 GeV for $\tan \beta = 35$. These limits are obtained assuming a universal scalar mass $m_0 \geq$ 500 GeV. The corresponding limits for all $m_0$ are 78.0 and 71.7 GeV. The 95% confidence level lower limits on the lightest neutralino mass, valid for any value of $\tan \beta$ are 32.8 GeV for $m_0 \geq 500$ GeV and 31.6 GeV for all $m_0$.