Search for Chargino and Neutralino Production at $\sqrt{s}$ = 181-184 GeV at LEP

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 cross-sections are obtained as a function of the chargino mass, the lightest neutralino mass, and the second lightest neutralino mass. 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 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 the lightest chargino-neutralino mass difference greater than 5 GeV the 95% C.L. lower limits on the lightest chargino mass 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.