Search for chargino and neutralino production at √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 (<(chi)over tilde>(+)(1)<(chi)over tilde>(-)(1)<(chi)over tilde>(0)(1)<(chi)over tilde>(0)(2)) cross-sections are obtained as a function of the chargino mass (m(<(chi)over tilde>1+/-)), the lightest neutralino mass (m(<(chi)over tilde>10)) and the second lightest neulralino mass (m(<(chi)over tilde>20)). 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, mo, implying heavy scalar fermions, and for the case of a small mo resulting in light scaler fermions and giving the worst-case Limits. Within this framework and for m(<(chi)over tilde>1+/-) - m(<(chi)over tilde>10) greater than or equal to 5 GeV the 95% C.L. lower limits on m(<(chi)over tilde>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 (25.2 GeV for all m(0)), with implications for experimental searches for the lightest neutralino as a dark matter candidate.