Searches for charginos and neutralinos in e+e- collisions at √s=161 and 172 GeV

The data recorded by the ALEPH detector at centre-of-mass energies of 161, 170, and 172 GeV are analysed for signals of chargino and neutralino production. No evidence of a signal is found, although candidate events consistent with the expectations from Standard Model processes are observed. Limits at 95% C.L. on the production cross sections are derived and bounds on the parameters of the Minimal Supersymmetric Standard Model are set. The lower limit on the mass of the lightest chargino is 85.5 GeV/c(2) for gaugino-like charginos (mu = -500 GeV/c(2)), and 85.0 GeV/c(2) for Higgsino-like charginos (M-2 = 500 GeV/c(2)), for heavy sneutrinos (M(<(nu)over tilde>) greater than or equal to 200 GeV/c(2)) and tan beta = root 2. The effect of light sleptons on chargino and neutralino limits is investigated. The assumptions of a universal slepton mass and a universal gaugino mass are relaxed, allowing less model-dependent limits to be obtained.