How light can the lightest neutralino be?

(1) respectively, with cβ = cosβ, sβ = sinβ and cθ = cos θw, sθ = sin θw, with the weak mixing angle θw. Besides the masses of the W and Z boson, MW and MZ , respectively, the neutralino and chargino sectors at tree level only depend on the U(1)Y and SU(2)L gaugino masses M1 and M2, respectively, the higgsino mass parameter μ, and the ratio tanβ = v2/v1 of the vacuum expectation values of the two Higgs fields. The neutralino (chargino) masses are the square roots of the eigenvalues of M0M†0 (M±M†±) [3]. The LEP limit on the chargino mass is mχ̃± 1 >∼ 100 GeV [3], from which follows that M2, |μ| >∼ 100 GeV. If the GUT relation M1 = 5/3 tan (θw)M2 ≈ 0.5M2 is assumed, then M1 >∼ 50 GeV, such that the lightest neutralino mass is constrained to mχ̃0 1 >∼ 50 GeV [3]. However, if one drops the GUT relation, M1 is an independent parameter, allowing to tune the neutralino mass determined from the lowest-order mass matrix M0 freely [4–8]. The neutralino mass is identically zero for [5]