Single-photon signal from neutralinos at LEP2

The production of invisible pairs of lightest neutralinos accompanied by a large-angle hard photon in the reaction e^+ e^- \to \chi^0_1 \chi^0_1 \gamma is studied at LEP2 energies. The most general gaugino/higgsino composition of the \chi^0_1 within the Minimal Supersymmetric Standard Model is assumed. The spectrum of the observed photon is derived within the framework of the p_t-dependent structure-function approach, whose accuracy is assessed to be within the foreseen experimental accuracy at LEP2. Higher-order QED corrections due to undetected initial-state radiation are also included. A comparison with the Standard Model main background from e^+ e^- \to \nu \bar \nu \gamma is performed for optimized photon kinematical cuts. Quantitative conclusions on the signal/background ratio are given for a wide range of values of the SUSY parameters.