Single-photon signal from neutralinos at LEP2

The production of invisible pairs of lightest neutralinos accompanied by a large-angle hard e(+)e(-) --> <(chi)over tilde>(0)(1)<(chi)over tilde>(0)(1) gamma is studied at LEP2 energies. The most general gaugino-higgsino composition of the <(chi)over tilde>(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(-) --> nu<(nu)over tilde>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.