G. Abbiendi et al., Search for anomalous photonic events with missing energy in e(+)e(-) collisions at root s = 130, 136 and 183 GeV, EUR PHY J C, 8(1), 1999, pp. 23-40
Photonic events with large missing energy have been observed in e(+)e(-) co
llisions at centre-of-mass energies of 130, 136 and 183 GeV collected in 19
97 using the OPAL detector at LEP. Results are presented for event topologi
es with a single photon and missing transverse energy or with an acoplanar
photon pair. Cross-section measurements are performed within the kinematic
acceptance of each selection. These results are compared with the expectati
ons from the Standard Model process e(+)e(-) --> v (v) over bar + photon(s)
. No evidence is observed for new physics contributions to these final stat
es. Using the data at root s = 183 GeV, upper limits on sigma(e(+)e(-) -->
XY) BR(X --> Y gamma) and sigma(e(+)e(-) --> XX) BR2 (X --> Y gamma) are de
rived for the case of stable and invisible Y. These limits apply to single
and pair production of excited neutrinos (X = nu*,Y = nu), to neutralino pr
oduction (X = <(chi)over tilde>(0)(2), Y = <(chi)over tilde>(0)(1)) and to
supersymmetric models in which X = <(chi)over tilde>(0)(1) and Y = (G) over
bar is a light gravitino.