Search for an LSP gluino at LEP with the DELPHI detector

Abstract.

In some supersymmetric models, the gluino ($\tilde {\rm g}$) is predicted to be light and stable. In that case, it would hadronize to form R-hadrons. In these models, the missing energy signature of the lightest supersymmetric particle is no longer valid, even if R-parity is conserved. Therefore, such a gluino is not constrained by hadron collider results, which looked for the decay $\tilde {\rm g}\to{\rm q \bar q}\tilde \chi^0_1$. Data collected by the DELPHI detector in 1994 at 91.2 GeV have been analysed to search for ${\rm q \bar q}\tilde {\rm g}\tilde {\rm g}$ events. No deviation from Standard Model predictions is observed and a gluino mass between 2 and 18 GeV/c² is excluded at the 95% confidence level in these models. Then, R-hadrons produced in the squark decays were searched for in the data collected by DELPHI at the centre-of-mass energies of 189 to 208 GeV, corresponding to an overall integrated luminosity of 609 Pb^-1. The observed number of events is in agreement with the Standard Model predictions. Limits at 95% confidence level are derived on the squark masses from the excluded regions in the plane ($m_{\tilde {\rm q} 1}, m_{\tilde {\rm g}}$)

$m_{\tilde {\rm t}1}>90$ GeV/c², and $m_{\tilde {\rm b}1}>96$ GeV/c² for purely left squarks.

$m_{\tilde {\rm t}1}>87$ GeV/c², and $m_{\tilde {\rm b}1}>82$ GeV/c² independent of the mixing angle.