Search for Scalar Top and Scalar Bottom Quarks at sqrt(s) = 189 GeV at LEP

TL;DR

This paper reports on searches for scalar top and bottom quarks at 189 GeV using OPAL data, setting new lower mass limits with no evidence of detection, thereby constraining supersymmetric models.

Contribution

First to set experimental mass limits on scalar top and bottom quarks at this energy with improved sensitivity and analysis methods.

Findings

No evidence for scalar top or bottom quarks was found.

Lower mass limit for scalar top quark is 90.3 GeV under certain conditions.

Lower mass limit for scalar bottom quark is 88.6 GeV for specific mass differences.

Abstract

Searches for a scalar top quark and a scalar bottom quark have been performed using a data sample of 182 pb-1 at a centre-of-mass energy of 189 GeV collected with the OPAL detector at LEP. No evidence for a signal was found. The 95% confidence level lower limit on the scalar top quark mass is 90.3 GeV if the mixing angle between the supersymmetric partners of the left- and right-handed states of the top quark is zero. In the worst case, when the scalar top quark decouples from the Z boson, the lower limit is 87.2 GeV. These limits were obtained assuming that the scalar top quark decays into a charm quark and the lightest neutralino, and that the mass difference between the scalar top quark and the lightest neutralino is larger than 10 GeV. The complementary decay mode of the scalar top quark decaying into a bottom quark, a charged lepton and a scalar neutrino has also been studied. From a search for the scalar bottom quark, a mass limit of 88.6 GeV was obtained if the mass difference between the scalar bottom quark and the lightest neutralino is larger than 7 GeV. These limits significantly improve the previous OPAL limits.