Search for single leptoquark and squark production in electron-photon scattering at $\sqrt{s_{\rm ee}}=189$ GeV at LEP

Abstract.

A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks ($\tilde{\mbox{q}}$) in R-parity violating SUSY models with the direct decay of the $\tilde{\mbox{q}}$ into Standard Model particles has been performed using e$^+$e$^-$ collisions collected with the OPAL detector at LEP at an e$^+$e$^-$ centre-of-mass energy $\sqrt{s_{\rm ee}}$ of 189 GeV. The data correspond to an integrated luminosity of about 160 pb$^{-1}$. The dominant process for this search is $\mbox{eq}\to\LQ/\tilde{\mbox{q}}\to\mbox{eq},\nu\mbox{q}$, where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95% confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings $\lambda$ to fermions larger than $\sqrt{4\pi\alpha_{\rm em}}$, the mass limits range from 121 GeV$/c^2$ to $175$ GeV$/c^2$ ($149$ GeV$/c^2$ to $188$ GeV$/c^2$) depending on the branching ratio $\beta$ of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings $\lambda$ for leptoquarks and $\lambda'_{1jk}$ for squarks, and on $\beta$ as a function of the scalar leptoquark/squark mass.