A Search for Third Generation Leptoquarks in p$\bar{p}$ Collisions at 1.8-TeV

Abstract

This thesis presents the results of a searcH for third generation leptoquarks in 72 $pb^{-1}$ of $p\overline{p}$ collisions at $\sqrt{s}$ = 1.8 TeV. The data were collected at the Collider Detector at Fermilab (CDF) during the 1992-1995 Collider runs. Leptoquarks (LQ) are spin- 0 or spin-1 particles which couple both to a quark and a lepton. Third generation leptoquarks are assumed to be produced in pairs and each to decay to a tau lepton $+b$ quark with a branching ratio $\beta$ The signature for leptoquarks investigated here is two taus plus two jets. Events with tau pairs are identified by the presence of a collimated high momentum jet, a high momentum electron or muon, and missing energy close to the lepton and transverse to the beam. At least two jets are required to reduce the background from QCD production of $Z^\circ$ bosons with associated jets and $Z\circ \to \tau^+\tau^-$. No evidence for a leptoquark signal is observed. Upper limits on $\sigma(p\overline{p} \to LQ \overline{LQ}) x \beta^2$ are obtained as a function of $M_{LQ}$ for scalar and vector leptoquarks. Using theoretical predictions for leptoquark pair production cross sections, scalar leptoquarks are excluded for $M_{LQ}$< 94 GeV/$c^2$ , non-gauge vector leptoquarks are excluded for $M_{LQ}$< 165 GeV/$c^2$ , and gauge vector leptoquarks are excluded for $M_{LQ}$ < 220 GeV /$c^2 $for $\beta$ = 100% at the 95% C.L. Non-gauge vector leptoquarks are excluded for $M_{LQ}$< 120 GeV/$c^2$ , and gauge vector leptoquarks are excluded for $M_{LQ}$< 178 GeV/$c^2$ for $\beta$ = 50% at the 95% C.L. The data do not constrain scalar leptoquarks for $\beta$ = 50% at the 95% C.L.