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

Abstract

This thesis presents the results of a searcJ<'[hird generation leptoquarks in 72 pb-1 of pp collisions at .y'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 (3. 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 zo bosons with associated jets and zo ~ r+r-. No evidence for a leptoquark signal is observed. Upper limits on u(pp ~ LQ LQ) x (3 2 are obtained as a function of MLQ for scalar and vector leptoquarks. Using theoretical predictions for leptoquark pair production cross sections, scalar leptoquarks are excluded for MLQ < 94 Ge VI c 2 , non-gauge vector leptoquarks are excluded for MLq < 165 Ge VI c 2 , and gauge vector leptoquarks are excluded for MLq < 220 GeV lc 2 for (3 = 100% at the 95% C.L. Non-gauge vector leptoquarks are excluded for MLq < 120 Ge VI c 2 , and gauge vector leptoquarks are excluded for MLq < 178 GeV lc 2 for (3 =50% at the 95% C.L. The data do not constrain scalar leptoquarks for (3 =50% at the 95% C.L. 11l Contents Acknowledgements 4 Leptoquark Search 4.1 Data Sets . . . . . . . . . 4.2 Lepton + Taujet Selection 4.3 Isolation Requirements . . 4.4 Z ---+ ee and Z ---+ p,p, Removal 4.5 Itt Direction: Di-tau Selection 4.6 Number of Jets: Leptoquark Selection 4. 7 Features of the Di-tau Candidate Sample

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