טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLandsman Hagar-Yael
SubjectA Measurement of Rb at LEP2 Energies
DepartmentDepartment of Physics
Supervisor Professor Yoram Rozen


Abstract

 Rb, The cross section of bottom quark pair’s production relative to the hadronic cross section in gamma or Z boson decays, is a sensitive probe of the standard model. The dependence of Rb on the center of mass energy and coupling constants is well described by the standard model. Since the b-quark is a member of the third fermion generation, Rb is sensitive to higher order correction from heavy particles.

A measurement of Rb at energies above the Z boson mass peak was preformed. The data were taken with the OPAL detector at LEP electrons-positrons collider, at energies ranging from 183 GeV to 207 GeV, during the LEP2 program. Because of the limited statistics it was impossible to use the double-tag technique. A sophisticated tagging technique based on lifetime, lepton and event shape information was used to identify bottom quarks. This technique is more efficient and background free than the one used in the previous OPAL measurement, where Rb was measured up to 189 GeV.  Both statistical and systematic uncertainties were successfully decreased.

The main background source is radiative events, where the electron (or positron) loose energy through initial state photon radiation. The cross section of events where the energy lose decreases the center of mass energy back to the Z boson mass peak are strongly enhanced. Another source of background is the decay of a pair of W bosons or Z bosons in the so-called 4-fermion event. Those backgrounds are removed using topological signatures, and corrected using Monte Carlo simulation.