Two complementary strategies for new physics searches at lepton colliders

In this thesis I present two complementary strategies for probing beyond-the-Standard Model physics using data collected in e+ e- collisions at lepton colliders. One strategy involves searching for effects at low energy mediated by new particles at the TeV mass scale, at which new physics is expected to manifest. Several new physics scenarios, including Supersymmetry and models with leptoquarks or compositeness, may lead to observable rates for charged lepton-flavor violating processes, which are forbidden in the Standard Model. I present a search for lepton-flavor violating decays of the Upsilon(3S) using data collected with the BABAR detector. This study establishes the 90% confidence level upper limits BF( Upsilon(3S) → etau) < 5.0 x 10--6 and BF(Upsilon (3S) → mutau) < 4.1 x 10-6, which are used to place constraints on new physics contributing to lepton-flavor violation at the TeV mass scale. An alternative strategy is to increase the collision energy above the threshold for new particles and produce them directly. I discuss research and development efforts aimed at producing a vertex tracker which achieves the physics performance required of a high energy lepton collider. A small-scale vertex tracker prototype is constructed using Silicon sensors of 50 mum thickness and tested using charged particle beams. This tracker achieves the targeted impact parameter resolution of sigmaIP = (5 ⊕ 10 GeV/pT) mum as well as a longitudinal vertex resolution of (260 +/- 10) mum, which is consistent with the requirements of a TeV-scale lepton collider. This detector research and development effort must be motivated and directed by simulation studies of physics processes. Investigation of a dark matter-motivated Supersymmetry scenario is presented, in which the dark matter is composed of Supersymmetric neutralinos. In this scenario, studies of the e+e - → H0A 0 production process allow for precise measurements of the properties of the A0 Supersymmetric Higgs boson, which improve the achievable precision on the neutralino dark matter candidate relic density to 8%. Comparison between this quantity and the dark matter density determined from cosmological observations will further our understanding of dark matter by allowing us to determine if it is of Supersymmetric origin.