A study of the decay?− → K{sub S}°?−?{sub?} (K{sub S}° →??−) using the BABAR detector is presented. Using 124.4 fb−1 of data we measure?(?− → {bar K}°?−?{sub {tau}}) = (0.830 ± 0.005(stat) ± 0.042(syst))%, which is the world's most precise measurement to date of this branching ratio, and is consistent with the current world average. This preliminary result, unlike most of the?({tau}− → {bar K}°?−?{sub {tau}}) measurements already published, is systematics dominated and so the biggest future improvement to this number should come from reducing the systematic uncertainties in the analysis. A study of the K? mass spectrum, from which the strange (K?) spectral function can be measured, reveals excess contributions above the K*(892) tail at higher K? mass. While in the past this has been thought to be due to K*(892) - K*(1410) interference, we find that the K*(1410), whose branching ratio to K? is approximately 7%, seems insufficient to explain the excess mass observed in the data. Instead, we perform a fit using a K*(892) - K*(1680) interference model and find better agreement. The discrepancy that remains could be due to an s-wave contribution to the interference that is not parameterized in the model used, and/or detector smearing that is not accounted for in our fit. We also attempt to find an s-wave contribution to the K? mass spectrum by searching for an sp-interference effect. While we find a hint that such an effect exists, we have neither the confidence in the statistics nor systematics in the higher K? mass region to announce an observation. We conclude that it would be a worthwhile study to pursue.

The {tau}{sup -} {yields} {pi}{sup -}{pi}{sup -}{pi}{sup +}{pi}{sup 0}{pi}{sup 0}{nu}{sub {tau}} and {tau}{sup -} {yields} 3h{sup -} 2h{sup +} {nu}{sub {tau}} decays have been studied using the BABAR experiment at the PEP-II e{sup +}e{sup -} storage ring. Preliminary branching fractions are given for the {tau}{sup -} {yields} {pi}{sup -}{pi}{sup -}{pi}{sup +}{pi}{sup 0}{pi}{sup 0}{nu}{sub {tau}} and to the sub-channels {tau}{sup -} {yields} {eta}{pi}{sup -} {pi}{sup 0}{nu}{sub {tau}} and {tau}{sup -} {yields} {omega}(782){pi}{sup -}{pi}{sup 0}{nu}{sub {tau}}. A preliminary upper limit is given on the branching fraction for the {phi}(1020){pi}{sup -}{pi}{sup 0}{nu}{sub {tau}} mode. In addition a preliminary measurement of the branching fraction of the {tau}{sup -} {yields} 3h{sup -}2h{sup +} {nu}{sub {tau}} decay (h = {pi}, K) is presented.

A search for the decay of the [tau] lepton to rare multi-pion final states is performed using the BABAR detector at the PEP-II asymmetric-energy e+e- collider. The analysis uses 232 fb-1 of data at center-of-mass energies on or near the [Upsilon](4S) resonance. In the search for the [tau]- 2!3[pi]-2[pi]+2[pi]°[nu]{sub [tau]} decay, we observe 10 events with an expected background of 6.5{sup +2.0}{sub -1.4} events. In the absence of a signal, we calculate the decay branching ratio upper limit [beta]([tau]- 2!3[pi]-2[pi]+2[pi]°[nu]{sub [tau]})

A preliminary measurement of the branching fraction of the [tau][sup -] --> 3h[sup -]2h[sup +][nu][sub [tau]] decay (h = [pi], K) with the BABAR detector is found to be (8.52 [+-] 0.09 [+-] 0.40) x 10[sup -4], where the first error is statistical and the second is systematic. The data show evidence that the [rho] resonance plays a strong role in the decay of the [tau] lepton to five charged hadrons.

The Standard Model of particle physics predicts that the branching ratio for the rare decay B{sup 0} {yields} {tau}{sup +}{tau}{sup -} is 3.1 x 10{sup -8}, though untested models which could supersede it predict large enhancements. This dissertation describes the search for this rare decay in 210.4 fb{sup -1} of B{sup 0}{bar B}{sup 0} data collected at the {Upsilon}(4S) resonance in the Babar detector at the Stanford Linear Accelerator Center. In the analysis, one neutral B meson is fully reconstructed in a hadronic mode and recoil events which are consistent with each tau decaying in a mode {tau} {yields} {pi}{nu}, {rho}{nu}, or l{nu}{bar {nu}} are selected. There is no evidence for signal. The result is consistent with a downward fluctuation by 1.1 statistical standard deviations of the expected Standard Model background. Taking the expected background, the number of observed events and the expected statistical and systematic errors into account yields 2.7 x 10{sup -3} as the upper limit for B{sup 0} {yields} {tau}{sup +}{tau}{sup -} at the 90% confidence level.