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We present a search for the decay B →?+?{sub?} using 288 fb−1 of data collected at the?(4S) resonance with the BABAR detector at the SLAC PEP-II B-Factory. A sample of events with one reconstructed semileptonic B decay (B− → D{sup o}l−{bar?}{sub {ell}}X) is selected, and in the recoil a search for B+ →?+?{sub?} signal is performed. The? is identified in the following channels:?+ → e+?{sub e}{bar?}{sub?},?+ →?+?{sub {mu}}{bar {nu}}{sub {tau}}, {tau}+ →?+?°{bar {nu}}{sub {tau}}. We measure a branching fraction of?(B+ → {tau}+{nu}{sub {tau}}) = 0.88{sub -0.67}{sup +0.68}(stat.) ± 0.11(syst.) x 10−4 and extract an upper limit on the branching fraction, at the 90% confidence level, of?(B+ → {tau}+{nu}{sub {tau}})
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.
Based on an 87-fb{sup -1} dataset collected by the Babar detector at the PEP-II asymmetric-energy B-Factory, a search for D{sup 0}-{bar D}{sup 0} mixing has been made using the semileptonic decay modes D*{sup +} {yields} {pi}{sup +}D{sup 0}, D{sup 0} {yields} Ke{nu} (+c.c.). The use of these modes allows unambiguous flavor tagging and a combined fit of the D{sup 0} decay time and D*{sup +}-D{sup 0} mass difference ({Delta}M) distributions. The high-statistics sample of unmixed semileptonic D{sup 0} decays is used to model the {Delta}M distribution and time-dependence of mixed events directly from the data. Neural networks are used to select events and reconstruct the D{sup 0}. A result consistent with no charm mixing has been obtained, R{sub mix} = 0.0023 {+-} 0.0012 {+-} 0.0004. This corresponds to an upper limit of R{sub mix}
A review of selected recent BaBar results are presented that illustrate the ability of the experiment to search for physics beyond the standard model. The decays B → [tau][nu] and B → s[gamma] provide constraints on the mass of a charged Higgs. Searches for Lepton Flavour Violation could provide a clear signal for beyond the standard model physics. Babar does not observe any signal for New Physics with the current dataset.
The authors present a search for the decay B → l+?{sub l} (l =?,?, or e) in (458.9 ± 5.1) x 106 B{bar B} pairs recorded with the BABAR detector at the PEP-II B-Factory. They search for these B decays in a sample of B+B− events where one B-meson is reconstructed as B− → D°{ell}−{bar?}X. Using the method of Feldman and Cousins, they obtain?(B+ →?+?{sub {tau}}) = (1.7 ± 0.8 ± 0.2) x 10−4, which excludes zero at 2.3?. They interpret the central value in the context of the Standard Model and find the B meson decay constant to be f{sub B}2 = (62 ± 31) x 103 MeV2. They find no evidence for B+ → e+?{sub e} and B+ →?+?{sub?} and set upper limits at the 90% C.L.?(B+ → e+?{sub e})
The authors present a search for the decay B -->?+?{sub?} in 124.1 x 106?(4S) decays recorded with the BABAR detector at the SLAC PEP-II B-Factory. A sample of events with one reconstructed exclusive semi-leptonic B decay (B− --> D*°l−{bar?}{sub {ell}}) is selected, and in the recoil a search for B+ -->?+?{sub?} signal is performed. The? is identified in the following channels:?+ --> e+?{sub e}{bar?}{sub?} -->?+?{sub {mu}}{bar?}{sub?},?+ -->?+{bar {nu}}{sub?}, {tau}+ -->?+?°{bar {nu}}{sub {tau}}, {tau}+ -->?+?−?+{bar {nu}}{sub {tau}}. The authors find no evidence of signal, and they set a preliminary upper limit on the branching fraction of B(B+ --> {tau}+{nu}{sub {tau}}) 4.3 x 10−4 at the 90% confidence level (CL). This result is then combined with a statistically independent BABAR search for B+ -- {tau}+{nu}{sub {tau}} to give a combined preliminary limit of B(B+ --> {tau}+{nu}{sub {tau}})
Recent results on hadronic decays of tau from BaBar are reviewed. The branching fraction of {tau}{sup -} {yields} 3h{sup -} 2h{sup +}{nu}{sub {tau}} is measured to be (8.56 {+-} 0.05{sub stat} {+-} 0.42{sub sys}) x 10{sup -4}. The underlaying substructure of this decay exposes strong contribution from {rho} resonance. The decay {tau}{sup -} {yields} f{sub 1}(1285){pi}{sup -} {nu}{sub {tau}} with f{sub 1}(1285) {yields} 2{pi}{sup -}2{pi}{sup +} is observed and the corresponding branching fraction is measured to be (3.9 {+-} 0.7{sub stat} {+-} 0.5{sub sys}) x 10{sup -4}. The search for {tau}{sup -} {yields} 4{pi}{sup -}3{pi}{sup +}({pi}{sup 0}){nu}{sub {tau}} decay was performed and without evidence of the signal the upper limit of B({tau}{sup -} {yields} 4{pi}{sup -}3{pi}{sup +}({pi}{sup 0}){nu}{sub {tau}}) is estimated to be 3.0 x 10{sup -7} at 90% confidence limit. The upper limits on branching fractions for the exclusive decays {tau}{sup -} {yields} 4{pi}{sup -}3{pi}{sup +}{nu}{sub {tau}} and {tau}{sup -} {yields} 4{pi}{sup -}3{pi}{sup +}{pi}{sup 0}{nu}{sub {tau}} are found to be 4.3 x 10{sup -7} and 2.5 x 10{sup -7} at 90% confidence limit, respectively.
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