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"This thesis presents the searches for two rare B meson decays: the radiative leptonic decay B+ -> l+ nu gamma (l = e, mu) and the flavor-changing neutral current B -> K(*) nu nubar. These searches use the full dataset collected by the BaBar experiment, which corresponds to almost 500 million BB pairs. After fully reconstructing the hadronic decay of one of the B mesons in Upsilon(4S) -> BB decays, evidence of B+ -> l+ nu gamma or B -> K(*) nu nubar is looked for in the rest of the event. No significant evidence of either signal decay is observed. Model-independent branching-fraction upper limits are set at BR(B ->e nu gamma)17 x10 {-6}, BR(B -mu nu gamma)24 x10 {-6}, and BR(B -l nu gamma)15.6 x10 {-6}, all at the 90% confidence level. These are currently the most stringent published upper limits for B+ - l+ nu gamma. In addition, branching-fraction upper limits are set at BR(B -> K+ nu nubar)3.7 x10 {-5}, BR(B - K0 nu nubar)8.0 x10 {-5}, BR(B - K*+ nu nubar)11.5 x10 {-5}, and BR(B - K*0 nu nubar)9.2 x10 {-5}, all at the 90% confidence level. For additional sensitivity to New Physics contributions, partial B - K(*) nu nubar branching-fraction upper limits are also determined over the full kinematic range." --
This thesis describes a search for the rare decay B + {yields} mu+numugamma, performed at the Stanford Linear Accelerator Center. The BABAR detector is used to analyze 81.65 fb{sup -1} of data, with (88.6 {+-} 0.6) x 10{sup 6} charged B meson decays, from the PEP-II e{sup +}e{sup -} collider running at the Upsilon(4S) resonance. The background estimate for the analysis is three events, and three signal candidates are observed. An upper limit on the branching fraction is set at B (B+ {yields} mu +numugamma)
"The document describes a search for the rare decay B[bar] [right arrow] K[bar]vv̄ in a data set of about 230 millions of BB[bar] pairs recorded by the BABAR detector at the PEP-II B Factory, via the process e+e- [right arrow] [Upsilon](4S) [right arrow] BB[bar]. The method used selects events in which the B+ meson is fully reconstructed, and looks for a K[bar] in the rest of the event. This is accomplished by applying kinematical and topological restrictions that select B[bar] [right arrow] K[bar]vv̄ events with the largest possible efficiency, while rejecting a maximum amount of background events. The number of background events is estimated from data and from simulated events, while the value of the selection efficiency relies solely on the simulation of B[bar] [right arrow] K[bar]vv̄ decays in the BABAR detector. These two numbers are required in order to set an upper limit on the branching ratio. We analyze a subset of the data sample and provide an interpretation of the result." --
This thesis describes a search for the rare radiative decay of a B meson to two photons. where the charged congugate mode is implied throughout. These decays are highly suppressed in the Standard Model where the branching fraction is expected to be of order 10^-8. In some new physics scenarios this could be enhanced by up to an order of magnitude to 10^-7. Therefore an observation of a significant signal above the Standard Model prediction could be a sign of new physics. The search for this rare decay was performed using the data collected with the BaBar detector at the SLAC National Accelerator Laboratory PEP-II storage ring operating at the Upsilon(4S) resonance. The analysis uses a dataset with an integrated luminosity of 425.7 fb-1 corresponding to 467 million BB pairs. A signal yield of 21.3 +12.8 -11.8 events with a significance of 1.88 sigma was measured using an unbinned extended maximum likelihood fit. An upper limit on the branching fraction is set at the 90% confidence level of less than 3.2 times 10^-7. This is about two times more stringent than the best upper limit of less than 6.2 times 10^-7 published by the Belle collaboration.
"We present two analyses of the rare B- -> Lambda pbar nu nubar decay. Within the Standard Model, the process B- -> Lambda pbar nu nubar is permitted but highly suppressed, with an expected branching fraction of (7.9 +/- 1.9) x 10^-7. This rarity makes it a sensitive probe for the existence of new physics, which may be discoverable in the form of a higher than predicted branching fraction.The first analysis is conducted on data collected at the BABAR experiment, which ran from 1999-2008. BABAR collected 433 fb^-1 of integrated luminosity at the Upsilon(4S) resonance; Upsilon(4S) decays almost exclusively into B-Bbar pairs, and this analysis uses hadronic tag reconstruction to fully reconstruct one of the B-mesons in this pair. The search for B- -> Lambda pbar nu nubar is then conducted among the decay products of the second, unreconstructed B-meson. Using both real data and Monte Carlo simulations, we develop and implement a signal selection that selects candidates conforming to the expected characteristics of B- -> Lambda pbar nu nubar decays. The result of the analysis is an upper limit on the branching fraction at the 90% confidence level of 3.02 x 10^-5. This is the world's first experimental result in the search for B- -> Lambda pbar nu nubar decays.The second analysis is conducted on Monte Carlo simulations of data that will be collected at Belle II, the world's only next-generation B-meson facility. Over its lifetime Belle II will collect 50 ab^-1 of data, approximately 100 times that of BABAR. This increase in data, along with more capable hardware and software, should allow Belle II to search for B- -> Lambda pbar nu nubar decays with greater sensitivity than BABAR. Using a simulated dataset equivalent in size to that used in our BABAR study, we employ Full Event Interpretation to fully reconstruct the decay of one B-meson in each B-Bbar pair, and conduct our signal selection in the remainder of the data. The result of the analysis is a predicted upper limit on the branching fraction at the 90% confidence level of 1.2 x 10^-4. A comparison of the Belle II and BABAR results highlights several areas where Belle II will need to improve in order to make best use of its projected dataset"--
We present a search for the rare decays B → K+?{bar?} and B° → K°?{bar?} using 459 million B{bar B} pairs collected with the BABAR detector at the SLAC National Accelerator Laboratory. Flavor-changing neutral-current decays such as these are forbidden at tree level but can occur through one-loop diagrams in the Standard Model (SM), with possible contributions from new physics at the same order. The presence of two neutrinos in the final state makes identification of signal events challenging, so reconstruction in the semileptonic decay channels B → D{sup (*)}l? of the B meson recoiling from the signal B is used to suppress backgrounds. We set an upper limit at the 90% confidence level of 1.3 x 10−5 on the total branching fraction for B+ → K+?{bar {nu}}, and 5.6 x 10−5 for B° → K°{nu}{bar {nu}}. We additionally report partial branching fractions in two ranges of di-neutrino mass squared for B+ → K+{nu}{bar {nu}}.
Abstract: We present a search for the decay of the charged B meson into a charged lepton and a neutrino 458.9 million Upsilon(4S) decays recorded with the Babar detector at the SLAC PEP-II B-Factory. A sample of events with one reconstructed exclusive semileptonic B decay is selected, and in the recoil a search for the signal decay is performed. The tau lepton is identified in decays to an electron and two neutrinos; a muon and two neutrinos; a charged pion and a neutrino; or a charged pion, a neutral pion, and a neutrino. The analysis strategy and the statistical procedure is set up for branching fraction extraction or upper limit determination. We determine from the data set a preliminary measurement of the branching fraction a charged B decaying to a tau lepton and a neutrino = (1.8 " 0.8 " 0.1)E-4, which excludes zero at 2.4 standard deviations. We extract the B meson decay constant = 255 " 58 MeV. Combination with the hadronically tagged measurement yields (1.8 " 0.6)E-4. We also set preliminary limits on the branching fraction of charged B decaying to an electron and a neutrino at 7.7E-6 and the charged B decaying to a muon and a neutrino at 11E-6. The limits are at the 90% confidence level.
"We present a sensitivity study for the search for the semi-leptonic, flavour-changing-neutral-current decay B- -> lambda pbar nu nubar. B- -> lambda pbar nu nubar is expected to be highly suppressed in the Standard Model and thus provides a sensitive probe to test for new physics. A branching fraction for B- -> lambda pbar nu nubar has never been experimentally measured.The analysis is conducted using data from the BABAR particle detector, based at the SLAC National Accelerator Laboratory, California, USA. The BABAR dataset comprises approximately 471 million BBbar pairs produced from e-e+ -> Upsilon(4S) collisions and subsequently Upsilon(4S) [right arrow] BBbar.Using hadronic tag reconstruction we separate the two B mesons in each event by fully reconstructing one B meson from known hadronic decay modes, we then conduct a search for B- -> lambda pbar nu nubar in the decay of the other B meson. Using Monte-Carlo simulations of B- -> lambda pbar nu nubar we develop a signal selection designed to isolate B- -> lambda pbar nu nubar while suppressing potential background processes.Data is blinded at later stages of the analysis and we provide a range of possible branching fraction upper limits as a function of the number of data events which might survive the signal selection. Assuming we observe no excess of data events over those predicted by Monte-Carlo, we predict branching fraction upper limits for B- -> lambda pbar nu nubar at the 90% confidence level of 2.64 x 10^-5 using the Barlow method and 3.10 x 10^-5 using the Feldman-Cousins method." --