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The flavour changing neutral current (FCNC) process, $B^+$ → $K^+ [tau]^+ [tau]^-$ highly suppressed in the Standard Model (SM). This decay is forbidden at tree level and only occurs at lowest order via one-loop diagrams.$B^+$ → $K^+ [tau]^+ [tau]^-$ thus has the potential to provide a stringent test of the SM and a fertile ground for new physics searches. Contributions due to virtual particles in the loop allow one to probe, at relatively low energies, new physics at large mass scales. We search for the rare FCNC process $B^+$ → $K^+ [tau]^+ [tau]^-$ using data collected by the BaBaR detector at the SLAC National Accelerator Laboratory. The BaBaR data sample corresponds to a total integrated luminosity, at the energy of the [Tau](4S) resonance, of 424.4 $fb^-1$ and 471 million $B\bar{B}$ pairs. For this search, hadronic $B_{tag}$ reconstruction is employed, where one B is exclusively reconstructed via one of many possible hadronic modes. The remaining decay products in an event are then attributed to the signal B, on which the search for $B^+$ → $K^+ [tau]^+ [tau]^-$ is performed. Each [tau] is required to decay leptonically, into either an electron or a muon and the lepton neutrinos. Furthermore, a multi-variate analysis technique (neural network) is used to select for signal events and suppress dominant background modes. No significant signal is observed. The resulting branching fraction is measured to be $\beta(B^+$ → $K^+ [tau]^+)$ = $1.31^{0:66}_{-0:61}$(stat.) $^{+0:35}_{-0:25}$(sys.) x 10$^{-3}$, which is consistent with zero at the 1.9[sigma] level, with an upper limit of 2.25 x 10$^{-3}$, at the 90% confidence level.
We present measurements of the branching fractions of 3-prong and 5-prong? decay modes using a sample of 430 million? lepton pairs, corresponding to an integrated luminosity of 468 fb−1, collected with the BABAR detector at the PEP-II asymmetric energy ee− storage rings. The?− → (3?)−??{sub?},?− → (3?)− →??{sub?} and {tau}− →?− f1(1285)?{sub {tau}} branching fractions are presented as well as a new limit on the branching fraction of the isospin-forbidden, second-class current {tau}− →?−????????(958)?{sub {tau}} decay. We find no evidence for charged kaons in these decay modes and place the first upper limits on their branching fractions.
This work presents measurements of the branching fractions for tau decays to 3-prong final states using a data set of 430 million tau lepton pairs, corresponding to an integrated luminosity of 468 fb-1, collected with the BaBar detector at the PEP-II asymmetric energy e+e- storage rings. The tau- -> (3pi)- eta nu_tau, tau^- ->pi- 2pi0 omega nu_tau and tau- -> f_1(1285) pi- nu_tau branching fractions are presented as well as a measurement of the non-resonant component of the tau- -> 3pi- 3pi0 nu_tau decay. In addition this work sets a new limit on the branching fraction of the isospin-forbidden, second-class current decay tau- -> pi- eta'(958) nu_tau.
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 flavour changing neutral current (FCNC) process , B+ → K+ [tau]+[tau]−, is highly sup-pressed in the Standard Model (SM). This decay is forbidden at tree level and only occursat lowest order via one-loop diagrams. B+ → K+ [tau]+[tau]− thus has the potential to providea stringent test of the SM and a fertile ground for new physics searches. Contributions dueto virtual particles in the loop allow one to probe, at relatively low energies, new physicsat large mass scales. We search for the rare FCNC process B+ → K+ [tau]+[tau]− using datacollected by the BABAR detector at the SLAC National Accelerator Laboratory. The BABARdata sample corresponds to a total integrated luminosity, at the energy of the [UPSILON](4S) reso-nance, of 424.4 fb−1 and 471 million BB pairs. For this search, hadronic Btag reconstructionis employed, where one B is exclusively reconstructed via one of many possible hadronicmodes. The remaining decay products in an event are then attributed to the signal B, onwhich the search for B+ → K+ [tau]+[tau]− is performed. Each [tau] is required to decay leptonically,into either an electron or a muon and the lepton neutrinos. Furthermore, a multi-variateanalysis technique (neural network) is used to select for signal events and suppress dominantbackground modes. No significant signal is observed. The resulting branching fraction ismeasured to be B(B+ → K+[tau]+[tau]−) = 1.310.66 (stat.)+0.35(sys.) × 10−3, which is consistent −0.61 −0.25with zero at the 1.9[sigma] level, with an upper limit of 2.25 ×10−3 at the 90% confidence level." --
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]})
This comprehensive work thoroughly introduces and reviews the set of results from Belle and BaBar - after more than two decades of independent and complementary work - all the way from the detectors and the analysis tools used, up to the physics results, and the interpretation of these results. The world’s two giant B Factory collaborations, Belle at KEK and BaBar at SLAC, have successfully completed their main mission to discover and quantify CP violation in the decays of B mesons. CP violation is a necessary requirement to distinguish unambiguously between matter and antimatter. The shared primary objective of the two B Factory experiments was to determine the shape of the so-called unitarity triangle, an abstract triangle representing interactions of quarks, the elementary constituents of matter. The area of the triangle is a measure of the amount of CP violation associated with the weak force. Many other measurements have been performed by the B Factories and are also discussed in this work.
Why didn't the matter in our Universe annihilate with antimatter immediately after its creation? The study of CP violation may help to answer this fundamental question. This book presents theoretical tools necessary to understand this phenomenon. Reflecting the explosion of new results over the last decade, this second edition has been substantially expanded. It introduces charge conjugation, parity and time reversal, before describing the Kobayashi-Maskawa (KM) theory for CP violation and our understanding of CP violation in kaon decays. It reveals how the discovery of B mesons has provided a new laboratory to study CP violation with KM theory predicting large asymmetries, and discusses how these predictions have been confirmed since the first edition of this book. Later chapters describe the search for a new theory of nature's fundamental dynamics. This book is suitable for researchers in high energy, atomic and nuclear physics, and the history and philosophy of science.