1,675 research outputs found
Search the decay of the B meson to two leptons at the Belle Experiment
No full textThe set of decays known as B⁰ → l⁺l⁻ are exceedingly rare. This thesis details procedures developed to optimize signal extraction and improve the upper limit on the measured branching fraction. When applied to 85 million BB events collected at the Belle experiment in Tsukuba, Japan, the resulting upper limits on branching fractions were 8.8×10⁻⁸ for B⁰ → e⁺e⁻, 1.0×10⁻⁷ for B⁰ → μ⁺μ⁻, and 8.5×10⁻⁸ for B⁰ → e±μ∓" at the 90% confidence level.Ph. D
Study of photon production in e + e - collisions at Tristan
No full textA study of photon production in multi-hadronic final states has been made at center-of-mass energies from 55 to 58 GeV with the AMY detector at the KEK e+e- storage ring TRISTAN.Ph. D
Bhabha scattering in e⁺e⁻ collisions at TRISTAN
No full textBhabha scattering, the process of e⁺e⁻ → e⁺e⁻, has been studied at center-of-mass energies from 50 to 58 GeV with the AMY detector at the KEK e⁺e⁻ storage ring TRISTAN. The study is based on a data sample of 79.7 pb⁻¹ integrated luminosity. The differential cross section of Bhabha scattering has been measured. The measured cross section is found to agree fairly well with the Standard Model of the electroweak theory. The measured cross section is also compared with various four-fermion contact interaction models, and confidence level lower limits on the composite scale, A, are determined. In addition, the limits on VV model are converted to SM-break-down scales, which indicate the validity of the SM down to the distance of order ~ 10⁻¹⁷ cm and the electron charge radius of ~ 10⁻¹⁶ cm. Attempts are made in searching for an additional boson Z'. No clear signal of the existence of a Z' boson is found up to energy of ~160 GeV/c². The effect of transverse beam polarization on Bhabha scattering is also studied. The ϕ dependence of Bhabha events are fitted to the QED prediction and found to agree with the theory. However, no quantitative conclusion on polarization effect can be drawn based on current data sample, which does not provide enough statistics. More data is being accumulated and further study should be carried out.Ph. D
Search for the lepton-flavor-violating τ− → e∓l±l− decays at Belle II
No full textWe present the result of a search for the charged-lepton-flavor violating decays τ− → e∓l±l−, where l is a muon or an electron, using a data sample with an integrated luminosity of 428 fb−1 recorded by the Belle II experiment at the SuperKEKB e+e− collider. The selection of e+e− → τ+τ− events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 × 10−8 are set at the 90% confidence level. These results are the most stringent bounds to date for four of the modes
Search for charged-lepton flavor violation in Υ(2S) → l∓τ± (l = e, μ) decays at Belle
No full textWe report a search for the charged-lepton flavor violation in Υ(2S) → l∓τ± (l = e, μ) decays using a 25 fb−1 Υ(2S) sample collected by the Belle detector at the KEKB e+e− asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions (B) at 90% confidence level. We obtain the most stringent upper limits: B(Υ(2S) → μ∓τ±) < 0.23 × 10−6 and B(Υ(2S) → e∓τ±) < 1.12 × 10−6
Measurement of bar{B0} Meson Properties Via Partial Reconstruction of the Decay bar{B0} -> D*+ l- ar{nu}
No full textUsing data recorded by the CLEO II detector operating at the Upsilon(4S) resonance at the Cornell Electron Storage Ring, several properties of B mesons are measured using a partially reconstructed tag of the decay mode bar{B0} -> D*+ l- bar{nu}. Using 2.38 fb**{-1} of on-resonance data and the averaged B meson semileptonic branching fraction through inclusive lepton momentum spectrum obtained by previous CLEO analysis, we measure the B0 and B- semileptonic branching fraction to be (10.78 +/- 0.60 +/- 0.69)% and (10.25 +/- 0.57 +/- 0.65)% respectively, which yields the lifetime ratio tau_+/tau_0 = 0.950 +0.117-0.080 +0.091-0.068, assuming the equality of semileptonic partial branching width for bar{B0} and B-. With a larger dataset of 3.1 fb**{-1}, we measured the B0-bar{B0} mixing parameter chi_d to be 0.189 +/- 0.019 +/- 0.006.Ph. D
Observation of b → dγ decays and determination of |V<sub>td</sub>/V<sub>ts</sub>|
No full textThe flavor changing neutral current process b → dγ is a sensitive probe to the Standard Model of elementary particle physics. Using a sample of 386 × 10⁶ B meson pairs accumulated by the Belle detector at the KEKB e⁺e⁻ collider, we measure the branching fractions for the exclusive modes B⁻ → ρ⁻γ, B̅⁰ → ρ⁰γ and B̅⁰ → ωγ as follows:
B(B⁻ → ρ⁻γ) = 0.55 −0.36+0.42−0.08+0.09
B(B̅⁰ → ρ⁰γ) = 1.25 −0.33+0.37−0.06+0.07
B(B̅⁰ → ωγ) = 0.56 −0.27+0.34−0.10+0.05
where the first error on each value is statistical and the second is systematic. Assuming that these three modes are related by isospin conservation rules, we find the combined branching fraction
B(B̅ → (ρ,ω)γ) = 1.32 −0.31+0.34−0.090.10.
This result is used to determine the ratio of CKM matrix elements,
|Vtd/Vts| = 0.199 −0.025+0.026−0.0150.018.Ph. D
Search for Bs0 → l∓ τ± with the semi-leptonic tagging method at Belle
No full textWe present a search for the lepton-flavor-violating decays Bs0 → l∓ τ±, where l = e, μ, using the full data sample of 121 fb−1 collected at the Υ(5S) resonance with the Belle detector at the KEKB asymmetric-energy e+ e− collider. We use Bs0B ̄s0 events in which one Bs0 meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for Bs0 → l∓ τ± decays and set upper limits on their branching fractions at 90% confidence level as B (Bs0 → e∓ τ±) < 14 × 10−4 and B (Bs0 → μ∓ τ±) < 7.3 × 10−4. Our result represents the first upper limit on the Bs0 → e∓ τ± decay rate. [Figure not available: see fulltext.
- …
