Semileptonic decays of the B c meson

The semileptonic decays of the B _c meson into heavy quarkonia J/ψ(η_c) and a pair of leptons are investigated on the basis of three-point sum rules of QCD and nonrelativistic QCD (NRQCD). Calculations of analytic expressions for the spectral densities of QCD and NRQCD correlation functions with allowance for Coulomb-like α_s/v terms are presented. At recoil momenta close to zero, generalized relations due to the spin symmetry of NRQCD are derived for the B _c → J/ψ(η_c)/ν_l form factors, with l denoting one of the leptons (e, μ, or τ). This allows one to express all NRQCD form factors in terms of a single universal quantity, an analog of the Isgur-Wise function at the maximal lepton-pair invariant mass. The gluon-condensate corrections to three-point functions are calculated both in full QCD in the Borel transform scheme and in NRQCD in the moment scheme. This enlarges the parametric-stability region of the sum-rule method, thereby rendering the results of the approach more reliable. Numerical estimates of the widths for the transitions B _c → J/ψ(η_c)lν_l are presented.     

B c-meson decays and lifetime within QCD sum rules

On the basis of three-point sum rules, the form factors for the semileptonic decays B _c ⁺ → B _s(B _s ^* )l ⁺ ν _l are calculated with allowance for α _s/v Coulomb corrections for the heavy quarkonium. Generalized relations associated with spin symmetry within the approach combining heavy-quark effective theory and nonrelativistic QCD are derived for form factors in the region of recoil momenta close to zero. The nonleptonic decays of the B _c meson are studied on the basis of the factorization hypothesis. By summing the main exclusive modes of c-quark decays and by using the results of a previous analysis of b-quark decays, the B _c-meson lifetime is estimated within QCD sumrules and within nonrelativistic QCD.     

The ντ-mass and the τ-decay modes

The behaviour of the different decay modes of the charged heavy lepton τ versus the neutrino ν_τ-mass is analyzed in detail. The τ→ν_τ A ₁ and τ→ν_τ K ^* decay rates have been evaluated using finite energy sum rules. The τ→ν_τ+ “Hadron Continuum” decay rate has been estimated within the framework of Quantum Chromodynamics (QCD). We find that the branching ratios of the semi-leptonic processes: τ→ν_τρ, τ→ν_τπ and τ→ν_τ+ “Hadron Continuum” are very sensitive to the value of the ν_τ-mass. Thus a more precise measurement of these branching ratios could provide an improved upper bound for the neutrino ν_τ-mass.     

Semileptonic B<Subscript>s</Subscript>→D<Subscript>sJ</Subscript>(2460)lν decay in QCD

Using three point QCD sum rule methods, the form factors relevant to semileptonic B_s→D_sJ(2460)ℓν decay are calculated. The q² dependences of these form factors are evaluated and compared with the heavy quark effective theory predictions. The dependence of the asymmetry parameter α, characterizing the polarization of the D_sJ meson, on q² is studied. The branching ratio of this decay is also estimated, and it is shown that it can easily be detected at LHC.     

Angular correlations for semileptonic D meson decays

We study the exclusive semileptonicD meson decaysD→K+π+l+ν. For this we develop the general formalism for the joint angular distribution of the charged lepton and theK meson and calculate the helicity dependent decay widths which determine the full angular distribution for several models withK ^*(892) andK ₀ ^* (1430) intermediate state with nonresonant background terms calculated from chiral Lagrangians.     

Evidence for the τ → νϱπ decay mode

Annihilation of e⁺e^? into final states with a single electron has been studied with the PLUTO detector at the DORIS storage ring at CMS energies from 3.6 to 5 GeV. In the sample of 4-prong events without any detected photon we observe 21 events which we assign to the reaction e⁺e^? → τ⁺τ^? → ννe + ν?⁰π. We obtain a branching ratio for τ⁺ → ν?⁰π⁺ of 0.050 ± 0.015 with an overall systematic uncertainty of 30%. The data are consistent with the ?π coming from an A₁ meson.     

QCD sum rules calculation ofB c decays

Leptonic and semileptonic decay widths of theB _c meson, as well as its mass, are evaluated by QCD sum rules. The possibility of discovering this meson at present and future high energy machines is also discussed.     

Investigation of the semileptonic weak decays of B (s) to S (Scalar Meson) via light-cone QCD sum rules

In this paper, the semileptonic weak decay process of B _(s) → S scalar mesons is investigated by using the light-cone QCD sum rules (LCSR) in the nonperturbative part. The corresponding transition matrix elements leading to form factors and the branching ratio of this process are determined.     

Study of J/Ψ three-body decays involving baryons and a pseudoscalar meson in the diquark model

The J/Ψ decay into a baryon pair and a pseudoscalar meson is computed, for some channels, in lowest order in perturbative QCD, modeling the baryon with a quark-diquark system. We use a set of parameters that has been proposed by some authors in order to fit the proton magnetic form factor G _M ^P, the angular distribution of protons in the processes γγ → pp?, γp → γp and the width of η_c → γγ.     

Bounds on a top quark with charged Higgs decay

If the top quark decays to a charged Higgs boson t→bH⁺, semileptonic decay signatures are suppressed and the mass bounds set by the UA1 top search are evaded. However the charged Higgs decay mode H⁺→τ⁺ν_τ gives a missing-ϱ_T signature; depending on the decay branching fraction and using the UA1 limits on new physics contributions to missing-ϱ_T, we set bounds on the top mass in this scenario. We also evaluate the feasibility of seeking visible-τ-jet signals for top at a pp̄ collider.     

Formfactors of semileptonic decaysB→D(D *)ev from QCD sum rules

Formfactors of semileptonic decaysB→D(D ^*)ev are calculated by means of QCD sum rules for three-point Green functions. Partial widths found essentially differ from values obtained in previous papers [1–5].     

Forward-backward asymmetry in top quark semileptonic decay

The semileptonic decay of the top quark t → bW⁺ → bl⁺ν_l is analyzed in the rest system of the W. The forward-backward asymmetry of the lepton l⁺ with respect to the heavy quark direction is defined and computed. It is argued that this observable will be an ideal tool to study top quark properties at Tevatron and LHC. Higher order QCD corrections are calculated and their structure is elucidated in some detail.     

Isospin violation in τ → 3πντ

Isospin violating signals in the τ^? → (3π)^?ν_τ decay mode are discussed. For the τ^? → π^?π^?π⁺ν_τ decay mode, isospin violation arises from the vector current contribution in the τ^? → ωπ^?ν_τ decay with the subsequent isospin violating ω decay into π⁺π^?. We demonstrate that such effects may be observed in presently available data through the measurement of the interference effects of these vector current contributions with the dominating axial vector current,i. e. through a measurement of the structure functionsW _F, WG, WH andW _I. In the case of the τ^? → π⁰π⁰π^?ν_τ decay mode, a vector current contribution is generated by ηπ⁰ mixing in the decay chain τ^? → η?^?ν_τ → π⁰π⁰π^?ν_τ. We find that this effect is rather small, the magnitude of the associated interference terms being too low for present statistics.     

Analysis of the semileptonic Bc→Bu *ℓ+ℓ- decay from QCD sum rules

We analyze the semileptonic B_c→B_u ^*ℓ⁺ℓ^- decay in the framework of the Standard Model. We calculate the B_c to B_u ^* transition form factors in QCD sum rules. Analytical expressions for the spectral densities and gluon condensates are presented. The branching ratio of the B_c→B_u ^*ℓ⁺ℓ^- decay is calculated, and it is obtained that this decay can be detectable at forthcoming LHC machines. PACS 12.60.-i, 13.30.-a., 13.88.+e     

The transition form factors for semi-leptonic weak decays of J/ψ in QCD sum rules

Within the standard model, we investigate the semi-leptonic weak decays of J/ψ. The various form factors of J/ψ making the transition to a single charmed meson (D^(*) _(d,s)) are studied in the framework of QCD sum rules. These form factors fully determine the rates of the weak semi-leptonic decays of J/ψ and provide valuable information on non-perturbative QCD effects. Our results indicate that the decay rate of the semi-leptonic weak decay mode J/ψ→D^(*)- _s+e⁺+ν_e is at the order of 10^-10. PACS 13.20.Gd; 13.25.Gv; 11.55.Hx     

Majorana neutrinos in rare meson decays

The rare meson decays K ⁺ → π ^??⁺?′⁺ and D ⁺ → K ^??⁺?′⁺ (?, ?′ = e, μ), which are induced by Majorana neutrino exchange and which are accompanied by lepton-number nonconservation, are considered. The effects of the meson structure are taken into account on the basis of the Gaussian model for the respective Bethe-Salpeter amplitudes. It is shown that existing direct experimental constraints on the decay branching ratios are overly lenient and therefore give no way to set realistic limits on effective Majorana masses. On the basis of the constraints on the lepton-mixing parameters and neutrino masses from precision measurements of electroweak processes, neutrino-oscillation experiments, searches for neutrinoless double-beta decay of nuclei, and cosmological data, indirect constraints on the branching ratios for the decays in question are obtained and found to be much more stringent than the above direct constraints.     

Radiative B* meson decays in QCD sum rules

The decay constant g _B* of the B* mesons and the transition amplitudes of the radiative decays $B_{a}^{?t}?ghtarrow B_{a}αmma$are calculated in the frame-work of QCD sum rules. It is shown that the decay rates B* → Bγ are one order smaller than the corresponding D* meson decays.     

τ− → π−π0ντ decay in the extended NJL model

The width of the decay τ^? → π^?π⁰ν_τ was calculated in the extended NJL model. Contact interaction of W boson with a pion pair as well as the contribution of the ρ mesons in the ground and first radialexcited states are taken into account. The sum of the contact diagram and diagram with intermediate ρ meson in the ground state leads to the result which coincides with the result of the vector-dominance model. Our results are in satisfactory agreement with experimental data.     

Measurements of tau decay modes and a precise determination of the mass

The cross sections for e⁺e^? → e⁺(μ⁺ + non showering track + any photons have been measured for cm energies between 3.1 GeV and 5.2 GeV. We observe τ-pair production below the thresholdfor charm production and determine the τ mass to be 1.807 ± 0.020 GeV from a fit to the energy dependence of the cross section. The ration of the leptonic branching ratios B_μ/B_e = 0.92 ± 0.32 is consistent with eμ-universality. The following branching ratios are determined for a V-A coupling: $\text{B(τ → ν}{}_{\mathrm{\tau }}\text{e}\text{ν}\text{) = B(τ → ν}{}_{\mathrm{\tau }}\text{μ}\text{ν}\text{) = 0.182 + 0.028}$. B(τ → ν_τ + charged hadron + any photons) = 0.29 ± 0.11, B(τ → ν_τ + three or more charged hadrons + any photons) = 0.35 ± 0.11.