Radiative Leptonic Decays Research Articles

Radiative leptonicBdecays in the instantaneous Bethe-Salpeter approach

Rare radiative leptonic decay $B\to l\bar \nu_l \gamma$ is studied in the instantaneous Bethe-Salpeter approach. The results are compared to other relativistic quark model predictions.

Lepton flavor violatingZdecays in the Zee model

We calculate lepton flavor violating (LFV) Z decays $\stackrel{\ensuremath{\rightarrow}}{Z}{e}_{i}^{\ifmmode\pm\else\textpm\fi{}}{e}_{j}^{\ensuremath{\mp}}$ $(i,j=e,\ensuremath{\mu},\ensuremath{\tau};i\ensuremath{\ne}j)$ in the Zee model keeping in mind the radiative leptonic decays ${e}_{i}\ensuremath{\rightarrow}{e}_{j}\ensuremath{\gamma}$ $(i=\ensuremath{\mu},\ensuremath{\tau};j=e,\ensuremath{\mu};i\ensuremath{\ne}j),$ $\ensuremath{\mu}$ decay, and the anomalous muon magnetic moment $(\ensuremath{\mu}\mathrm{AMM}).$ We investigate three different cases of Zee ${f}_{\mathrm{ij}}$ coupling: (A) ${f}_{e\ensuremath{\mu}}^{2}{=f}_{\ensuremath{\mu}\ensuremath{\tau}}^{2}{=f}_{\ensuremath{\tau}e}^{2},$ (B) ${f}_{e\ensuremath{\mu}}^{2}\ensuremath{\gg}{f}_{\ensuremath{\tau}e}^{2}\ensuremath{\gg}{f}_{\ensuremath{\mu}\ensuremath{\tau}}^{2},$ and (C) ${f}_{\ensuremath{\mu}\ensuremath{\tau}}^{2}\ensuremath{\gg}{f}_{e\ensuremath{\mu}}^{2}\ensuremath{\gg}{f}_{\ensuremath{\tau}e}^{2},$ subject to the neutrino phenomenology. Interestingly, we find that, although case (C) satisfies the large excess value of the $\ensuremath{\mu}\mathrm{AMM},$ it is unable to explain the solar neutrino experimental result, whereas case (B) satisfies the bimaximal neutrino mixing scenario, but in contrast with the results of $\ensuremath{\mu}\mathrm{AMM}$ experiments. We also find that, of all three cases, case (C) gives rise to the largest contribution to the ratio $B(\stackrel{\ensuremath{\rightarrow}}{Z}{e}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\tau}}^{\ensuremath{\mp}})/B(\stackrel{\ensuremath{\rightarrow}}{Z}{\ensuremath{\mu}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\mu}}^{\ensuremath{\mp}})\ensuremath{\simeq}{10}^{\ensuremath{-}8},$ which is still two orders less than the value accessible to be probed by future linear colliders, whereas, for the other two cases, this ratio is too low to be observed even in the near future for all possible LFV Z decay modes.

Long-distance effects inB→Vγradiative weak decays

A systematic approach to long-distance effects in exclusive radiative weak B decays is presented, based on a combination of the heavy quark limit with perturbative QCD. The dominant long-distance effects, connected with weak annihilation and W-exchange topologies, can be computed in a model-independent way using experimental data on B radiative leptonic decays. Nonfactorizable corrections vanish in the chiral limit and to leading twist. The left-handed photon amplitudes are shown to be enhanced relative to the right-handed ones, both in the long- and short-distance parts of the $\overline{B}$ decay amplitudes. Recent CLEO data on $\stackrel{\ensuremath{\rightarrow}}{B}{K}^{*}\ensuremath{\gamma}$ decays are consistent with standard model estimates of the long-distance contributions, and disfavor an enhanced gluonic penguin contribution. We discuss the implications of our results for the extraction of $|{V}_{\mathrm{td}}|.$

Radiative leptonic decays ofBmesons in QCD

We compute the form factors parametrizing radiative leptonic decays of heavy mesons ${B}^{+}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}\ensuremath{\nu}$ for photon energies much larger than ${\ensuremath{\Lambda}}_{\mathrm{QCD}},$ where perturbative QCD methods for exclusive processes can be combined with the heavy quark effective theory. The form factors can be reliably obtained in this region in an expansion in powers of $\ensuremath{\Lambda}{/E}_{\ensuremath{\gamma}}.$ The leading term in this expansion displays an additional spin symmetry manifested in the equality of form factors of vector and axial vector currents. The leading twist form factors can be written as the convolution of the B meson light-cone wave function with a hard scattering amplitude, which is explicitly calculated to one-loop order. The Sudakov double logarithms of the form $[({\ensuremath{\alpha}}_{s}/\ensuremath{\pi}){\mathrm{log}}^{2}{(2E}_{\ensuremath{\gamma}}/\ensuremath{\Lambda}){]}^{n}$ are resummed to all orders. As an application we present a method for determining the CKM matrix element $|{V}_{\mathrm{ub}}|$ from a comparison of photon spectra in B and D radiative leptonic decays.

Observation of radiative leptonic decay of the tau lepton

Using 4.68 fb(-1) of e(+)e(-) annihilation data collected with the CLEO II detector at the Cornell Electron Storage Ring, we have studied tau radiative decays tau(-)-->nu(tau)&mgr;(-)nu;(&mgr;)gamma and tau(-)-->nu(tau)e(-)nu;(e)gamma. For a 10 MeV minimum photon energy in the tau rest frame, the branching fraction for radiative tau decay to a muon or electron is measured to be (3.61+/-0.16+/-0. 35)x10(-3) or (1.75+/-0.06+/-0.17)x10(-2), respectively. The branching fractions are in agreement with standard model theoretical predictions.

RADIATIVE LEPTONIC Bc DECAYS

We analyze the radiative leptonic Bc decay mode: Bc→ℓνγ(ℓ=e,μ) using a QCD-inspired constituent quark model. The prediction: ℬ(Bc→ℓνγ)≃3 × 10-5 confirms that this channel is experimentally promising in view of the large number of Bc mesons which are expected to be produced at the future hadron facilities.

Radiative leptonicBcdecays in effective field theory

The recent discovery of the B_c meson by the CDF collaboration and proposed new experiments at Fermilab and CERN motivate new theoretical studies of the B_c system. Here we investigate the radiative leptonic decay B_c -> gamma l nu. This process is an important background to the annihilation process B_c -> l nu, which will be used to extract the B_c decay constant. We perform a model-independent calculation, based on QCD, of the partial width and various kinematic distributions. We also examine the decay within the framework of NRQCD, an effective field theory of nonrelativistic quarks, generalizing the NRQCD Lagrangian to include external sources for the weak and electromagnetic currents. Finally, we comment on the role of the B_c* intermediate state for very soft photon energies.

Radiative leptonic decays of Bc meson

To the leading order, the radiative leptonic decays B c→γℓ ν ̄ (ℓ= e, μ ) are studied carefully. In the study, a non-relativistic constituent quark model and the effective Lagrangian for the heavy flavour decays are used. As a result, the branching ratios turn out to be of the orders of 10 −5 for B c→γμ ν ̄ or for B c→γe ν ̄ . Based on the study, we point out the decays being accessible experimentally at the future LHC, and the possibility to determine the decay constant f B c through the radiative decays.

Radiative leptonic decays of heavy mesons.

We compute the photon spectrum and the rate for the decays $B(D)\to l\nu_l\gamma$ These photonic modes constitute a potentially large background for the purely leptonic decays which are used to extract the heavy meson decay constants. While the rate for $D\to l\n\g$ is small, the radiative decay in the $B$ meson case could be of comparable magnitude or even larger than $B\to\m\n$. This would affect the determination of $f_B$ if the $\tau$ channel cannot be identified. We obtain theoretical estimates for the photonic rates and disscuss their possible experimental implications.

On-Shell Current Algebra and the Radiative Leptonic Decay ofK+

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On-Shell Current Algebra and the Radiative Leptonic Decay ofK+

Dispersion relations and the hard-meson method of Schnitzer and Weinberg are used to study the radiative leptonic decay of the ${K}^{+}$. It is shown that in the pole-dominance approximation the relevant form factor in the axial-vector amplitude cannot be unsubtracted. Possible alterations of our results arising from relaxing a smoothness approximation are estimated to be small. We discuss and compare various symmetry-breaking schemes for the evaluation of necessary coupling constants. The branching ratio for ${K}^{+}\ensuremath{\rightarrow}\ensuremath{\gamma}{e}^{+}\ensuremath{\nu}$ is calculated to be \ensuremath{\sim}2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ for interesting structure-dependent decays. This is comparable to that for $K\ensuremath{\rightarrow}e\ensuremath{\nu}$ and two orders of magnitude larger than one would expect from the usual estimates for electromagnetic decays. The feasibility of experimentally observing the decay is discussed, as are the possible effects of electromagnetic violations of time-reversal invariance. From these results, a soft-pion estimate of ${F}_{4}$, the vector form factor in ${K}_{l4}$, yields $|{F}_{4}|\ensuremath{\approx}6.9$.

Extension of Cabibbo's theory to radiative leptonic decays of pseudoscalar mesons

We propose to extend the Cabibbo theory to the radiative leptonic decays. As an application, we study the π(K)→l+νl+γ decay. The axial part is determined by using conservation of electromagnetic current and assuming A1-dominance. For the vector part, assuming ρ-dominance, we extend Vaks and Ioffe's relation for π→l+νl+γ showing that it is a good approximation for all physical values of the momentum transfer. Furthermore, we show that the relative phase between the vector and axial parts can be determined. The available experimental data on π→e+νe+γ are compared with the predictions.