Radiative decays of the Z boson

We calculate the Stokes parameters of the photons produced in the decays of neutral vector bosons Z, Z 1+¯1+ and Z q+¯q+, wherel=e, , or , and q is a quark. In the decays of unpolarized Z bosons (with the production of unpolarized leptons or quarks) the nonzero Stokes parameters for ₂ (circularly polarized photons) and ₃ (linearly polarized photons). The magnitude of ₃ does not depend on the parameters of the netural weak current of the leptons and the quarks (if their mass is neglected). The anomalous magnetic moment of the Z boson is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 38–43, February, 1986.     

Radiative decays of mesons in the NJL model

We revisit the theoretical predictions for anomalous radiative decays of pseudoscalar and vector mesons. Our analysis is performed in the framework of the Nambu–Jona–Lasinio model, introducing adequate parameters to account for the breakdown of chiral symmetry. The results are comparable with those obtained in previous approaches. Received: 14 September 2000 / Revised version: 1 March 2001 / Published online: 8 June 2001     

Radiative decays of vector mesons in the chiral bag model

A new model of radiative -meson decays of vector mesons in the chiral bag model is proposed. The quark--meson interaction has the form of a pseudoscalar coupling and is located on the bag surface. The vector meson decay width depends on the quark masses, the -meson decay constant, the radius of the bag, and the free parameter Z², which specifies the disappearance of the bag during the decay. The obtained results for the -and p-decay widths are in satisfactory agreement with the experiment.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 71–76, February, 1988.     

Radiative decays of radially excited pseudoscalar mesons in the extended Nambu-Jona-Lasinio model

In the extended NJL model the radiative decay widths of the radially excited states of the pseudo-scalar π, η, and η′ mesons are calculated. The predictions for the decay widths of the processes π(1300) → (ρ⁰, ω)γ, η(1295) → (ρ⁰, ω, φ)γ, and η(1475) → (ρ⁰, ω, φ)γ are given. Nowadays, there are no solid experimental data for these processes. The comparison of the results obtained in the framework of the standard and the extended NJL models for decays of the ground states of mesons is given. It is shown that these calculations correspond to each other and are also in satisfactory agreement with experimental data. This allows one to expect that the extended NJL can give reliable predictions for the excited states of mesons.     

Radiative leptonic decays of pseudoscalar mesons in the bilocal relativistic potential model

Radiative leptonic decays of the pseudoscalar mesons are studied in the bilocal relativistic potential model. Partial and total widths of the pion decay are calculated. It is shown that, in the model under consideration, the decay widths can be determined by calculating the vector (f _v ) and axial-vector (f _a ) form factors. These form factors can be found by jointly solving the Salpeter and Schwinger-Dyson equations with a specified interquark potential.     

Radiative decays at LHCb

Precise measurements on rare radiative B decays are performed with the LHCb experiment at LHC. The LHCb results regarding the ratio of branching fractions for two radiative decays, B ⁰ → K ^*0 γ and B _s → ?γ, the direct CP asymmetry in B ⁰ → K ^*0 γ decay channel and the observation of the photon polarization in the B ^± → K ^±π^?π^± γ decay, are included. The first two measurements were performed in 1 fb^–1 of pp collisions data and the third one in 3 fb^–1 of data, respectively.     

Flavour-changing Z decays

We compute neutral Higgs and Z⁰-boson couplings to quarks in a model with three quark-lepton families and one mirror family. Mixing between ordinary and mirrorquarks leads to flavour-changing couplings which are negligible for the known quarks. However, t-quark couplings are sizeable and the branching ratio for the decay can be as large as 10⁻⁴.     

Radiative decays and the nature of heavy quarkonia

We argue that the photon spectra in radiative decays of various heavy quarkonium states provide important information on their nature. If two of these states are in the strong coupling regime, we are able to produce a parameter-free model-independent formula, which holds at next-to-leading order and includes both direct and fragmentation contributions. When the formula is checked against recent CLEO data it favors Y(2S) and Y(3S) in the strong coupling regime and disfavors Y(1S) in it.     

Radiative decays and transition form factors of strange mesons in the relativistic constituent quark model

Motivated by the recent lattice QCD results indicating that the topological charge contribution to the flavor singlet axial vector current can be traded off by the constituent quark masses, we investigate the radiative decays of pseudoscalar (π,K, η, η′), vector (ρ,K*, ω, ?) and axial vector (A ₁) mesons using a simple relativistic constituent quark model. For both simplicity and relativity, we take advantage of the distinguished features in the light-cone quantization method: (1) the Fock-state expansion of meson wavefunctions are not contaminated by the vacuum fluctuation, (2) the assignment of meson quantum numbers are given by the Melosh transformation. Except the well-known constituent quark masses of (u,d,s) quarks and the spin-averaged meson masses, the only parameter in the model is the gaussian parameter β which determines the broadness (or sharpness) of radial wavefunction. The computed decay widths and the transition form factors of ρ, ω → π(η)γ*,K* →Kγ* andA ₁ → πγ* at 0≤Q ²≤5 GeV² and π⁰(η) → γ*γ at 0≤Q ²≤3 GeV² are in a remarkably good agreement with the experimental data and the result forA ₁ ⁺ → π₊ γ* transition is quite consistent with the experiments of pion scattering on a nucleus using Primakoff effect. This model is potentially useful in the cocktail analyses of the dilepton productions in proton-proton, proton-nucleus and nucleus-nucleus collisions at SPS energies and a little above.     

Radiative corrections to pseudoscalar meson decays

Recoupling matrix elements are evaluated in the harmonic oscillator approximation for all possible angular and radial excitations in processes where quarks recombine. A diagrammatic representation is given. Their use is demonstrated in calculating the transition potential for ρ→2π in a pair creation model.     

Radiative decays of charged leptons in arbitrary order

In radiative decays of charged leptons induced by non-degenerate neutrino masses and consequent lepton flavour-mixing, the dominant suppression factor at the 1-loop level is (Σ m _a ² )/m _W ² ,m _a being the mass of theath neutrino. We show that this suppression factor is presentin all orders in a class of models including the standard model.     

Radiative decays of quarkonia into Goldstone fermions

We consider the decay of³ S ₁ quarkonia into a photon and two Goldstone fermions in a model realizing supersymmetry in a nonlinear way. We derive a lower bound of 3.5 GeV for the scale of spontaneous supersymmetry breaking from Υ-decays. We show that the decay of a³ S ₁ quarkonium into two Goldstone fermions is forbidden in the order we consider.