Semileptonic decays of D mesons involving production of a heavy neutral lepton E/sup 0/ having small mass (0.5 GeV/c/sup 2/ ..KeE/sup 0/ and D..-->..KeE/sup 0/ (..-->..e..pi..) has specific properties which make it possible to search for the E/sup 0/ in a more purposeful manner.

The processes of quasielastic production of the charmed baryons ..lambda../sup +//sub c/, ..sigma../sup +//sub c/, and ..sigma../sup + +//sub c/ in neutrino-nucleon reactions are considered. Calculations are made of the differential and total cross sections for two sets of the parameters M/sub V/ and M/sub A/ which determine the form factors of weak transitions of nucleons into charmed baryons. The chosen parameter values make it possible to obtain for the cross sections results which completely span the currently existing range of uncertainties in the theoretical predictions. Attention is drawn to an important kinematic effect which restricts the emission angles of the charmed baryons to the range theta

The possibilities of obtaining information on the tau-neutrino mass from study of the cascade decays of D/sup + -/ and F/sup +/ mesons via the channels tau/sup + -/+..nu../sub tau/(nu-bar/sub tau/) and tau/sup + -/..--> pi../sup + -/(rho/sup + -/) +nu-bar/sub tau/(..nu../sub tau/) are analyzed. It is shown that the most favorable conditions for observing these processes occur in e/sup +/e/sup -/ annihilation processes at energies corresponding to the D(F)-meson factory. It is possible in principle to lower the upper bound on the ..nu../sub tau/ mass by roughly an order of magnitude.

A study is made of the possibility of measuring the lifetime of the ..sigma../sup 0/ hyperon produced in the process ..lambda..A..--> sigma../sup 0/A by separating the Coulomb peak from the strong-interaction background in the differential cross section for small momentum transfers to the nucleus. The strong-interaction contribution to the process ..lambda..N..--> sigma../sup 0/N is estimated by the method of complex angular momenta with allowance for cuts. The cross section for the nuclear process ..lambda..A..--> sigma../sup 0/A is evaluated using the Glauber formalism. It is shown that in the case of heavy nuclei the electromagnetic interactions dominate over the entire region of coherence for laboratory energies of the incident ..lambda.. hyperon in the range E> or approx. =10 GeV. The ranges of energies and emission angles of the ..lambda.. hyperons and photons from the decay of coherently produced ..sigma../sup 0/ hyperons are determined.

Coherent production of two pions in high-energy collisions of pions with heavy nuclei at small momentum transfers to the nucleus is considered. The contributions from photon and ..omega..-Reggeon exchange are analyzed. The incoherent background is estimated. An expression for the differential cross section which is valid in a wide range of effective masses of the pions produced is obtained. It is shown that for incident-pion energies of 20--50 GeV in the lab frame it is better to extract data on the reaction ..gamma pi --> pi pi.. from the transition ..pi --> pi pi.. in heavy nuclei rather than on protons. The rho- and rho' (1250) -meson production cross sections are estimated and the angles characterizing the distributions of the pion and photon emitted in the ..pi../sup 0/ -meson decay are given.

Expressions are obtained for the inclusive distributions of rho/sup 0/ mesons from cascade decays of heavy leptons tau +- ..--> nu../sub t/A/sub 1/ +- (..-->..rho/sup 0/..pi.. +- ) formed in the e/sup +/e/sup -/ annihilation process. The possibilities are studied of determining the characteristics of the tau lepton from the energy and angular distributions of rho/sup 0/ mesons.