[en] An approximate, but non-perturbative, RG equation is derived for theories involving scalars and fermions, in d dimensions with n_f flavours. The approximation consists in restricting the parameter space to interactions without derivatives. In a numerical study of the equation in d = 3 and d = 4, in the range of parameter space explored, no evidence is found of new fixed points generated by the inclusion of fermions. (orig.)

[en] We study the role of multi-instanton configurations in the multiparticle production at high energies in the standard model. A critical point s_∞ is found where one, two, three, etc. instanton-anti-instanton pairs give comparable contributions to the total cross section. For energies above this point the instanton approximation collapses. If is found that at the critical point the total cross section is still exponentially small. (orig.)

[en] By comparing the Bethe Ansatz solution with perturbation theory we obtain the exact results m=8/e Λsub(anti Manti S) and m=(32/πe)^1/2 Λsub(anti Manti S) for the O(3) and O(4) non-linear σ-models, respectively. Here Λsub(anti Manti S) is the renormalization group invariant scale in the anti Manti S scheme. (orig.)

[en] The production of consecutive accelerated bunches with a separation time of up to 200 ns and a width of 500 ps FWHM, is the goal of this new chopping beam system. The chopper 500 should be cut the present length of the accelerated beam bunches, delivered from the superconducting cyclotron, from 1.5/2 ns to 0.5 ns. Up to now all the components of the system are ready. The 1/4 λ resonator, the coupling capacitor, the 3''1/8 coaxial line, the 50 kW final stage RF amplifier, the driver preamplifier, the DDS frequency synthesizer, are on site. Low and high power test are in progress on the components of the chopper system. All the technical performances, the experimental results, together with the numerical simulations, will be presented in this paper. (author)

[en] We review the motivations for the search for stochastic backgrounds of gravitational waves and we compare the experimental sensitivities that can be reached in the near future with the existing bounds and with the theoretical predictions. (author)

[en] We discuss the issue of the momentum dependence of instanton-induced amplitudes. We consider multi-loop graphs in gφ⁴, which on the one hand can be computed through the Lipatov approach using instantons and on the other hand are given by the usual Feynman graphs. Using this correspondence we can understand the peculiar momentum dependence. We find that the momentum dependence characteristic to the one-instanton amplitudes corresponds to low-multiplicity cuts, while the genuine ultraviolet behavior is dominated by high-multiplicity cuts which are not reproduced in the one-instanton approximation and presumably could be related to multi-instantons. The contribution of the multi-instanton configurations is estimated, and a boundary value of the momentum transfer where the regimes are changed is found. (orig.)

[en] We study the amplitudes with a large number of external legs in gφ⁴ theory within the Lipatov approach. This approach has been originally suggested to determine how the amplitudes with a fixed number of external lines depend on the order of perturbation theory, at larger orders. We discuss the generalization of this method to amplitudes with a large number of external legs, N, and few or no loops at all (tree-level multiparticle amplitudes). We show that the N-dependence in this limit is under theoretical control and we reproduce the factorial growth discussed in the literature. We elucidate the relation between the alleged onset of the strong-coupling regime in multiparticle production and the well-known problem of the divergence of perturbation theory at large orders. We argue that if the number of particles produced exceeds a critical value the cross sections are bounded by exp(-const./g). (orig.)

[en] The design of a compact superconducting cyclotron able to deliver light ions with Q/A=0.5 at 250 A MeV is presented. This cyclotron is optimised to accelerate H₂⁺ molecules to be extracted by stripping. The extraction by stripping allows to extract high beam power to drive a facility of radioactive ion beams. Simultaneously a low intensity beam could be extracted to perform radiotherapy treatment and interdisciplinary research. The main characteristics of the accelerator are presented

[en] We investigate the role of multi-instantons at high energies in weakly coupled theories. We perform an explicit computation of their effect and we find that they become as important as the one-instanton contribution at energies where the latter is still exponentially small (''premature unitarization''). The computation relies on various simplifying assumptions, and we discuss them. We argue, however, that premature unitarization is a phenomenon of a very general nature, which should survive our simplifying assumptions. If this is the case, there exists a critical energy where a change of regime is forced, and the dilute-instanton-gas approximation totally collapses. Possible scenarios in the critical and supercritical domain are discussed at the qualitative level

[en] We calculate the relationship between the coupling constants of the simple hypercubic and the body-centered hypercubic lattice, to one-loop order, using the background-field method. This corresponds to determining the ratio of the renormalization scales Λ for the two schemes. For SU(2) we find Λ/sub BCH//Λ/sub H/ = 0.29, in good agreement with existing Monte Carlo results