Pion decay constant at finite temperature

The values of the pion decay constant, and of the bilinear fermionic condensate, for temperatures from zero up to the critical temperature for chiral phase transition, are calculated using our composite operator approach to finite temperature QCD. For small temperatures our results agree with those found with other approaches. Near the critical temperature we recover the universal behaviour typical of second order phase transition. We have assumed that effects responsible for deconfinement do not strongly perturb our chiral approach for two and possibly three flavours.     

Direct neutron decay of the isoscalar giant dipole resonance

The direct and statistical neutron decay of the isoscalar giant dipole resonance has been studied in ⁹⁰Zr, ¹¹⁶Sn, and ²⁰⁸Pb using the (α, α’ n) reaction at a bombarding energy of 200 MeV. The spectra of fast decay neutrons populating valence hole states of the Z, N − 1 nuclei were analyzed, and estimates for the branching ratios were determined. The observation of the nucleon-direct-decay channels helped to select giant-resonance strengths and suppress the underlying background and continuum, which led to an indication of the existence of a new mode with L = 2 character, presumably the overtone of the isoscalar giant quadrupole resonance. The text was submitted by the authors in English.     

Pion decay site spectroscopy in copper

With the aim of obtaining direct information on the sites occupied by positive pions (π⁺), channelling experiments have been performed on the decay muons (μ⁺) from π⁺ implanted into high-purity copper crystals. At 100 K virtually exclusive occupancy of octahedral interstitial sites (0) is found in good accordance with earlier μSR results. At room temperature a significant fraction of the implanted pions decay on metastable tetrahedral interstitial sites (T). The data suggest strongly that above about 100 K both μ⁺ and π⁺ diffuse by phonon-assisted tunnelling between stable 0 sites and metastable T sites. The enthalpy difference between these two sites is estimated to be about 50 meV for μ⁺ and of the same order of magnitude for π⁺. These values are much smaller than the available theoretical predictions. The implications of low-lying metastable interstitial sites for the μ⁺ stopping sites of low temperatures are discussed.     

Isobar giant resonance formation in self-conjugate nuclei

The production of isobars with concomitant giant resonance excitations due to peripheral collisions of relativistic heavy ions is investigated. The interaction is described by a modified form of the central term in the one-pion-exchange potential (OPEP) where the projectile ordinary spin operator is replaced by a transition spin operator which describes the creation of an isobar from a nucleon. The scattering is analyzed using time-dependent harmonic perturbation theory to determine the reaction total cross sections. The results obtained, which are valid for reactions involving self-conjugate nuclei, are applied to the specific collison of 2.1 $\text{GeV}\text{nucleon}$¹⁶O projectiles with ¹²C targets at rest. Cross sections are investigated using two different models for the nuclear spin states. In the first model, the many-body nuclear spin state is reduced, in the spirit of a particle-hole state, to an equivalent two-body state called a particle-core state. In the second model, the many-body spin states are described by unsymmetrized products of individual particle spins. Properties of the spin giant resonance and isobar giant resonance states are investigated. Finally, isobar decay and isobar/pion absorption effects are discussed.     

On direct proton decay of the isovector spin-flip giant monopole resonance

A semimicroscopic approach based on both the continuum-RPA method and a phenomenological treatment of the spreading effect is applied to describe the direct proton decay and gross properties of the charge-exchange isovector spin-flip giant monopole resonance. Calculation results are compared with the available experimental data. The text was submitted by the authors in English.     

Pion decay constants in a strong magnetic field

Decay constants of the charged and neutral pions in magnetic field are considered in the framework of the effective quark-antiquark lagrangian respecting Gell-Mann–Oakes–Renner (GOR) relations at zero field. The \(\sqrt {\frac{{{e_q}B}}{\sigma }} \) dependence is found in strong fields e_qB ? σ for the neutral pion, while the charged pion constant decreases as \(\sqrt {\frac{\sigma }{{{e_q}B}}} \).     

Pion absorption in the resonance region

Pion absorption in the resonance region is studied microscopically in terms of the elementary coupling of mesons to nucleons and isobars. Particular emphasis is put on calculating the contribution from genuine two-body absorption and three-body absorption. At low energies the two-body absorption is the most important channel, but around resonance and beyond the three-body absorption becomes important and can account for about 50% of the total absorption cross section. Genuine multiparticle absorption by more than three particles is estimated to be a small fraction of the total. Comparison with the experimental data is made and good agreement with experiment is found for different nuclei and different energies.     

Direct and statistical gamma decay of the giant quadrupole resonance of 208Pb

The gamma decay of the giant quadrupole resonance of ²⁰⁸Pb is discussed. The relative contribution of the decay via the compound nucleus is calculated from the statistical theory. It is found that the compound decay is as important as the direct decay. The summed contribution of the direct and compound decay modes, however, is a factor of 2 smaller than the observed gamma branching ratio GQR → gs.     

Pion production in nuclear collisions

The production of pions in nuclear collisions is analyzed in terms of a microscopic reaction model, where the free energy of the entrance channel is transferred to the final channel through the coupling of the relative motion to the internal excitation of N^*-resonances. Such a model allows quite naturally for a consistent analysis of the exclusive and the inclusive production of pions. It turns out that the pionic fusion cross section is determined predominantly by the spectroscopic parentage between the initial target projectile combination and the final nucleus, whereas the inclusive part is dominated by the energy dependence of the phase space factor, i.e. by the number of the degrees of freedom which are relevant in a given kinematical situation. This model is applied to the analysis of available experimental data from threshold to the (1232)-region. A consistent analysis of both the inclusive and the exclusive part of the pion spectrum is presented.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Cross sections for isobar production in neutrino-nucleon collisions

The production of isobars in collisions of neutrinos and antineutrinos with nucleons is analyzed. The ordinary Hofstadter form factors as well as exponentially decaying form factors are used. Graphs of the cross sections as functions of the neutrino (or antineutrino) energy are shown for the production of the isobars N*_3/2 and Y*_3/2.Translated from Izvestiya VUZ. Fizika, No. 8, pp. 82–86, August, 1970.     

Pion pair production in photon-photon collisions

The process γγ→π⁺π^? was measured using the detector MD-1 at VEPP-4. The two-photon reactionse ⁺ e ^?, μ⁺ μ^? and π⁺ π^? pair production were separated using scintillation counters, Cherenkov counters and shower-range chambers. A radiation widthГ _γγ(f ₂(1270))=3.1±0.35±0.35 keV was obtained.     

Pion production in two-body pd collisions

The reactions d(p, π⁺)t and d(p, π^o)³ He are analyzed in terms of the pp → π⁺d sub-process. The exclusion principle induces an additional exchange contribution involving the πd scattering amplitude. Acceptable deuteron and tritron (³H) wave functions which reproduce the charge form factors yield results consistent with experimental angular distributions at 470, 590, 754 and 1192 proton lab. energies. The forward peak is not sensitive to details of the wave functions. Both the direct and exchange form factors develop a zero which is imposed by the dip in the ³He charge form factor. The associated interference pattern governs the backward cross section.     

Pion production at large transverse momentum

A constituent interchange model is used to give a natural explanation of the observed energy dependence of ISR data for π° production at 90°. It is predicted that at large fixed p_T and sufficiently high energy the cross section will become constant, with a p_T dependence of (p_T²)^?2.5. It is also tentatively suggested that the e⁺e^? annihilation structure function $\text{ν}\text{W}{}_2$ should behave like ω^?1.5 near ω = 0.