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Register a new userA phenomenological analysis of antiproton interactions at low energies
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We present an optical potential analysis of the antiproton-proton interactions at low energies. Our optical potential is purely phenomenological, and has been parametrized on data recently obtained by the Obelix Collaboration at momenta below 180 MeV/c. It reasonably fits annihilation and elastic data below 600 MeV/c, and allows us for an evaluation of the elastic cross section and rho-parameter down to zero kinetic energy. Moreover we show that the mechanism that depresses antiproton-nucleus annihilation cross sections at low energies is present in antiproton-proton interactions too.
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The recent experimental data obtained by the OBELIX group on $bar{p}$D and $bar{p}^4$He total annihilation cross sections are analyzed. The combined analysis of these data with existing antiprotonic atom data allows, for the first time, the imaginary parts of the S-wave scattering lengths for the two nuclei to be extracted. The obtained values are: $Im a^{sc}_0 = [- 0.62 pm 0.02 ({stat}) pm 0.04 ({sys})] fm$ for $bar{p}$D and $Im a^{sc}_0 = [- 0.36pm 0.03({stat})^{+0.19}_{-0.11}({sys})] fm$ for $bar{p}^4$He. This analysis indicates an unexpected behaviour of the imaginary part of the $bar{p}$-nucleus S-wave scattering length as a function of the atomic weight A: $|Im a^{sc}_0|$ ($bar{p}$p) > $|Im a^{sc}_0|$ ($bar{p}$D) > $|Im a^{sc}_0|$ ($bar{p}^4$He).
Antiproton scattering off $^3He$ and $^4He$ targets is considered at beam energies below 300 MeV within the Glauber-Sitenko approach, utilizing the $bar N N$ amplitudes of the Julich model as input. A good agreement with available data on differential $bar p ^4He$ cross sections and on $bar p ^3He$ and $pbar ^4He$ reaction cross sections is obtained. Predictions for polarized total $bar p ^3$He cross sections are presented, calculated within the single-scattering approximation and including Coulomb-nuclear interference effects. The kinetics of the polarization buildup is discussed.
We set up a plane wave impulse approximation (PWIA) formalism for the analysis of the annihilation cross sections of antinucleons on nuclear targets at very low momenta (below 100 MeV/c), where semiclassical approximations cant be applied. Since, as we test here, PWIA fails in reproducing the unexpected ``inversion behavior of the $bar{p}p$ and $bar{p}-$nucleus annihilation cross sections found in a recent experimentcite{obe1,obe2} we discuss some further possibilities, with a special attention to the optical potential model.
An enhancement of antiprotons produced in p+d reaction in comparison with ones in p+p elementary reaction is investigated. In the neighborhood of subthreshold energy the enhancement is caused by the difference of available energies for antiproton production. The cross section in p+d reaction, on the other hand, becomes just twice of the one in elementary p+p reaction at the incident energy far from the threshold energy when non-nucleonic components in deuteron target are not considered.
We analyze virtual Compton scattering off the nucleon at low energies in a covariant, model-independent formalism. We define a set of invariant functions which, once the irregular nucleon pole terms have been subtracted in a gauge-invariant fashion, is free of poles and kinematical zeros. The covariant treatment naturally allows one to implement the constraints due to Lorentz and gauge invariance, crossing symmetry, and the discrete symmetries. In particular, when applied to the $epto epgamma$ reaction, charge-conjugation symmetry in combination with nucleon crossing generates four relations among the ten originally proposed generalized polarizabilities of the nucleon.
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