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Register a new userAnnihilation of antiprotons on nuclear targets at low energies: a simple analysis
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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.
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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).
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.
The goal of the AE$mathrm{bar{g}}$IS experiment at the Antiproton Decelerator (AD) at CERN, is to measure directly the Earths gravitational acceleration on antimatter. To achieve this goal, the AE$mathrm{bar{g}}$IS collaboration will produce a pulsed, cold (100 mK) antihydrogen beam with a velocity of a few 100 m/s and measure the magnitude of the vertical deflection of the beam from a straight path. The final position of the falling antihydrogen will be detected by a position sensitive detector. This detector will consist of an active silicon part, where the annihilations take place, followed by an emulsion part. Together, they allow to achieve 1$%$ precision on the measurement of $bar{g}$ with about 600 reconstructed and time tagged annihilations. We present here, to the best of our knowledge, the first direct measurement of antiproton annihilation in a segmented silicon sensor, the first step towards designing a position sensitive silicon detector for the AE$mathrm{bar{g}}$IS experiment. We also present a first comparison with Monte Carlo simulations (GEANT4) for antiproton energies below 5 MeV
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.
Background $alpha$-nucleus potentials play an essential role for the calculation of $alpha$-induced reaction cross sections at low energies in the statistical model. Uncertainties of these calculations are related to ambiguities in the adjustment of the potential parameters to experimental elastic scattering angular distributions (typically at higher energies) and to the energy dependence of the effective $alpha$-nucleus potentials. Purpose The present work studies cross sections of $alpha$-induced reactions for $^{64}$Zn at low energies and their dependence on the chosen input parameters of the statistical model calculations. The new experimental data from the recent Atomki experiments allow for a $chi^2$-based estimate of the uncertainties of calculated cross sections at very low energies. Method The recent data for the ($alpha$,$gamma$), ($alpha$,$n$), and ($alpha$,$p$) reactions on $^{64}$Zn are compared to calculations in the statistical model. A survey of the parameter space of the widely used computer code TALYS is given, and the properties of the obtained $chi^2$ landscape are discussed. Results The best fit to the experimental data at low energies shows $chi^2/F approx 7.7$ per data point which corresponds to an average deviation of about 30% between the best fit and the experimental data. Several combinations of the various ingredients of the statistical model are able to reach a reasonably small $chi^2/F$, not exceeding the best-fit result by more than a factor of 2. Conclusions The present experimental data for $^{64}$Zn in combination with the statistical model calculations allow to constrain the astrophysical reaction rate within about a factor of 2. However, the significant excess of $chi^2/F$ of the best-fit from unity asks for further improvement of the statistical model calculations and in particular the $alpha$-nucleus potential.
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