Measurement of the antiproton-nucleus annihilation cross section at 5.3 MeV

Antiproton annihilation cross sections on medium-heavy and heavy nuclear targets are measured for the first time at 5.3 MeV kinetic energy at the Antiprotons Decelerator facility of CERN. The results agree with the expected behavior from the black-disk model with the Coulomb corrections.     

Antiproton-deuteron annihilation at low energies

Recent experimental data obtained by the OBELIX collaboration on ˉpD and ˉp⁴ 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 ± 0.02 stat) ± 0.04 (sys)] fm for ˉpD and Im a^sc ₀= [−0.36 ± 0.03 (stat)^+0.19 _−0.11 (sys)] fm for ˉp ⁴He. This analysis indicates an unexpected behaviour of the imaginary part of the ˉp-nucleus S-wave scattering length as a function of the atomic weight A: |Im a^sc ₀| (macr;pp) > |Im a^sc ₀| (ˉpD) > |Im a^sc ₀ (ˉp⁴ He). Received: 3 November 1999 / Revised version: 15 December 1999     

Measurement of the spectral flux density of negative muons at very low energies

The spectral flux density of negative muons has been measured down to 5 eV.     

In-flight antiproton annihilation on nuclei at low energies

The results of the annihilation cross sections measurement of 5.3 MeV antiprotons on nickel, tin, platinum and Mylar targets performed by the ASACUSA Collaboration at CERN are presented and compared with the existing data and models. From the experimental point of view the presented data are the first measurement of antinucleon annihilation cross sections at low energies obtained with a pulsed beam. This results open the road for the next measurements at the very low energies of the order of 100 keV that are in progress by the ASACUSA Collaboration. The experimental method foreseen for the 100 keV measurement is illustrated.     

Shape of pion multiplicity in antineutron-proton annihilation at very low momentum

The relation between the amount ofp-wave in antineutron-proton annihilation into pions at very low momenta (≦ 0.3 GeV/c) and the even or odd character of the produced pion number is underlined. A simple calculation of the magnitude of the effect suggests the possible interest of measurements.     

On the treatment of the annihilation interaction in antiproton-nucleus collisions

We discuss how to treat the annihilation interaction in antiproton-nucleus collisions within the framework of the multiple scattering model under the constraint that real annihilation of the projectile can take place only once.     

Total electron-scattering cross-section for molecular hydrogen at low energies by photoelectron spectroscopy

The relative total electron-scattering cross-section for molecular hydrogen has been measured in the energy region 0.02–1.14 eV. In general the Ramsauer technique is used for such a measurement, but in the present experiment the relative cross-sections have been measured by photoelectron spectroscopy. This is the first time that this technique has been put to such a use. The results in this energy region seem to match better the results of Ramsauer and Kollath than those of Golden et al.     

Influence of magnetic fields on electron-Ion recombination at very low energies

Radiative recombination (inverse photoionization) is believed to be well understood since the beginning of quantum mechanics. Still, modern experiments consistently reveal excess recombination rates at very low electron-ion center-of-mass energies. In a detailed study on recombination of F6+ and C6+ ions with magnetically guided electrons we explored the yet unexplained rate enhancement, its dependence on the magnetic field B, the electron density n(e), and the beam temperatures T( perpendicular) and T( ||). The excess scales as T(-1/2)( perpendicular) and, surprisingly, as T(-1/2)( ||), increases strongly with B, and is insensitive to n(e). This puts strong constraints on explanations of the enhancement.     

Electron-positron annihilation into hadrons at high energies

In the limit of extremely high energies with the transverse momentum of each hadron fixed, the reaction e⁺ + e⁻ → hadrons proceeds by the annihilation of this pair into two virtual photons which turn into two fireballs of hadrons with negligible final-state interaction.     

Recombination of electrons with highly charged heavy ions at very low energies

Recombination of highly charged ions with free electrons is studied in merged-beams experiments at the UNILAC accelerator in Darmstadt and at the heavy-ion storage ring TSR in Heidelberg. Unexpected high recombination rates are observed for a number of ions at very low energiesE _cm in the electron-ion center-of-mass frame. In particular, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates of U²⁸⁺ ions nearE _cm=0 eV. The observations point to a general phenomenon in electron ion recombination depending onE _cm, on the ion charge state, and possibly also on electron density, electron beam temperature, and strength of external magnetic fields.     

Measurement of np elastic scattering at high energies and very small momentum transfers

The np elastic differential cross section has been measured for incident neutron momenta 100–400 GeV/c in the |t| range 6 · 10^?6 ? 5 · 10^?1 (GeV/c)². The np data of this experiment provide a first direct measurement of the hadronic amplitude for |t| < 10^?2 (GeV/c)², which is consistent with the extrapolations from higher |t| values. Our data for |t| < 10^?4 (GeV/c)² are consistent with a rise which can be attributed to Schwinger scattering, caused by the interaction of the neutron magnetic moment with the proton.     

Bethe-Heitler cross-section for very high photon energies and large muon scattering angles

The cross-section for the process $\gamma + A \rightarrow \mu^+ + \mu^- + X$ is studied where the photon energy is of the order of several hundreds of GeV and where one of the leptons is scattered to large angles. This is of practical importance for muon shielding calculations at future linear electron colliders. In addition to the photon pole contribution which was previously considered especially by Y.S.Tsai, we identify another component due to the muon pole (equivalent photon and equivalent muon approximation). This is discussed following the usual Bethe-Heitler formula. Then we give practically useful formulae for inclusive lepton (muon) production along with some numerical examples. Received: 22 January 1999 / Published online: 15 April 1999     

Measurement of the electron-positron annihilation cross-section into ππ,KK-pairs at the total energy 1.18−1.34 GeV

Using the electron-position storage ring VEPP-2 an experiment has been performed in which the cross-sections of the reactions e⁺e⁻→π⁺π⁻ and e⁺e⁻→K⁺K⁻ were measured in the energy regi on 1.18–1.34 GeV. The experimental values of the formfactors lie higher than curves extrapolated from the ?- and ?-meson region.     

Quantum gravity at very high energies

The problem of time and the quantization of three-dimensional gravity in the strong coupling regime is studied following path integral methods. The time is identified with the volume of spacetime. We show that the effective action describes an infinite set of massless relativistic particles moving in a curved three-dimensional target space, i.e., a tensionless 3-brane on a curved background. If the cosmological constant is zero the target space is flat and there is no ‘‘ graviton" propagation (i.e., G[g_ij(2),g_ij(1)]=0). If the cosmological constant is different from zero, 3D gravity is both classical and quantum mechanically soluble. Indeed, we find the following results: (i) the general exact solutions of the Einstein equations are singular at t=0 showing the existence of a big-bang in this regime and (ii) the propagation amplitude between two geometries 〈g_ij(2),t₂|g_ij(1),t₁〉 vanishes as t→0, suggesting that big-bang is suppressed quantum mechanically. This result is also valid for D>3.     

Two-photon processes at very high energies

Formulae are presented by which a numerically stable computer program for two-photon processes above $\text{s}\text{= 100}\text{GeV}$ can be constructed. For this purpose a new method for evaluating matrix elements is introduced. It is then shown that two-photon processes should be no background to Drell-Yan signals in pp collisions. A comparison is made between two-photon physics at e⁺e^?, ep and pp collisions.     

On parity-violating three-nucleon interactions and the predictive power of few-nucleon EFT at very low energies

We address the typical strengths of hadronic parity-violating three-nucleon interactions in “pion-less” Effective Field Theory (EFT) in the nucleon-deuteron (iso-doublet) system. By analysing the superficial degree of divergence of loop diagrams, we conclude that no such interactions are needed at leading order, O(eQ^-1)\ensuremath {O}(\epsilon Q^{-1}) . The only two distinct parity-violating three-nucleon structures with one derivative mix ²S_\frac12\ensuremath ^2S_{\frac{1}{2}} and ²P_\frac12\ensuremath ^2P_{\frac{1}{2}} waves with iso-spin transitions D \Delta I = 0 or 1. Due to their structure, they cannot absorb any divergence ostensibly appearing at next-to-leading order, O(eQ⁰)\ensuremath {O}(\epsilon Q^0) . This observation is based on the approximate realisation of Wigner’s combined SU(4) spin-isospin symmetry in the two-nucleon system, even when effective-range corrections are included. Parity-violating three-nucleon interactions thus only appear beyond next-to-leading order. This guarantees renormalisability of the theory to that order without introducing new, unknown coupling constants and allows the direct extraction of parity-violating two-nucleon interactions from three-nucleon experiments.