F. Güttner

Measurement of the p̄p → n̄n cross section at low p̄ momenta

First measurements of the differential cross section are presented for pp → nn at LEAR in the momentum range between 180 and 600 MeV/c. The differential cross sections show a forward peaking followed by a smooth drop-off. No indication of the “bump-dip” structure reported at higher momenta has been observed. The angle-integrated charge-exchange cross sections are determined down to Enncm = 6.6 MeV.

Study of high energetic electrons emitted during heavy ion collisions

Double differential cross sectionsd2σ/dΩdE were measured for high energetic electrons emitted in heavy ion collisions. Electrons were detected in the energy range of 60 keV-500 keV for various target projectile combinations 66≦Zu=Zt+Zp≦145 and projectile velocities between 7 % and 10 % of the speed of light. Clear evidence was found that these electrons stem from the united atom formed during the collision. Slope and height of the spectra are discussed with respect to the momentum distribution of strongly bound states (i.e. theL-shell) at momenta far above the mean value. In addition for the systems S, Ni, Br→Pb electrons were detected in coincidence withK x-rays of Pb. By this method the contribution of theK-shell of the combined system to the total spectrum could be separated. Binding energies of theK-shell were estimated by a slope comparison between the coincident and single spectrum. The resulting values are close to the united atom limit.

Search for a narrow resonance in antiproton-proton annihilation cross sections in the beam momentum range between 400 and 600 MeV/c

Abstract A measurement of the p p annihilation cross section in the beam momentum region between 400 and 600 MeV/c has been performed with a mass resolution of 1 MeV/c2 and statistical precision generally better than 1%. No evidence for the X(1935) resonance is found. The upper limit at the 90% confidence level is 5 mb MeV/c2 for a width of 2 MeV/c2.

Measurements of the antiproton-proton annihilation cross-section in the beam momentum range between 180 and 600 MeV/c

The differential cross section for antiproton-proton elastic scattering has been measured for the beam momenta between 180 and 600 MeV/c. The real-to-imaginary ratio of the forward elastic scattering amplitude is derived from the Coulomb-nuclear interference. The ratio is found to be close to zero between 180 and 500 MeV/c with a minimum ofρ=−0.14 at 260 MeV/c. This result is contrary to model predictions. The phase shifts for thes-,p- andd-waves are extracted. The partial wave compositions of the elastic and inelastic cross sections have been determined. A large contribution of thep-wave is identified in the antiproton-proton interactions at small momenta.

An apparatus for the measurement of antiproton-proton cross sections at low momenta

An experiment which aims at a precise measurement of pp cross sections at low momenta is presented. Its design and performance are described with emphasis on the particle identification capabilities. A calorimeter-like detector array which serves for the detection of antineutrons is discussed in some detail. Its properties were measured in a test experiment and are compared to Monte Carlo calculations.

Antiproton-Proton Cross Sections at Small Momenta

Since the startup of LEAR in 1983 we have undertaken a series of measurements of antiproton-proton cross sections for annihilation (pp → mesons)1, charge-exchange reaction (pp → nn)2, and elastic scattering (pp → pp)3,4. We have focussed our efforts of the measurements on the beam momentum range between 160 and 600 MeV/c. Our detector is optimized to handle the antiproton beam at very small momenta5. The liquid hydrogen target is placed in the vacuum tank, and the vacuum is directly connected to the beam line. The incident beam is defined by thin scintillators also placed in the vacuum. With the apparatus of these unique designs we have explored the beam momentum region below 300 MeV/c with a good precision.