A. Adamo

Meson spectroscopy with antineutrons

Abstract First results on meson spectroscopy using antineutrons as projectiles are reported. Three-body (π + π + π − ) and five-body (π + π + π + π − π − ) final states following the annihilation of 100 to 290 MeV/ c antineutrons on hydrogen were analyzed. The (π + π − ) invariant mass spectrum in the three-body reaction is accounted for by the ϱ 0 anf f 2 (1270) resonances, together with an enhancement around 1540 MeV, in the region of the AX (1565) and the f 2 (1515) resonances, whereas the best fit for the (π + π + π − π − ) invariant mass in five-body events shows the already reported structure ζ(1480) and suggests the presence of a second structure around 1650 MeV.

A measurement of the ratio from annihilation in deuterium and hydrogen gas

Abstract We have measured the K + K - /π + π - ratio R from p annihilations in gaseous D 2 and H 2 at atmospheric pressure. The measurement was performed with the OBELIX spectrometer. From the measured value in gaseous D 2 (0.27±0.02) we infer a P wave contribution to p −p annihilation in D 2 of (18±7)%.

Protonium annihilation in P-wave using low-density (ϱ/ϱ∼10−3) hydrogen targets. Measurements of cascade times and widths

Abstract The use of low-density hydrogen targets is demonstrated experimentally to be a new way of selecting protonium ( p p atom) annihilations with initial angular momentum L = 1. The mean value and the width of the time distributions which occur between protonium formation and annihilation have been measured at room temperature at 8.2, 4.1, 2.1 mbar. They are (110±10; 45±5), (145±15; 82±9) and (210±25;125±9) ns, respectively.

An experimental study of antiproton-4He annihilation at rest

This paper reports new experimental results on the annihilation of antiprotons stopped in a4He gas target. The measurement was carried out with the Obelix spectrometer installed at the LEAR machine of CERN. Main results can be summarized as follows, (i) The invariant mass distribution of 2π+2π− produced at rest is similar to those from hydrogen and deuterium targets: the peak seen in recent experiments around 1150 MeV/c2 is absent, (ii) 2π angular correlations are very close to those found for hydrogen. The nuclear structure does not modify remarkably the pion production and emission, (iii) πp and pp angular correlations are strongly peaked close to 180°, in qualitative agreement with INC model predictions, (iv) The pp invariant mass distribution reveals events which may be due to η(ω) absorption; the estimated upper limit for probability of π and ω absorption is about 3 × 10−3. (v) Events having features compatible with two-nucleon annihilations with production of heavy mesonic systems are detected both in the 2π+ 2π− and the 2π−π+ channels. The production rate of the 2π+ 2π− channel is of the order of 5 × 10−3.

A new measurement of the Pontecorvo reaction anti-p+d --> pi- +p with the OBELIX spectrometer at LEAR

Abstract Antinucleon-nucleus annihilations into two-body final states containing only one or no meson are unusual annihilations (Pontecorvo reactions), practically unexplored experimentally, with the exception of the channel p d → π − p , for which only two low-statistics measurements exist. Their physical interest lies in the possibility of exploring small-distance nuclear dynamics, in which an important role can be played by non-nucleonic degrees of freedom. A new measurement of the p d → π − p reaction rate at rest, performed with the OBELIX spectrometer at LEAR, with the best statistics up to now and a careful evaluation of systematic effects is reported, together with a critical analysis of the existing theoretical models. The measured branching ratio, which confirms the previous results, can represent a reference point for the studies in the field.

New results on meson spectroscopy from Obelix

Abstract A preliminary final state analysis of a sample of n p in-flight annihilations into three and five charged pions is presented. The partial-wave analysis of three-prong data confirms previous interpretations of the Ax ƒ 2 (1520) resonance, from p p annihilations at rest. Concerning five-prong events our analysis favours spin 0 for the already reported ζ (1480) state, but more refined calculations are underway. At high n momenta, the five-prong data show some evidence for a possible spin 2 state at 1640 MeV/c 2 decaying in ρ ° ρ °.

Anti-p stopping power in hydrogen below 1 MeV and the anti-p 4He metastable state

Abstract The stopping power of H, for p S was measured in the p energy interval 1 keV + 120 keV using a low density gas target ( p pρ ≈ l0 −3 + 10 −2 ÷ pρ being the Hz density at STP) with the OBELIX spectrometer at the LEAR of CEBN. Furthermore, the formation of metastable states in the p 4 He interaction at rest, having a mean life of the order of 3 μs, was investigated at six target gas pressures (3 atm and 400,250,200,150,50 mbar). The influence of contaminants (like Ha, Ne, Ar, N2 and Xe) on the life of such metastable states was examined at the target gas pressure of 3 atm.

Antinucleon-nucleon annihilation cross section measurements at low energies

Abstract First results concerning p -p and n -p annihilation cross section and n -p total cross sections at low energies ( 2.25 ≤ E P ≤ 22.76 MeV and 3.4 E n MeV ) obtained by the OBELIX spectrometer are reported. The agreement with the few existing data in this energy range is good; the comparison with some models gives interesting indications on the S and P wave contribution at very low energy and on the range of the elastic interaction.

Annihilation cross-sections of antineutrons on C, Al, Cu, Sn and Pb at low momenta (180–280 MeV/c) with the OBELIX spectrometer

SummaryAnnihilation cross-sections σann for antineutrons on some nuclei (C, Al, Cu, Sn and Pb) at three antineutron momenta (180, 240 and 280 MeV/c) were measured at LEAR (CERN) with the OBELIX spectrometer. A behaviour σann=σ0Aν has been found withν≈2/3. The data are discussed following some models for antineutron-nucleus interaction.

Nuclear physics with OBELIX

Abstract Recent experimental results in antinucleon-nucleus interaction obtained by the OBELIX collaboration at LEAR are presented. Preliminary results concerning the total antineutron-nucleus annihilation cross section at low energy are reported. They represent the first complete set of measurements on medium and heavy nuclei available in the momentum region below 300 MeV/c. A new measurement of the exotic one-meson annihilation at rest p d → π − p (Pontecorvo reaction) with the better statistics and precision up to now, is reported. Finally, the measurement