M. Poli

p̄p annihilation cross section at very low energy

Abstract The pp total annihilation cross section has been measured, with the Obelix apparatus at LEAR, at ten values of the antiproton incident momentum between 43 and 175 MeV/c. The values of the cross section show that the well known 1 p behaviour of the annihilation cross section is drastically modified at very low momenta, which demonstrates the important role of the Coulomb force in low energy pp interaction. Moreover, they do not present any explicit resonant behaviour. Finally, when compared to potential model calculations, the data suggest that the percentage of P-wave in pp interaction around 50 MeV/c antiproton incident momentum is less than 5%.

New measurements of the p̄p annihilation cross section at very low energy

Abstract The p p total annihilation cross section has been measured at four values of the p incident momentum, between 70 MeV/c and 38 MeV/c, with the Obelix apparatus at LEAR. The new measurements are in agreement with the trend of previous measurements of the p p total annihilation cross section at low energy, performed by the Obelix experiment [A. Bertin et al., Phys. Lett. B 369 (1996) 77; A. Benedettini et al., Nucl. Phys. B (Proc. Suppl.) 56A (1997) 58], as well as with a fit of the latter data based on a low energy expansion of the scattering amplitude [J. Carbonell, K.V. Protasov, A. Zenoni, Phys. Lett. B 397 (1997) 345]. The departure of the annihilation cross section from a smooth behaviour, suggested by a previous measurement of the cross section around 44 MeV/c [A. Bertin et al., Phys. Lett. B 369 (1996) 77], is not confirmed by the new data.

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.

P̄P partial cross sections at low energy

New measurements of the pp total annihilation cross-section at very low momenta (around 50 MeV/c) confirm the previous results obtained by the OBELIX collaboration. The measured values are in general agreement with a scattering length approximation of the annihilation cross-section where the pp scattering lengths are obtained from measurement of the protonium shifts and widths and which accounts for the Coulomb interaction. Partial annihilation cross-sections and annihilation frequencies for some channels have been measured too. The values of the total and partial cross sections at the lowest p momentum show an unexpected behaviour

Study of φ and f′2(1525) meson production in anti-p p annihilation at rest

Abstract The reaction p p→K + K − π 0 was analysed for antiproton annihilations at rest at three hydrogen target densities. A strong dependence of the p p→φπ 0 yield on the quantum numbers of the initial state is observed. The branching ratio of the φπ0 channel from the 3S1 initial state is more than 15 times larger than the one from the 1P1 state. A large apparent violation of the OZI rule for tensor meson production from p p -annihilations from the P-waves (1+++2++) is observed: Rexp(f′2π0/f2π0)=(149±20)·10−3, significantly exceeding the OZI-rule prediction R=16·10−3.

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.

Measurements of the pd annihilation at rest

The measurement of different reactions of d annihilation at rest in a gaseous target has been performed using the OBELIX spectrometer at LEAR (CERN). A strong deviation from the OZI-rule prediction was found from the measurement of the ratio in two regions of proton momenta, P 400 MeV/c: and (113 ± 30) × 10−3, respectively. These values are about 30 times greater than the theoretical prediction. For the first time the excitation of the †-resonance was observed among the final-state products of d annihilation. The existence of a broad enhancement in the 4π invariant mass at m ≈ 1480 MeV, seen in previous experiments, was confirmed. A ≈ 100 MeV downward shift of the bump position, when the proton momentum increased up to P > 400 MeV/c, was also observed, while the positions of ω, ϱ and f2(1270) did not change with the proton momentum. The following branching ratios were measured: ..

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.