P. Boccaccio

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.

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.

Measurement of the pp annihilation cross-sections at very low energies

Abstract The p p annihilation cross-sections at 66.4–68.9–71.4 MeV/c have been measured using the presently instrumented part of the OBELIX spectrometer. The results have been obtained at p energies ( ≃ 2.5 MeV) by one order of magnitude smaller than in any existing experiment concerning the in-flight antiproton-proton annihilation. The average value obtained over the total interval 65.1–72.5 MeV/c, βσanT = 33.2 ± 1.7 (stat.) ±1.8 (syst.) mb, in agreement with a specific calculation, would favour for the spin-averaged S-wave scattering length the value (−0.7 + i0.8) fm. The branching ratios for the annihilations in two, four and six charged mesons turn out to be quite similar to those for annihilation at rest in liquid hydrogen.

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 °.

Measurement of the frequency of the annihilation reaction anti-p p -> pi0 pi0 at rest in a NTP hydrogen target

Abstract The annihilation frequency of the reaction p p → π 0 π 0 has been measured, for the first time by stopping antiprotons in a NTP hydrogen target. The obtained result ƒ(π 0 π 0 ) NTP = (12.7±0.5±2.0)×10 −4 is discussed in the frame of the study of the initial angular momentum of the pp system.

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.