U. Gastaldi

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.

Observation of muonic X-rays in helium gas and measurement of the two-photon decay rate of the 2s metastable state

SummaryWe have measured the yield of theK andL lines of the muonic helium system stopping negative muons in a gaseous helium target (7 atm, 293 °K). We have also observed the two-photon decay of the ionie metastable 2s state and we found that this system is formed in (3.4 ± 0.7)% of the stopped muons.RiassuntoAbbiamo misurato le intensità delle lineeK edL del sistema muonico eccitato (μHe) fermando muoni negativi in elio gassoso (7 atm, 293 °K). Abbiamo anche osservato il decadimento, via due fotoni, del sistema ionico metastabile (μHe)2s+ ed è stato trovato che questo si forma nel (3.4 ± 0.7)% dei casi in cui un muone è fermato.РеэюмеМы иэмерили интенсивностьK- иL-линий системы меэоатомов гелия, обраэуюшихся при остановке отрицательных мюонов в мищени гаэообраэного гелия (при давлении 7 атмосферы и температуре 293 °К). Мы также наблюдали двух-фотонный распад метастабильного 2s состояния иона меэоатома гелия (μНе)+ и обнаружили, что зта система обраэуется в (3.4 ± 0.7)% случаев остановки мюонов.

φ and ω meson production in n¯p annihilation and the OZI rule

Abstract The φπ + /ωπ + ratio from n ¯ p annihilations on a liquid hydrogen target, for n ¯ momenta between 64 and 297 MeV/ c , was measured using the OBELIX spectrometer at LEAR. The ratio R ( ϕ π / ω π ) = σ ( n ¯ p → ϕ π + ) / σ ( n ¯ p → ω π + ) turned out 0.110±0.015 stat ±0.006 syst . Implications of this result on the OZI rule are discussed.

p̄D and p̄ 4He annihilation cross sections at very low energy

Abstract The p D and p 4 He total annihilation cross sections have been measured with the Obelix apparatus at LEAR at, respectively, three and two values of the p incident momentum between 70 MeV/c and 36 MeV/c. The values of the p D annihilation cross section, at such low energies, are in agreement with a surprising result obtained in a recent low statistics measurement of the antiprotonic deuterium K α X-rays [M. Augsburger et al., Phys. Lett. B 461 (1999) 417] (see following article), concerning the unexpected narrow width of the antiprotonic deuterium 1s level.

Measurement of the 2P3/2−S1/2 energy difference in the (μ−4He)+ muonic ion by means of a tunable infrared dye-laser

Abstract Results of an experiment to measure the 2P 3/2 −2S 1/2 energy difference δ 1 in the muonic (μ −4 H e) + ion are presented; we found δ 1 exp = 1.5274 ± 0.0009 eV. Comparison with QED predictions shows that there is agreement between theory and experiment within 0.0087 eV; this limitation is mainly due to the uncertainty with which the r.m.s. charge radius of the 4 He nucleus is known.

Spin-parity analysis of the final state π+π−π0 from pp annihilation at rest in hydrogen targets at three densities

Abstract The partial wave analysis of the reaction p p → π + π − π 0 at rest has been performed for the first time by using high-statistics data sets collected in hydrogen targets at three different densities. The different mixtures of partial waves corresponding to our samples allow a reliable determination of each contribution. This technique is crucial in the identification of the exotic candidate f2(1565) which can be produced by 1S0, 3P1 and 3P2 initial states. The amplitude analysis was performed in the frame of the K-Matrix and P-Vector approach. It requires presence of the exotic candidates f0(1500) and f2(1565) with the following masses and total widths: (1449 ± 20)MeV, (114 ± 30)MeV and (1507 ± 15)MeV, (130 ± 20)MeV respectively. In addition to f0(980), f0(1300) f2(1270) and ϱ(770), a clear evidence of I = 1, JP = 1− signal, which we identify with ϱ(1450), is obtained.

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.