O.E. Gorchakov

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.

Observations of atoms consisting of π+ and π− mesons

Abstract In the experiment 272±49 atoms consisting of π + and π + − mesons were observed. The atoms were produced in a Ta target by 70 GeV protons and π + and π − mesons were detected from the atom break-up in the same target.

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.

Experimental estimation of the lifetime of atoms formed by π+ and π− mesons

Abstract Based on observation [L.G. Afanasyev et al., Phys. Lett. B 308 (1993) 200] of 272±49 atoms formed by π + and π − an estimation of the atom lifetime in the ground (1S) state is obtained: τ 1 = (2.9 +∞ −2.1 ) · 10 −15 s or τ 1 > 0.6·10 −15 s at 90% confidence level. Atoms were produced in inclusive p Ta interactions at 70 GeV.

Antineutron-proton total cross section from 50 to 400 MeV/c

The antineutron-proton total doss section has been measured in the low momentum range 50-400 MeV/c (below 100 MeV/c for the first time). The measurement was performed at LEAR (CERN) by the OBELIX experiment, thanks to its unique antineutron beam facility. A thick target transmission technique has been used. The measured total cross section shows an anomalous behaviour below 100 MeV/c. A dominance of the isospin I = 0 channel over the I = 1 one at low energy is clearly deduced

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.

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.

Evidence for two pseudoscalar states in the 1.4–1.5 GeV mass region

Abstract A confirming evidence for two pseudoscalar states with masses M 1 =1405±5, Γ 1 =50±4 MeV/ c 2 and M 2 =1500±10, Γ 2 =100±20 MeV/ c 2 is reported. The second pseudoscalar fills the nonet and leaves the first as an outsider supporting the hypothesis for its exotic nature.

Study of the isovector scalar mesons in the channel p̄p→K±K0Sπ∓ at rest with initial angular momentum state selection

Abstract The study of the I =1, J PC =0 ++ states with the OBELIX detector in the channel p p→K ± K 0 S π ∓ at three different target densities is reported. The data show the evidence for an extra scalar state with mass 1.29±0.01 GeV/ c 2 and width 0.080±0.005 GeV/ c 2 .