E. Botta

First results on 12ΛC production at DAΦNE

Lambda-hypernuclei are produced and studied, with the FINUDA spectrometer, for the first time at an e+e- collider: DAPHNE, the Frascati phi-factory. The slow negative kaons from phi(1020) decay are stopped in thin (0.2 g/cm^2) nuclear targets, and Lambda-hypernuclei formation is detected by measuring the momentum of the outgoing pi^-. A preliminary analysis on 12Lambda-C shows an energy resolution of 1.29 MeV FWHM on the hypernuclear levels, the best obtained so far with magnetic spectrometers at hadron facilities. Capture rates for the ground state and the excited ones are reported, and compared with previous experiments.

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%.

Evidence for heavy hyperhydrogen (Lambda)H-6

Evidence for the neutron-rich hypernucleus H-6(Lambda) is presented from the FINUDA experiment at DA Phi NE, Frascati, studying (pi(+), pi(-)) pairs in coincidence from the (K) over bar (-)(stop) + Li-6 -> H-6(Lambda) + pi(+) production reaction followed by H-6(Lambda) -> He-6 + pi(-) weak decay. The production rate of H-6(Lambda) undergoing this two-body pi(-) decay is determined to be (2.9 +/- 2.0) x 10(-6)/K-stop(-). Its binding energy, evaluated jointly from production and decay, is B-Lambda(H-6(Lambda)) = (4.0 +/- 1.1) MeV with respect to 5H + Lambda. A systematic difference of (0.98 +/- 0.74) MeV between B-Lambda values derived separately from decay and from production is tentatively assigned to the H-6(Lambda) 0(g.s)(+) excitationEvidence for the neutron-rich hypernucleus 6{\Lambda}H is presented from the FINUDA experiment at DA{\Phi}NE, Frascati, studying ({\pi}+, {\pi}-) pairs in coincidence from the K- +6Li \rightarrow 6 H+{\pi}+ production reaction followed by 6{\Lambda}H \rightarrow 6He + {\pi}- weak decay. The production rate of 6{\Lambda}H undergoing this two-body {\pi}- decay is determined to be (2.9\pm2.0)\cdot10-6/K-. Its binding energy, evaluated jointly from production and decay, is B{\Lambda}(6{\Lambda}H) = (4.0\pm1.1) MeV with respect to 5H+{\Lambda}. A systematic difference of (0.98 \pm 0.74) MeV between B{\Lambda} values derived separately from decay and from production is tentatively assigned to the 6{\Lambda}H 0+g.s. \rightarrow 1+ excitation.

Evidence for 6{\Lambda}H

Evidence for the neutron-rich hypernucleus H-6(Lambda) is presented from the FINUDA experiment at DA Phi NE, Frascati, studying (pi(+), pi(-)) pairs in coincidence from the (K) over bar (-)(stop) + Li-6 -> H-6(Lambda) + pi(+) production reaction followed by H-6(Lambda) -> He-6 + pi(-) weak decay. The production rate of H-6(Lambda) undergoing this two-body pi(-) decay is determined to be (2.9 +/- 2.0) x 10(-6)/K-stop(-). Its binding energy, evaluated jointly from production and decay, is B-Lambda(H-6(Lambda)) = (4.0 +/- 1.1) MeV with respect to 5H + Lambda. A systematic difference of (0.98 +/- 0.74) MeV between B-Lambda values derived separately from decay and from production is tentatively assigned to the H-6(Lambda) 0(g.s)(+) excitationEvidence for the neutron-rich hypernucleus 6{\Lambda}H is presented from the FINUDA experiment at DA{\Phi}NE, Frascati, studying ({\pi}+, {\pi}-) pairs in coincidence from the K- +6Li \rightarrow 6 H+{\pi}+ production reaction followed by 6{\Lambda}H \rightarrow 6He + {\pi}- weak decay. The production rate of 6{\Lambda}H undergoing this two-body {\pi}- decay is determined to be (2.9\pm2.0)\cdot10-6/K-. Its binding energy, evaluated jointly from production and decay, is B{\Lambda}(6{\Lambda}H) = (4.0\pm1.1) MeV with respect to 5H+{\Lambda}. A systematic difference of (0.98 \pm 0.74) MeV between B{\Lambda} values derived separately from decay and from production is tentatively assigned to the 6{\Lambda}H 0+g.s. \rightarrow 1+ excitation.

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.

First observation of the hyper superheavy hydrogen HΛ6

Abstract Three candidate events of the neutron-rich hypernucleus H Λ 6 were uniquely identified in the FINUDA experiment at DAΦNE, Frascati, by observing π + mesons from the ( K stop − , π + ) production reaction on 6 Li targets, in coincidence with π − mesons from H Λ 6 → He 6 + π − weak decay. Details of the experiment and the analysis of its data are reported, leading to an estimate of ( 2.9 ± 2.0 ) ⋅ 10 − 6 / K stop − for the H Λ 6 production rate times the two-body π − weak decay branching ratio. The H Λ 6 binding energy with respect to H 5 + Λ was determined jointly from production and decay to be B Λ = ( 4.0 ± 1.1 ) MeV , assuming that 5 H is unbound with respect to H 3 + 2 n by 1.7 MeV. The binding energy determined from production is higher, in each one of the three events, than that determined from decay, with a difference of ( 0.98 ± 0.74 ) MeV here assigned to the 0 g.s. + → 1 + excitation. The consequences of this assignment to Λ hypernuclear dynamics are briefly discussed.

Proton spectra from Non-Mesonic Weak Decay of p-shell Λ-hypernuclei and evidence for the two-nucleon induced process

Abstract New spectra from the FINUDA experiment of the Non-Mesonic Weak Decay (NMWD) proton kinetic energy for Be Λ 9 , B Λ 11 , C Λ 12 , C Λ 13 , N Λ 15 and O Λ 16 are presented and discussed along with the published data on He Λ 5 and Li Λ 7 . Exploiting the large mass number range and the low energy threshold (15 MeV) for the proton detection of FINUDA, an evaluation of both Final State Interactions (FSI) and the two-nucleon induced NMWD contributions to the decay process has been done. Based on this evaluation, a linear dependence of FSI on the hypernuclear mass number A is found and for the two-nucleon stimulated decay rate the experimental value of Γ 2 / Γ p = 0.43 ± 0.25 is determined for the first time. A value for the two-nucleon stimulated decay rate to the total decay rate Γ 2 / Γ NMWD = 0.24 ± 0.10 is also extracted.

Hypernuclear spectroscopy with K− at rest on 7Li, 9Be, 13C and 16O

Abstract The FINUDA experiment collected data to study the production of hypernuclei on different nuclear targets. The hypernucleus formation occurred through the strangeness-exchange reaction K stop − + Z A → Z Λ A + π − . From the analysis of the momentum of the emerging π − , binding energies and formation probabilities of Li Λ 7 , Be Λ 9 , C Λ 13 and O Λ 16 have been measured and are here presented. The behavior of the formation probability as a function of the atomic mass number A is also discussed.

Measurement of the proton spectra from non-mesonic weak decay of 5ΛHe, 7ΛLi and 12ΛC

Abstract The results of a measurement of the proton spectra following the non-mesonic weak decay of 5ΛHe, 7ΛLi and 12ΛC are presented and discussed. The experiment was performed at the ( e + e − ) collider DAΦNE at Laboratori Nazionale di Frascati of INFN. It is the first measurement for 7ΛLi, and for all the spectra the lower limit on the energy of the protons is 15 MeV, never reached before. All the spectra show a similar shape, namely a peak at around 80 MeV as expected for the free Λ p → n p weak reaction, with a low energy rise that should be due to final state interactions and/or two-nucleon induced weak processes. The decay spectrum of 5ΛHe is somehow similar to the ones reported by previous measurements and theoretical calculations, but the same does not happen for the 12ΛC one.

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.