arXiv subjects
Publications and source records attributed to A. Filippi.
Search for the neutron-rich hypernucleus 9LHe is reported by the FINUDA experiment at DAFNE, INFN-LNF, studying (pi+, pi-) pairs in coincidence from the K-stop + 9Be --> 9LHe + pi+ production reaction followed by 9LHe --> 9Li + pi- weak decay. An upper limit of the production rate of 9LHe undergoing this two-body pi- decay is determined to be (2.3 +/- 1.9) 10-6/K-stop at 90% confidence level.
The search of nuclear bound states of anti-K in few-body nuclear systems such as K-pp, can be extended from the nuclear medium to the vacuum, using the glue-rich Y(1S) decays at B-factories. Here the possibility for such a measurement at the future SuperB factory is discussed.
Three candidate events of the neutron-rich hypernucleus 6ΛH were uniquely identified in the FINUDA experiment at DAΦNE, Frascati, by observing π+ mesons from the (K-stop,π+) production reaction on 6Li targets, in coincidence with π-mesons from 6ΛH \rightarrow 6He+π- weak decay. Details of the experiment and the analysis of its data are reported, leading to an estimate of (2.9\pm2.0)\cdot10-6/K- stop for the 6ΛH production rate times the two-body π- weak decay branching ratio. The 6ΛH binding energy with respect to 5H + Λ was determined jointly from production and decay to be BΛ = (4.0 \pm 1.1) MeV, assuming that 5H is unbound with respect to 3H + 2n 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 \pm 0.74) MeV here assigned to the 0+g.s. \rightarrow 1+ excitation. The consequences of this assignment to Λ hypernuclear dynamics are briefly discussed.
A direct experimental evidence of the occurrence of the weak reaction $Λnp\rightarrow nnp$ in nuclei has been obtained by the FINUDA experiment. Three events have been found that can be attributed to $^{7}_Λ$Li and $^{9}_Λ$Be two nucleon-induced non mesonic weak decays. The kinematic analysis of such events is presented here.
This paper reports results from a study of the reaction pp->pK0Σ+ at beam momenta of p_{beam} = 2950, 3059, and 3200 MeV/c (excess energies of ε= 126, 161, and 206 MeV). Total cross sections were determined for all energies; a set of differential cross sections (Dalitz plots; invariant mass spectra of all two-body subsystems; angular distributions of all final state particles; distributions in helicity and Jackson frames) are presented for ε= 161 MeV. The total cross sections are proportional to the volume of available three-body phase-space indicating that the transition matrix element does not change significantly in this range of excess energies. It is concluded from the differential data that the reaction proceeds dominantly via the N(1710)P_{11} and/or N(1720)P_{13} resonance(s); N(1650)S_{11} and Δ(1600)P_{33} could also contribute.
Evidence for the neutron-rich hypernucleus 6ΛH is presented from the FINUDA experiment at DAΦNE, Frascati, studying (π+, π-) pairs in coincidence from the K- +6Li \rightarrow 6 H+π+ production reaction followed by 6ΛH \rightarrow 6He + π- weak decay. The production rate of 6ΛH undergoing this two-body π- decay is determined to be (2.9\pm2.0)\cdot10-6/K-. Its binding energy, evaluated jointly from production and decay, is BΛ(6ΛH) = (4.0\pm1.1) MeV with respect to 5H+Λ. A systematic difference of (0.98 \pm 0.74) MeV between BΛ values derived separately from decay and from production is tentatively assigned to the 6ΛH 0+g.s. \rightarrow 1+ excitation.
This letter is concerned with the study of the $K^-_{stop}A\rightarrow π^\pmΣ^\mp A'$ reaction in p-shell nuclei, i.e., $^{6,7}Li$, $^9Be$, $^{13}C$ and $^{16}O$. The $π^\pmΣ^\mp / K^-_{stop}$ emission rates are reported as a function of $A$. These rates are discussed in comparison with previous findings. The ratio $π^-Σ^+/π^+Σ^-$ in p-shell nuclei is found to depart largely from that on hydrogen, which provides support for large in-medium effects possibly generated by the sub-threshold $Λ(1405)$. The continuum momentum spectra of prompt pions and free sigmas are also discussed as well as the $π^\pmΣ^\mp$ missing mass behavior and the link with the reaction mechanism. The apparatus used for the investigation is the FINUDA spectrometer operating at the DA$Φ$NE $ϕ$-factory (LNF-INFN, Italy).
The Non-Mesonic (NM) decay of $^4_Λ{\mathrm{He}}$ and $^5_Λ{\mathrm{He}}$ in two-body channels has been studied by the FINUDA Collaboration. Two-body NM decays of hypernuclei are rare and the existing observations and theoretical calculations are scarce. The $^4_Λ{\mathrm{He}}\rightarrow d+d,\; p+t$ decay channels simultaneously observed by FINUDA on several nuclei are compared: the $pt$ channel is the dominant one. The decay yields for the two decay channels are assessed for the first time: they are $(1.37\pm 0.37)\times 10^{-5}/K^-_{stop}$ and $(8.3\pm 1.0)\times 10^{-5}/K^-_{stop}$, respectively. Due to the capability of FINUDA of identifying $^5_Λ{\mathrm{He}}$ hypernuclei, a few $^5_Λ{\mathrm{He}}\rightarrow d+t$ decay events have also been observed. The branching ratio for this decay channel has been measured for the first time: $(3.0\pm 2.3)\times 10^{-3}$.
The FINUDA experiment collected data to study the production of hypernuclei on different nuclear targets. The hypernucleus formation occurred through the strangeness-exchange reaction $K^-_{stop} + \; ^AZ \rightarrow \; ^A_ΛZ + π^-$. From the analysis of the momentum of the emerging $π^-$, binding energies and formation probabilities of $^7_Λ$Li, $^9_Λ$Be, $^{13}_Λ$C and $^{16}_Λ$O 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.
The decay of $Λ$-hypernuclei without pion emission, known as Non Mesonic Weak Decay (NMWD), gives an effective tool to investigate $Δ$S=1 four-baryon interactions. It was theoretically suggested that the two-nucleon induced mechanism could play a substantial role in reproducing the observed NMWD decay rates and nucleon spectra, but at present no direct evidence of such a mechanism has been obtained. The FINUDA experiment, exploiting the possibility to detect both charged and neutral particles coming from the hypernucleus decay, has allowed us to deduce the relative weight of the two nucleon induced decay rate to the total NMWD rate. The value of $Γ_{2N}$/$Γ_{NMWD}$=0.24$\pm$${0.03_{stat}}^{+0.03_{sys}}_{-{0.02_{sys}}}$ has been deduced, with an error reduced by a factor more than two compared with the previous assessment.
This paper reports results on simultaneous measurements of the reaction channels pp -> pK+Λand pp -> pK+Σ^0 at excess energies of 204, 239, and 284 MeV (Λ) and 127, 162, and 207 MeV (Σ^0). Total and differential cross sections are given for both reactions. It is concluded from the measured total cross sections that the high energy limit of the cross section ratio is almost reached at an excess energy of only about 200 MeV. From the differential distributions observed in the overall CMS as well as in the Jackson and helicity frames, a significant contribution of interfering nucleon resonances to the Λproduction mechanism is concluded while resonant Σ^0-production seems to be of lesser importance and takes place only through specific partial waves of the entrance channel. The data also indicate that kaon exchange plays a minor role in the case of Λ- but an important role for Σ^0-production. Thus the peculiar energy dependence of the Λ-to-Σ^0 cross section ratio appears in a new light as its explanation requires more than mere differences between the pΛand the pΣ^0 final state interaction. The data provide a benchmark for theoretical models already available or yet to come.
Hyperon production in the threshold region was studied in the reaction pp -> K+Lp using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering the full phase-space, were taken at the three different beam momenta of p_beam=2.95, 3.20 and 3.30 GeV/c, corresponding to excess energies of epsilon=204, 285 and 316 MeV, respectively. Total cross-sections were deduced for the three beam momenta to be 23.9+/-0.8 +/-2.0 ub, 28.4+/-1.3 +/-2.2 ub and 35.0+/-1.3 +/-3.0 ub. Differential observables including Dalitz plots were obtained. The analysis of the Dalitz plots reveals a strong influence of the N(1650)-resonance at p_beam=2.95 GeV/c, whereas for the higher momenta an increasing relative contribution of the N(1710)- and/or of the N(1720)-resonance was observed. In addition, the pL-final-state interaction turned out to have a significant influence on the Dalitz plot distribution.
New spectra from the FINUDA experiment of the Non Mesonic Weak Decay (NMWD) proton kinetic energy for 9(Lambda)Be, 11(Lambda)B, 12(Lambda)C, 13(Lambda)C, 15 (Lambda)N and 16(Lambda)O are presented and discussed along with the published data on 5(Lambda)He and 7(Lambda)Li. 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 Gamma2/Gammap=0.43+-0.25 is determined for the first time. A value for the two nucleon stimulated decay rate to the total decay rate Gamma2/GammaNMWD=0.24+-0.10 is also extracted.
A systematic study of the production of omega-mesons in proton-proton-collisions was carried out in a kinematically complete experiment at three excess energies(epsilon= 92, 128, 173MeV). Both protons were detected using the large-acceptance COSY-TOF spectrometer at an external beam line at the Cooler Synchrotron COSY at Forschungszentrum Jülich. The total cross section, angular distributions of both omega-mesons and protons were measured and presented in various reference frames such as the overall CMS, helicity and Jackson frame. In addition, the orientation of the omega-spin and invariant mass spectra were determined. We observe omega-production to take place dominantly in Ss and Sp final states at epsilon = 92, 128 MeV and, additionally, in Sd at epsilon= 173 MeV. No obvious indication of resonant omega-production via N^*-resonances was found, as proton angular distributions are almost isotropic and invariant mass spectra are compatible with phase space distributions. A dominant role of ^3P_1 and ^1S_0 initial partial waves for omega-production was concluded from the orientation of the decay plane of the omega-meson. Although the Jackson angle distributions in the omega-p-Jackson frame are anisotropic we argue that this is not an indication of a resonance but rather a kinematical effect reflecting the anisotropy of the omega angular distribution. The helicity angle distribution in the omega-p-helicity frame shows an anisotropy which probably reflects effects of the omega angular momenta in the final state; this observable may be, in addition to the orientation of the omega decay plane, the most sensitive one to judge the validity of theoretical descriptions of the production process.
The FINUDA experiment performed a systematic study of the charged mesonic weak decay channel of $p$-shell $Λ$-hypernuclei. Negatively charged pion spectra from mesonic decay were measured with magnetic analysis for the first time for ${^{7}_ΛLi}$, ${^{9}_ΛBe}$, ${^{11}_ΛB}$ and ${^{15}_ΛN}$. The shape of the $π^{-}$ spectra was interpreted through a comparison with pion distorted wave calculations that take into account the structure of both hypernucleus and daughter nucleus. Branching ratios $Γ_{π^{-}}/Γ_{tot}$ were derived from the measured spectra and converted to $π^{-}$ decay rates $Γ_{π^{-}}$ by means of known or extrapolated total decay widths $Γ_{tot}$ of $p$-shell $Λ$-hypernuclei. Based on these measurements, the spin-parity assignment $1/2^+$ for ${^{7}_ΛLi}$ and $5/2^+$ for ${^{11}_ΛB}$ ground-state are confirmed and a spin-parity $3/2^+$ for ${^{15}_ΛN}$ ground-state is assigned for the first time.
Data accumulated recently for the exclusive measurement of the $pp\to ppπ^+π^-$ reaction at a beam energy of 0.793 GeV using the COSY-TOF spectrometer have been analyzed with respect to possible events from the $pp \to nnπ^+π^+$ reaction channel. The latter is expected to be the only $ππ$ production channel, which contains no major contributions from resonance excitation close to threshold and hence should be a good testing ground for chiral dynamics in the $ππ$ production process. No single event has been found, which meets all conditions for being a candidate for the $pp \to nn π^+π^+$ reaction. This gives an upper limit for the cross section of 0.16 $μ$b (90% C.L.), which is more than an order of magnitude smaller than the cross sections of the other two-pion production channels at the same incident energy.
The single-pion production reactions $pp\to dπ^+$, $pp\to npπ^+$ and $pp\to ppπ^0$ were measured at a beam momentum of 0.95 GeV/c ($T_p \approx$ 400 MeV) using the short version of the COSY-TOF spectrometer. The central calorimeter provided particle identification, energy determination and neutron detection in addition to time-of-flight and angle measurements from other detector parts. Thus all pion production channels were recorded with 1-4 overconstraints. Main emphasis is put on the presentation and discussion of the $npπ^+$ channel, since the results on the other channels have already been published previously. The total and differential cross sections obtained are compared to theoretical calculations. In contrast to the $ppπ^0$ channel we find in the $npπ^+$ channel a strong influence of the $Δ$ excitation already at this energy close to threshold. In particular we find a $(3 cos^2Θ+ 1)$ dependence in the pion angular distribution, typical for a pure s-channel $Δ$ excitation and identical to that observed in the $dπ^+$ channel. Since the latter is understood by a s-channel resonance in the $^1D_2$ $pn$ partial wave, we discuss an analogous scenario for the $pnπ^+$ channel.
The two-pion production reaction $\vec{p}p\to ppπ^+π^-$ was measured with a polarized proton beam at $T_p \approx$ 750 and 800 MeV using the short version of the COSY-TOF spectrometer. The implementation of a delayed pulse technique for Quirl and central calorimeter provided positive $π^+$ identification in addition to the standard particle identification, energy determination as well as time-of-flight and angle measurements. Thus all four-momenta of the emerging particles could be determined with 1-4 overconstraints. Total and differential cross sections as well as angular distributions of the vector analyzing power have been obtained. They are compared to previous data and theoretical calculations. In contrast to predictions we find significant analyzing power values up to $A_y$ = 0.3.