The experimental data obtained from the reaction of 6 Li projectiles at 2A GeV on a fixed graphite target were analyzed to study the invariant mass distributions of d + π − and t + π − . Indications of a signal in the d + π − and t + π − invariant mass distributions were observed with significances of 5.3 σ and 5.0 σ , respectively, when including the production target, and 3.7 σ and 5.2 σ , respectively, when excluding the target. The estimated mean values of the invariant mass for d + π − and t + π − signal were 2059.3 ± 1.3 ± 1.7 MeV/c 2 and 2993.7 ± 1.3 ± 0.6 MeV/c 2 respectively. The lifetime estimation of the possible bound states yielding to d + π − and t + π − final states were deduced to be as 181 +30 −24 ± 25 ps and 190 +47 −35 ± 36 ps, respectively. Those final states may be interpreted as the two-body and three-body decay modes of a neutral bound state of two neutrons and a hyperon, 3 n.A hypernucleus, a subatomic system with at least one bound hyperon, is studied in order to deduce the information about fundamental hyperon (Y )-nucleon (N) and Y -Y interactions. Hypernuclei have been mainly studied by means of the missing-mass experiments with secondary-meson and primary-electron beams [1] and earlier with emulsion techniques and bubble chambers [2]. In these experiments, a variety of hypernuclei with the lightest hyperon, the hyperon, were produced and identified. However, the isospin of the produced hypernuclei is similar to that of the target nucleus in these experiments, since they are produced by the elementary process of converting one nucleon in the target nucleus into a hyperon.Information on the Λ-N interaction was already inferred from the hypernuclei in the vicinity of the β stability line * c.rappold@gsi.de † t.saito@gsi.de [3][4][5][6]. The nature of the Λ-N interaction for neutron-rich hypernuclei, in which the ΛN -ΣN coupling three-body force may play a role as described theoretically in Refs. [7-11], has not yet been studied in detail since only a few cases were observed, 10 Li [12], 7 He [13], and 6 H [14]. We thus search for other neutron-rich hypernuclei by means of induced reactions of heavy-ion beams.Neutron-and proton-rich hypernuclei can be indeed studied by using projectile fragmentation reactions of heavy-ion beams. In such reactions, a projectile fragment can capture a hyperon produced in the hot participant region to produce a hypernucleus [15][16][17][18][19]. They might also be produced in a multistage process, such as through a Fermi breakup decay of excited heavier hypernuclear spectators, possibly formed in peripheral collisions [19][20][21].We, the HypHI Collaboration, have proposed a series of experiments at the GSI Helmholtz Centre for Heavy Ion Research that would use induced reactions of stable heavy-ion beams and rare-isotope beams to produce 041001-1 0556-2813/2013/88(4)/041001 (6)
A novel experiment, aiming at demonstrating the feasibility of hypernuclear spectroscopy with heavy-ion beams, was conducted. Using the invariant mass method, the spectroscopy of hypernuclear products of 6 Li projectiles on a carbon target at 2 A GeV was performed. Signals of the Λ-hyperon and 3 Λ H and 4 Λ H hypernuclei were observed for final states of p+π − , 3 He+π − and 4 He+π − , respectively, with significance values of 6.7, 4.7 and 4.9σ. By analyzing the proper decay time from secondary vertex distribution with the unbinned maximum likelihood fitting method, their lifetime values were deduced to be 262 +56 −43 ± 45 ps for Λ, 183 +42 −32 ± 37 ps for 3 Λ H, and 140 +48 −33 ± 35 ps for 4 Λ H.
At the Mainz Microtron MAMI, the first high-resolution pion spectroscopy from decays of strange systems was performed by electron scattering off a 9 Be target in order to study the Λ binding energy of light hypernuclei. Positively charged kaons were detected by a short-orbit spectrometer with a broad momentum acceptance at zero degree forward angles with respect to the beam, efficiently tagging the production of strangeness in the target nucleus. In coincidence, negatively charged decay-pions were detected by two independent high-resolution spectrometers. About 10 3 pionic weak decays of hyperfragments and hyperons were observed. The pion momentum distribution shows a monochromatic peak at p π ≈ 133 MeV/c, corresponding to the unique signature for the two-body decay of hyperhydrogen 4 Λ H → 4 He + π − , stopped inside the target. Its Λ binding energy was determined to be B Λ = 2.12 ± 0.01 (stat.) ± 0.09 (syst.) MeV with respect to the 3 H + Λ mass.
Hypernuclear production cross sections have been deduced for the first time with induced reaction of heavy ion beam on fixed target and by means of the invariant mass method by the HypHI Collaboration exploiting the reaction of 6 Li + 12 C at 2 A GeV or √ s N N = 2.70 GeV. A production cross section of 3.9 ± 1.4 μb for 3 H and of 3.1 ± 1.0 μb for 4 H respectively in the projectile rapidity region was inferred as well as the total production cross section of the hyperon was measured and found to be equal to 1.7 ± 0.8 mb. A global fit based on a Bayesian approach was performed in order to include and propagate statistical and systematic uncertainties. Production ratios of 3 H/ 4 H, 3 H/ and 4 H/ were included in the inference procedure. The strangeness population factors S 3 and S 4 of 3 H and 4 H respectively were extracted. In addition, the multiplicities of the hyperon, 3 H, and 4 H together with the rapidity and transversal momentum density distributions of the observed hypernuclei were extracted and reported.
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