Lifetime Measurement ofΛ12C,Λ28

Abstract

We have measured lifetimes of L hypernuclei over a broad mass range explicitly identifying L hypernuclear production in the ͑p 1 , K 1 ͒ reaction. The obtained results are t͑ 12 L C͒ 231 6 15 ps, t͑ 28 L Si͒ 206 6 12 ps, and t͑ L Fe͒ 215 6 14 ps. The lifetimes of L hypernuclei over the mass region from carbon to iron are found almost constant at about 80% of that of free L within the statistical uncertainties. The short-range nature of the nonmesonic weak decay process, which is dominant in heavy L hypernuclei, is possibly responsible for the observed weak hypernuclear mass dependence.[S0031-9007(98)07604-2] PACS numbers: 21.80. + a, 13.30.Eg, 13.75.Ev, 21.10.Tg A L hyperon bound in a nucleus eventually decays from the ground state either through a p-mesonic or a nonmesonic weak decay (NMWD) process. Through a p-mesonic process, a L in a hypernucleus decays to a nucleon and a pion, i.e., L ! Np, as a free L does. However, the p-mesonic decay is strongly suppressed in hypernuclei, except very light ones, due to the small energy release in the process and the Pauli blocking. Instead, the NMWD process, which is the strangeness changing baryon-baryon weak interaction process ͑L 1 N ! N 1 N͒, becomes dominant in most hypernuclei due to the much larger momentum of the final nucleon, ϳ400 MeV͞c, than the typical Fermi momentum, ϳ270 MeV͞c.The total decay width of a L hypernucleus, G, is the sum of the partial decay widths; 1 n, respectively, and G 2p2h is that of two-nucleon-induced NMWD ͑LNN ! NNN͒. The branching ratio for the NMWD process already reaches to about 3͞4 by 12 L C [1,2]. Therefore, the decay of heavy L hypernuclei provides us with valuable and clean information on the NMWD process, henceforth the baryon-baryon weak interaction and the short-range correlations in nuclear matter.However, the understanding of the NMWD process is still in a primitive stage. Although there have been several microscopic model calculations for NMWD processes, which are based on either various meson exchange interactions [3][4][5] or quark-hadron hybrid interactions [6,7], none of them have successfully reproduced both the total ͑G͒ and partial decay widths (G n and G p ) simultaneously and particularly their ratio ͑G n ͞G p ͒.So far the experimental data for the partial decay widths had large errors and provided only limited constraints. Among the weak decay observables, the lifetime can be determined most accurately free from nuclear final state interactions and material effects. Therefore its accurate data for heavy L hypernuclei are very much awaited to understand the NMWD mechanism.The most reliable measurements of hypernuclear lifetimes were carried out for relatively light hypernuclei from 4 L H [8] to 12 L C [9] by direct measurements of the production and decay times of the L hypernuclei. There have been a few other lifetime measurements for heavier L hypernuclei [10][11][12][13]. However, in these measurements the produced hypernuclei were not explicitly identified. The reliable measurement for heavier hypernuclear l...

Meson exchange in the weak decay of Λ hypernuclei and the Γn/Γp ratio

“…Λ B 1.37 ± 0.17 [6,47] (K − , π − ) 1.25 ± 0.08 [48] (π + , K + ) 12 Λ C 1.25 ± 0.19 [6,47] (K − , π − ) 1.14 ± 0.08 [49] (π + , K + ) …”

Hypernuclear weak decay puzzle

“…The data are taken from Refs. [39][40][41][42][43], and large experimental errors are due to the low efficiencies and large backgrounds in neutron detection. The calculations were performed for both b = 1.51 fm and b = 1.75 fm, being the later given parenthetically.…”

“…To find out in which way should these parameters be varied we perform 2 The FSI also make hard the extraction of the n/p ratio from the experimental data [39][40][41][42][43], and to surmount this difficulty Hashimoto et al [43] have quite recently combined the Monte Carlo FSI internuclear cascade models from Ref. [14] with the geometry of the detectors.…”

Weak decay of Λ in nuclei: quarks versus mesons

“…Thus the weak decay of Λ hypernucleus will be one of the clue of research for the ΛN → NN reaction. Many theoretical and experimental efforts have been devoted to the nonmesonic decays of light and heavy hypernuclei [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The mechanism of nonmesonic weak decay is still not clear, especially the theoretical prediction of the n/p ratio, which is the ratio of the neutron-induced decay to the proton-induced one, is not compatible with the experimental one.…”