Nonmesonic weak decay of light hypernuclei with coherent Σ mixing

Abstract

Nonmesonic weak decays of the A = 4, and 5 hypernuclei are studied. The short range parts of the hyperon-nucleon weak interactions are described by the direct quark (DQ) weak transition potential, while the longer range interactions are given by the π and K meson exchange processes. Virtual Σ mixings of the coherent type are found to give significant effects on the decay rates of 4 Λ He. A large violation of the ∆I = 1/2 rule is predicted in the J = 0 transition amplitudes. many-body dynamics and nonperturbative QCD effects on hadronic weak interaction are important.Nonmesonic weak decay of hypernucleus is unique as a new type of weak interaction process, which involves only baryons. In weak decays of heavy hypernuclei, pion emissions via Λ → pπ − , nπ 0 are hindered due to the Pauli blocking on the final state nucleon. Instead, the decay is mainly induced by a two-body transition, ΛN → NN, and does not emit pions. This nonmesonic weak baryon-baryon interaction is analogous to the parity violating part of the nuclear force |∆S| = 1 sector.The mechanism of nonmesonic decay of hypernuclei is a long standing problem due to some disagreements between theory and experiment. One of them is the Γ nn /Γ pn ratio, i.e., the ratio of Γ nn = Γ(Λn → nn) and Γ pn = Γ(Λp → pn) transitions. Another, to the validity of ∆I = 1/2 rule. Many studies have been done. The one-pion-exchange (OPE) is the simplest to describe the ΛN → NN transition. It has a similar structure as the OPE in nuclear force and in both of them the tensor transition is strong. As it enhances Γ pn , the Γ nn /Γ pn ratio is suppressed. Typical prediction for OPE is Γ nn /Γ pn ∼ 0.1, while most experimental data indicate Γ nn /Γ pn ∼ 1 or larger [1].In the nonmesonic decay, the Λ-N mass difference is reflected in a high momentum transfer between the baryons, and therefore, short range interaction effects must be important. The lifetime measurements of heavy hypernuclei show saturation at large A and therefore suggest importance of short range interactions [2].An attempt was made in refs. [3,4,5] by considering the effect of the polarization of the pion propagator, which seems to enhance Γ nn /Γ pn slightly. The ρ meson contribution was calculated in refs. [6,7], but it could not reproduce the experimental data. The two-pion exchange and the effective σ meson exchanges [8,9,10] may play an important role because it increases the central attraction which enhances the Γ nn /Γ pn ratio through the enhancement of the J = 0 channels. The pseudoscalar and vector mesons (π, K, η, ρ, ω, and K * ) are considered in [11,12]. The contribution of each meson comes out large, especially for kaon exchange, and significant cancellations between the different contributions are found [13]. The final result is 4-6 times larger than the OPE one [14].Apart from the meson exchange model, several authors pointed out the importance of quark degrees of freedom in baryons [15,16,17,18,19]. Recently we proposed to treat the short range part using the valence quark picture of the baryon and...

Nonmesonic weak decay spectra of HeΛ4

“…This might indicate that, neither the FSI, nor the two-nucleon induced decay processes play a significant role in the s-shell, at least not for 4 Λ He. As a byproduct we have found that the π + K exchange potential with soft cutoffs (SPKE) is capable of accounting for the experimental values related to Γ p and Γ n in all three [30].…”

“…The most relevant issue here is, however, that there is a region of rather soft Λ π and Λ K where all the Γ N are reproduced fairly well. 2 In Table I are shown the results for Λ π = 0.7 GeV and Λ K = 0.9 GeV, labelled as P3, which we call the soft π + K exchange (SPKE) potential, and which will be used in the evaluation of the NMWD spectra We are aware that the OME models predict a too large and negative asymmetry parameter a Λ in 5 Λ He [13,21,22,23,30,32], and also that there are two recent proposals to bring this value into agreement with experiments by going beyond the OME model and incorporating [27]; T2 -(π +4BP I) [28]; T3 -(PKE) [30]; T3' -(π +K +DQ) [30]; T4 -(π + 2π/σ + 2π/ρ + ω) [14]; T5 -(PSVE) [16]; T6 -(π + K + σ + DQ) [31], T7 -(PSVE) [33]; T7' -(PSVE +2π + 2π/σ) [33]. C) Present Results: P1 -(PSVE); P2 -(PKE); P3 -(SPKE).…”

Nonmesonic Weak Decay Spectra of Light Hypernuclei

“…In the evaluation of Γ N M , and the corresponding spectra, are used the Shell Model (SM) and the Fermi Gas Model (FGM) for Γ 1 (within the two-particle phase space) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][20][21][22][23], while for Γ 2 (within the three-particle phase space) only the later model has been employed so far [20][21][22][23].…”

The ΛΛ→YN weak transition in double-Λ hypernuclei

“…where Γ pp stands for the Σ + p → pp transition which is available when the virtual Σ exists (7). The Γ Y N represents the ΛΛ → Y N transition, which is a characteristic transition for the multi-Λ system.…”