Nonmesonic weak decay dynamics from proton spectra of Λ-hypernuclei

Abstract

A novel comparison between the data and the theory is proposed for the nonmesonic (NM) weak decay of hypernuclei. Instead of confronting the primary decay rates, as is usually done, we focus attention on the effective decay rates that are straightforwardly related with the number of emitted particles. Proton kinetic energy spectra of 5 Λ He, 7 Λ Li, 9 Λ Be, 11 Λ B, 12 Λ C, 13 Λ C, 15 Λ N and 16 Λ O, measured by FINUDA, are evaluated theoretically. The Independent Particle Shell Model (IPSM) is used as the nuclear structure framework, while the dynamics is described by the One-Meson-Exchange (OME) potential. Only for the 5 Λ He, 7 Λ Li, and 12 Λ C hypernuclei is it possible to make a comparison with the data, since for the rest there is no published experimental information on number of produced hypernuclei. Considering solely the one-nucleon-induced (1N -NM) decay channel, the theory reproduces correctly the shapes of all three spectra at medium and high energies (Ep > ∼ 40 MeV). Yet, it greatly overestimates their magnitudes, as well as the corresponding transition rates when the full OME (π + K + η + ρ + ω + K * ) model is used. The agreement is much improved when only the π + K mesons with soft dipole cutoff parameters participate in the decay process. We find that the IPSM is a fair first order approximation to disentangle the dynamics of the 1N -NM decay, the knowledge of which is indispensable to inquire about the baryon-baryon strangeness-flipping interaction. It is shown that the IPSM provides very useful insights regarding the determination the 2N -NM decay rate. In a new analysis of the FINUDA data, we derive two results for this quantity with one of them close to that obtained previously.

Nonmesonic weak decay of charmed hypernuclei

“…From previous works of Refs. [54,55,58,65,75,76], we know that to describe the NMWD in Λ hypernuclei with open-shell cores within the IPSM it is enough to make the replacementĴ 2 /ĵ 2 Λ → F j N J in the decay-rate expression with the DCSC, where F j N J is a spectroscopic factor. Making the same replacements here, i.e.Ĵ 2 /ĵ 2…”

“…[53]. Recently, it was suggested that the π + K meson-exchange model with soft monopole form factors could be a good starting point to describe this type of interaction in light and medium systems [54,55]. The lifetime of the free charmed baryon Λ + c is τ Λ + c = (2.00 ± 0.06) × 10 −13 s, which corresponds to the decay rate Γ(Λ + c → all) = (3.29 ± 0.10) × 10 −9 MeV.…”

Relativistic model for the nonmesonic weak decay of single-lambda hypernuclei

“…In what follows we will make use of analogies with nonrelativistic calculations. From previous works [21][22][23][24][25][26][27] done by our group, we know that to describe the hypernuclei with open-shell cores within the IPSM it is enough to do the following replacement in Eq. ( 48)…”

“…The NMWD dynamics is frequently handled by onemeson-exchange (OME) models. Such models are motivated by the fact that the N N interaction at long distance is due to the one-pion-exchange, but with the difference that in NMWD the exchange processes occur with one strong and one weak vertex and can include other mesons in addition to the π, like the pseudoscalar (K, η) and vector (ρ, ω, K * ) mesons [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The coupling constants at the strong vertices can be taken from different OME models for the N N interaction, while those at the weak vertices can be extracted from free Λ decay data and making use of soft meson theorems and SU (6) W symmetry [11,12].…”

“…One consequence of such large energy releases is that nonrelativistic approaches, like those of Refs. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], become inapplicable for the evaluation of NMWD transition matrix elements in charmed hypernuclei. In addition, a large energy release also implies that nuclear recoil cannot be neglected in the calculation of decay rates, particularly for light-weight nuclei.…”