σ\sigma meson exchange effect on nonmesonic hypernuclear weak decay observables

We analyze the influence of $\sigma$ meson exchange on the main nonmesonic hypernuclear weak decay observables: the total rate, $\Gamma_{NM}$, the neutron-to-proton branching ratio, $\Gamma_{n/p}$, and the proton asymmetry parameter, $a_\Lambda$. The $\sigma$ meson exchange is added to the standard strangeness-changing weak $\Lambda N\to NN$ transition potential, which includes the exchange of the complete pseudoscalar and vector mesons octet ($\pi$, $\eta$, $K$, $\rho$, $\omega$, $K^*$). Using a shell model formalism, the $\sigma$ meson weak coupling constants are adjusted to reproduce the recent $\Gamma_{NM}$ and $\Gamma_{n/p}$ experimental data for $^5_{\Lambda}He$. Numerical results for the remaining observables of $^5_{\Lambda}He$ and all the observables of $^{12}_{\Lambda}C$ decays are presented. They clearly show that the addition of the $\sigma$ meson, in spite of improving some observables values, is not enough to reproduce simultaneously all the measurements, and the puzzle posed by the experimental data remains unexplained.Comment: 4 pages, 1 figure. Submitted for publication in Phys. Re

Interpopulational variation in costs of reproduction related to pregnancy in a viviparous lizard

Interpopulational variation in reproductive costs may affect variation in life history traits including reproductive investment (i.e. clutch mass relative to either maternal body mass or length). While the relationships between reproductive investment and costs of reproduction, especially costs to mobility, have been well studied in squamate reptiles, how these costs relate to investment and explain patterns within and between populations is not always straightforward. In the present study, we examined the relationship between reproductive investment and costs of reproduction (gravid and postpartum sprint speeds and maternal postpartum body condition) in two populations of a viviparous skink, Pseudemoia entrecasteauxii living in different habitat types. We found that costs of reproduction (i.e. impact on gravid and postpartum sprint speeds) depended on the interaction between relative reproductive burden (RRB) and population. There was no link between relative clutch mass (RCM) and maternal sprint speeds. Maternal postpartum body condition was not related to either RRB or RCM for either population. Gravid females living in the open habitat population showed significantly slower sprint speed compared with the same females immediately postparturition, and other gravid females living in a closed habitat population. Such females are likely to experience a higher cost of reproduction in terms of changes in sprint speed as well as exposure to predators and may show a behavioural shift to crypsis in order to compensate for locomotor impairment and to reduce the risk of predation. We suggest that factors which relate to costs of reproduction (i.e. sprint speeds) are complex and may involve multiple factors such as reproductive investment and habitat characteristics

Pionic Decay of a Possible d'-Dibaryon

The pionic decay of a possible d'-dibaryon in the process d' --> pi + N + N is studied in the microscopic quark shell model and with a single-quark transition operator describing the transition q --> pi + q'. For the d' with quantum numbers J^P=0^-, T=0, we employ a six-quark shell-model wave function with a spatial s^5p [51]_X-configuration with N=1 harmonic oscillator quanta. It is shown that the pionic decay width depends strongly on the mass and size of the d'. In the case that the calculated d' mass is close to the experimental one a small pionic decay width of 0.04 MeV is obtained. This is an order of magnitude smaller than the experimentally suggested value of 0.5 MeV. Two possibilities to improve the calculated width are suggested. The effect of the nonstatic correction term in the transition operator and the influence of the form factor at the decay vertex on the decay width are also discussed.Comment: Latex, 15 pages, 1 postscript figure, accepted for publication in Nucl. Phys.

Sigma Exchange in the Nonmesonic Decays of Light Hypernuclei and Violation of the Delta I=1/2 Rule

Nonmesonic weak decays of s-shell hypernuclei are analyzed in microscopic models for the Lambda N to NN weak interaction. A scalar-isoscalar meson, sigma, is introduced and its importance in accounting the decay rates, n/p ratios and proton asymmetry is demonstrated. Possible violation of the Delta I=1/2 rule in the nonmesonic weak decay of Lambda is discussed in a phenomenological analysis and several useful constraints are presented. The microscopic calculation shows that the current experimental data indicate a large violation of the Delta I=1/2 rule, although no definite conclusion can be derived due to large ambiguity of the decay rate of {^4_Lambda H}.Comment: 13 pages, 5 figure

The ΛΛ→YN\Lambda \Lambda \to YN Weak Transition in Double-Λ\Lambda Hypernuclei

We calculate the $\Lambda \Lambda \to YN$ transition rate of ${^{\phantom{\Lambda}6}_{\Lambda \Lambda}}$He by the hybrid picture, the $\pi$ and $K$ exchanges plus the direct quark processes. It is found that the hyperon-induced decay is weaker than the nucleon-induced decay, but the former may reveal the short-range mechanism of the weak transition and also give a clear signal of the strong $\Delta I=3/2$ transition. The $\Lambda \Lambda \to Y N$ transition in double-$\Lambda$ hypernucleus is complement to the $\Lambda N \to NN$ transition as it occurs only in the J=0 channel, while the J=1 transition is dominant in the $\Lambda N \to NN$ case.Comment: 9 page

Neutron and proton spectra from the decay of Λ\Lambda hypernuclei

We have determined the spectra of neutrons and protons following the decay of $\Lambda$ hypernuclei through the one- and two-nucleon induced mechanisms. The momentum distributions of the primary nucleons are calculated and a Monte Carlo simulation is used to account for final state interactions. From the spectra we calculate the number of neutrons ($N_n$) and protons ($N_p$) per $\Lambda$ decay and show how the measurement of these quantities, particularly $N_p$, can lead to a determination of $\Gamma_n / \Gamma_p$, the ratio of neutron to proton induced $\Lambda$ decay. We also show that the consideration of the two-nucleon induced channel has a repercussion in the results, widening the band of allowed values of $\Gamma_n / \Gamma_p$ with respect to what is obtained neglecting this channel.Comment: 30 pages, 12 Postscript figures, uuencoded file, ReVTeX, epsf.st

Asymmetries in the Non-Mesonic Weak Decay of Polarized Lambda-Hypernuclei

The non-mesonic weak decay of polarized Lambda-hypernuclei is studied for the first time by taking into account, with a Monte Carlo intranuclear cascade code, the nucleon final state interactions. A one-meson-exchange model is employed to describe the Lambda N-> n N processes in a finite nucleus framework. The relationship between the intrinsic Lambda asymmetry parameter a_\Lambda and the asymmetry a^M_\Lambda accessible in experiments is discussed. A strong dependence of a^M_\Lambda on nucleon final state interactions and detection threshold is obtained. Our results for a^M_\Lambda are consistent with ^{11}_\Lambda B and ^{12}_\Lambda C data but disagree with observations in ^5_\Lambda He.Comment: 4 RevTeX pages, 2 figure