225 research outputs found

### On the role of ground state correlations in hypernuclear non-mesonic weak decay

The contribution of ground state correlations (GSC) to the non--mesonic weak
decay of $^{12}_\Lambda$C and other medium to heavy hypernuclei is studied
within a nuclear matter formalism implemented in a local density approximation.
We adopt a weak transition potential including the exchange of the complete
octets of pseudoscalar and vector mesons as well as a residual strong
interaction modeled on the Bonn potential. Leading GSC contributions, at first
order in the residual strong interaction, are introduced on the same footing
for all isospin channels of one-- and two--nucleon induced decays. Together
with fermion antisymmetrization, GSC turn out to be important for an accurate
determination of the decay widths. Besides opening the two--nucleon stimulated
decay channels, for $^{12}_\Lambda$C GSC are responsible for 14% of the rate
$\Gamma_1$ while increasing the $\Gamma_{n}/\Gamma_{p}$ ratio by 4%. Our final
results for $^{12}_\Lambda$C are: $\Gamma_{\rm NM}=0.98$,
$\Gamma_{n}/\Gamma_{p}=0.34$ and $\Gamma_2/\Gamma_{\rm NM}=0.26$. The
saturation property of $\Gamma_{\rm NM}$ with increasing hypernuclear mass
number is clearly observed. The agreement with data of our predictions for
$\Gamma_{\rm NM}$, $\Gamma_n/\Gamma_p$ and $\Gamma_2$ is rather good.Comment: 32 pages, 9 figure

### Relationships between nonmesonic-weak-decays in different hypernuclei

Using as a tool the s-wave approximation (sWA), this work demonstrates that
the nonmesonic weak decay transition rates $\Gamma_{n}$ and $\Gamma_{p}$ can be
expressed in all hypernuclei up to $^{29}_\Lambda$Si (and very likely in
heavier ones too) in the same way as in the s-shell hypernuclei, i.e. as a
linear combination of only three elementary transition rates. This finding
leads to the analytic prediction that, independently of the transition
mechanism, all hypernuclei that are on the stability line (N = Z), i.e.
$^5_\Lambda$He, $^7_\Lambda$Li, $^9_\Lambda$Be, $^{11}_\Lambda$B,
$^{13}_\Lambda$C, $^{17}_\Lambda$O, $^{29}_\Lambda$Si, etc should roughly
have the same ratio $\Gamma_{n}/\Gamma_{p}$, the magnitude of which rapidly
increases when one approaches the neutron drip-line (N >> Z), and opposite
happens when one goes toward the proton drip-line (N << Z).Comment: 7 pages, 1 figur

### A systematic study on the binding energy of $\Lambda$ hypernuclei

In this paper, we calculated the binding energy per baryon of the $\Lambda$
hypernuclei systemically, using the relativistic mean field theory (RMF) in a
statistic frame. Some resemble properties are found among most of the
hypernuclei found in experiments. The data show that a $\Lambda$ hypernucleus
will be more stable, if it is composed of a $\Lambda$ hyperon adding to a
stable normal nuclear core, or a $\Lambda$ hyperon replacing a neutron in a
stable normal nuclear core. According to our calculations, existences of some
new $\Lambda$ hypernuclei are predicted under the frame of RMF.Comment: 8 pages, 6 figures, 3 table

### 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

### Inclusive pion double charge exchange on O-16 above the delta resonance

The forward inclusive pion double charge exchange reaction,
^{16}O(pi^-,pi^+)X, at T_0 = 0.50 and 0.75 GeV has been studied in the
kinematical region where an additional pion production is forbidden by
energy-momentum conservation. The experiment was performed with the SKS
spectrometer at KEK PS. The measured ratio of double charge exchange
cross-section for these energies dsigma(0.50 GeV)/dOmega / dsigma(0.75
GeV)/dOmega = 1.7 +/- 0.2, disagrees with the value of 7.2 predicted within the
conventional sequential single charge exchange mechanism. Possible reasons for
the disagreement are discussed in connection with the Glauber inelastic
rescatterings.Comment: 4 pages, 5 figure

### $\Lambda N$ correlations from the stopped $K^-$ reaction on ${}^4$He

We have investigated correlations of coincident $\Lambda N$ pairs from the
stopped $K^-$ reaction on ${}^4$He, and clearly observed $\Lambda p$ and
$\Lambda n$ branches of the two-nucleon absorption process in the $\Lambda N$
invariant mass spectra. In addition, non-mesonic reaction channels, which
indicate possible exotic signals for the formation of strange multibaryon
states, have been identified.Comment: 5 pages, 3 figures, submitted to Physical Review Letter

### Exclusive Measurement of the Nonmesonic Weak Decay of ^{5}_{\Lambda}He Hypernucleus

We performed a coincidence measurement of two nucleons emitted from the
nonmesonic weak decay (NMWD) of ^{5}_{\Lambda}He formed via the
^{6}Li(\pi^+,K^+) reaction. The energies of two nucleons and the pair number
distributions in the opening angle between them were measured. In both np and
nn pairs, we observed a clean back-to-back correlation coming from the two-body
decay of \Lambda p --> n p and \Lambda n --> n n, respectively. The ratio of
the nucleon pair numbers was N_{nn}/N_{np}=0.45 \pm 0.11(stat)\pm 0.03(syst) in
the kinematic region of cos(theta_{NN}) < -0.8. Since each decay mode was
exclusively detected, the measured ratio should be close to the ratio of
\Gamma(\Lambda p --> np)/\Gamma(\Lambda n --> nn).
The ratio is consistent with recent theoretical calculations based on the
heavy meson/direct quark exchange picture.Comment: Submitted to Phys. Rev. lett., 4 pages, 3 figure

### Proton asymmetry in non-mesonic weak decay of light hypernuclei

We have obtained the decay asymmetry parameters in non-mesonic weak decay of
polarized Lambda-hypernuclei by measuring the proton asymmetry. The polarized
Lambda-hypernuclei, 5_Lambda-He, 12_Lambda-C, and 11_Lambda-B, were produced in
high statistics via the (pi^+,k^+) reaction at 1.05 GeV/c in the forward
angles. Preliminary analysis shows that the decay asymmetry parameters are very
small for these s-shell and p-shell hypernuclei.Comment: 4pages, 4figures, International Conference on Hypernuclear and
Strange Particle Physics (HYP2003

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