The Li(K, π)ΣH reaction and narrow Σ states

Small-angle cross sections are calculated for ⁶Li(K⁻, π⁺)_Σ⁶H at 720 MeV/c, in order to intepret the data of the preceding paper. Due to the spin-isospin selectivity of ΣN → ΛN conversion, the substitution 1s_N → 1s_Σ leads to a narrow resonance high in the Σ continuum. Further tests of selectivity with ^3,4He targets are suggested. The energy shifts of Σ states which arise from the spin-isospin dependence of the ΣN interaction are discussed.     

High-resolution gamma-ray spectroscopy of ΛLi

We report on the first observation of hypernuclear γ transitions using germanium detectors. Using a large-acceptance Ge detector system, we observed two γ transitions in _Λ⁷Li, the spin-flip transition at 689±4keV and the transition at 2050±2 keV (preliminary). The strength of the ΛN spin-spin interaction is derived from the energy of the former transition. As for the latter transition, B(E2) was measured to be 3.9±0.6±0.4 e²fm⁴ (preliminary), which indicates a shrinkage of the nuclear size of _Λ⁷Li from ⁶Li and confirms “glue-like role” of Λ.     

Mesonic and non-mesonic decay widths of ΛC

Results are reported on the precise measurement of the lifetime and the weak-decay branching ratios with charged particle(π⁻ and proton) emission from _Λ¹²C formed via ¹²C(π⁺,K⁺) reaction. The π⁻ mesonic decay width is consistent with the calculations including the pion wave function distortion effect. We observed large contribution of the neutron-stimulated decay widths, Γ(Λ + n → n + n), in the non-mesonic weak decay, which cannot be explained by the current theoretical calculations.     

Four-body calculation of ΛH and ΛHe with realistic YN and NN interactions

We carried out four-body calculations of _Λ⁴H and _Λ⁴He taking both the 3N + Λ and 3N + Σ channels explicity with the use of realistic NN and YN interactions. The Σ-channel component plays an important role in binding energies of the A = 4 hypernuclei though the admixture is approximately 1%. The ΛN - ΣN coupling is found to be of central-force type in the Nijmegen model D and of tensor-force type in the model F.     

Escape of charged particles at stopping annihilation of antiprotons in the heavy nuclei of nuclear photoemulsion

“Rare” annihilation channels for antiprotons stopping on heavy (Ag, Br) nuclei of photoemulsion, have been sought; 4872 stops of antiprotons on photoemulsion nuclei are analysed. Events of formation and decay of the hyperfragment _Λ⁴H, escape of ⁸He and ⁸Li nuclei, one-prong stars with the mean range 79.5±5.1 μm of secondary slow “b” particles are found among the annihilation stars at capture on nuclei (Ag, Br). The lower limits for the production probability of _Λ⁴H and ⁸He, ⁸Li nuclei per antiproton stopping in the nuclei (Ag, Br) are

W_Λ⁴H2×10⁻⁴ and W_⁸He,⁸Li=(1.3±0.6)×10⁻³.

The branching ratio for the production of one-prong stars with the secondary “b” particles is at least (1.3±0.6)×10⁻³. Possible mechanisms for a production of these events in annihilation processes are considered.     

Study of impulse model in reaction Kd → pΛπ+ neutrals

In an analysis of 1114 events of the type K⁻d → pΛ⁰π⁻+ neutrals with ‘spectator’ proton momentum in excess of 0.11 GeV/c we have found that the number of protons having a momentum in the range 0.25–1.2 GeV/c exceeds that predicted on the basis of the impulse model using the Hulthén distribution by 3 : 1. Final-state interactions in which the proton does not form resonances with the other final-state particles are mainly held responsible for this effect.     

Experimental study of the time-dependent rate of K → πππ

The time-dependence of the decay rate of initially pure K⁰ into the final state (π⁺π⁻π⁰) has been studied in search for the decay k_S⁰→π⁺π⁻π⁰. No evidence is found in a sample of 384 observed events. The ratio of the CP -violating K_S⁰ amplitude and the K_L⁰ amplitude is η₊₋₀ = (0.13_−0.20^+0.17) + i(0.17_−0.26^+0.27); the ratio of the CP-conserving K_S⁰ amplitude and the K_L⁰ amplitude is < 0.4. The energy dependence of the K⁰→π⁺π⁻π⁰ matrix element is found to be a₊₋₀ = −0.31 ± 0.03.     

Σ hypernuclear spectra from (K, π) inclusive reactions

We report on DWIA calculations of the pion inclusive spectra related to Σ-formation in (K⁻, π⁺) reactions on nuclei. Realistic distorted waves are used to describe the incoming kaon and outgoing pion. The Σ wave function is calculated in a real Woods-Saxon potential, the depth of which provides information about the underlying Σ N effective interaction. The absorptive effect due to the Σ-Λ conversion process in the nuclear medium is taken into account by effective two-channel coupled equations. Comparisons are made with the available data on ¹²C and ¹⁶O. Using a weak Σ-nucleus potential the overall agreement is satisfactory for the spectrum derived from kaon-in-flight experiments. Concerning the three peaks reported in a stopped kaon experiment on ¹²C, the lowest peak structure can be generated by increasing the depth of the Σ potential in ¹²C. However, the remaining two narrow structures cannot be reproduced as Σ-particle-proton-hole states in our continuum treatment of the Σ spectrum. The difficulties in extracting the strength of the Σ-nucleus spin-orbit potential are also discussed.     

Search for a light neutral particle in the decay of the 3.68 MeV state in C

A search is made for a short lived neutral particle, ø, in the decay of the 3.68 MeV (3/2^-) state in ¹³C. No evidence for such a particle with a rest mass in the region of 1.7–2.0 MeV/c² is found with a limit on the branching ratio Γ _ø/Γ_γ7×10 ⁻⁵. An upper limit of 10⁻⁶ is placed for the coupling of such a particle to proton/neutron.     

Structure of a Λ-hypernucleus ΛY measured with the SKS spectrometer

We have measured an excitation energy spectrum of a Λ-hypernucleus _Λ⁸⁹Y via the (π⁺, K⁺) reaction by using the SKS spectrometer at KEK. The best quality data with the energy resolution of 1.63 MeV(FWHM) was obtained in the highest statistics so far. In the spectrum, we can find a splitting of the f-orbital of a Λ particle with the separation of 1.6±0.15 MeV(statistical error only), which may be attributed to the spin-orbit splitting.     

Determination of the K(1800) spin parity

A spherical harmonic moment analysis of the reactions K⁻p → K⁻π⁺n and K⁺p → K⁺π⁻Δ⁺⁺ at 13 GeV/c demonstrates the existence of a broad K^* state with mass in the vicinity of 1800 MeV and spin parity 3⁻.     

Simultaneous measurements of the p+d→(A=3)+π reactions

A stack of annular detectors made of high-purity germanium was used to measure simultaneously pd → ³H π⁺ and pd → ³He π⁰ differential cross sections at beam momenta of 750 MeV/c, 800 MeV/c, and 850 MeV/c over a large angular range. The extracted total cross sections for the pd → ³He π⁰ reactions bridge a gap between near threshold data and those in the resonance region. The ratio of the cross sections for the two reaction channels taken at the same η=p^cm_π/m_π yields 2.11±0.08 indicating that a deviation from isospin symmetry is very small.     

Hypernuclei with sigma particles

The production of Σ particles in the (K⁻, π⁻) on ⁹Be at 720 MeV/c has been studied at the CERN proton synchrotron (PS). The missing-mass spectrum shows structures in the Σ region, which have been interpreted as belonging to hypernuclei with sigma particles. An upper limit of 8 MeV for the width of these states has been estimated.     

The contribution of 2N photon absorption in C(γ,2N) reactions for Eγ = 150–400 MeV

The ¹²C(γ,pn) and ¹²C(γ,pp) reactions have been studied using tagged photons of energy E_γ = 150–400 MeV. Recoil momentum distributions are compared to the results of Monte Carlo calculations based on a two-nucleon photon absorption model and two different phase space models. The ¹²C(γ,pn) data at low missing energy are consistent with absorption on 1p² and 1s1p nucleon pairs.     

Electro-pion production on O to bou nd states of the residual nuclei

Differential cross sections for the reactions 16O(γ, π−π+−)16F16F to the sum of the four lowest lying states in 16F and 16N have been measured as a function of angle for pions with a kinetic energy of 30 MeV. The extracted ratios R = σ(γ, π−)/σ(γ, π+), the first ones to discrete final states as a function of angle, are in fair agreement with results obtained for the nucleon. For positive pions the energy dependence of the cross section has been measured at the angles of 45° and 90°. Distorted wave impulse approximation calculations fail to describe the energy dependence.     

Structure determination, magnetic and optical properties of a new chromium(II) thioantimonate, [Cr((NH2CH2CH2)3N)]Sb4S7

The chromium(II) antimony(III) sulphide, [Cr((NH₂CH₂CH₂)₃N)]Sb₄S₇, was synthesised under solvothermal conditions from the reaction of Sb₂S₃, Cr and S dissolved in tris(2-aminoethyl)amine (tren) at 438 K. The products were characterised by single-crystal X-ray diffraction, elemental analysis, SQUID magnetometry and diffuse reflectance spectroscopy. The compound crystallises in the monoclinic space group P2₁/n with a=7.9756(7), b=10.5191(9), c=25.880(2) Å and β=90.864(5)°. Alternating SbS₃³⁻ trigonal pyramids and Sb₃S₆³⁻ semi-cubes generate Sb₄S₇²⁻ chains which are directly bonded to Cr(tren)²⁺ pendant units. The effective magnetic moment of 4.94(6)μ_B shows a negligible orbital contribution, in agreement with expectations for Cr(II):d⁴ in a ⁵A ground state. The measured band gap of 2.14(3) eV is consistent with a correlation between optical band gap and framework density that is established from analysis of a wide range of antimony sulphides.     

Spectroscopy of light Λ hypernuclei

We recently measured high-statistics and high-resolution spectra of p-shell Λ hypernuclei produced by the (π⁺, K⁺) reaction. In this article, cross sections and their angular distributions of _Λ¹²C and _Λ¹⁶O are reported. The obtained cross sections are well reproduced by DWIA calculations.     

Self energies of the pion and the Δ isobar from the He(e,e′π)H reaction

In a kinematically complete experiment at the Mainz microtron MAMI, pion angular distributions of the ³He(e,e′π⁺)³H reaction have been measured in the excitation region of the Δ resonance to determine the longitudinal (L), transverse (T), and the LT interference part of the differential cross section. The data are described only after introducing self-energy modifications of the pion and Δ-isobar propagators. Using Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as inferred from the measurement on the mass shell, we deduce a reduction of the π⁺ mass of MeV/c² in the neutron-rich nuclear medium at a density of fm⁻³. Our data are consistent with the Δ self energy determined from measurements of π⁰ photoproduction from ⁴He and heavier nuclei.     

The contribution of Bc mesons to the search for B → τ ντ decays at LEP

We study the contribution of B_c mesons to the search for B → τν_τ decays. We find that at LEP the contributions from B_u and B_c mesons can be comparable. This observation can have a relevant impact on the extraction of constraints on new physics (such as charged-Higgs contributions) from current LEP limits on B → τν final states. Inclusion of the B_c contribution can reduce the current L3 limit on tan β/M_H from 0.38 GeV⁻¹ (90% CL) down to 0.27 GeV⁻¹ (90% CL).     

Search for narrow NNπ resonances in exclusive pp→ppππ measurements

Narrow structures in the range of a few MeV have been searched for in ppπ⁺ and ppπ⁻ invariant mass spectra (M_ppπ⁺ and M_ppπ⁻) obtained from exclusive measurements of the pp→ppπ⁺π⁻ reaction at T_p=725,750 and 775 MeV using the PROMICE/WASA detector at CELSIUS. The selected reaction is particularly well suited for the search for dibaryon resonances decoupled from NN and/or NΔ. In the mass range 2020 MeV/c²<m_dibaryon<2085 MeV/c² no narrow structures could be identified on the 3σ level of statistical significance neither in M_ppπ⁻ nor in M_ppπ⁺ giving an upper limit (95% C.L.) for dibaryon production in this reaction of σ<20 nb.